Return spring

In drum brakes, return springs ensure that the brake shoes return to the correct rest position as soon as the driver releases the brake pedal. These tension springs pull the brake shoes away from the brake drum and back towards the backing plate, providing sufficient clearance and preventing the brake lining from dragging. They are particularly relevant for drum brake axles on MAN F8, F90, F2000, M2000, L2000, TGL, TGM and off-road/construction vehicles such as TGS, as well as on Mercedes LN, SK, Atego, Axor and Arocs. Drum brakes are also used in buses, including MAN Lion’s City, Lion’s Coach, Mercedes Citaro and Neoplan coaches, depending on the axle configuration and operating profile.

A return spring must always be identified based on the vehicle chassis number. Visual comparison or selection based solely on the model name is never sufficient. Spring wire diameter, free length, winding direction, hook shape, installation position, brake type, drum diameter and brake shoe design may differ depending on the axle configuration. Left and right, primary and secondary brake shoes, or upper and lower springs are also not always interchangeable.

Return springs are highly stressed spring steel components, generally manufactured from heat-treated spring steel with a corrosion-resistant surface treatment. Their spring force must be sufficient to reliably retract the brake shoes without unnecessarily loading the actuation system via the S-camshaft, brake chamber and brake adjuster. An incorrect spring rate or free length may result in insufficient retraction, increased pedal travel or uneven brake response.

In heavy-duty pneumatic drum brake systems, the springs work together with brake shoe rollers, S-cams, anchor pins and the automatic brake adjuster. Use only springs with the correct wire diameter, hook geometry and rated tensile force. A universal spring, an overstretched used spring or a mixed installation of old and new springs causes unpredictable brake balance.

Return springs can fatigue, corrode or wear locally around the hooks after thousands of braking cycles. Typical fault symptoms include dragging brakes, overheated brake drums, burnt brake linings, the vehicle pulling to one side, uneven wear between the brake shoes and increased fuel consumption due to rolling resistance. In the event of severe spring breakage, a brake shoe may remain partially in contact with the drum, potentially causing blue discolouration, hairline cracks in the brake drum and wheel bearing damage due to heat exposure.

Secure the vehicle, depressurise the pneumatic brake circuit in accordance with workshop instructions, and remove the wheel and brake drum. First check whether the S-camshaft moves freely and whether the brake adjuster is not adjusted too tightly; a dragging brake is not solely a spring issue. Then compare the free length and hook position of both return springs with the specified dimension or with a new reference spring. Check for permanent elongation, broken coils, rust pitting, worn spring hooks and contact marks on the brake shoes or backing plate. Also check the radial play of the brake shoe pivot points and the condition of the brake shoe rollers and S-camshaft bushes.

Replace return springs per brake set and preferably per axle if one spring is fatigued, deformed or broken. Also fit new springs when renewing brake shoes, brake linings, brake drums or S-camshaft components. Use suitable brake spring tools: damage to the hook or twisting of the spring during installation reduces its effective spring force. Never lubricate springs, brake linings or the drum with grease or oil; only the specified sliding and bearing points of the brake actuation system may be treated with the correct heat-resistant lubricant.

After installation, the drum must be able to rotate without dragging, while the brake shoes engage evenly when applied. Adjust the brake adjuster in accordance with the manufacturer's instructions and check the temperature of both wheels or brake drums with an infrared thermometer after a short test drive. A clear temperature difference indicates remaining drag, incorrect adjustment or a fault in the brake actuation system. Braem can provide technical support for the correct identification of brake components based on the chassis number.