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2026

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 to determine their failure.

Before starting the main text, I want to emphasize my point of view: the braking system is an integrated system, and 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 problems of the self-adjusting arm, one cannot just treat the symptoms individually.

卡车的自调臂该如何正确使用和维修？

▎Analysis of Reasons for Brake Clearance Nonconformity

We categorize the various reasons for brake clearance nonconformity into four main parts: self-adjusting arm, brake drum, brake assembly, and vehicle/axle:

Regarding the self-adjusting arm: improper matching of clearance, inaccurate preset clearance values; poor consistency of self-adjusting arm products, unstable adjustment accuracy; self-adjusting arm not correctly installed on the vehicle;

Regarding the brake drum: improper processing accuracy of the brake drum, inner control out of round, excessive 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.

Regarding the brake assembly: poor rigidity of the brake assembly, including camshaft and brake shoes; abnormal thermal expansion of the friction pad, abnormal expansion when heated; abnormal fit between camshaft, brake shoes, and brake backing plate, leading to out-of-round condition of the outer diameter of the friction surface.

Regarding vehicle/axle: abnormal coaxiality of the axle housing, poor overall rigidity; excessive play in the wheel end bearing, the hub cannot rotate normally around the vehicle 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 once; if the minimum torque measured is less than 18 Nm, it indicates that the self-adjusting arm is damaged and must be replaced in time;

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.

卡车的自调臂该如何正确使用和维修？

▎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 hex head 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.

卡车的自调臂该如何正确使用和维修？

Secondly, the vehicle exhibits slow brake reset and dragging brake issues, which are judged to be caused by internal sticking of the automatic adjusting arm, and it is required to replace the automatic adjusting arm.

Misconception guidance: In the vehicle brake system, the automatic adjusting arm only serves to transmit the chamber force and ensure the brake clearance between the brake shoes and the brake drum. The automatic adjusting arm is mounted on the camshaft and has no reset force after braking. Brake reset relies on the return springs of the brake shoes and the return springs of the chamber.

The correction method is: When the vehicle exhibits the above issues, it is necessary to promptly check whether the installation of the automatic adjusting arm is correct (especially the axial clearance), and whether there are problems with the return springs of the brake caliper, the return springs of the brake, and the fit between the S camshaft and the roller.

▎Simple Method to Determine Failure of Self-Adjusting Lever Function

Excluding improper installation of the self-adjusting lever, use a SW12 wrench to turn the worm hex head counterclockwise by more than 180° to enlarge the gap between the brake drum and the brake shoe (do not use pneumatic or air tools).

Then, with the vehicle's brake air pressure maintained at ≥0.6 MPa, apply the brakes several times. If the worm hex head is observed to rotate clockwise, it indicates that the self-adjusting lever is functioning normally and can be used.

卡车的自调臂该如何正确使用和维修？

▎Analysis of Common Causes of Self-Adjusting Arm Faults

Here we only analyze two typical fault modes: brake overheating and brake fading. To make it easier for everyone to read, the editor has drawn two flowcharts to help everyone understand.

Flowchart for Analyzing Causes of Brake Overheating

卡车的自调臂该如何正确使用和维修？

Flowchart of Causes Analysis for Brake Fade

卡车的自调臂该如何正确使用和维修？

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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