Low frequency angular vibrations in the roll mode on farm tractors

J. agric. Engng Res. (1972) 17, 22-32

Low Frequency Angular Vibrations in the Roll Mode on Farm Tractors L. SWFLOT*; C. W. SUGGst The dynamic behaviour in the roll mode of a tractor, seat, and driver versus dead mass, is studied on a test track by help of motion picture from behind and linear acceleration components measured on the seat. The 1·6 and 6 in high track obstacles and the driving speed were adjusted to give the worst conditions in the side to side mode using the inflation pressures 8, 11, and 17 lb/in 2 in the rear tyres. A comparison of seats and effects of roll vibration on man was carried out on a vibration simulation device using the excitation frequencies 1, 1'7, 2'5, and 4 Hz. The natural frequency in the roll mode was in the range 1·5-1 '9 Hz. The rotational point for seat roll displacement was found to be 46--66 in below the centre of gravity of the tractor . Two spring suspended seats had a considerable rotation of their own, creating worse conditions than a rigid seat under exposure to roll vibrations. The human reactions, recorded as heart rate, tracking performance, subjectiverating and linear acceleration on hips and shoulders, showed marked differences in the quality of seats concerning roll mode excitation. 1. Introduction Vertical vibrations on farm tractors have been considered in several earlier investigations . 1 . 2 . 3 . 4 .5 .6 .7 Such studies have led to improvement of seats by the development of proper damping suspension for vertical movements. Vibrations in horizontal direction (horizontal linear components) have also been reported studied." 4. 6. 8 These results have not yet led to any direct improvement of the seating conditions. It seems now to be agreed that the human tolerance to vibrations is lower for horizontal directions than for vertical. 9 Angular vibrations, in the roll and pitch mode, are also reported to be worse than pure linear vertical vibrations. 10 The evaluation criteria are based on pure linear horizontal or vertical vibrations. This type of vibration is rarely found on farm tractors, but rather a complexity of all forms of vibrations. I I Angular vibrations on tractors are not easy to study. Matthews: as the only known work on this matter, has found maximum angular oscillations of 3° for both the pitch and the roll mode when driving over rough pasture. Crossing newly ploughed land obliquely the maximum angular displacement was 6-7° in the pitch mode and 3·5-4° in the roll mode. Crossing the same land perpendicularly the angular roll displacement was up to 8°. The main frequencies for the angular oscillations were ",4 Hz in the pitch mode on the pasture and < 2 Hz for the roll mode in all experiments and for the pitch mode on ploughed land. The linear acceleration components were calculated, assuming that the rotational axis goes through the centre of gravity. Several works based on models and computer simulation have also given some insights into the behaviour of tractors" 12, 13 . '4. There are, however, many difficulties in establishing useful technical and mathematical models of tractor-seat/driver systems for the angular and horizontal modes which are valid for practical conditions. This study aims at giving a simple picture of what happens on a current "good" tractor seat (good for vertical vibrations) in the side to side mode . This roll mode seems to be most important, with respect to the magnitude of vibrations and to the vibration influence on man. The main goal is to clarify what to stress in further research and how best to evaluate the effects of vibrations acting on tractor dri vers. The results should also give some data and ideas for further theoretical and simulation studies . • The Norwegian Institute of Agricultural Engin eering , 1432 As-NLH, Norway

t North Carolina State Universit y. Raleigh . 27607 N.C.• U.S.A.

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L. SJ0FLOT; C. W . SUGGS

2.

Method and equipment 2.1. Tractor and seat The tractor used had a 48 h.p . engine and was equipped with tyres 6·00--16 (front) and 13·6-28 (rear). The total weight was 3860 Ib with 1630 Ib on the front axle and 2230 lb on the rear axle standing on horizontal ground. The centre of gravity was in the median plane 31 in in front of the rear axle centre and 29·5 in above the ground (inflation pressure l l lb/in" in the rear tyres). For the experiments the inflation pressure was 30 lb/in" in the front tyres and varied in the steps 8, II and 17 lb/in 2 in the rear tyres. The centre distance between the rear wheels was 56·3 in. The seat had parallelogram suspension, torsion spring and hydraulic shock absorber. The upper part formed a seat bowl 18 in wide and 13·5 in deep ending in an II in high back rest. The upholstery was 1·5 in thick. The seat was located >- 13 in in front of the rear axle centre and ,..,42·5 in above the ground (driver loaded). The natural angular frequency in the roll mode was found from acceleration recordings by forcing the tractor manually with a long beam from the left wheel. 2.2. Driving experiments on test track For the basic experiments a test track provided with obstacles for the rear wheels was used. The front wheel distance was extended as wide as possible so that the front wheels could pass outside the obstacles. For one part of the experiments the track consisted of a series of 6 in high obstacles 31·5 in apart in one wheel track only; and in another part of 1·6 in high obstacles in both tracks, 31·5 in apart, but with one row staggered 15·75 in compared to the other, giving the worst conditions in the roll mode. The driving speed (,..,2·5 mile/h) was adjusted to give the worst conditions, i.e. to pass the obstacles with a frequency close to the natural angular frequency ofthe tractor in order to get resonance. The observed variables were the linear acceleration in the three main directions on the tractor body and on the seat cushion under the driver. Motion pictures from behind were taken to study the displacement behaviour of the tractor, the seat and the driver. The basic reference for the evaluation of the motion pictures was a frame on a slide behind the tractor, towed from the front end (Fig. 1).

Fig. I . Schematic view of the situation making the movie shots, showing the camera (A), the towed slide (B) carrying the reference fram e (C) , and the bar on the tractor (D) and on the seat (E)

2.3. Laboratory experiments on vibration platform A comparison of seats and vibration effects on man was made on an electrohydraulic vibration simulation device (Fig. 2). Because of the joints A, B, and C the bar D will create angular movements around the point C when the piston E moves up and down. The kind of movements depends on the location of the seat F, as shown by arrows for the indicated positions I and 2. In position 3 the seat was directly attached to the piston and had therefore only vertical movements. The radius of the angular movements in these studies was 62 in in Pos . I and 67 in in Pos. 2. The excitation force on the piston is electronically controlled. In these experiments only a sine wave input from a wave generator was used. The acceleration measured on the seat just under the "driver" was maintained at the desired level for each experiment by adjusting the voltage input to the servo valve G .

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LOW FREQUENCY ANGULAR VIBRATIONS ON FARM TRACTORS

Fig. 2. Electro-hydraulic vibration simulation device for studying effects of angular movements on tractor seats and tractor drivers

Two spring suspended seats were used in comparison with the rigid seat (Seat I) shown in Fig. 2. One of them (Seat 2) was the tractor seat already described. The other one (Seat 3) also had parallelogram spring suspension and hydraulic shock absorber. The seating area was 19 in broad and 16 in deep, and the back rest, mounted separately from the seating area, was 20 in high. The upholstery was 3 in thick. The preloading of the spring was adjustable to the "operator's" weight for both seats 2 and 3. The recorded variables were heart rate , tracking error, linear acceleration on the hips and shoulders and subjective judgement of two male subjects over a vibration exposure period of 10 min. The vibration intensity was set to 0·5 g linear component (horizontal in Pos. I and 2, vertical in Pos. 3) at the frequencies I, 1'7, 2'5, and 4 Hz. The tracking task consisted of following a stepwise varying pattern pre viously drawn on a paper recorder chart. The recorder pen was electrically connected to a control lever (Fig. 2, H) so that it responded linearly to motion of the control in the fore and aft direction. The tracking error was evaluated as response/settling time for eight step responses and as deviation from the given straight line between the step variations.

3. Results and discussion 3.1. Natural frequency in the roll mode By forcing the tractor manually in side to side angular motion the natural roll frequency from recordings on the seat loaded with a 100 Ib weight was found to be 1·68 Hz for tyre pressures of 8 and l l Ib/in", and 1·85 Hz for 17Ib/in 2 • The natural frequencies in the vertical mode at the same time were 3·3 Hz and 3·7 Hz respectively. This agrees well with other reports on such natural frequencies. The 100 Ib weight was selected because it is the approximate seat load imposed by the driver in these tests. The driving experiments on the test track gave the main roll frequency 1·5 Hz for 8 and 11 Ib/ in 2 and 1·6 Hz for 17 Ib/in 2 inflation pressure. 3.2. Linear acceleration components in vertical and side to side direction The ratio of linear horizontal, side to side acceleration to vertical acceleration measured on the seat when forcing the tractor manually was within 3·8-4 ·8 for all the three tyre pressures used. In

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L. SJ0FLOT; C. W. SUGGS

the driving experiments this ratio was found to be 3·1-4·0 using the 100 Ib weight on the seat and 1,8-3,2 with a 150-lb driver on the seat. This decrease in the ratio is due to lower side to side acceleration under the driver than under the dead mass. The vertical component is generally about the same for the driver and for the weight. The driver seems to apply some active damping in the horizontal direction. On the tractor body beneath the seat the horizontal side to side/vertical acceleration ratio was about I for all inflation pressures and conditions used in these driving experiments. Here the side to side component is about t that of the seat and the vertical component about twice as high as on the seat. The seat reduces the vertical vibrations and amplifies vibrations in the roll mode and side to side direction. The acceleration level was in the range of 0·4---0·8 g* in both the vertical and the side to side direction on the tractor body, and 0·3---0·5 g vertical and 0·6-1,6 g side to side on the seat.

3.3. Static measurements and dynamic behaviour of the tractor, seat, and driver Having the tractor standing with the left rear wheel on a plane of the same height as the obstacles used (Fig. 3), the angular displacements could be determined by static measurements of T

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the location of the bar fastened to the tractor body A and to the seat B. At a height of 1·6 in the angular displacement of both bars was 1.8 for 8 lb/in 2 inflation pressure and I .2 for 17 lb/in 2. At a height of 6 in the angular displacement was 6.90 and 6.20 respectively. The larger angles associated with the lower tyre pressure were due to sideways deflection of the tyres. The seat, loaded with a 100 Ib weight, followed the tractor quite well. No noticeable relative movements between seat and tractor were observed. The rotation point C appeared to be ""25 in below the centre of gravity or '" 37·5 in below the mass-loaded seat. The linear displacement components on the seat for the highest obstacle were 5 in and 4·4 in side to side, and 3· I in and 3·5 in vertical, for the inflation pressures 8 and 17 Ib/in 2, respectively. This gives a horizontal/vertical components ratio of 1·6 and 1·3. 0

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LOW FREQUENCY ANGULAR VIBRATIONS ON FARM TRACTORS

The dynamic behaviour is studied by redrawing every two frames of the motion pictures, adjusting the size and space orientation to the frame on the towed slide (Fig. 1). Examples of the extreme movements over one "cycle", i.e. passing one obstacle or one on each side, are shown with the driver in the seat in Figs 4 and 5. From such drawings the angular and linear displacement are measured and averaged for 3-4 "cycles" in each experiment.

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The motion of the seat as the tractor passes over the 1·6 in obstacles is given in Table 1. The total displacement increases with increasing inflation pressure; the opposite of what was found under static conditions. The horizontal side to side component in all cases is much larger than the vertical, giving a H/V-ratio of 6-10. The rotational point appears to be ",65,5 in below the centre of gravity for 8 lb/in 2 inflation pressure, '" 57·7 in below the centre of gravity for II lb/in 2 and ",45,7 in below the centre of gravity for 17Ib/in 2 • The rotational radius from the seat will then be '" 78 in, 70 in and 58·3 in respectively, for these three inflation pressures. The lowering of the rotational point from the centre of gravity, also found in the static situation, is due to sideways tyres deflection together with the real rotation around the centre of gravity. There were some smaller, but hardly measurable relative angular movements between the seat and the tractor. For passing the 6 in obstacles under the left rear wheel (Fig. 5), the results in Table II show the same tendency as for the other condition. The displacement is larger, but the H/V-ratio, in the

27

L. SJ0FLOT; C. W. SUGGS

range of 3-7, is a little lower. The rotational radius is about the same as in the other series. The relative movements of the seat are clear under these conditions, especially in the vertical direction. The larger angular displacement of the seat, measured to 1--4° more than on the tractor, is due to the seat's own rotation in the suspension. /

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TABLE

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Displacement at the tractor seat passing 1·6 in high obstacles placed on both sides, in this way:

Linear seat displacement components Inflation pressure (lbfin 2 )

Max. angular seat displacement (deg)

8

2-4 2-6 3-6

11

17

Side to side (in) 2·5 3'2 3-6

Vertical (in) 0-4 0'4 0·3

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LOW FREQUENCY ANGULAR VIBRATIONS ON FARM TRACTORS TABLE

II

Displacement at the tractor seat passing 6 in high obstacles placed on one side, in this way: _ _-+

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Linear seat displacement components Inflation pressure (lbjin 2 )

Max. angular seat displacement (deg)

Side to side (in)

Vertical (in)

8 11 17

3·3 3-6 4·0

4·3 4·4 4'7

0·6 1·2 1·5

Using the 100 lb weight on the seat instead of the driver some changes in the seat movements appeared in both test series. Care should be taken if a dead mass, only, is used to study such kind of vibrations on machine seats. The linear side to side motion of the driver is given in Table III. The hips have generally the same movements as the seat. There is a clear attenuation of the displacement up to the shoulders and generally still more up to the head. The vertical movements of the head were very small in all experiments. TABLE

III

Max. side to side displacement of the driver's body Linear side to side displacement (in) Inflation pressure (lb/in 2 )

Hips Shoulders Head

J'6 in obstacle series

6 in obstacle series

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3.4. Properties of seats concerning side to side angular motion compared to vertical motion Concerning the seats already described Seat 1 (rigid: Fig. 2) was covered with a l-in thick cushion. Seat 2 showed a natural frequency in the vertical mode of ,..., 2·5 Hz, measured with the 100 lb weight placed on the upholstery of the complete seat. Seat 3 showed no marked natural frequency (vertical), but had an amplification considering the mass/platform displacement over the whole frequency range 2,5-5,8 Hz. Separated the spring suspension part had a natural frequency of ,...,2·3 Hz and the 3 in thick upholstery of 4-5 Hz. Seat 3, and especially Seat 2, had a rather strong damping contributing masking any resonance in the frequency response experiments. The location of the seats on the vibration device (Fig. 2) were chosen to get conditions close to what was found in the driving experiments. The seating area of a rigid seat will have almost pure angular motion in Pos. 1 and a horizontal/vertical displacement ratio of 2·4 in Pos. 2, giving angular displacement for the actual experiment conditions as shown for Seat 1 in Table IV. The further results in the table are based on recordings on the seat pans, averaged for two "drivers" of weight 170 and 210 lb. The excitation acceleration of 0·5 g linear component was about the largest feasible for the experiment period of 10 min.

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L. SJ0FLOT; C. W. SUGGS

IV

TABLE

Angular displacement of the seats in the experiments on vibration platform using excitation acceleration of 0·5 g linear component side to side

Total angular displacement (deg) Excitation frequency

Position 1

Position 2

(Hz)

I 1·7 2·5 4

Seat I (rigid)

Seat 2

Seat 3

Seat J (rigid)

Seat 2

Seat 3

2·90 1·04 0·56 0·24

3·04 1·14 0·56 0·26

3·18 \·16 0·58 0·26

2·94 1·10 0'56 0·24

3·14 1-20 0·58 0·22

3·06 1·14 0·54 0·24

The pan of the spring suspended seats shows generally larger angular movements than the rigid seat, especially for the lower frequencies. Seat 3 appears to be poorer than Seat 2 for the pure angular movements (Pos. I), but better than Seat 2 for the combination of side to side and vertical motion in Pos. 2 Both the seats 2 and 3 have marked roll movements of their own around an axis through the seating area (Fig. 6). These movements are largest for Seat 2, and larger on both seats for the heaviest "driver".

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LOW FREQUENCY ANGULAR VIBRATIONS ON FARM TRACTORS

The linear displacement components of the seat pans are similar to the angular displacement. For the frequencies 1 and 1·7 Hz in Pos. 2 the conditions on Seat 2 and 3 are worst for the heaviest "driver", and in Pos. 1 at the same frequencies for the lightest. When exposed to vertical vibrations (Pos, 3) both Seats 2 and 3 show a considerable amplification of seat pan/platform displacement at 1·7 Hz. Acceleration measurements on the subjects hips and shoulders, Table Y, show that the spring suspended seats (2 and 3) are better than the rigid seat concerning vertical vibrations (Pos. 3). For vibrations in the roll mode (Pos. 1 and 2) the hip and shoulder acceleration is higher using the Seats 2 and 3 than the rigid seat. It seems somewhat easier to stabilize the shoulders on Seat 3 than on Seat 2. TABLE

V

Average linear acceleration measured on the hips and shouders of two subjects in experiments giving the seats an accIeration of 0·5 g (linear component) at the frequencies 1, 1'7, 2'5, and 4 Hz Linear acceleration component (g) Seat 1 (rigid)

Seat 2

Seat 3

Vertical

Side to side

Vertical

Side to side

Vertical

Side to side

Hips: Pas. 1 Pas. 2 Pas. 3

0·16 0·21 0·51

0·46 0·40 0·07

0·19 0·24 0'41

0'51 0'53 0·03

0·19 0·27 0·39

0-45 0·44 0·05

Shoulders: Pas. 1 Pas. 2 Pas. 3

0·08 0·33 0·65

0·24 0·27 0·04

0·09 0·26 0·42

0·33 0·43 0·02

0·14 0·33 0·54

0·16 0·15 0·01

Averaged for the experiments using the frequencies 1, 1'7, 2'5, and 4 Hz the results of the "drivers' " reactions on the different seats are given in Fig. 7. The heart rate (top left) is generally low and is not a very good parameter here. It is significantly highe-r for the pure angular motion in Pos. 1 than for the pure vertical in Pos. 3 for all seats. The combination of side to side and vertical motion in Pos. 2 has clearly given the highest heart rate on the rigid seat. The heart rate is surprisingly low on the rigid seat for vertical vibrations. The highest heart rate appears at 1·7 Hz in Pos. 1 and 2, and is especially marked when using the Seats 1 and 2. The tracking settling time (top right) is a measure of the "drivers' " response time for eight step variations in the tracking pattern. Averaged for all frequencies used this response time is shortest when using Seat 3 and longest using Seat 2, while the rigid seat fits between. These results are hard to explain and a more comprehensive paper concerning the method and human reactions as a whole is given elsewhere." When exposed to pure angular vibrations (Pos. I) the tracking error measured as deviation in mm- from following a straight line between the step variations (bottom left) is largest on Seat 3, followed by Seat 2. Under the other conditions (Pos. 2 and 3) the tracking performance is most affected using the rigid seat. The combination of side to side and vertical motion in Pos. 2 seems to create the worst condition on all seats. The poorest tracking in Pos. 1 appeared at 1 Hz and was most pronounced for Seat 2. In Pos. 2 1·7 Hz seemed to be a bad frequency for all the seats. Large tracking error also appeared at 2·5 Hz for the rigid seat and at 1 Hz for Seat 2 in this Pos. A tendency of the interaction short response time-bad tracking, or longer response time-better tracking can be seen. The "drivers' " subjective rating of the conditions in each experiment (bottom right) using a rating scale 0-5 (O=no effects of vibrations, 5=unbearable effects) is not sensitive enough to give

31

L. SJ0FLOT; C. W. SUGGS

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Fig. 7. Heart rate, tracking performance and subjective rating of two "drivers" in the laboratory experiments with the three seats in the three test positions (Pos. 3-pure vertical movements) _ . , Rest level; 0--0, seat 1; X - -- X, seat 2; ,II. .... 1\, seat 3

any useful results. Seat 2 is definitely judged worst under the combined horizontal/vertical vibration exposure in Pos. 2, and Seat 3 as worst under exposure to pure angular vibrations (Pos. 1). For vertical vibrations (Pos. 3) all the seats are judged about the same, averaged for all frequencies used in these experiments. The "drivers" have preferred the vertical movements in Pos. 3 over the angular roll movements in Pos. I and 2 without regard to seat.

4. Conclusions Angular motion in the roll mode on pneumatic tyred tractors is caused partly by rotation around the centre of gravity and partly by sideways tyre deflection; together giving a long radius of rotation depending on the inflation pressure. The instant rotational point appeared to be about 46 and 66 in below the centre of gravity for 17 and 8 Ib/in>, respectively, in driving experiments on test track. The natural frequency in the roll mode of this 3860 Ib tractor appeared to be in the range of 1,5-1,9 Hz for the inflation pressure 8-17Ib/in 2 in the rear tyres. The tractor seat had 1-4° angular displacement of its own around an axis just under the seating level. The driver applies some active damping of the side to side movements, shown by lower side to side acceleration with a driver on the seat as compared to a 100 Ib dead mass on the seat. Experiments on the vibration device show that spring suspended seats designed for attenuating vertical vibrations may create worse conditions than a rigid seat when exposed to vibrations in the roll mode. Roll movements with a relatively large vertical component, often appearing on tractors, seem to create especially bad conditions for the "drivers". Pure angular roll movements of the seat also create worse conditions than the pure vertical movements. There are clear differences in the quality of "good" seats concerning the roll vibrations. It should be possible to determine this quality by testing the seats on a rather simple vibration simulation device. The effects of roll vibration on man should be further investigated in order to establish tolerance criteria and standardized methods for testing of seats and evaluation of practical conditions.

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LOW FREQUENCY ANGULAR VIBRATIONS ON FARM TRACTORS

Acknowledgements The authors wish to acknowledge the W. K. Kellogg Foundation for offering fellowship and other financial aid making these studies possible. Our thanks are also due to the Biological and Agricultural Engineering Department at the North Carolina State University, especially the Human Engineering group, for the opportunity to carry out these experiments and for the support and participation in this project. REFERENCES 1

2 3

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5

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11

12

13

14

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Haack, U. Ober die giinstigste Gestaltung der Schleppersitzfederung bei luftbereiften Ackerschleppern mit starrer Hinterachse. Landtech. Forsch., 1963,3 (1) 1 Morrison, C. S.; Harrington, R. E. Tractor seating for operator comfort. Agric. Engng, 1962, 43 632 Dupuis, H. Senkrechte schwingbeschleunigungen von fahrern in kraftfahrzeugen, auf ackerschleppern und selbstfahrenden arbeitsmachinen. Grund. Landtech., 1963, 169 Matthews, J. Ride comfort for tractor operators: II. Analysis of ride vibrations on pneumatic-tyred tractors. J. Agric, Engng Res., 1964,9 147 Bjerninger, S. Vibrations of tractor driver. Acta polytech. Scand., Mechanical Engineering Series No. 23, 1966 Huang, B. K.; Suggs, C. W. Vibration studies of tractor operators. Trans. Am. Soc. agric. Engrs, 1967, 10 (4) 478 Wendeborn, J. D. Ein beitrag zur verbesserung des fahrkomforts auf ackerschleppern. Fortschr.-Ber. VDI-Zeitschrift, Reihe 14, Nr. 8, 1968 Pleszczynski,W. Proba ergonomicznej oceny srodowiska drganiowego na ciagniku rolniczym (The trial of ergonomic estimation of vibration environment on the agriculture tractor). Roczn. naukroln., 1970, t. 68-C-3, 489 ISO (International Organization for Standardization). Guide for the Evaluation of Human Exposure to Whole-body Vibration. Document ISO/TC 108/WG 7 (Secretariat-19) 36, June 1970 Pradko, F.; Lee, R.; Kaluza, V. Theory of Human Vibration Response. ASME Paper 66-WA/BHF-16, 1966 Sjoflot, L. Measuring and evaluating low frequency vibrations (0'3-110 Hz) acting on machine operators in agriculture and forestry. Norwegian Institute of Agricultural Engineering, Research Report No. 19, 1970 Raney, J. P.; Liljedahl, J. B.; Cohen, R. The dynamic behaviour offarm tractors. Trans. Am. Soc. Agric. Engrs, 1961,4 (2) 215 Matthews, J.; Talamo, J. C. D. Ride comfort for tractor operators: II/. Investigations of tractor dynamics by analogue computer simulation. J. Agric. Enging Res., 1965, 10 (2) 93 Goering, G. E.; Buchele, W. F. Computer simulation ofan unsprung vehicle. Part I and I/. Trans. Am. Soc. Agric. Engrs, 1967, 10 (2) 272 Sjoflot, L.; Suggs, C. W. Human reactions to whole-body angular vibrations side to side compared to linear vertical vibrations. Ergonomics (in press)