| Tractor is main mechanical equipment for agricultural production in China.Due to its advantages of simple operation,large traction and convenient maintenance,it is still a main tool for agricultural production,transportation and field.Since the world's first tractor has been invented in 1851,wheeled tractor technology has matured.The front wheel steering system it uses,it has always been the subject of research by researchers and universities.According to my own observations and a large number of domestic and foreign materials,I found that the following problems are worth exploring:(1)When the tractor is working in the field or other harsh terrain,cases where the tractor cannot be turned off smoothly and the crops are repeatedly crushed;(2)When the tractor is turning,the Ackermann steering geometry cannot be satisfied,resulting in shortened tire life;(3)When the tractor is working on steep slopes,the stability of the tractor is reduced,and dangerous situations such as rollover are prone to occur.Steering performance of existing tractors cannot fully meet our needs.Therefore,it isnecessary to improve a tractor that can reduce the turning radius and improve the flexibility during cornering.The development of four-wheel steering technology will solve this problem.At present,the research on four-wheel steering technology has developed rapidly at domestic and abroad,but most of them focus on control theory.However,the design of the rear wheel steering mechanism still needs to be improved.On this basis,this thesis intends to use computer software to assist the design,analysis and simulation of experimental processes.Based on the front wheel steering device,this paper designs the double-trapezoidal steering mechanism of the rack and pinion as well as the disconnected steering trapezoidal mechanism.The simulation is carried out by MATLAB and ADAMS to obtain the reasonable parameters of the mechanism.At the same time,in order to ensure the steering stability of the tractor,the front and rear suspension systems are optimized to improve the flexibility,stability and safety of the tractor.The main research contents of this paper are as follows:(1)Based on Ackerman's steering geometry theorem,the steering characteristics of the four-wheel steering system at low speed and medium speed are analyzed.The left and right steering wheel angle relationship and the turning radius of the front wheel steering and the four-wheel steering under ideal conditions are compared by drawing.By analyzing and comparing several different four-wheel steering systems,the design idea of using the motor direct drive design rear wheel steering mechanism is obtained.(2)The design requirements of the steering trapezoidal mechanism are proposed,and the main parameters affecting the steering trapezoid are analyzed.The relationship between the length of crossbar and the steering trapezoidal angle,the length of the trapezoidal arm and the steering trapezoidal angle are derived.(3)Establish a mathematical model of the rack and pinion double trapezoidal steering mechanism.Through the constraint conditions,the corner trapezoid is selected near 65°,and the trapezoidal arm is optimized in multiple values in the range of 142~178mm.The values of each design variable under different trapezoidal arm lengths and different trapezoidal bottom angles are obtained.In view of the superiority of the disconnected steering trapezoid and the space limitation of the rack and pinion steering gear,the rack and pinion steering gear is replaced with a steering rod,and the design idea of the disconnected steering trapezoid is proposed.(4)Considering space limitation of double-trapezoidal steering device of the rack and pinion,this paper proposes a disconnected rear wheel steering mechanism,and relation between input angle and output angle is derived by the rotation matrix method.Taking trapezoidal bottom angle and trapezoidal arm length as design variables,the minimum value of the difference between the left steering arm angle and the theoretical angle is taken as the evaluation accuracy index.The optimized trapezoidal arm length is 137 mm,and the bottom angle of the steering trapezoid is 69°,Under the constraint condition,left and rightsteering arm angle relation curves and angle difference curve are obtained.In order to verify whether the design is reasonable,the kinematics simulation was carried out in ADAMS.In the range of the left steering arm-10~10°,the right steering arm angle is-9.57~10.33 °,which verified the design and simulation results,the proposed rear wheel disconnect steering mechanism meets the design requirements.(5)Based on the rear wheel steering mechanism,the steering stability of the four-wheel steering tractor is studied.The influences of the transmission mechanism and the steering gear,the tire and the front and rear suspension parameters on the turning stability are analyzed.The main way to improve the turning stability of the tractor is obtained.(6)In order to enhance the steering stability of the tractor,in ADAMS,front and rear suspension models are established according to hard point coordinates.After setting the parameters,the wheel positioning parameters of front and rear suspensions of the tractor are analyzed by the wheel jump simulation experiment.In the suspension parameters that do not meet the design requirements,the multi-objective function simulation analysis is established in ADAMS/Insight.The variation range of the rear wheel camber angle is changed from the original 4.5° to 1.8°;the range of the caster back rake angle is changed from the original 6.7° to 2.5°;the range of the rear wheel toe angle changes from4.0° to 0.49°,and the wheel variation The variation range is from 14.7mm to6.1mm.The experimental results all meet the design requirements,and thesuspension system is optimized,so that the suspension system can better adapt to the rear wheel steering mechanism. |
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