| In recent years,with the rapid and steady development of domestic economy,heavy duty tractors as a production tool have achieved excellent achievement in production and sales.Meanwhile,the users also put forward higher requirements for the comfort performance of heavy duty tractors.Because the domestic transportation environment is complex and long-time driving is more common,the comfort of the vehicle directly affects the driver’s health,and may even lead to traffic accidents caused by fatigue driving.Therefore,improving the ride comfort of heavy duty tractors to reduce driver fatigue has become a hot issue in the domestic heavy duty tractors industry.A type of 6×2 dual front axle steering tractor developed by Shaanxi automobile group co.has better steering stability and with a light weight and low price.After mass production,it quickly became the main best-selling tractor in the domestic medium and long distance transportation market.However,the cab floor vibration problem during the high-speed driving seriously affects the ride comfort and handling safety of the drivers,which has always troubled the engineers and technicians.This paper focuses on thecab floor abnormal vibration of the6×2 dual front axle steering tractor and the vibration comfort of the driver’s feet.Using the large dataof trucktraffic flow on expressway in recent years,this paper analyzes the running situation of this type of tractor.The main vibration source that causes the cab floor abnormal vibration of this type of tractor is found by testing,it is the dynamics transmission system.Then the influence of the vehicle transmission system on the vibration response of the cab floor is studied by using the vehicle chassis dynamometer.The rigid-flexible coupling multi-body dynamics model is established by ADAMS/CAR software.The influence of the main parameters of the vehicle transmission system on the vibration of the cab floor is also analyzed by using the finite element method and the multi-body dynamics method.Finally,using the electromagnetic vibration system combined with psychology evaluation method,the feetvertical vibration comfort for Chinese people with sitting position has been studied,The vibration limit of the feet for Chinese driver under 4 hours continuous driving is proposed.In order to clarify the position and development trend of the 6×2 double front axle steering tractor in domestic freight market status and development trend.Tractors axle structure and its corresponding traffic volume,driving quantity and freight turnover of heavy-duty tractors on expressway in recently years are studied.By comparing the ride comfort of dual front steering axle tractor and single front steering axle tractor,it is concluded that the current feet vibration frequency weighting curve in the national standard cannot effectively reflect the response of Chinese driver’s feet to the floor vibration of heavy duty tractors.Through the diagnosis of the typical fault tractor,the main source that causes the abnormal vibration of the cab floor is the vehicle transmission system.In order to study the influence of vehicle power transmission system on the vibration of cab floor,the designed test scheme is using the chassis dynamometer,the sensitivity of mainly parameters of the power transmission system to the vibration response of the cab floor is studied.The results shows that the dynamic imbalance of the transmission shaft should not be greater than 60g while the modal frequency of the middle hanger shall not be less than 200Hz and the radial stiffness of the shaft support should be controlled at1500±30N/mm.Multi-body dynamics simulation analysis can be carried out before data freezing by using rigid-flexible coupling multi-body dynamic vehicle model,so it is very important to establish a correct multi-body dynamics model.In the process of building the vehicle model,the main parameters,such as the mass center and moment of inertia of cab,the stiffness and damping of cab suspension are obtained by experiments.The finite element model is used for the core components such as frame,transmission shaft and the mode of them is compared by means of experiment results.After completing assembly of the vehicle model,the variation rule of the driver’s floor vibration is analyzedunder usual speed,and the accuracy of the vehicle model is verified by comparing the test results.Moreover,the relationship between the parameters such as unbalance of transmissionshaft,the stiffness of middle hangerand so on and the vibration acceleration of the cab floor is analyzed.The results show that the simulation results are consistent with the test results.Cab floor vibration can be transmitted directly through the driver’s feet to the legs and body,which is an important factor affecting the driver’s ride comfort,so it is very important to establish a reasonable criteriato evaluate driver’s feet comfort.Current national standard GB/4970-2009"Method of running test-Automobile ride comfort"directly adopts the recommended values in the international standard ISO2631-1997“Mechanical vibration and shock-Evaluation of human exposure to whole-body vibration"for the feet vibration frequency weighting coefficient,the characteristics of Chinese people are not fully considered.In order to study the response characteristics of Chinese feet under mechanical vibration,a vibration evaluation experiment system was designed and builted.Thefrequency weighted curve of feet vibration in the range of 6.3Hz~80Hz was obtained by using the psychological evaluation method,the results show that the frequency weighting coefficient of Chinese driver’s feet subjected to mechanical vibration in the range of 6.3Hz~10Hz is basically equivalent to the recommended value of ISO2631-1997.When the vibration frequency exceeds 10Hz,the Chinese driver’sfeet are more sensitive to vibration,which is obviously higher than the corresponding value of ISO2631-1997.The cab floor vibration not only affects the ride comfort of the driver,but also easily affects the driver’s handling when the feet is in the vibration environment for a long time.In order to study the influence of driver’s feet fatigue on driving safety in vibration environment,the driver’s braking reaction time was selected as the assessment index.Firstly,through the human body vibration evaluation system,the mean value of vibration accelerationon feet of Chinese driver in sitting posture corresponding to the feet vibration fatigue limit is obtained.Then the changes in braking reaction timeof driver’s before and after feet vibration fatigueare obtained by using the braking reaction time test system.The experimental results show that after 2 minutes of feet vibration,the braking reaction time of volunteer was prolonged for0.33s on average.Moreover,the phenomenon of hysteresis increased obviously during the braking process after two minutes of vibration while the volunteer feeling of the feetwas numb and the movement was slow.It is proved that the vibration fatigue of the feet has a significant effect on the braking reaction time of the drivers.Meanwhile,the vibration limit value of the vertical vibration of the cab floor under the continuous four hours driving condition is 0.59m/s~2 based on the relationship between the vibration energy and vibration response of the human body on ISO 2631. |
