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Operator protection in rollover events of articulated narrow track tractors
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/issn/15375110
Research Paper
Operator protection in rollover events of articulated narrow track tractors Andrew L. Guzzomi a, Valda Rondelli b,*, Enrico Capacci b a Department of Mechanical Engineering, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia b Department of Agricultural and Food Sciences, University of Bologna, Viale Fanin 50 Bologna, BO 40127, Italy
article info
Articulated steering arrangements, wheeled or tracked, are commonly used on agricultural
Article history:
tractors and earth moving machines. In Continental Europe and Italy in particular, this
Published online xxx
configuration is typical of narrow track tractors. Despite their widespread use, there is little research that specifically investigates articulated tractor rollover protective structure
Keywords:
(ROPS) in-field performance with respect to the ROPS test procedures. The aim was to
virtual analysis
investigate the relevance of the standardised ROPS test procedures regarding articulation
testing procedure
joint mobility. The shortcomings of the procedures concerning the Clearance Zone and
ROPS
ROPS testing are evidenced and discussed. It is a recommendation of this work that an
clearance zone
update of the international testing procedure be developed for articulated tractor types.
CAD
The paper indicates how this may be accomplished and presents remedial measures that can be implemented in the interim until this latter goal is achieved. © 2019 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Tractors and machines in agriculture and earth-moving sector are a large family of machine types that differ in size, mass and shape. Diversity in the vehicles available on the market is necessary to cope with the wide range of applications required. Within this family of machines a notable type of tractor is the articulated variant, since such designs cover a tenfold range of mass from small articulated agricultural tractors and mowers with mass less than 1000 kg to massive articulated frame dumpers and large tractors with mass of more than 10,000 kg. Mechanisation of intense farming environments such as vineyards and orchards led to the development of specialised
compact tractors with reduced track widths to aid manoeuvrability in such restrictive working environments. In many parts of Europe, intensive farming is often coupled with farming on fertile undulating soil and consequently presents a heightened risk of tractor rollover. As the number of narrow tractors has increased, researchers in Europe have reflected on the rollover research that was conducted approximately 40 years ago (Chisholm, 1979aed; Schwanghart, 1973, 1982) and realised that the narrow-track wheeled agricultural tractors should be considered a separate category from standard wheeled tractors (Guzzomi & Rondelli, 2013). The corresponding rollover protective structure (ROPS) testing procedures of the Organisation for the Economic Development and Co-operation (OECD) were thus devised: Code 6 for front
* Corresponding author. E-mail address: [email protected] (V. Rondelli). https://doi.org/10.1016/j.biosystemseng.2019.04.020 1537-5110/© 2019 IAgrE. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Nomenclature CAD DOF E F FEA ISO M OECD ROPS SIP
Computer Aided Design Degree Of Freedom Energy (J) Force (N) Finite Element Analysis International Standardisation Organisation Reference mass (kg) Organisation for the Economic Co-operation and Development RollOver Protective Structure Seat Index Point
mounted and Code 7 for rear mounted and cab devices (OECD Code 6 and 7, 2018). The continued occurrence of rollover accidents (Laurendi, Gattamelata, & Vita, 2010; Rondelli, Casazza, & Martelli, 2018; € Ozdes ‚ , Berber, & Celik, 2011) has led to renewed interest in rollover research; in particular, recent research has focused on narrow track tractor rollover (Franceschetti et al, 2014, 2016; Guzzomi, Rondelli, Guarnieri, Molari, & Molari, 2009; Guzzomi & Rondelli, 2013; Rondelli & Guzzomi, 2010). It is common in Europe for narrow track tractors to be articulated; a quarter of the ROPSs fitted on Narrow Track tractors tested at the OECD Test Station at the University of Bologna over the period from 1993 to 2008 were articulated (Guzzomi & Rondelli, 2013). Due to the observable differences in some physical parameters, plus recognition that kinematically articulated tractors behave differently to the fixed chassis tractors, it was a recommendation of Guzzomi and Rondelli (2013) that further research into articulated tractor ROPS safety be performed. The purpose of this paper is to highlight the limitations of the standardised ROPS testing procedures with articulated tractor types. Particular attention and reference are given to the clearance zone and applicability of an exposure and intrusion criteria similarly to that proposed by Ayers, Dickson, and Warner (1994) for ROPS mounted on fixed chassis tractors. The exposure criteria describes a failure condition in which the clearance zone is exposed to the ground plane during a tractor overturn while the intrusion criteria refers to a failure condition in which the ROPS, loaded and deformed, enters the clearance zone prior to absorbing the appropriate energy requirement. The paper is organised as follows: section 2 provides further context of the work and gives the relevant specifications of ROPS testing procedures with specific reference to the OECD Codes 6 and 7 for two-post ROPS mounted on Narrow Track tractors (N.B. International Standardisation Organisation (ISO) testing procedures ISO 12003-1 (2008) and ISO 120032 (2008) are harmonised to OECD codes for the same ROPS and tractor types); section 3 highlights the main shortcomings of the test procedures by identifying the differences between articulated chassis and standard fixed chassis narrow track tractors; section 4 suggests remedial measures which can be implemented in the short-term and demonstrates an approach to safer design through a prototype development
with the support of an Italian manufacturer specialised in Narrow Track tractors. In section 5 conclusions are drawn and the future directions for useful research and development are suggested.
2.
Rops testing procedure specifications
Since 1974 homologation procedures mandate that new tractor designs in Europe have to be fitted with ROPS (European Directive 74/150/EEC, 1974, currently replaced by the European Regulation 167/2013, 2013) and that the ROPS has to pass a compulsory certification process. Tractor ROPSs are designed to protect the operator by absorbing energy resulting from the impact of the tractor with the ground surface during rollover while preserving the operator clearance zone (OECD ROPS Codes, 2018) Accordingly, the OECD Tractor ROPS Codes (OECD, 2018) prescribe tests to determine ROPS energy absorption and force resistance requirements while maintaining a clearance zone for the driver. Today these tests are conducted according to the OECD coded procedures and can be performed by 19 authorised test stations dispersed across the globe. In this section key definitions and techniques given in OECD Codes 6 and 7 (2018) are summarised enabling implicit limitations of such techniques and definitions regarding articulated tractors to be evidenced. OECD Codes 6 and 7 are applicable to wheeled or tracked agricultural tractors that have a fixed or adjustable minimum track width, with one of the axles that is less than 1150 mm fitted with tyres of a larger size. For a ROPS comprising a two-post frame mounted behind the driver seat Code 7 (which also covers a four-post frame or enclosed cab) is applicable, whilst Code 6 refers to a two-post ROPS mounted in front of the driver seat.
2.1.
Definitions
ROPS are devised to avoid or limit the risks to the driver resulting from rollover of the tractor during normal operation. The concept of tractor normal operation is a key consideration because other uses of the tractor are not covered by the rollover protection provided. The ROPS is characterised by the provision of space, termed the clearance zone, which is large enough to protect the driver when seated either inside the envelope of the structure, or within a space defined by a series of straight lines from the outer edges of the structure to any part of the tractor that might come into contact with flat ground and that is capable of supporting the tractor in that position if the tractor overturns (OECD Code 6 and 7, 2018). The reference position for designing the operator clearance zone is the seat index point (SIP) determined in accordance with the procedure given in ISO 5353 (1995). The Codes specify how the seat should be adjusted for the test. The clearance zone (Fig. 1) is defined on the basis of the reference plane and the SIP for both the front and the rear mounted ROPS (Codes 6 and 7, OECD 2018). The determination of the clearance zone is dependent on a reference geometry defined in the codes by a reference plane and line. It is assumed that the tractor is on a horizontal surface, the seat, where adjustable, is adjusted to its rear uppermost position,
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 1 e Clearance zone for front and rear mounted ROPS according to Codes 6 (a) and 7 (b). Envelope of clearance zones for tractors with reversible driving position according to Codes 6 (c) and 7 (d). (OECD ROPS Codes, 2018).
and the steering wheel, where adjustable, is adjusted to the mid position for seated driving. The clearance zones for tractors with a non-reversible seat are pictorially shown in Fig. 1a and b respectively for front and rear mounted ROPS. For tractors with a reversible driving position (reversible seat and steering wheel), the clearance zone is the envelope of the two clearance zones defined by the different positions of the steering wheel and the seat. This envelope is depicted respectively for front and rear mounted ROPS in Fig. 1c and d.
2.2.
ROPS testing procedures
The ROPS testing procedures are based on a sequence of loadings where the values for the forces and energies depend on the mass of the tractor considered. OECD Codes use the reference tractor mass in all calculations. There is no
consideration of partitioning mass between the anterior and posterior bodies. Additionally, during each test no part of the protective structure can enter the clearance zone (OECD ROPS Codes, 2018). This latter condition, as introduced by Ayers et al. (1994), represents the intrusion criterion which must be satisfied. In addition the clearance zone is considered outside the protection of the ROPS if any part of it would have come into contact with the ground plane if the tractor had overturned in the direction from which the impact occurred (Fig. 2). For this purpose, the front and rear tyres and track setting are assumed to be the minimum configuration specified by the manufacturer (OECD ROPS Codes, 2018). Adopting the same reference definition of Ayers et al. (1994), this second failure criteria is defined as the exposure criterion. The Codes outline a specific treatment for articulated tractors with respect to anchoring the tractor to the ground during the loading sequence. During the ROPS tests the tractor
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 2 e Imaginary ground plane corresponding to simplified real overturning event. chassis has to be firmly lashed to the ground so that the hinge is locked with no articulation. Test stations typically therefore orientate the articulated tractor in a straight configuration (front and rear bodies collinear) and secure each body rigidly to the foundation therefore reproducing the behaviour of a fixed chassis tractor in the loading sequence. It is interesting to note that the ISO 3471 (2008) ROPS Standard for earthmoving machinery treats articulated machines independently of their fixed chassis counterparts and specifies that
the machine hinge shall be locked with no articulation if both frames are used during the evaluation. However, if only the frame on which the ROPS is mounted is secured for the test, the connections shall be at or near the articulation joint and axle support or at the outside end of the frame and only the mass of this machine part is considered in energy calculation. Therefore, both OECD tractor ROPS Codes 6 and 7 and the equivalent ISO standards 12003e1 (2008), together with ISO 3471 (2008) standard for earth-moving machines, do not
Fig. 3 e Comparison of two-post front mounted (left) and rear mounted (right) ROPS type: (a) and (b) fixed chassis tractor; (c) and (d) articulated tractors. Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 4 e Decoupled movement of articulated joint: (a) straight; (b) yaw axis limit; (c) roll axis limit.
contain special provisions for articulated tractors and machines with respect to the intrusion or exposure criteria. On the contrary, the ISO 21299: 2009 ROPS Standard for powered ride-on turf care equipment requires the clearance zone remaining protected for any angle of articulation of the machine when overturned.
3.
Test method shortcomings
The fundamental difference between a fixed chassis conventional tractor and an articulated one, is that, unlike a fixed chassis tractor (Fig. 3, a and b), it is possible that the articulated tractor two-post ROPS is not mounted on the same rigid part of the chassis as the seat and, more importantly, the rigid body defining the other contact points during rollover (Fig. 3, c and d). Many of the Code's deficiencies concerning articulated tractor ROPS testing appear to have arisen due to the Code being applied outside of the limiting assumptions implicit in
the model derivations and evident in the initial testing. Artificially forcing an articulated tractor to remain straight with the articulated parts collinear and axles rigidly secured during ROPS testing is not representative of what may arise during a real rollover event. Checking the non-interference of the clearance volume for the straight configuration when the twopost ROPS and the driver seat are mounted to separate bodies of the tractor is problematic since in reality the effective clearance volume changes as a function of articulation if the articulation 2 degrees of freedom (DOF) roll and yaw joint of the tractor is not fixed. Both of modes are shown separately in Fig. 4 for a two-post rear mounted ROPS type articulated tractor. An analogous situation also arises for two-post front mounted ROPS tractors (although in that case the ROPS would have moved with the anterior body). It should be noted that the roll and yaw motions do not have to be independent. These problems are specific to articulated type tractors and there is no acknowledgement of this in the Codes. It should be apparent that, just as the model/method developed for fixed chassis tractors arose from consideration
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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tractor, attached to the ROPS. The front hard point or other structural member that defines the clearance zone is also on the same, nominally rigid, body. Although for a fixed chassis tractor the front axle is free to rotate about its pivot until the axle stop, it is likely to influence the inertial loading and hence energy to a lesser extent than the mass of the tractor since its inertia is typically orders of magnitude smaller. The error associated with fixing the axle should thus be small for the light mechanical front ends typical of two-wheel drive tractors; those used in many of the original studies from which the codes were derived (Guzzomi, 2012). This error could increase for the more massive front ends with larger roll angles typical of 4-wheel-drive tractors (Baker & Guzzomi, 2013; Rondelli & Guzzomi, 2010). However, it is more important that this front axle pivot articulation does not significantly influence the clearance zone during a typical real rollover event nor does the steering angle. The movement of the ROPS chassis part with respect to the seat chassis part affects the exposure of the clearance zone in a real rollover of an articulated tractor.
1) 3D CAD of articulated tractor
2) Generate 3D Clearance Zone Volume 3) Simulate roll-plane to determine deformed ROPS limits 4) Design ROPS with limits in mind (possible to use FEA and Codes to guide design)
5) Test ROPS according to existing Codes. Measure and record deflection information.
4.
6) Use CAD to check interference of Clearance Zone in all possible articulated positions
Yes
Does plane infringe on the clearance volume at any articulation angle?
No
ROPS design feasible Fig. 5 e Possible flow diagram for integrated experimentalsimulation approach to articulated tractor ROPS design and certification.
of the physical process of real rollover events, the model/ method for articulated tractors has to encompass those situations which may occur in real articulated tractor operation and not corresponding to a unique artificial testing procedure. Codes 6 and 7 specify that the axles must be firmly secured to the ground. For fixed chassis tractors with front axle pivot, the chassis becomes one rigidly secured body. Thus, the subsequent testing of the ROPS then demonstrates its absorption capabilities and the suitability of its anchorage to the chassis. The clearance volume is then checked accordingly. For a fixed chassis tractor this makes physical sense as the ROPS is mounted to this rigid body (either rigidly or via flexible members i.e. silent blocks). In a rollover, when the ROPS contacts the ground, there is effectively a rigid mass, i.e. the
Test method remedial measures
The biggest shortcoming in the Codes for articulated tractors relates to the potential exposure and intrusion of the clearance zone. Methods to check exposure and intrusion of the clearance zone may be purely experimental or involve a combination of experimental and theoretical approaches. Ayers et al. (1994) proposed the use of a rigid plywood (opaque) board to simulate the ground plane that could be used to check contact points and distances to the clearance zone which may arise during rollover. They also developed a mathematical model to simulate the contacting plane(s) and tractor geometry to check clearance as the ROPS deforms. They assumed the ROPS length remained constant during deformation and noted that better accuracy could be obtained by including actual ROPS height in the model. They also noted that front-axle-pivot mobility can change the contact point position for a fixed chassis tractor. It is advisable to physically check that the clearance volume is maintained during and after the ROPS certification test. Using the method proposed by Ayers et al. (1994), a rigid plane made out of a transparent material, for example Polymethyl Methacrylate (PMMA), could be held by a gantry cable attached to the centrally located lifting lug. The plane could be manoeuvred appropriately and placed on the deformed ROPS and tractor contact points to physically indicate where the tractor clearance zone would be exposed. Given the more complicated nature of articulated tractor mobility, its transparent properties would aid checking and design. However, physical checking would be difficult and time consuming during the loading sequence because the two rotational DOFs of the articulated joint would require many set-ups and possibly additional instruments together with wires/cord to reproduce the imaginary ground plane in all possible articulation configurations. The articulated joint has two rotational DOFs, thus there is relative movement about these axes (until their angle movement stops, Fig. 4) and the associated inertia of each part is significant. Thus, not only does the significant
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Table 1 e Tractor data for CAD evaluation. Rear tyre diameter Rear tyre width Rear track width Front tyre diameter Front tyre width Front track width Wheelbase Horizontal and vertical distances to SIP from centre of rear axle Horizontal and vertical distances from SIP to the forward upper edge of the steering wheel Horizontal and vertical distances from SIP to the rear upper edge of the seat back rest Horizontal and vertical distances from SIP to the front part of the tractor capable of supporting the mass of the tractor when overturned Width of the front part of the tractor capable of supporting the mass of the tractor when overturned Width of ROPS Horizontal and vertical distances from SIP to the front upper edge of ROPS Radius of curvature of ROPS upper part Horizontal distance to the articulation pivot from centre of rear axle Articulation angle Vertical distances to oscillation pivot from centre of rear axle Oscillation angle
Table 2 e ROPS test deflections for CAD evaluation. Loading and crushing tests
ROPS side
Maximum displacement x
Rear loading First crushing Front loading Side loading Second crushing
Right Left Right Left Right Left Right Left Right Left
y
Plastic displacement
z
x
* *
y
z
* * * *
* *
* *
* *
* *
* * * *
* *
* Direction to be assessed, x transversal, y longitudinal, z vertical.
inertial loading of each part during an actual rollover result in the rollover dynamics being affected but the clearance zone is also influenced by the articulation angle. Although it possible to develop a parametric mathematical model, similar to that proposed by Ayers et al. (1994), Computer-aided design (CAD) capabilities and their user friendliness today could offer an alternate route. A hybrid experimental-simulated approach could use CAD simulated virtual interference tests involving planes defined by three contact points on the deformed ROPS and articulated tractor. These checks could be performed before initiating the tests, at locations of maximum ROPS test deflection and also the final, plastically deformed, positions at each stage of the loading sequence. As the contact of the ground plane on the tractor occurs at three points which result in three lines connecting the points, it is also possible to add taut wires between these points on the physical tractor to track the imaginary ground plane. Visualising intrusion/exposure of the clearance zone is thus achieved by viewing the tractor from a position that enables the imaginary ground plane to be revealed. A video recording of the ROPS test could permit checking of the physical sense of the data to be used in the CAD software model. Such an approach could be integrated in the articulated tractor design-
certification process as shown in the flow diagram of Fig. 5 using the data recorded in Tables 1 and 2. The methods proposed here should serve as a guide to this process and effectively require that the traditionally defined, i.e. when straight, clearance zone shall remain protected for any angle of articulation, in line with the recent ISO 21299 (2009) requirement for powered ride-on turf care equipment.
4.1.
Case study
The feasibility of this hybrid approach was investigated through a case study with an Italian tractor manufacturer who performed the first 4 steps of Fig. 5. The prototype two-post ROPS was designed by updating an already existing two-post ROPS mounted on a commercial articulated narrow track tractor fitted with equal sized wheels and reversible seat and steering wheel. The main tractor characteristics are given in Fig. 6. The clearance zone for the case study (Fig. 7) was described by the envelope of the two clearance zones defined by the different positions of the steering wheel and the seat (Fig. 1d). The aim was to maintain the survival volume at any angle of articulation of the tractor when overturned.
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 6 e Main tractor-case study characteristics. All dimensions in mm.
Fig. 7 e Clearance zone considered for the case study, a tractor designed with reversible seat and steering wheel and mounted with a rear two-post ROPS. Prior to the test, the articulated chassis was mounted with a jig to replicate the front part of the tractor capable of supporting the mass of the tractor when overturned
(defined by the radiator in front of the engine) and two jigs to replicate the front wheels to permit physical checking of the results. Moreover, joint mobility was considered
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 8 e Tractor chassis mounted with the two-post ROPS and the jigs added to permit physical checking of the clearance zone during the loading sequence.
together with wires reproducing the imaginary ground planes (Fig. 8). OECD Code 7 was considered for the evaluation of the strength behaviour of the case study ROPS. The standard provides the following sequence of loading and crushing: 1) 2) 3) 4) 5)
Loading at the rear of the structure First crushing test Loading at the front of the structure Loading at the side of the structure Second crushing test
The loading and crushing requirements are functions of the reference mass and the wheelbase of the tractor. The reference mass is at least the mass of the tractor excluding optional accessories but including coolant, oils, fuel and the protective structure. The following equations were considered for the tractor with a reversible driver's position (reversible seat and steering wheel): 1) 2) 3) 4) 5)
Where M is the reference mass of the tractor, which was 3000 kg. The points of application of the load tests and the required energy (E) and force (F) to be sustained values are listed below.
E F E E F
¼ 500 þ 0:5 M ¼ 20 M ðNÞ ¼ 500 þ 0:5 M ¼ 1:75 M ðJÞ ¼ 20 M ðNÞ
ðJÞ ðJÞ
The horizontal loadings were applied using a hydraulic cylinder fitted with a load cell and a linear displacement transducer to measure the loading force and the ROPS deflection under load. The crushing force was exerted by means of a beam connected at the ends with two hydraulic cylinders and fitted with two load cells. Additional linear displacement transducers were located on the ROPS to track the deformation of the whole structure. The loading sequence was video recorded by two cameras positioned at convenient angles to ensure appropriate fields of view and a standardlength bar was included in each camera arrangement in order to calibrate CAD measurements. After the loading sequence, the final permanent deflection was measured directly on the upper part of the protective structure.
5. Table 3 e Energy, force and deflection obtained for each loading. Loading and crushing tests Rear loading to the left First crushing Front loading to the right
Side loading to the right Second crushing
Rear left: 2.000 kJ First crushing force: 60.000 kN Front right: 2.000 kJ Side right: 5.250 kJ Second crushing force: 60.000 kN
Results and discussion
The prototype two-post ROPS, rear mounted on a Narrow Track tractor, was tested in accordance with Code 7 at the
Test results Energy: 2.041 kJ Force: 15.110 kN Deflection: 252 mm Force: 60.976 kN Energy: 2.012 kJ Force: 13.493 kN Deflection: 295 mm Energy: 5.338 kJ Force: 46.897 kN Deflection: 205 mm Force: 60.954 kN
Table 4 e Final permanent deflections of the upper part of the ROPS. Direction
Way
Longitudinal
forwards
Sideway
to the left
Vertical
downwards
Position
Displacement [mm]
left-hand right-hand left-hand right-hand left-hand right-hand
5 - 20 55 55 17 - 17
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 9 e Representative photos during the loading sequence of the OECD Code 7 test. Columns: (a) rear loading, (b) front loading, (c) side loading. Rows: (1) initial ROPS position, (2) maximum deflection of the ROPS, (3) permanent deflection of the ROPS.
Fig. 10 e Representative test results for the loadings according to the OECD Code 7 test. (a) rear loading, (b) front loading, (c) side loading.
University of Bologna OECD Test Station. The results in terms of energy, force and deflection for each loading are shown in Table 3. The permanent deflections of the upper part of the ROPS, measured directly after the loading sequence, are reported in Table 4.
A reference video of the complete test was taken along with photos prior to each test (first row, Fig. 9) to verify the exposure criteria requirement for the clearance zone in all articulation angles of the tractor chassis and at the point corresponding to satisfaction of the requisite force/energy of
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 11 e Sample CAD generated images used in the experimental-simulation approach. The different stages are: (a) undeformed ROPS, straight configuration; (b) undeformed ROPS, articulated configuration; (c) longitudinal tests; (d) lateral test, straight configuration; (e) lateral test, articulated configuration.
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Fig. 12 e (a) Original two-post ROPS; (b) Case study two-post ROPS.
each test (second row, Fig. 9), the intrusion/exposure criteria were checked. The third row of the Fig. 9 shows the plastic deformation. The graphs depict the corresponding force with respect to the deflection for each loading (Fig. 10). Key photos and data (Table 1) were imported into CAD software where the displacements were reproduced. The measured data was then used in the CAD software to deform the two-post ROPS in accordance with Fig. 10. The integrity of the clearance zone was then checked for all articulation angles at each stage of the ROPS test. Figure 11 depicts the case study example which aims to mitigate the exposure and intrusion criteria for a plane contacting the ROPS and the front part of the tractor capable of supporting its mass when overturned. The clearance zone corresponds to Fig. 1d, removing any concave portions (i.e. connecting the top of the seat to the corner of the normal seat position clearance zone). From top to bottom in Fig. 11 are depicted the un-deformed ROPS in the straight (a) and articulated configurations (b) used for interference checks; the longitudinal tests, from rear left and front right shown in straight configuration (c); the lateral test in the straight (d) and articulated configurations (e). Figure 12a shows the original two-post ROPS which satisfies the current requirement of the Code while Fig. 12b corresponds to the two-post ROPS resulting from the evaluation of the intrusion and exposure of the clearance zone for all articulation angles (Fig. 11). The latter procedure necessitated a greater than 35% increase in ROPS width and 16% in ROPS height. The perceived advantages of the proposed hybrid approach could be recognised by:
the possibility of simulating clearance zone interference at any articulation angle and at any time together with the simulated plane stretched to the desired dimension (area), the minimal additional time compared to the existing testing procedure, the easy measurement of clearance margins, the minimal danger associated with the loaded structure the possibility for tractor manufacturer to verify the exposure criteria fulfilment at the design stage.
6.
Conclusions
This paper highlights shortcomings of the OECD Codes 6 and 7 ROPS testing procedures regarding articulated tractors and presents remedial measures related to the evaluation of the intrusion and exposure of the clearance zone during the ROPS strength test. It has been shown that, while it is a straightforward matter to determine a functional clearance zone for a fixed chassis tractor, the functionality of an articulated tractor clearance zone, as currently accepted by OECD Codes 6 and 7, to provide protection for the operator in some configurations of roll and yaw seems questionable. This is because for articulated tractors, the seat, the two-post ROPS and the rear or front hard point, respectively for Code 6 and 7, are not on the same rigid chassis. Hence, although it may be found that the ROPS design can surpass the required energy and force requirements, it may be the case that the real clearance zone has been intruded or exposed by an articulated joint rotation (i.e. either yaw or roll DOF or both).
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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The clearance zone should be protected in all possible articulation angles. The paper proposed using a combination of experimental and simulation to check this. Although the proposed hybrid CAD based method is a step in the right direction, the development of separate international standard testing procedures specific to articulated tractor types is deemed necessary. In order to define these procedures, further testing and research on articulated tractor rollover events need to be simulated and experimentally verified.
Acknowledgements The authors are grateful to all University of Bologna e DISTAL, OECD Testing Station staff. The authors acknowledge the support of the UWA ECM Research Development Grant provided to Guzzomi in support of the research collaboration with Rondelli and Capacci and the University of Bologna.
references
Ayers, P. D., Dickson, M., & Warner, S. (1994). Model to evaluate exposure criteria during roll-over protective structures (ROPS) testing. Transactions of the ASAE, 37(6), 1763e1768. Baker, V., & Guzzomi, A. L. (2013). A model and comparison of 4wheel-drive fixed-chassis tractor rollover during Phase I. Biosystems Engineering, 116(2), 179e189. Chisholm, C. J. (1979a). A mathematical model of tractor overturning and impact behaviour. Journal of Agricultural Engineering Research, 24, 375e394. Chisholm, C. J. (1979b). Experimental validation of a tractor overturning simulation. Journal of Agricultural Engineering Research, 24, 395e415. Chisholm, C. J. (1979c). Analysis of rigid-body motion from cine film measurements. Journal of Agricultural Engineering Research, 24, 441e446. Chisholm, C. J. (1979d). The effect of parameter variation on tractor overturning and impact behaviour. Journal of Agricultural Engineering Research, 24, 417e440. European Directive 74/150/EEC. (1974). Directive on the approximation of the laws of the Member States relating to the typeapproval of wheeled agricultural or forestry tractors. Available at: http://eur-lex.europa.eu/. European Regulation 167/2013. (2013). Regulation on the approval and market surveillance of agricultural and forestry vehicles. Available at: http://eur-lex.europa.eu/. Franceschetti, B., Capacci, E., & Rondelli, V. (2016). Effects of rubber tracks on narrow-track tractors on the non-continuous rolling prediction model. Journal of Agricultural Safety and Health, 22, 262e273. Franceschetti, B., Roland, L., & Rondelli, V. (2014). Comparison between a rollover tractor dynamic model and actual lateral tests. Biosystems Engineering, 127, 79e91. Guzzomi, A. L. (2012). A revised kineto-static model for Phase I tractor rollover. Biosystems Engineering, 113(1), 65e75.
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Guzzomi, A., & Rondelli, V. (2013). Narrow-track wheeled agricultural tractor parameter variation. Journal of Agricultural Safety and Health, 19(4), 237e260. Guzzomi, A. L., Rondelli, V., Guarnieri, A., Molari, G., & Molari, P. G. (2009). Available energy during the rollover of narrow track wheeled agricultural tractors. Biosystems Engineering, 104(3), 318e323. ISO 12003-1. (2008). Agricultural and forestry tractors - roll-over protective structures on narrow-track wheeled tractors - Part 1: Front-mounted ROPS. Switzerland: International Standard. Available at: www.iso.org. ISO 12003-2. (2008). Agricultural and forestry tractors - roll-over protective structures on narrow-track wheeled tractors - Part 2: Rear-mounted ROPS. Switzerland: International Standard. Available at: www.iso.org. ISO 21299. (2009). Powered ride-on turf care equipment - roll-over protective structures (ROPS) - test procedures and acceptance criteria. Switzerland: International Standard. Available at: www.iso.org. ISO 3471. (2008). Earth-moving machinery - roll-over protective structures - laboratory tests and performance requirements. Switzerland: International Standard. Available at: www.iso.org. ISO 5353. (1995). Earth-moving machinery, and tractors and machinery for agriculture and forestry – Seat index point. Switzerland: International Standard. Available at: www.iso.org. Laurendi, V., Gattamelata, D., & Vita, L. (2010). Safety level investigation of front mounted roll-over protective structures on narrow-track wheeled agricultural and forestry tractors. Proceedings of SHWA, 16e18. September 2010, Ragusa, Italy. OECD Code 6. (2018). OECD Standard Code for the official testing of front mounted rollover protective structures on narrow-track wheeled agricultural and forestry tractors. Paris: Organisation for the Economic Co-operation and Development. Available at: www.oecd.org. OECD Code 7. (2018). OECD Standard Code for the official testing of rear mounted rollover protective structures on narrow-track wheeled agricultural and forestry tractors. Paris: Organisation for the Economic Co-operation and Development. Available at: www. oecd.org. OECD ROPS Codes. (2018). OECD Standard Codes for the official testing of rollover protective structures on agricultural and forestry tractors. Paris: Organisation for the Economic Co-operation and Development. Available at: www.oecd.org. € Ozdes ‚ , T., Berber, G., & Celik, S. (2011). Death cases related to tractor overturns. Tu¨rkiye Klinikleri Tıp Bilimleri Dergisi, 31(1), 133e141. Rondelli, V., Casazza, C., & Martelli, R. (2018). Tractor rollover fatalities, analyzing accident scenario. Journal of Safety Research, 67, 99e106. Rondelli, V., & Guzzomi, A. L. (2010). Selecting ROPS safety margins for wheeled agricultural tractors based on tractor mass. Biosystems Engineering, 105(3), 402e410. Schwanghart, H. (1973). Berechnungsmethode Fur das unsturzverhalten eines achrschleppers am hang [Method for calculating the overturning behaviour of tractors on slopes]. Grundl. Landtechnik, 23(6), 170e176. Schwanghart, H. (1982). Umsturzverhalten von traktoren und auswirkungen auf die schutzvorrichtungen und die sicherheit [Rollover tractor behaviour, impact on the protective devices and safety]. Munich, Germany: TU.
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/issn/15375110
Research Paper
Operator protection in rollover events of articulated narrow track tractors Andrew L. Guzzomi a, Valda Rondelli b,*, Enrico Capacci b a Department of Mechanical Engineering, The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia b Department of Agricultural and Food Sciences, University of Bologna, Viale Fanin 50 Bologna, BO 40127, Italy
article info
Articulated steering arrangements, wheeled or tracked, are commonly used on agricultural
Article history:
tractors and earth moving machines. In Continental Europe and Italy in particular, this
Published online xxx
configuration is typical of narrow track tractors. Despite their widespread use, there is little research that specifically investigates articulated tractor rollover protective structure
Keywords:
(ROPS) in-field performance with respect to the ROPS test procedures. The aim was to
virtual analysis
investigate the relevance of the standardised ROPS test procedures regarding articulation
testing procedure
joint mobility. The shortcomings of the procedures concerning the Clearance Zone and
ROPS
ROPS testing are evidenced and discussed. It is a recommendation of this work that an
clearance zone
update of the international testing procedure be developed for articulated tractor types.
CAD
The paper indicates how this may be accomplished and presents remedial measures that can be implemented in the interim until this latter goal is achieved. © 2019 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Tractors and machines in agriculture and earth-moving sector are a large family of machine types that differ in size, mass and shape. Diversity in the vehicles available on the market is necessary to cope with the wide range of applications required. Within this family of machines a notable type of tractor is the articulated variant, since such designs cover a tenfold range of mass from small articulated agricultural tractors and mowers with mass less than 1000 kg to massive articulated frame dumpers and large tractors with mass of more than 10,000 kg. Mechanisation of intense farming environments such as vineyards and orchards led to the development of specialised
compact tractors with reduced track widths to aid manoeuvrability in such restrictive working environments. In many parts of Europe, intensive farming is often coupled with farming on fertile undulating soil and consequently presents a heightened risk of tractor rollover. As the number of narrow tractors has increased, researchers in Europe have reflected on the rollover research that was conducted approximately 40 years ago (Chisholm, 1979aed; Schwanghart, 1973, 1982) and realised that the narrow-track wheeled agricultural tractors should be considered a separate category from standard wheeled tractors (Guzzomi & Rondelli, 2013). The corresponding rollover protective structure (ROPS) testing procedures of the Organisation for the Economic Development and Co-operation (OECD) were thus devised: Code 6 for front
* Corresponding author. E-mail address: [email protected] (V. Rondelli). https://doi.org/10.1016/j.biosystemseng.2019.04.020 1537-5110/© 2019 IAgrE. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Nomenclature CAD DOF E F FEA ISO M OECD ROPS SIP
Computer Aided Design Degree Of Freedom Energy (J) Force (N) Finite Element Analysis International Standardisation Organisation Reference mass (kg) Organisation for the Economic Co-operation and Development RollOver Protective Structure Seat Index Point
mounted and Code 7 for rear mounted and cab devices (OECD Code 6 and 7, 2018). The continued occurrence of rollover accidents (Laurendi, Gattamelata, & Vita, 2010; Rondelli, Casazza, & Martelli, 2018; € Ozdes ‚ , Berber, & Celik, 2011) has led to renewed interest in rollover research; in particular, recent research has focused on narrow track tractor rollover (Franceschetti et al, 2014, 2016; Guzzomi, Rondelli, Guarnieri, Molari, & Molari, 2009; Guzzomi & Rondelli, 2013; Rondelli & Guzzomi, 2010). It is common in Europe for narrow track tractors to be articulated; a quarter of the ROPSs fitted on Narrow Track tractors tested at the OECD Test Station at the University of Bologna over the period from 1993 to 2008 were articulated (Guzzomi & Rondelli, 2013). Due to the observable differences in some physical parameters, plus recognition that kinematically articulated tractors behave differently to the fixed chassis tractors, it was a recommendation of Guzzomi and Rondelli (2013) that further research into articulated tractor ROPS safety be performed. The purpose of this paper is to highlight the limitations of the standardised ROPS testing procedures with articulated tractor types. Particular attention and reference are given to the clearance zone and applicability of an exposure and intrusion criteria similarly to that proposed by Ayers, Dickson, and Warner (1994) for ROPS mounted on fixed chassis tractors. The exposure criteria describes a failure condition in which the clearance zone is exposed to the ground plane during a tractor overturn while the intrusion criteria refers to a failure condition in which the ROPS, loaded and deformed, enters the clearance zone prior to absorbing the appropriate energy requirement. The paper is organised as follows: section 2 provides further context of the work and gives the relevant specifications of ROPS testing procedures with specific reference to the OECD Codes 6 and 7 for two-post ROPS mounted on Narrow Track tractors (N.B. International Standardisation Organisation (ISO) testing procedures ISO 12003-1 (2008) and ISO 120032 (2008) are harmonised to OECD codes for the same ROPS and tractor types); section 3 highlights the main shortcomings of the test procedures by identifying the differences between articulated chassis and standard fixed chassis narrow track tractors; section 4 suggests remedial measures which can be implemented in the short-term and demonstrates an approach to safer design through a prototype development
with the support of an Italian manufacturer specialised in Narrow Track tractors. In section 5 conclusions are drawn and the future directions for useful research and development are suggested.
2.
Rops testing procedure specifications
Since 1974 homologation procedures mandate that new tractor designs in Europe have to be fitted with ROPS (European Directive 74/150/EEC, 1974, currently replaced by the European Regulation 167/2013, 2013) and that the ROPS has to pass a compulsory certification process. Tractor ROPSs are designed to protect the operator by absorbing energy resulting from the impact of the tractor with the ground surface during rollover while preserving the operator clearance zone (OECD ROPS Codes, 2018) Accordingly, the OECD Tractor ROPS Codes (OECD, 2018) prescribe tests to determine ROPS energy absorption and force resistance requirements while maintaining a clearance zone for the driver. Today these tests are conducted according to the OECD coded procedures and can be performed by 19 authorised test stations dispersed across the globe. In this section key definitions and techniques given in OECD Codes 6 and 7 (2018) are summarised enabling implicit limitations of such techniques and definitions regarding articulated tractors to be evidenced. OECD Codes 6 and 7 are applicable to wheeled or tracked agricultural tractors that have a fixed or adjustable minimum track width, with one of the axles that is less than 1150 mm fitted with tyres of a larger size. For a ROPS comprising a two-post frame mounted behind the driver seat Code 7 (which also covers a four-post frame or enclosed cab) is applicable, whilst Code 6 refers to a two-post ROPS mounted in front of the driver seat.
2.1.
Definitions
ROPS are devised to avoid or limit the risks to the driver resulting from rollover of the tractor during normal operation. The concept of tractor normal operation is a key consideration because other uses of the tractor are not covered by the rollover protection provided. The ROPS is characterised by the provision of space, termed the clearance zone, which is large enough to protect the driver when seated either inside the envelope of the structure, or within a space defined by a series of straight lines from the outer edges of the structure to any part of the tractor that might come into contact with flat ground and that is capable of supporting the tractor in that position if the tractor overturns (OECD Code 6 and 7, 2018). The reference position for designing the operator clearance zone is the seat index point (SIP) determined in accordance with the procedure given in ISO 5353 (1995). The Codes specify how the seat should be adjusted for the test. The clearance zone (Fig. 1) is defined on the basis of the reference plane and the SIP for both the front and the rear mounted ROPS (Codes 6 and 7, OECD 2018). The determination of the clearance zone is dependent on a reference geometry defined in the codes by a reference plane and line. It is assumed that the tractor is on a horizontal surface, the seat, where adjustable, is adjusted to its rear uppermost position,
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 1 e Clearance zone for front and rear mounted ROPS according to Codes 6 (a) and 7 (b). Envelope of clearance zones for tractors with reversible driving position according to Codes 6 (c) and 7 (d). (OECD ROPS Codes, 2018).
and the steering wheel, where adjustable, is adjusted to the mid position for seated driving. The clearance zones for tractors with a non-reversible seat are pictorially shown in Fig. 1a and b respectively for front and rear mounted ROPS. For tractors with a reversible driving position (reversible seat and steering wheel), the clearance zone is the envelope of the two clearance zones defined by the different positions of the steering wheel and the seat. This envelope is depicted respectively for front and rear mounted ROPS in Fig. 1c and d.
2.2.
ROPS testing procedures
The ROPS testing procedures are based on a sequence of loadings where the values for the forces and energies depend on the mass of the tractor considered. OECD Codes use the reference tractor mass in all calculations. There is no
consideration of partitioning mass between the anterior and posterior bodies. Additionally, during each test no part of the protective structure can enter the clearance zone (OECD ROPS Codes, 2018). This latter condition, as introduced by Ayers et al. (1994), represents the intrusion criterion which must be satisfied. In addition the clearance zone is considered outside the protection of the ROPS if any part of it would have come into contact with the ground plane if the tractor had overturned in the direction from which the impact occurred (Fig. 2). For this purpose, the front and rear tyres and track setting are assumed to be the minimum configuration specified by the manufacturer (OECD ROPS Codes, 2018). Adopting the same reference definition of Ayers et al. (1994), this second failure criteria is defined as the exposure criterion. The Codes outline a specific treatment for articulated tractors with respect to anchoring the tractor to the ground during the loading sequence. During the ROPS tests the tractor
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Fig. 2 e Imaginary ground plane corresponding to simplified real overturning event. chassis has to be firmly lashed to the ground so that the hinge is locked with no articulation. Test stations typically therefore orientate the articulated tractor in a straight configuration (front and rear bodies collinear) and secure each body rigidly to the foundation therefore reproducing the behaviour of a fixed chassis tractor in the loading sequence. It is interesting to note that the ISO 3471 (2008) ROPS Standard for earthmoving machinery treats articulated machines independently of their fixed chassis counterparts and specifies that
the machine hinge shall be locked with no articulation if both frames are used during the evaluation. However, if only the frame on which the ROPS is mounted is secured for the test, the connections shall be at or near the articulation joint and axle support or at the outside end of the frame and only the mass of this machine part is considered in energy calculation. Therefore, both OECD tractor ROPS Codes 6 and 7 and the equivalent ISO standards 12003e1 (2008), together with ISO 3471 (2008) standard for earth-moving machines, do not
Fig. 3 e Comparison of two-post front mounted (left) and rear mounted (right) ROPS type: (a) and (b) fixed chassis tractor; (c) and (d) articulated tractors. Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 4 e Decoupled movement of articulated joint: (a) straight; (b) yaw axis limit; (c) roll axis limit.
contain special provisions for articulated tractors and machines with respect to the intrusion or exposure criteria. On the contrary, the ISO 21299: 2009 ROPS Standard for powered ride-on turf care equipment requires the clearance zone remaining protected for any angle of articulation of the machine when overturned.
3.
Test method shortcomings
The fundamental difference between a fixed chassis conventional tractor and an articulated one, is that, unlike a fixed chassis tractor (Fig. 3, a and b), it is possible that the articulated tractor two-post ROPS is not mounted on the same rigid part of the chassis as the seat and, more importantly, the rigid body defining the other contact points during rollover (Fig. 3, c and d). Many of the Code's deficiencies concerning articulated tractor ROPS testing appear to have arisen due to the Code being applied outside of the limiting assumptions implicit in
the model derivations and evident in the initial testing. Artificially forcing an articulated tractor to remain straight with the articulated parts collinear and axles rigidly secured during ROPS testing is not representative of what may arise during a real rollover event. Checking the non-interference of the clearance volume for the straight configuration when the twopost ROPS and the driver seat are mounted to separate bodies of the tractor is problematic since in reality the effective clearance volume changes as a function of articulation if the articulation 2 degrees of freedom (DOF) roll and yaw joint of the tractor is not fixed. Both of modes are shown separately in Fig. 4 for a two-post rear mounted ROPS type articulated tractor. An analogous situation also arises for two-post front mounted ROPS tractors (although in that case the ROPS would have moved with the anterior body). It should be noted that the roll and yaw motions do not have to be independent. These problems are specific to articulated type tractors and there is no acknowledgement of this in the Codes. It should be apparent that, just as the model/method developed for fixed chassis tractors arose from consideration
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tractor, attached to the ROPS. The front hard point or other structural member that defines the clearance zone is also on the same, nominally rigid, body. Although for a fixed chassis tractor the front axle is free to rotate about its pivot until the axle stop, it is likely to influence the inertial loading and hence energy to a lesser extent than the mass of the tractor since its inertia is typically orders of magnitude smaller. The error associated with fixing the axle should thus be small for the light mechanical front ends typical of two-wheel drive tractors; those used in many of the original studies from which the codes were derived (Guzzomi, 2012). This error could increase for the more massive front ends with larger roll angles typical of 4-wheel-drive tractors (Baker & Guzzomi, 2013; Rondelli & Guzzomi, 2010). However, it is more important that this front axle pivot articulation does not significantly influence the clearance zone during a typical real rollover event nor does the steering angle. The movement of the ROPS chassis part with respect to the seat chassis part affects the exposure of the clearance zone in a real rollover of an articulated tractor.
1) 3D CAD of articulated tractor
2) Generate 3D Clearance Zone Volume 3) Simulate roll-plane to determine deformed ROPS limits 4) Design ROPS with limits in mind (possible to use FEA and Codes to guide design)
5) Test ROPS according to existing Codes. Measure and record deflection information.
4.
6) Use CAD to check interference of Clearance Zone in all possible articulated positions
Yes
Does plane infringe on the clearance volume at any articulation angle?
No
ROPS design feasible Fig. 5 e Possible flow diagram for integrated experimentalsimulation approach to articulated tractor ROPS design and certification.
of the physical process of real rollover events, the model/ method for articulated tractors has to encompass those situations which may occur in real articulated tractor operation and not corresponding to a unique artificial testing procedure. Codes 6 and 7 specify that the axles must be firmly secured to the ground. For fixed chassis tractors with front axle pivot, the chassis becomes one rigidly secured body. Thus, the subsequent testing of the ROPS then demonstrates its absorption capabilities and the suitability of its anchorage to the chassis. The clearance volume is then checked accordingly. For a fixed chassis tractor this makes physical sense as the ROPS is mounted to this rigid body (either rigidly or via flexible members i.e. silent blocks). In a rollover, when the ROPS contacts the ground, there is effectively a rigid mass, i.e. the
Test method remedial measures
The biggest shortcoming in the Codes for articulated tractors relates to the potential exposure and intrusion of the clearance zone. Methods to check exposure and intrusion of the clearance zone may be purely experimental or involve a combination of experimental and theoretical approaches. Ayers et al. (1994) proposed the use of a rigid plywood (opaque) board to simulate the ground plane that could be used to check contact points and distances to the clearance zone which may arise during rollover. They also developed a mathematical model to simulate the contacting plane(s) and tractor geometry to check clearance as the ROPS deforms. They assumed the ROPS length remained constant during deformation and noted that better accuracy could be obtained by including actual ROPS height in the model. They also noted that front-axle-pivot mobility can change the contact point position for a fixed chassis tractor. It is advisable to physically check that the clearance volume is maintained during and after the ROPS certification test. Using the method proposed by Ayers et al. (1994), a rigid plane made out of a transparent material, for example Polymethyl Methacrylate (PMMA), could be held by a gantry cable attached to the centrally located lifting lug. The plane could be manoeuvred appropriately and placed on the deformed ROPS and tractor contact points to physically indicate where the tractor clearance zone would be exposed. Given the more complicated nature of articulated tractor mobility, its transparent properties would aid checking and design. However, physical checking would be difficult and time consuming during the loading sequence because the two rotational DOFs of the articulated joint would require many set-ups and possibly additional instruments together with wires/cord to reproduce the imaginary ground plane in all possible articulation configurations. The articulated joint has two rotational DOFs, thus there is relative movement about these axes (until their angle movement stops, Fig. 4) and the associated inertia of each part is significant. Thus, not only does the significant
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Table 1 e Tractor data for CAD evaluation. Rear tyre diameter Rear tyre width Rear track width Front tyre diameter Front tyre width Front track width Wheelbase Horizontal and vertical distances to SIP from centre of rear axle Horizontal and vertical distances from SIP to the forward upper edge of the steering wheel Horizontal and vertical distances from SIP to the rear upper edge of the seat back rest Horizontal and vertical distances from SIP to the front part of the tractor capable of supporting the mass of the tractor when overturned Width of the front part of the tractor capable of supporting the mass of the tractor when overturned Width of ROPS Horizontal and vertical distances from SIP to the front upper edge of ROPS Radius of curvature of ROPS upper part Horizontal distance to the articulation pivot from centre of rear axle Articulation angle Vertical distances to oscillation pivot from centre of rear axle Oscillation angle
Table 2 e ROPS test deflections for CAD evaluation. Loading and crushing tests
ROPS side
Maximum displacement x
Rear loading First crushing Front loading Side loading Second crushing
Right Left Right Left Right Left Right Left Right Left
y
Plastic displacement
z
x
* *
y
z
* * * *
* *
* *
* *
* *
* * * *
* *
* Direction to be assessed, x transversal, y longitudinal, z vertical.
inertial loading of each part during an actual rollover result in the rollover dynamics being affected but the clearance zone is also influenced by the articulation angle. Although it possible to develop a parametric mathematical model, similar to that proposed by Ayers et al. (1994), Computer-aided design (CAD) capabilities and their user friendliness today could offer an alternate route. A hybrid experimental-simulated approach could use CAD simulated virtual interference tests involving planes defined by three contact points on the deformed ROPS and articulated tractor. These checks could be performed before initiating the tests, at locations of maximum ROPS test deflection and also the final, plastically deformed, positions at each stage of the loading sequence. As the contact of the ground plane on the tractor occurs at three points which result in three lines connecting the points, it is also possible to add taut wires between these points on the physical tractor to track the imaginary ground plane. Visualising intrusion/exposure of the clearance zone is thus achieved by viewing the tractor from a position that enables the imaginary ground plane to be revealed. A video recording of the ROPS test could permit checking of the physical sense of the data to be used in the CAD software model. Such an approach could be integrated in the articulated tractor design-
certification process as shown in the flow diagram of Fig. 5 using the data recorded in Tables 1 and 2. The methods proposed here should serve as a guide to this process and effectively require that the traditionally defined, i.e. when straight, clearance zone shall remain protected for any angle of articulation, in line with the recent ISO 21299 (2009) requirement for powered ride-on turf care equipment.
4.1.
Case study
The feasibility of this hybrid approach was investigated through a case study with an Italian tractor manufacturer who performed the first 4 steps of Fig. 5. The prototype two-post ROPS was designed by updating an already existing two-post ROPS mounted on a commercial articulated narrow track tractor fitted with equal sized wheels and reversible seat and steering wheel. The main tractor characteristics are given in Fig. 6. The clearance zone for the case study (Fig. 7) was described by the envelope of the two clearance zones defined by the different positions of the steering wheel and the seat (Fig. 1d). The aim was to maintain the survival volume at any angle of articulation of the tractor when overturned.
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Fig. 6 e Main tractor-case study characteristics. All dimensions in mm.
Fig. 7 e Clearance zone considered for the case study, a tractor designed with reversible seat and steering wheel and mounted with a rear two-post ROPS. Prior to the test, the articulated chassis was mounted with a jig to replicate the front part of the tractor capable of supporting the mass of the tractor when overturned
(defined by the radiator in front of the engine) and two jigs to replicate the front wheels to permit physical checking of the results. Moreover, joint mobility was considered
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Fig. 8 e Tractor chassis mounted with the two-post ROPS and the jigs added to permit physical checking of the clearance zone during the loading sequence.
together with wires reproducing the imaginary ground planes (Fig. 8). OECD Code 7 was considered for the evaluation of the strength behaviour of the case study ROPS. The standard provides the following sequence of loading and crushing: 1) 2) 3) 4) 5)
Loading at the rear of the structure First crushing test Loading at the front of the structure Loading at the side of the structure Second crushing test
The loading and crushing requirements are functions of the reference mass and the wheelbase of the tractor. The reference mass is at least the mass of the tractor excluding optional accessories but including coolant, oils, fuel and the protective structure. The following equations were considered for the tractor with a reversible driver's position (reversible seat and steering wheel): 1) 2) 3) 4) 5)
Where M is the reference mass of the tractor, which was 3000 kg. The points of application of the load tests and the required energy (E) and force (F) to be sustained values are listed below.
E F E E F
¼ 500 þ 0:5 M ¼ 20 M ðNÞ ¼ 500 þ 0:5 M ¼ 1:75 M ðJÞ ¼ 20 M ðNÞ
ðJÞ ðJÞ
The horizontal loadings were applied using a hydraulic cylinder fitted with a load cell and a linear displacement transducer to measure the loading force and the ROPS deflection under load. The crushing force was exerted by means of a beam connected at the ends with two hydraulic cylinders and fitted with two load cells. Additional linear displacement transducers were located on the ROPS to track the deformation of the whole structure. The loading sequence was video recorded by two cameras positioned at convenient angles to ensure appropriate fields of view and a standardlength bar was included in each camera arrangement in order to calibrate CAD measurements. After the loading sequence, the final permanent deflection was measured directly on the upper part of the protective structure.
5. Table 3 e Energy, force and deflection obtained for each loading. Loading and crushing tests Rear loading to the left First crushing Front loading to the right
Side loading to the right Second crushing
Rear left: 2.000 kJ First crushing force: 60.000 kN Front right: 2.000 kJ Side right: 5.250 kJ Second crushing force: 60.000 kN
Results and discussion
The prototype two-post ROPS, rear mounted on a Narrow Track tractor, was tested in accordance with Code 7 at the
Test results Energy: 2.041 kJ Force: 15.110 kN Deflection: 252 mm Force: 60.976 kN Energy: 2.012 kJ Force: 13.493 kN Deflection: 295 mm Energy: 5.338 kJ Force: 46.897 kN Deflection: 205 mm Force: 60.954 kN
Table 4 e Final permanent deflections of the upper part of the ROPS. Direction
Way
Longitudinal
forwards
Sideway
to the left
Vertical
downwards
Position
Displacement [mm]
left-hand right-hand left-hand right-hand left-hand right-hand
5 - 20 55 55 17 - 17
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Fig. 9 e Representative photos during the loading sequence of the OECD Code 7 test. Columns: (a) rear loading, (b) front loading, (c) side loading. Rows: (1) initial ROPS position, (2) maximum deflection of the ROPS, (3) permanent deflection of the ROPS.
Fig. 10 e Representative test results for the loadings according to the OECD Code 7 test. (a) rear loading, (b) front loading, (c) side loading.
University of Bologna OECD Test Station. The results in terms of energy, force and deflection for each loading are shown in Table 3. The permanent deflections of the upper part of the ROPS, measured directly after the loading sequence, are reported in Table 4.
A reference video of the complete test was taken along with photos prior to each test (first row, Fig. 9) to verify the exposure criteria requirement for the clearance zone in all articulation angles of the tractor chassis and at the point corresponding to satisfaction of the requisite force/energy of
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Fig. 11 e Sample CAD generated images used in the experimental-simulation approach. The different stages are: (a) undeformed ROPS, straight configuration; (b) undeformed ROPS, articulated configuration; (c) longitudinal tests; (d) lateral test, straight configuration; (e) lateral test, articulated configuration.
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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Fig. 12 e (a) Original two-post ROPS; (b) Case study two-post ROPS.
each test (second row, Fig. 9), the intrusion/exposure criteria were checked. The third row of the Fig. 9 shows the plastic deformation. The graphs depict the corresponding force with respect to the deflection for each loading (Fig. 10). Key photos and data (Table 1) were imported into CAD software where the displacements were reproduced. The measured data was then used in the CAD software to deform the two-post ROPS in accordance with Fig. 10. The integrity of the clearance zone was then checked for all articulation angles at each stage of the ROPS test. Figure 11 depicts the case study example which aims to mitigate the exposure and intrusion criteria for a plane contacting the ROPS and the front part of the tractor capable of supporting its mass when overturned. The clearance zone corresponds to Fig. 1d, removing any concave portions (i.e. connecting the top of the seat to the corner of the normal seat position clearance zone). From top to bottom in Fig. 11 are depicted the un-deformed ROPS in the straight (a) and articulated configurations (b) used for interference checks; the longitudinal tests, from rear left and front right shown in straight configuration (c); the lateral test in the straight (d) and articulated configurations (e). Figure 12a shows the original two-post ROPS which satisfies the current requirement of the Code while Fig. 12b corresponds to the two-post ROPS resulting from the evaluation of the intrusion and exposure of the clearance zone for all articulation angles (Fig. 11). The latter procedure necessitated a greater than 35% increase in ROPS width and 16% in ROPS height. The perceived advantages of the proposed hybrid approach could be recognised by:
the possibility of simulating clearance zone interference at any articulation angle and at any time together with the simulated plane stretched to the desired dimension (area), the minimal additional time compared to the existing testing procedure, the easy measurement of clearance margins, the minimal danger associated with the loaded structure the possibility for tractor manufacturer to verify the exposure criteria fulfilment at the design stage.
6.
Conclusions
This paper highlights shortcomings of the OECD Codes 6 and 7 ROPS testing procedures regarding articulated tractors and presents remedial measures related to the evaluation of the intrusion and exposure of the clearance zone during the ROPS strength test. It has been shown that, while it is a straightforward matter to determine a functional clearance zone for a fixed chassis tractor, the functionality of an articulated tractor clearance zone, as currently accepted by OECD Codes 6 and 7, to provide protection for the operator in some configurations of roll and yaw seems questionable. This is because for articulated tractors, the seat, the two-post ROPS and the rear or front hard point, respectively for Code 6 and 7, are not on the same rigid chassis. Hence, although it may be found that the ROPS design can surpass the required energy and force requirements, it may be the case that the real clearance zone has been intruded or exposed by an articulated joint rotation (i.e. either yaw or roll DOF or both).
Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020
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The clearance zone should be protected in all possible articulation angles. The paper proposed using a combination of experimental and simulation to check this. Although the proposed hybrid CAD based method is a step in the right direction, the development of separate international standard testing procedures specific to articulated tractor types is deemed necessary. In order to define these procedures, further testing and research on articulated tractor rollover events need to be simulated and experimentally verified.
Acknowledgements The authors are grateful to all University of Bologna e DISTAL, OECD Testing Station staff. The authors acknowledge the support of the UWA ECM Research Development Grant provided to Guzzomi in support of the research collaboration with Rondelli and Capacci and the University of Bologna.
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Please cite this article as: Guzzomi, A. L et al., Operator protection in rollover events of articulated narrow track tractors, Biosystems Engineering, https://doi.org/10.1016/j.biosystemseng.2019.04.020