Research On Key Technology Of Drum Brakes

Drum brake, which applies friction to assuring parking and slowing down the running vehicles, is widely installed in automobiles because of high brake efficiency, simple structure and low price. Afterwards, its operating environment is complex because the braking process is constantly accompanied with friction contact and mechanical vibration. In order to make deeper study on this problem, some theories, such as multibody dynamics, tribology and contact mechanics, have been done on this field. Due to the above reason, it has great value in research on the braking process of drum brake, especially about how to enhance brake efficiency. To study the contact principle of the friction plates-drum, the contact angle equations are established when the center of the drum coincides with that of friction plates or not. Based on virtual prototype technology and finite element theory, dynamic simulation of traditional drum brake is discussed by utilizing the multi-body dynamics simulation software ADAMS and general finite element software ANSYS. The mechanical principle of brake is further studied, and a floating drum brake structure improved method is proposed. The main work is listed as follows:(1)The present research situation of brake at home and abroad is discussed. Combined with the subject requirements, the main research contents of drum brake is determined. Then the technical route is proposed and the key pointes of the study are recognized clearly.(2) By means of the kinematics analysis on drum brake, the contact angle equations are established when the center of the drum coincides with that of friction plates or not. The contact angle equations of drum brake used for the heavy vehicles are also simulated by Matlab. It can be concluded that the contact angle of friction plates at the top is smaller than that at the bottom when the center of friction plates coincide with the center of drum. Because of the elastic deformation of actual structures, the top friction plates contact with drum easily. When the offset from the center of drum to the center of the friction plates equal to0.1mm, the leading shoe and trailing shoe both have the smaller contact angle at the top of the friction plates than that at the bottom. And the contact angle at the bottom of friction plates have great changes as the eccentric angle changes. Thus there is great difference in contact state between the actual structures as a result of random eccentric quantity. Although the structure elastic deformation is not taken in to account while the contact angel equation of friction lining is established, the changing regularity between friction lining and drum brake contact zone during braking process can be analyzed from geometric scope, which provide a new approach for the analysis of braking process of complicated drum brake; apart from this, the article also represented a novel construction of floating drum brake:the brake shoe has a radial movement along the support pin position, and built improved contact angle motion equations, which provided a theoretical model for the following analyses.(3) In order to study the brake process accurately for the leading-trailing brake system with an opening device by a cam, the paper presents that constituting brake shoes, brake drum with cam as a system, applying brake torque on the cam, analysising opening force, brake torque and change rule of vibrations by ADAMS dynamic simulation software. Analysis holds that brake torque of leading shoe is not equal to the one of trailing shoe, and brake torque of leading shoe is much larger than the one of trailing shoe in the brake process of leading-trailing shoe brake system with an opening device by a cam, the result is also proved by FE model; Owing to the nonlinear friction factor and other reasons, the leading shoe and the trailing shoe have been in a state of random vibration and their vibration intensity increases with the raising of the vehicle speed. The distribution regulation of the tangential friction (or normal force) on the brake lining of the leading shoe is that the middle part is greater than the both sides. The tangential friction on the trailing shoe is far less than the one of leading shoe, and its distribution law is that the upper part is large and the lower part is small. The above conclusions are also validated by FEA results. The opening force of the trailing shoe is greater than the one of the leading shoe, and both opening force remains unchanged though the vehicle speed is different in the condition that input torque applied on the cam is equal.(4) The finite element model of floating-shoe brake is builded by ANSYS finite element software by appllying input torque on the cam axis to simulate brake process of brake system, then friction contact between brake lining and drum is analysised. Contact stress cloud-chart and distribution curve of brake torque are obtained.The motion rule of floating leading shoe and trailing shoe’s contact angular are analysed, and at last the correctness of the simulation analysis is verified through the braking performance platform test of the two kinds of brakes.The analysises indicate that:The contact stress of floating-shoe is less than that of tradition structure in the condition of brake torque keeping invariant because of leading-shoe’s larger contact area of floating shoe brake, The trailing shoe floats upward, which results in that the trailing shoe’s friction torque of floating-shoe brake is larger than the tradition’s. The total brake torque of floating-shoe brake is larger than the one of traditional brake. When the floating brake works, the contact angular’s variation is decreased through the regulation of hoof support pin’s DOF, and the contact condition is improved, which goes against the trailing’s improvement(5) The influence of the floating-shoe and tradional brake’s eccentric on torque is analysised through ANSYS finite element software in this paper, and also the influence of the floating-shoe brake’s eccentric on contact state of friction is analysised through the motion equation of contact angle.Last, the bench tests of vehicle are performed in order to verify the correctness of the theoretical analysis.It can be concluded that the floating hoof drum brake can make its own total friction torque unchangeable basically when the offset is changed. For this reason the vehicles have lower the brakes’ influence on vehicles’ offset in the process of braking. When the offset changes, the total friction torque of a traditional hoof drum brake always changes a lot, which easily makes the braking torques of left and right wheels unequal and causes braking deviation. Floating-leading shoe structure can improve contact condition and enlarge contact areas of brake lining obviously, also optimal adjustment movement value of leading shoe can be obtained by contact motion equation, while brake lining contact condition can not be improved by floating-trailing shoe structure. Through the vehicle deviation test of traditional drum brake and floating drum brake on the test bed, the differences of brake forces for the traditional drum brake are larger than those for the floating drum brake. The result above explains that braking force of left wheel and right wheel on vehicles with traditional drum brake have serious deviation, which agree with the simulation result.