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On the Definition of Narrow-track Wheeled Agricultural Tractors
ARTICLE IN PRESS Available online at www.sciencedirect.com
Biosystems Engineering (2004) 88 (1), 75–80 doi:10.1016/j.biosystemseng.2004.02.001 PM–Power and Machinery
On the Definition of Narrow-track Wheeled Agricultural Tractors G. Molari; V. Rondelli Department of Agricultural Economics and Engineering, Bologna University, viale G. Fanin, 50, 40127, Italy; e-mail of corresponding author: [email protected] (Received 19 July 2003; accepted in revised form 10 February 2004; published online 9 April 2004)
The narrow-track agricultural tractor has been defined in the field of application of International Organisation for Standardisation (ISO), European Community (EC) and Organisation for Economic Co-operation and Development (OECD) Standards. The field of application does not allow narrow-track tractors to be fitted with tyres set at a minimum track width wider than 1150 mm, enabling tyres to be fitted with adjustable track width which, still having a minimum track width of not more than 1150 mm, can be set at greater track widths during normal operation of the tractor. Tests showed that tyre types which can be fitted on a narrow-track tractor and always determine a tractor track width wider than 1150 mm, have the same behaviour as the tyre types currently allowed in the field of application of the Standards, when set at the allowed maximum track widths. In the light of these results, it is recommended to define a narrow-track tractor as an agricultural tractor that, when fitted with tyres of the greatest diameter indicated by the manufacturer, has a minimum track width of not more than 1150 mm. # 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction
the OECD (OECD, 1990a, 1990b) developed specific Directives and Codes to evaluate the strength of the protective structures mounted on these tractors. The International Organisation for Standardisation (ISO) has also recently developed a new standard test for the narrow-track wheeled tractors (ISO, 2002). At present, OECD test procedures are published in eight Codes (OECD, 2002a) and among these, OECD Codes 6 and 7 are specific to narrow-track wheeled agricultural and forestry tractors, with front and rear mounted ROPS, respectively. In detail, OECD Code 6 and the corresponding EC Directive 87/402/EEC, in the field of application, specify the following characteristics for narrow-track tractors:
1.1. General considerations and problem description The large number of agricultural tractors overturning every year, widely reported in studies since 1931 (Arndt, 1971; Myers, 2000), focused attention on the public health problem of protecting the tractor operator from the risk of an overturn and supported the introduction of a roll-over protective structure (ROPS) on tractors. Firstly Sweden, in 1959, and then all the other European countries, adopted a regulation requiring ROPS on all new tractors. Also in 1959, the Organisation for Economic Co-operation and Development (OECD) developed a test procedure to evaluate the strength of the structures and established energy criteria (OECD, 1959). At the end of the 1980s, as a consequence of the expanding market for narrow-track tractors, mainly for use in orchards and vineyards, where the space above or at the sides of the tractor is often limited, first the European Community (EC) (EEC, 1986, 1987) and then 1537-5110/$30.00
(a) ground clearance of not more than 600 mm beneath the lowest points of the front and rear axles, allowing for the differential; (b) fixed or adjustable minimum track width with one of the axles less than 1150 mm fitted with tyres of a larger size; it is assumed that the axle mounted with the wider tyres is set at a track width of not more 75
# 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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G. MOLARI; V. RONDELLI
than 1150 mm; it must be possible to set the track width of the other axle in such a way that the outer edges of the narrower tyres do not go beyond the outer edges of the tyres of the other axle; where the two axles are fitted with rims and tyres of the same size, the fixed or adjustable track width of the two axles must be less than 1150 mm; (c) mass greater than 600 but less than 3000 kg, corresponding to the unladen weight of the tractor, including the roll-over protective structure and tyres of the largest size recommended by the manufacturer; and (d) being fitted with roll-over protective structures of the dual-pillar type mounted in front of the driver’s seat and characterised by a reduced zone of clearance attributable to the tractor silhouette, thus rendering it inadvisable, under any circumstances, to impede access to the driving position but worthwhile retaining these structures (fold-down or not) in view of their undoubted ease of use. The ISO Standard 12003-1, equivalent to OECD Code 6, is applicable to tractors equipped with front mounted ROPS, having the following characteristics: (i) a ground clearance of not more than 600 mm beneath the lowest points of the front and rear axles, housing; (ii) a fixed or adjustable minimum track width of one of the two axles of less than 1150 mm fitted with the widest specified tyres and with the overall width of the other axle being less than that of the first axle; and (iii) a mass greater than 600 kg but less than 3000 kg, unladen, including the ROPS and tyres of the largest size recommended by the manufacturer. On the basis of the protective structure type, the reduced zone of clearance for the driver and track width, these Standards foresee two preliminary tests for evaluating the lateral stability and the non-continuous rolling behaviour of the machine. These preliminary tests must be carried out with ‘the tractor fitted with tyres having the greatest diameter indicated by manufacturer and the smallest cross-section for tyres of that diameter’. For the lateral stability the angle of inclination of the tractor must be at least 388 at the moment when it is resting in a state of unstable equilibrium of the wheel touching the ground, while the non-continuous rolling test refers to a slope of 1/15 (about 348). The manufacturers of narrow-track tractors very often allow the tractors to be fitted with a wide range of tyres to match the tractor configurations with different field conditions occurring during normal
tractor operation. In particular, for forage harvesting on mountain and hill lands, extra-wide low-pressure flotation tyres, known with the familiar name of grassland tyres, are widely used that make the minimum track width of the narrow tractors, mainly in the type with four wheels of the same size, always wider than 1150 mm. As Standards are currently defined these tyre types are not allowed and the tractor in this configuration is not a narrow-track tractor. This appears to be in contradiction with another point of the Standards field of application. In fact the Standards allow the tractor track width to be adjusted, because in the field use of the tractor there must be the need to exceed the width of 1150 mm. The only compulsory point is that the minimum track width has to be less than 1150 mm. The track width definition in the field of application of the Standards for narrow-track wheeled tractors is currently under discussion and a clear demonstration of this are not only some proposals for amending the OECD Code 6 (OECD, 2001, 2002b), but also the differences between the fields of application of the OECD and ISO Standards.
1.2. Objective of the research The objective of this study is to evaluate if a narrowtrack tractor can be fitted with tyres that cause the minimum track width to exceed 1150 mm, because these configurations allow the same level of driver protection ascertained by the current Standards. This would allow the field of application of the Standards to be extended to tyre types widely used on narrow-track tractors and manufactured for matching the tractors to normal operating conditions for these tractors.
2. Materials and methods Eight narrow-track tractors, two and four wheel drive, available on the market have been selected (Table 1). Tests on the protective structures, according to OECD Code 6, have been carried out at the Laboratory of the Department of Agricultural Economics and Engineering of the University of Bologna. Preliminary tests were done on the tractors fitted with tyres having the greatest diameter indicated by the manufacturer and the smallest cross-section width for tyres of that diameter in the minimum track width, as foreseen by the OECD Code. The maximum track width with the same tyres was also tested. Tests were repeated with all the other tyres specified for the tractors, in the
ARTICLE IN PRESS NARROW-TRACK WHEELED AGRICULTURAL TRACTORS
configurations of minimum and maximum track width per tyre type. For the lateral stability test the tractors were placed on a horizontal plane so that the tractor front-axle pivot point could move freely. The part of the tractors rigidly connected to the axle bearing more than 50 per cent of the tractor weight was tilted, using a reclining plane, to
Table 1 Characteristics of the two wheel drive (2WD) and four wheel drive (4WD) tractors tested Tractor identification T1 T2 T3 T4 T5 T6 T7 T8
4WD 2WD 4WD 2WD 4WD 4WD 4WD 2WD
Tractor
Mass, kg
Number of specified tyre types
isodiametric vineyard reversible orchard orchard isodiametric isodiametric vineyard
1848 1270 700 1580 2408 1678 1130 1952
9 7 4 4 5 3 5 9
Fig. 1. Lateral stability test; a reclining platform for tilting the tractor up to a state of unstable equilibrium on the wheels touching the ground
77
determine the angle at the moment when the tractors were resting in a state of unstable equilibrium on the wheels touching the ground (Fig. 1). The test was performed once with the steering wheel turned to full right lock and once with it turned to full left lock. The non-continuous rolling test was done using the calculation method with the computer programme listed in OECD Code 6, by previous evaluation of the characteristic of the tractors required for the calculation and measurement of the height of the centre of gravity and the moment of inertia at the longitudinal axis through the centre of gravity. These last two parameters were determined by means of an oscillating platform which can carry the tractors (Fig. 2), according to the method studied by Casini-Ropa (1976). This method is, currently considered the most reliable, even if it involves laborious and not completely certain measurements (Fabbri and Molari, 1999).
3. Results In the results presented below, reference is made mainly to the data for two narrow-track tractors as they are representative of the entire group of eight tractors tested in terms of lateral stability and non-continuous rolling behaviour. Tables 2 and 3 show that the tractor configuration foreseen for the preliminary tests by Standards was the most stringent for lateral stability, with a measured angle of 53 and 508, respectively. All other tyre types, in the different tractor configurations tested, gave higher angles for the tractor state of unstable equilibrium, always much higher than the 388 set in the Standards. Similar results were obtained in all tractors tested (Table 4) and this indicates that the tyres with the greatest diameter and the minimum cross-section width, when the tractor minimum track width is set, allowed the minimum angle for lateral stability. The grassland tyre types, with fixed track width of over 1150 mm (last two tyre types in Table 2 and last one in Table 3), did not cause lateral stability problems when fitted on the narrow-track tractors. These tyre types showed a lateral stability limit angle comparable to the angles measured for the other tyre types tested, when set at maximum track width. The results of the tests on non-continuous rolling behaviour (Tables 2 and 3) show that the tractor preparation foreseen by the Standards effectively prevented the tractor’s continuous rolling; on the contrary all the other tractor preparations tested gave negative results, with the exception of some preparations, in any case referring to track width of less than 1150 mm.
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G. MOLARI; V. RONDELLI
Fig. 2. Determination of centre of gravity co-ordinates
Table 2 Results of the tests on the four wheel drive, isodiametric tractor T1; lateral stability angle and behaviour of the tractor in terms of non-continuous rolling referred to all the tyres recommended by the manufacturer, in the configuration of minimum and maximum track width for each one Tyre size *
112 R20 112 R20y 275/80 R18 280/70 R18 300/70 R20 320/70 R20 11 LR 16 320/65 R18 400/55–175z 31 1550–15z
Track width, mm
Tyre width, mm
Lateral stability angle, degree
Non-continuous rolling behaviour
1130 1485 1140–1400 1140–1400 1125–1410 1140–1485 1140–1400 1140–1400 1285 1295
275 275 280 280 295 320 280 320 400 395
53 63 55–63 56–63 54–64 53–64 57–63 55–64 62 63
Positive Negative Negative Negative Positive only with 1125 mm track width Positive only with 1140 mm track width Negative Negative Negative Negative
*
Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the minimum track width. Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the maximum track width. z Grassland tyre types. y
It can be observed that the tractor preparations that allowed the best safety conditions for tractor lateral stability did not coincide with those that must be preferred in terms of the non-continuous behaviour of the tractor in the case of an accidental overturn laterally on a slope. For the tractor preparations that did not give positive results in the non-continuous rolling tests, including those fitted with grassland tyre types, a new geometry of the protective structures should be studied.
4. Discussion The two preliminary tests, on lateral stability and non-continuous behaviour, have been considered separately for presenting the results even if they are part of a single test procedure. These two tests are two welldefined stages that must be satisfactorily completed in the preparation foreseen by the Standards. The static lateral stability of the tractor must first be verified, in a tractor configuration which was shown to be the most stringent for the group of tractors analysed; then the
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NARROW-TRACK WHEELED AGRICULTURAL TRACTORS
Table 3 Results of the tests on the two wheel drive, vineyard tractor T8; lateral stability angle and behaviour of the tractor in terms of noncontinuous rolling referred to all the tyres recommended by the manufacturer, in the configuration of minimum and maximum track width for each one Front and rear tyre size
Track width, mm
Tyre width, mm
Lateral stability angle, degree
Non-continuous rolling behaviour
825–168 112 R28*
1135 1040
167 286
50
Positive
825–168 112 R28y
1495 1470
167 286
62
Negative
280/70 R18 360/70 R28
1241–1439 1100–1470
280 352
51–62
Positive only with 1100 mm rear track width
250/80 R18 124 R24
1150–1495 1100–1470
250 375
52–62
Negative
280/70 R18 420/70 R24
1241–1439 1100–1470
282 415
52–64
Negative
210/80 R16 320/70 R24
1182–1438 1104–1448
214 316
52–63
Negative
280/70 R16 380/70 R24
1192–1448 1102–1450
284 370
52–61
Negative
200/70 R16 320/70 R24
1182–1438 1104–1448
208 316
53–63
Negative
240/70 R16 320/70 R24
1182–1438 1104–1448
242 316
53–63
Negative
1350 1307
395 462
67
Negative
z
29 1250–15 44 1800–20z *
Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the minimum track width. Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the maximum track width. z Grassland tyre types). y
non-continuous rolling test must be performed. In the trial this test did not give positive results in the tractor preparations that represented the best conditions in terms of lateral stability. These results, apparently contradictory, are logical if the two tests are considered strictly linked together. First of all tractor lateral stability is verified in the worst tractor preparation for this aspect, and then, only if the angle of inclination is at least 388 at the moment when a tractor is resting in a state of unstable equilibrium on the wheels touching the ground, the non-rolling behaviour of the machine has to be checked, with the same tractor preparation, in the event of its overturning laterally on a 348 slope. It seems unlikely to be fortuitous that the angles considered in the two tests are so close. The type of protective structure allowed for the narrow-track tractor, when it is mounted in front of the driver’s seat and is the dual-pillar type to be folded down if necessary, has been studied working in field conditions where the space around the tractor is often limited. This is necessary for working requirements but is also recognised as being less effective than the other
Table 4 Lateral stability angle of the eight tractors tested statically Tractor identification
T1 T2 T3 T4 T5 T6 T7 T8
Lateral stability angle, degree Tractor configuration foreseen by Standards
Tractor configuration allowed by specified tyres
53 39 48 46 40 51 42 50
54–64 39–51 50–64 47–60 41–56 52–69 44–56 51–67
types of protective structures (frame, cab, rear nonfolding roll-bar). It is thus necessary to verify not just the strength of the structure itself but also the tractor behaviour in the most severe preparation for lateral stability and, because the dual-pillar structure is
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G. MOLARI; V. RONDELLI
considered unable to protect the driver during a continuous roll-over of the tractor if it overturns laterally on a slope, in the same tractor preparation, it must also be tested if the protective structure is designed to prevent continuous rolling over. Evaluation of the non-continuous rolling behaviour must thus be performed in the most severe tractor preparation for lateral stability. If all the above provides an acceptable safety level for the driver of a narrow-track tractor, taking into consideration that the field of application of the Standards allow a fixed or adjustable track width, it means that in the field use of the tractor it would be possible to exceed the minimum track width of 1150 mm. It also appears that the tyre types, which often cause a fixed track width of more than 1150 mm, can be fitted on the narrow-track tractor because their behaviour is comparable to that of the allowed tyre types when the maximum track width is set.
5. Conclusions In the light of the previous considerations it would be advisable to modify the field of application of the Standards for agricultural narrow-track wheeled tractors in the point referring to tractor minimum track width as follows: fixed or adjustable minimum track width of one of the two axles of less than 1150 mm, when fitted with tyres of the greatest diameter indicated by the manufacturer, and the overall width of the other axle being less than that of the first axle. Adopting this amendment, it would be advisable to include in the tractor test report the list of tyres recommended by the manufacturer in order to avoid any subsequent alteration of specified tyre types. The proposal modifies one of the main parameters for defining a narrow-track tractor. However, this tractor is still specific and well-defined, with overall dimension, mass, clearance zone for the driver and clearance zone around the power take off differing from those of other tractor types. The proposal eliminates the anomaly which foresees fitting tyres on the narrow tractor that, even if allowing a minimum track width of less than 1150 mm, a wider track width can be set with a driver safety level exactly corresponding to that of tyres not allowed by the Standards because they cannot be set to a track of less than 1150 mm. The proposal, if accepted by all the regulating Organisations, would unify the field of application of the Standards for agricultural narrow-track wheeled tractors and define these tractors clearly and uniformly.
A possible alternative, in terms of increasing driver safety and minimising the frequency and severity of driver injury resulting from accidental overturning during normal tractor operation, is to limit the narrow-track tractor to a set track width always less than 1150 mm, even if this would reduce the normal use of this type of tractor.
References Arndt J F (1971). Roll-over protective structure for farm and construction tractors}a 50 year rewiew. SAE Technical Paper Series, 710508, Warrendale, PA Casini-Ropa G (1976). Attrezzatura e metodo per il rilievo dell’altezza da terra del baricentro delle macchine agricole. [Equipment and methods for surveying the height above the ground of the centre of gravity of agricultural machinery.] Rivista di Ingegneria Agraria, 2, 81–85 EEC (1986). Directive 86/298/EEC. European Economic Community, Strasbourg EEC (1987). Directive 87/402/EEC. European Economic Community, Strasbourg Fabbri A; Molari G (1999). La propagazione degli errori nella misura dell’altezza da terra del baricentro dei trattori agricoli. [Error propagation in measuring height from the ground of the tractors centre of gravity.] Rivista di Ingegneria Agraria, 4, 224–229 ISO 12003-1 (2002). Agricultural and Forestry Tractors} Narrow-track Wheeled Tractors}Part 1: Front-mounted Roll-over Protective Structures. International Organisation for Standardisation, Geneva Myers M L (2000). Prevention effectiveness of rollover protective structure. Journal of Agricultural Safety and Health, 6, 29–70 OECD (1959). Standard Codes for the Official Testing of Protective Structure Mounted on Agricultural Tractors. Organisation for Economic Co-operation and Development, Paris OECD (1990a). CODE 6}Standard Codes for the Official Testing of Front-mounted Protective Structures on Narrowtrack Wheeled Agricultural and Forestry Tractors. Organisation for Economic Co-operation and Development, Paris OECD (1990b). CODE 7}Standard Codes for the Official Testing of Rear-mounted Protective Structures on Narrow-track Wheeled Agricultural and Forestry Tractors. Organisation for Economic Co-operation and Development, Paris OECD (2001). Applicability of code 4 or 6 and 7 when 1150 mm tractor minimum track width may be exceeded, AGR/ CA/T(2001)3. Organisation for Economic Co-operation and Development, Paris OECD (2002a). Standard Codes for the Official Testing of Agricultural and Forestry Tractors. Organisation for Economic Co-operation and Development, Paris OECD (2002b). Applicability of code 6 or 7 when 1150 mm track width may be exceeded, AGR/CA/T(2002)28. Organisation for Economic Co-operation and Development, Paris
Biosystems Engineering (2004) 88 (1), 75–80 doi:10.1016/j.biosystemseng.2004.02.001 PM–Power and Machinery
On the Definition of Narrow-track Wheeled Agricultural Tractors G. Molari; V. Rondelli Department of Agricultural Economics and Engineering, Bologna University, viale G. Fanin, 50, 40127, Italy; e-mail of corresponding author: [email protected] (Received 19 July 2003; accepted in revised form 10 February 2004; published online 9 April 2004)
The narrow-track agricultural tractor has been defined in the field of application of International Organisation for Standardisation (ISO), European Community (EC) and Organisation for Economic Co-operation and Development (OECD) Standards. The field of application does not allow narrow-track tractors to be fitted with tyres set at a minimum track width wider than 1150 mm, enabling tyres to be fitted with adjustable track width which, still having a minimum track width of not more than 1150 mm, can be set at greater track widths during normal operation of the tractor. Tests showed that tyre types which can be fitted on a narrow-track tractor and always determine a tractor track width wider than 1150 mm, have the same behaviour as the tyre types currently allowed in the field of application of the Standards, when set at the allowed maximum track widths. In the light of these results, it is recommended to define a narrow-track tractor as an agricultural tractor that, when fitted with tyres of the greatest diameter indicated by the manufacturer, has a minimum track width of not more than 1150 mm. # 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
1. Introduction
the OECD (OECD, 1990a, 1990b) developed specific Directives and Codes to evaluate the strength of the protective structures mounted on these tractors. The International Organisation for Standardisation (ISO) has also recently developed a new standard test for the narrow-track wheeled tractors (ISO, 2002). At present, OECD test procedures are published in eight Codes (OECD, 2002a) and among these, OECD Codes 6 and 7 are specific to narrow-track wheeled agricultural and forestry tractors, with front and rear mounted ROPS, respectively. In detail, OECD Code 6 and the corresponding EC Directive 87/402/EEC, in the field of application, specify the following characteristics for narrow-track tractors:
1.1. General considerations and problem description The large number of agricultural tractors overturning every year, widely reported in studies since 1931 (Arndt, 1971; Myers, 2000), focused attention on the public health problem of protecting the tractor operator from the risk of an overturn and supported the introduction of a roll-over protective structure (ROPS) on tractors. Firstly Sweden, in 1959, and then all the other European countries, adopted a regulation requiring ROPS on all new tractors. Also in 1959, the Organisation for Economic Co-operation and Development (OECD) developed a test procedure to evaluate the strength of the structures and established energy criteria (OECD, 1959). At the end of the 1980s, as a consequence of the expanding market for narrow-track tractors, mainly for use in orchards and vineyards, where the space above or at the sides of the tractor is often limited, first the European Community (EC) (EEC, 1986, 1987) and then 1537-5110/$30.00
(a) ground clearance of not more than 600 mm beneath the lowest points of the front and rear axles, allowing for the differential; (b) fixed or adjustable minimum track width with one of the axles less than 1150 mm fitted with tyres of a larger size; it is assumed that the axle mounted with the wider tyres is set at a track width of not more 75
# 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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G. MOLARI; V. RONDELLI
than 1150 mm; it must be possible to set the track width of the other axle in such a way that the outer edges of the narrower tyres do not go beyond the outer edges of the tyres of the other axle; where the two axles are fitted with rims and tyres of the same size, the fixed or adjustable track width of the two axles must be less than 1150 mm; (c) mass greater than 600 but less than 3000 kg, corresponding to the unladen weight of the tractor, including the roll-over protective structure and tyres of the largest size recommended by the manufacturer; and (d) being fitted with roll-over protective structures of the dual-pillar type mounted in front of the driver’s seat and characterised by a reduced zone of clearance attributable to the tractor silhouette, thus rendering it inadvisable, under any circumstances, to impede access to the driving position but worthwhile retaining these structures (fold-down or not) in view of their undoubted ease of use. The ISO Standard 12003-1, equivalent to OECD Code 6, is applicable to tractors equipped with front mounted ROPS, having the following characteristics: (i) a ground clearance of not more than 600 mm beneath the lowest points of the front and rear axles, housing; (ii) a fixed or adjustable minimum track width of one of the two axles of less than 1150 mm fitted with the widest specified tyres and with the overall width of the other axle being less than that of the first axle; and (iii) a mass greater than 600 kg but less than 3000 kg, unladen, including the ROPS and tyres of the largest size recommended by the manufacturer. On the basis of the protective structure type, the reduced zone of clearance for the driver and track width, these Standards foresee two preliminary tests for evaluating the lateral stability and the non-continuous rolling behaviour of the machine. These preliminary tests must be carried out with ‘the tractor fitted with tyres having the greatest diameter indicated by manufacturer and the smallest cross-section for tyres of that diameter’. For the lateral stability the angle of inclination of the tractor must be at least 388 at the moment when it is resting in a state of unstable equilibrium of the wheel touching the ground, while the non-continuous rolling test refers to a slope of 1/15 (about 348). The manufacturers of narrow-track tractors very often allow the tractors to be fitted with a wide range of tyres to match the tractor configurations with different field conditions occurring during normal
tractor operation. In particular, for forage harvesting on mountain and hill lands, extra-wide low-pressure flotation tyres, known with the familiar name of grassland tyres, are widely used that make the minimum track width of the narrow tractors, mainly in the type with four wheels of the same size, always wider than 1150 mm. As Standards are currently defined these tyre types are not allowed and the tractor in this configuration is not a narrow-track tractor. This appears to be in contradiction with another point of the Standards field of application. In fact the Standards allow the tractor track width to be adjusted, because in the field use of the tractor there must be the need to exceed the width of 1150 mm. The only compulsory point is that the minimum track width has to be less than 1150 mm. The track width definition in the field of application of the Standards for narrow-track wheeled tractors is currently under discussion and a clear demonstration of this are not only some proposals for amending the OECD Code 6 (OECD, 2001, 2002b), but also the differences between the fields of application of the OECD and ISO Standards.
1.2. Objective of the research The objective of this study is to evaluate if a narrowtrack tractor can be fitted with tyres that cause the minimum track width to exceed 1150 mm, because these configurations allow the same level of driver protection ascertained by the current Standards. This would allow the field of application of the Standards to be extended to tyre types widely used on narrow-track tractors and manufactured for matching the tractors to normal operating conditions for these tractors.
2. Materials and methods Eight narrow-track tractors, two and four wheel drive, available on the market have been selected (Table 1). Tests on the protective structures, according to OECD Code 6, have been carried out at the Laboratory of the Department of Agricultural Economics and Engineering of the University of Bologna. Preliminary tests were done on the tractors fitted with tyres having the greatest diameter indicated by the manufacturer and the smallest cross-section width for tyres of that diameter in the minimum track width, as foreseen by the OECD Code. The maximum track width with the same tyres was also tested. Tests were repeated with all the other tyres specified for the tractors, in the
ARTICLE IN PRESS NARROW-TRACK WHEELED AGRICULTURAL TRACTORS
configurations of minimum and maximum track width per tyre type. For the lateral stability test the tractors were placed on a horizontal plane so that the tractor front-axle pivot point could move freely. The part of the tractors rigidly connected to the axle bearing more than 50 per cent of the tractor weight was tilted, using a reclining plane, to
Table 1 Characteristics of the two wheel drive (2WD) and four wheel drive (4WD) tractors tested Tractor identification T1 T2 T3 T4 T5 T6 T7 T8
4WD 2WD 4WD 2WD 4WD 4WD 4WD 2WD
Tractor
Mass, kg
Number of specified tyre types
isodiametric vineyard reversible orchard orchard isodiametric isodiametric vineyard
1848 1270 700 1580 2408 1678 1130 1952
9 7 4 4 5 3 5 9
Fig. 1. Lateral stability test; a reclining platform for tilting the tractor up to a state of unstable equilibrium on the wheels touching the ground
77
determine the angle at the moment when the tractors were resting in a state of unstable equilibrium on the wheels touching the ground (Fig. 1). The test was performed once with the steering wheel turned to full right lock and once with it turned to full left lock. The non-continuous rolling test was done using the calculation method with the computer programme listed in OECD Code 6, by previous evaluation of the characteristic of the tractors required for the calculation and measurement of the height of the centre of gravity and the moment of inertia at the longitudinal axis through the centre of gravity. These last two parameters were determined by means of an oscillating platform which can carry the tractors (Fig. 2), according to the method studied by Casini-Ropa (1976). This method is, currently considered the most reliable, even if it involves laborious and not completely certain measurements (Fabbri and Molari, 1999).
3. Results In the results presented below, reference is made mainly to the data for two narrow-track tractors as they are representative of the entire group of eight tractors tested in terms of lateral stability and non-continuous rolling behaviour. Tables 2 and 3 show that the tractor configuration foreseen for the preliminary tests by Standards was the most stringent for lateral stability, with a measured angle of 53 and 508, respectively. All other tyre types, in the different tractor configurations tested, gave higher angles for the tractor state of unstable equilibrium, always much higher than the 388 set in the Standards. Similar results were obtained in all tractors tested (Table 4) and this indicates that the tyres with the greatest diameter and the minimum cross-section width, when the tractor minimum track width is set, allowed the minimum angle for lateral stability. The grassland tyre types, with fixed track width of over 1150 mm (last two tyre types in Table 2 and last one in Table 3), did not cause lateral stability problems when fitted on the narrow-track tractors. These tyre types showed a lateral stability limit angle comparable to the angles measured for the other tyre types tested, when set at maximum track width. The results of the tests on non-continuous rolling behaviour (Tables 2 and 3) show that the tractor preparation foreseen by the Standards effectively prevented the tractor’s continuous rolling; on the contrary all the other tractor preparations tested gave negative results, with the exception of some preparations, in any case referring to track width of less than 1150 mm.
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Fig. 2. Determination of centre of gravity co-ordinates
Table 2 Results of the tests on the four wheel drive, isodiametric tractor T1; lateral stability angle and behaviour of the tractor in terms of non-continuous rolling referred to all the tyres recommended by the manufacturer, in the configuration of minimum and maximum track width for each one Tyre size *
112 R20 112 R20y 275/80 R18 280/70 R18 300/70 R20 320/70 R20 11 LR 16 320/65 R18 400/55–175z 31 1550–15z
Track width, mm
Tyre width, mm
Lateral stability angle, degree
Non-continuous rolling behaviour
1130 1485 1140–1400 1140–1400 1125–1410 1140–1485 1140–1400 1140–1400 1285 1295
275 275 280 280 295 320 280 320 400 395
53 63 55–63 56–63 54–64 53–64 57–63 55–64 62 63
Positive Negative Negative Negative Positive only with 1125 mm track width Positive only with 1140 mm track width Negative Negative Negative Negative
*
Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the minimum track width. Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the maximum track width. z Grassland tyre types. y
It can be observed that the tractor preparations that allowed the best safety conditions for tractor lateral stability did not coincide with those that must be preferred in terms of the non-continuous behaviour of the tractor in the case of an accidental overturn laterally on a slope. For the tractor preparations that did not give positive results in the non-continuous rolling tests, including those fitted with grassland tyre types, a new geometry of the protective structures should be studied.
4. Discussion The two preliminary tests, on lateral stability and non-continuous behaviour, have been considered separately for presenting the results even if they are part of a single test procedure. These two tests are two welldefined stages that must be satisfactorily completed in the preparation foreseen by the Standards. The static lateral stability of the tractor must first be verified, in a tractor configuration which was shown to be the most stringent for the group of tractors analysed; then the
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NARROW-TRACK WHEELED AGRICULTURAL TRACTORS
Table 3 Results of the tests on the two wheel drive, vineyard tractor T8; lateral stability angle and behaviour of the tractor in terms of noncontinuous rolling referred to all the tyres recommended by the manufacturer, in the configuration of minimum and maximum track width for each one Front and rear tyre size
Track width, mm
Tyre width, mm
Lateral stability angle, degree
Non-continuous rolling behaviour
825–168 112 R28*
1135 1040
167 286
50
Positive
825–168 112 R28y
1495 1470
167 286
62
Negative
280/70 R18 360/70 R28
1241–1439 1100–1470
280 352
51–62
Positive only with 1100 mm rear track width
250/80 R18 124 R24
1150–1495 1100–1470
250 375
52–62
Negative
280/70 R18 420/70 R24
1241–1439 1100–1470
282 415
52–64
Negative
210/80 R16 320/70 R24
1182–1438 1104–1448
214 316
52–63
Negative
280/70 R16 380/70 R24
1192–1448 1102–1450
284 370
52–61
Negative
200/70 R16 320/70 R24
1182–1438 1104–1448
208 316
53–63
Negative
240/70 R16 320/70 R24
1182–1438 1104–1448
242 316
53–63
Negative
1350 1307
395 462
67
Negative
z
29 1250–15 44 1800–20z *
Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the minimum track width. Tyres with the greatest diameter and smallest width for tyres of that diameter, set at the maximum track width. z Grassland tyre types). y
non-continuous rolling test must be performed. In the trial this test did not give positive results in the tractor preparations that represented the best conditions in terms of lateral stability. These results, apparently contradictory, are logical if the two tests are considered strictly linked together. First of all tractor lateral stability is verified in the worst tractor preparation for this aspect, and then, only if the angle of inclination is at least 388 at the moment when a tractor is resting in a state of unstable equilibrium on the wheels touching the ground, the non-rolling behaviour of the machine has to be checked, with the same tractor preparation, in the event of its overturning laterally on a 348 slope. It seems unlikely to be fortuitous that the angles considered in the two tests are so close. The type of protective structure allowed for the narrow-track tractor, when it is mounted in front of the driver’s seat and is the dual-pillar type to be folded down if necessary, has been studied working in field conditions where the space around the tractor is often limited. This is necessary for working requirements but is also recognised as being less effective than the other
Table 4 Lateral stability angle of the eight tractors tested statically Tractor identification
T1 T2 T3 T4 T5 T6 T7 T8
Lateral stability angle, degree Tractor configuration foreseen by Standards
Tractor configuration allowed by specified tyres
53 39 48 46 40 51 42 50
54–64 39–51 50–64 47–60 41–56 52–69 44–56 51–67
types of protective structures (frame, cab, rear nonfolding roll-bar). It is thus necessary to verify not just the strength of the structure itself but also the tractor behaviour in the most severe preparation for lateral stability and, because the dual-pillar structure is
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G. MOLARI; V. RONDELLI
considered unable to protect the driver during a continuous roll-over of the tractor if it overturns laterally on a slope, in the same tractor preparation, it must also be tested if the protective structure is designed to prevent continuous rolling over. Evaluation of the non-continuous rolling behaviour must thus be performed in the most severe tractor preparation for lateral stability. If all the above provides an acceptable safety level for the driver of a narrow-track tractor, taking into consideration that the field of application of the Standards allow a fixed or adjustable track width, it means that in the field use of the tractor it would be possible to exceed the minimum track width of 1150 mm. It also appears that the tyre types, which often cause a fixed track width of more than 1150 mm, can be fitted on the narrow-track tractor because their behaviour is comparable to that of the allowed tyre types when the maximum track width is set.
5. Conclusions In the light of the previous considerations it would be advisable to modify the field of application of the Standards for agricultural narrow-track wheeled tractors in the point referring to tractor minimum track width as follows: fixed or adjustable minimum track width of one of the two axles of less than 1150 mm, when fitted with tyres of the greatest diameter indicated by the manufacturer, and the overall width of the other axle being less than that of the first axle. Adopting this amendment, it would be advisable to include in the tractor test report the list of tyres recommended by the manufacturer in order to avoid any subsequent alteration of specified tyre types. The proposal modifies one of the main parameters for defining a narrow-track tractor. However, this tractor is still specific and well-defined, with overall dimension, mass, clearance zone for the driver and clearance zone around the power take off differing from those of other tractor types. The proposal eliminates the anomaly which foresees fitting tyres on the narrow tractor that, even if allowing a minimum track width of less than 1150 mm, a wider track width can be set with a driver safety level exactly corresponding to that of tyres not allowed by the Standards because they cannot be set to a track of less than 1150 mm. The proposal, if accepted by all the regulating Organisations, would unify the field of application of the Standards for agricultural narrow-track wheeled tractors and define these tractors clearly and uniformly.
A possible alternative, in terms of increasing driver safety and minimising the frequency and severity of driver injury resulting from accidental overturning during normal tractor operation, is to limit the narrow-track tractor to a set track width always less than 1150 mm, even if this would reduce the normal use of this type of tractor.
References Arndt J F (1971). Roll-over protective structure for farm and construction tractors}a 50 year rewiew. SAE Technical Paper Series, 710508, Warrendale, PA Casini-Ropa G (1976). Attrezzatura e metodo per il rilievo dell’altezza da terra del baricentro delle macchine agricole. [Equipment and methods for surveying the height above the ground of the centre of gravity of agricultural machinery.] Rivista di Ingegneria Agraria, 2, 81–85 EEC (1986). Directive 86/298/EEC. European Economic Community, Strasbourg EEC (1987). Directive 87/402/EEC. European Economic Community, Strasbourg Fabbri A; Molari G (1999). La propagazione degli errori nella misura dell’altezza da terra del baricentro dei trattori agricoli. [Error propagation in measuring height from the ground of the tractors centre of gravity.] Rivista di Ingegneria Agraria, 4, 224–229 ISO 12003-1 (2002). Agricultural and Forestry Tractors} Narrow-track Wheeled Tractors}Part 1: Front-mounted Roll-over Protective Structures. International Organisation for Standardisation, Geneva Myers M L (2000). Prevention effectiveness of rollover protective structure. Journal of Agricultural Safety and Health, 6, 29–70 OECD (1959). Standard Codes for the Official Testing of Protective Structure Mounted on Agricultural Tractors. Organisation for Economic Co-operation and Development, Paris OECD (1990a). CODE 6}Standard Codes for the Official Testing of Front-mounted Protective Structures on Narrowtrack Wheeled Agricultural and Forestry Tractors. Organisation for Economic Co-operation and Development, Paris OECD (1990b). CODE 7}Standard Codes for the Official Testing of Rear-mounted Protective Structures on Narrow-track Wheeled Agricultural and Forestry Tractors. Organisation for Economic Co-operation and Development, Paris OECD (2001). Applicability of code 4 or 6 and 7 when 1150 mm tractor minimum track width may be exceeded, AGR/ CA/T(2001)3. Organisation for Economic Co-operation and Development, Paris OECD (2002a). Standard Codes for the Official Testing of Agricultural and Forestry Tractors. Organisation for Economic Co-operation and Development, Paris OECD (2002b). Applicability of code 6 or 7 when 1150 mm track width may be exceeded, AGR/CA/T(2002)28. Organisation for Economic Co-operation and Development, Paris