Vehicle rigid motion effects on whole body vibration Giovanni Molari1*, Michele Mattetti1, Matteo Badodi1, Enrico Sedoni2 1
DEIAGRA, University of Bologna, Viale Fanin, 50 – 40127 Bologna, ITALY 2 CNH, Viale delle Nazioni, 55 – 41122 Modena, ITALY *Corresponding author. E-mail:
[email protected]
Abstract The vibrations transmitted to the driver are dangerous to his safety because they can cause temporary and permanent injuries to the body. Tractors are characterized by vibrations with high intensity and low frequency that increase the difficulties in the design of solutions able to reduce vibration transmissibility to the driver. The new suspension systems diffused in recent years have been designed to reduce the vertical acceleration, despite the fact that numerous studies have highlighted how the most critical directions are represented by the horizontal ones. This is caused by the angular accelerations, amplified in modern tractors with high dimensions, by the high distance from the seat and the tractor rolling axis. Despite manufacturers’ efforts in the last decade, the level of the vibration transmitted to the driver in different tasks exceeds the exposure levels established by the standards. The goal of the paper is to evaluate the influence of each vehicle’s rigid motion (pitch, roll and bounce) on the vibration level transmitted to the driver in a tractor of about 170 kW power with axle suspension, cab and seat suspension. Two inertial measuring units have been fitted on the cab and frame of a tractor to measure the roll angular velocity and the bounce motion respectively of the cab and frame. A triaxial accelerometer has been fitted on the driver seat to measure the driver acceleration. The tractor has been used in different working conditions to evaluate the influence of the mass distribution on each rigid motion and with different driving velocities. The power spectrum densities have been analyzed to evaluate the predominant frequencies for each mode. These tests have permitted a better comprehension of the whole body’s vibrations caused by a tractor. In particular, the distribution of the acceleration on the three axis has been defined with respect to different working conditions. In the on road transport the longitudinal acceleration is prevalent. The vibration level on the road is more influenced by the pitch and the bounce. The suspension systems and, in particular, the front axle suspensions are able to reduce the pitch of the tractor, but do not reduce the bounce due to the absence of the rear suspensions. 1. Introduction The agricultural sector is one of the most dangerous sectors with respect to injuries on the workplace (Eurostat, 2012). One of the most relevant problems is the exposure of the human body to vibrations. Indeed the vibrations can cause temporary and permanent damage to the body (Hulshof & Van Zanten, 1987; Griffin, 1996). In any case, in the short term, the vibrations cause a reduction of the productivity, attention level, comfort and, as a consequence, an increase in accident risks (Fairley, 1995). In this context, vibration reduction is one of the main aspects for tractor producers, not only for safety problems but also for market strategy. Indeed, one of the main consequences connected with vibration reduction is the possibility to increase the working velocity and productivity. To reduce the problems related to vibration, limits on the vibration level transmitted to the tractor driver have been fixed by the directive 2002/44/EC (EC, 2002), and, in particular, a limit value and an action value have been set. However, recent studies have shown that a high number of tractors exceed, in different working conditions, not only the action limit but also the maximum limit (Scarlett, Price, & Stayner, 2007). The vibrations transmitted to the tractor driver have in general a high intensity and a low frequency (Rakheja & Sankar, 1984). Recent studies have highlighted that the vibration level measured on the seat is higher than that measured on the cab (Scarlett, Price, Semple, & Stayner, 2005), this is due to the angular movement of the tractor. In particular, the roll increases the lateral accelerations
and the pitch increases the longitudinal ones. This is more evident in the modern tractor caused by an increasing of the distance between the drive and the roll axle and the pitch axle. Despite the fact that the vertical acceleration is higher with respect to the others, the standards have fixed a higher scaling factor for the horizontal accelerations (ISO, 1997). As a consequence, the horizontal accelerations are more influential with respect to the vertical ones on the global acceleration level as demonstrated by recent studies (Scarlett 2007). Inspite of this, the manufacturers have concentrated their efforts on the vertical acceleration reduction, due to its facility. It is also difficult to isolate the driver from the horizontal accelerations due to the necessity of a suspension with a very low natural frequency to obtain an effective isolation (Griffin, 1996). The solution currently adopted on the longitudinal suspension is designed to increase the confort, moving the seat in phase with the driver (Donati, 2002), while the cab suspension is desiged to reduce the pitch effects on the cab due to the frame. In this context it is fondamental to know all the rigid motions of the tractor: roll, pitch, and bounce and to evaluate their effect on the seat. The goal of the paper is to evaluate the influence of each vehicle rigid motion (pitch, roll and bounce) on the vibration level transmitted to the driver in a medium-high power tractor. 2. Materials and Methods A tractor with a PTO power of about 170 kW with a maximum speed of 50 kmh-1 was used. The configurations of the tractor are described in Table 1. TABLE 1: Tested tractor Configuration
Value
Width (mm)
2490
Wheelbase (mm)
2818
Total mass (kg)
8374
Cab mass (kg)
850
Load on the front axle (%)
39
Front tire Front tire inflation pressure (kPa) Rear tire Rear tire inflation pressure (kPa)
Goodyear Optitrac DT818 540/65 R30 120 Goodyear Optitrac DT818 650/65 R42 120
The tractor was fitted with a pneumatic seat suspension and a front axle suspension constituted by one hydropneumatic link. All the tests were performed with and without the front axle plugged. The tractor was driven in a road and in an ISO rough track (ISO, 2002). In the on road driving and on an ISO track the tractor was configurated without implements connected to a three point linkage with a speed respectively of about 38 kmh-1 and 5 kmh-1. The roll velocity ( ), pitch velocity ( ), respectively of the frame (subscript f) and of the cab (subscript c), were measured correspondingly in the longitudinal and lateral directions, referred to a vehicle axis system defined by ISO standard 8855 (ISO, 1991). The measurements were taken with two inertial measuring units (Land-Mark 10 VG LN, Gladiator Technologies, Snoqualmie, Wash) the first fixed on the first step of the cab and the second fixed on the cab platform near the seat. The accelerations
( , respectively of the frame (subscript f) and of the cab (subscript c), were measured with two tiaxial accelerometers (Model 4630, Measurment specialities, Hampton, VA, United States) fixed near the inertial platforms. The accelerations on the seat ( were measured with a triaxial seat accelerometer (Model 4515-B, Bruel & Kjaer, Nærum, Denmark). The vehicle axis system (x, y, z) is defined by ISO standard 8855 (ISO, 1991). All the acquired signals have been filtred with a low pass filter with a cutting frequency of about 5Hz to remove high frequency noise. Five ripetitions for each condition were acquired. The power spectrum density (Stoica & Moses, 1997) was calculated to evaluate the main frequency for each rigid motion. The influence of each proper motion on the horizontal accelerations of the seat was evaluated assuming that the vehicle turns around the dynamic center of the pitch and roll (Genta & Morello, 2009), due to the fact that the seat is rigidly connected to the cab flat in the horizontal direction. Therefore, the contribution of the pitch on the longitudinal acceleration of the seat (ax,p) and of the roll on the transversal acceleration of the seat (ay,r) are defined by: 1
where: lps: distance between the pitch center and the seat lrs: distance between the roll center and the seat The RMS values of ax,p and ay,r were calculated and compared with the RMS values of ax,s, ay,s to evaluate the contribution of the roll and pitch to the whole body vibration. 3. Results In figure 1 the power spectral density (PSD) of the pitch, roll, and bounce of the cab tractor with the front axle suspension in the active and disabled position are reported. All the PSD present a main peak in the same frequency of about 2.25 Hz, this implies that all the rigid motions of the tractor are coupled. The power of the pitch is lower with respect to the power of the bounce, this effect is positive because the bouncing is more confortable with respect to the pitching. The axial suspension permits a reduction of the peaks of the PSD, on the pitch and on the bounce with a reduction of about 50% of the amplitute of the main peak. The roll is mainly caused by the radial deformation of the rear wheels, however the front suspension permits a reduction of the roll and, in particular, of the peaks as previously analysed (Mattetti et al., 2012). To increase the confort of the drivers, it is better to have the main frequencies of the rigid motions between 0.3 Hz and 1.3 Hz. However it is difficult to modify the solutions presented on the front axle of the tractors due to the necessity to have a rigid suspension able to sustain the load. The coupling of the rigid motions is due to the different wheels of the front axle with respect to the rear ones (Lines & Murphy, 1991), with a consequence that a roll rotation causes a radial deformation different on the axles and therefore a pitch. The same effect is similar for the bounce. The coupling of the rigid motion is in any case positive. To obtain a good ride it is advisable to have a difference between the main frequency of the pitch and of the bounce lower than 20% and the main frequency of the roll similar to the main frequency of the bounce (Gillespie, 1992).
-4
PSD [(°/s 2)2/Hz]
4
CAB PITCH
x 10
Axle suspension OFF Axle suspension ON
3 2 1 0
0
1
2
3
4 Frequency [Hz] CAB ROLL
5
6
7
8
0
1
2
3
4 Frequency [Hz] CAB BOUNCE
5
6
7
8
0
1
2
3
4 Frequency [Hz]
5
6
7
8
-5
PSD [(°/s 2)2/Hz]
2
x 10
1.5 1 0.5 0
PSD [(m/s 2)2/Hz]
0.4 0.3 0.2 0.1 0
FIGURE 1: PSD cab movement comparison between the front axle suspension in active and passive position. In Figure 2 the effect of the angular acceleration of the seat is reported. The roll effect on the transversal acceleration is reduced with respect to the effect of the pitch, with a contribution around 11% on the seat acceleration.
Angular acceleration effect [%]
12 10 8 6
Front axle suspension OFF
4
Front axle suspension ON
2 0 Roll effect [%]
Pitch effect [%]
FIGURE 2: Effect of the pitch and the roll on the seat acceleration.
4. Conclusions The tests showed in the paper have permitted a better comprehension of the whole body vibrations caused by a tractor, an analysis of the effect of the axle suspension on the global vibration level, and the effect of the angular rigid motion on the seat acceleration. In the tractror used for the tests, the main frequency of the pitch spectrum is higher with respect to the maximum limits suggested. This is due to the stiffness of the tractor suspension system, costituted by the wheels and the suspension on the front axle. The roll, the pitch, and the bounce are coupled. The effect of the angular movements on the acceleration in the seat is evident, in particular in the tests with the axle suspension disabled. It would be interesting to analyze the behavor on the field in different works conditions, and in different load conditions, where the effects of the rigid motions could be higher. References Donati, P. (2002). Survey of thechnical preventative measures to reduce whole-body vibration effects when designing mobile machinery. Journal of Sound and Vibration, 253(1), 169–183. EC. (2002). Directive 2002/44/EC of the European Parliament and of the Council (Page) (pp. 1–7). Strasbourg (France): European Commission. Eurostat. (2012). Database health and safety at work. Eurostat. Fairley, T. E. (1995). Predicting the discomfort caused by tractor vibration. Ergonomics, 38(10), 2091–2106. Genta, G., & Morello, L. (2009). The Automotive Chassis: System design. Springer. Gillespie, T. D. (1992). Fundamentals of Vehicle Dynamics. Society of Automotive Engineers. Griffin, M. J. (1996). Handbook of Human Vibration. Amsterdam: Elsevier. Hulshof, C., & Van Zanten, B. V. (1987). Whole-body vibration and low-back pain. A review of epidemiologic studies. International Archives of Occupational and Environmental Health, 59(3), 205–220. ISO. (2002). Agricultural wheeled tractors and field machinery -- Measurement of whole-body vibration of the operator ( No. 5008:2002) (pp. 1–24). Lines, J. A., & Murphy, K. (1991). The stiffness of agricultural tractor tyres. Journal of Terramechanics, 28(1), 49–64. Mattetti, M., Molari, G., Pesce, M., Grillo, M., Forte, M., & Sedoni, E. (2012). Evaluation of the frequency response of tractor cab angular movements. Transactions of the ASABE. doi:In press Rakheja, S., & Sankar, S. (1984). Suspension Designs to Improve Tractor Ride: II. Passive Cab Suspension. SAE Technical Paper, 841108. Retrieved from http://papers.sae.org/841108 Scarlett, A. J., Price, J. S., & Stayner, R. M. (2007). Whole-body vibration: Evaluation of emission and exposure levels arising from agricultural tractors. Journal of Terramechanics, 44(1), 65–73. Scarlett, A. J., Price, R. S., Semple, D. A., & Stayner, R. M. (2005). Whole-body vibration on agricultural vehicles: evaluation of emission and estimated exposure levels ( No. RR321) (pp. 1– 249). Health and Safety Executive. Stoica, P., & Moses, R. L. (1997). Introduction to spectral analysis. Jersey: Prentice Hall.
