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2026
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How should the self-adjusting arm of a truck be properly used and maintained?
Today, the editor will start from the perspective of the practical application of self-adjusting arms, focusing on introducing the precautions in the actual use, maintenance, and inspection processes of self-adjusting arms, as well as simple methods for determining their failure.
Before starting the main text, I want to emphasize my own viewpoint: the braking system is an integrated system. Components such as the brake caliper, self-adjusting arm, camshaft, brake, and brake drum are interdependent and influence each other. When analyzing and dealing with various issues of the self-adjusting arm, one cannot just treat the symptoms separately.
▎Analysis of Causes for Brake Clearance Being Non-Compliant
We categorize the various causes of non-compliant brake clearance into four main parts: self-adjusting arm, brake drum, brake assembly, and vehicle/axle:
Self-adjusting arm: improper clearance matching, inaccurate preset clearance values; poor consistency of self-adjusting arm products, unstable adjustment accuracy; self-adjusting arm not correctly installed on the vehicle.
Brake drum: unsuitable machining accuracy of the brake drum, internal roundness out of control, excessive circular runout; inappropriate material and structure of the brake drum, abnormal thermal expansion, poor overall rigidity, prone to deformation; improper installation of the brake drum, whether it is an internal or external brake drum, it rotates synchronously with the hub coaxially. If installed improperly, the brake drum will be abnormally unbalanced.
Brake assembly: poor overall stiffness of the brake assembly, including the camshaft and brake shoes; abnormal thermal expansion of the friction pad, abnormal expansion when heated; abnormal fit between the camshaft, brake shoes, and brake backing plate, causing the outer circle of the friction pad braking surface to be out of round.
Vehicle/axle: abnormal coaxiality of the axle housing, poor overall rigidity; excessive wheel end bearing clearance, the hub cannot rotate normally around the axle centerline.
▎Daily Maintenance Requirements for the Self-Adjusting Arm
1. Every 20,000 kilometers of vehicle operation, grease should be applied to the lubrication nozzle of the self-adjusting arm (No. 2 lithium-based grease);
2. During daily use, if brake softness is felt, it is recommended to check the counterclockwise torque of the hex head of the self-adjusting arm worm. Rotate it once; if the measured minimum torque is less than 18 Nm, it indicates that the self-adjusting arm is damaged and must be replaced promptly;
3. Check whether the control arm is securely fixed. Excessive wear of the control arm bushing or loosening of the control arm locking bolt can lead to failure of the self-adjusting function.
▎Typical Misconceptions in Using Self-Adjusting Arms
First, during regular maintenance, when a vehicle experiences issues like excessively small brake clearance or overheating, it is common to use a wrench to turn the worm gear 'hexagon' counterclockwise in order to increase the brake clearance.
Misconception Guidance: The manual for self-adjusting arms specifies that the worm gear hexagon may only be manually turned during installation, removal, or inspection. Otherwise, it will directly cause premature wear of the internal mechanism of the self-adjusting arm and lead to weak braking performance during subsequent vehicle use.
Correction Method: During regular maintenance, try to avoid turning the worm gear 'hexagon' to extend the service life of the self-adjusting arm. If the vehicle experiences excessively small brake clearance, promptly check whether the self-adjusting arm model is correct, whether the installation is proper, and whether there are any issues with the brake components.
Secondly, the vehicle exhibits slow brake reset and dragging brakes, which is judged to be caused by internal sticking of the self-adjusting arm, and replacement of the self-adjusting arm is required.
Misconception Guidance: In the vehicle's braking system, the self-adjusting arm only serves to transmit the thrust of the brake chamber and ensure the braking clearance between the brake shoes and the brake drum. The self-adjusting arm is mounted on the camshaft and has no power to reset the brakes by itself. Brake reset relies on the return springs of the brake shoes and the return springs of the brake chamber.
Correction Method: When the above problem occurs in the vehicle, the installation of the self-adjusting arm should be checked in a timely manner (especially the axial clearance), as well as the return springs of the brake wheel cylinder, the return springs of the brakes, and the fit between the S-cam shaft and the roller.
▎Simple Method to Determine Self-Adjusting Arm Failure
After ruling out improper installation of the self-adjusting arm, use an SW12 wrench to turn the hex head of the worm gear counterclockwise by more than 180° to enlarge the clearance between the brake drum and the brake shoes (do not use pneumatic or air tools).
With the vehicle brake air pressure ≥0.6MPa, apply several braking operations. If the hex head of the worm gear rotates clockwise, the self-adjusting arm is judged to be functioning normally and can be used normally.
▎Analysis of Common Causes of Self-Adjusting Arm Failures
Here, we only analyze two typical failure modes: brake overheating and brake fading. To make it easier for everyone to read, I have created two flowcharts to help with understanding.
制动发热故障原因分析流程图
制动疲软故障原因分析流程图
The above are some tips and insights I have summarized regarding the daily use, maintenance, inspection, and failure analysis of self-adjusting arms. The explanation may still not be very comprehensive, and I hope fellow truck drivers can offer valuable suggestions. Thank you.
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