The function of a hand brake is clear: it must reliably secure a parked vehicle against accidently rolling away, as well as assist with hill starts. Firstly, the safe functioning of a hand brake requires that it can be fully released at any time. This can be critical if the hand brake lever is taken out of the parking position but the driver does not notice that the parking brake has not fully released the rear wheel. This leads to the brake shoes on the rear axle overheating. This can cause not only a strong odour, but it can even lead to the failure of the brake system itself. This is often caused by blocked expanding locks.
Secondly, material wear or fatigue in the expanding lock can lead to the brake shoes no longer coming into full contact. Due to the lack of sufficient pressure, the vehicle may roll away despite the hand brake being applied. In contrast to electric parking brakes or brake systems that have a brake piston parking brake, expanding locks transfer the force of the hand brake to the brake shoes in a purely mechanical process. They do so by combining two functions: they lock the lever to securely lock the rear wheels; and they act as an adjuster to compensate for wear in the friction material.
Vehicle manufacturers therefore recommend replacing the expanding lock every time the brake shoes are replaced. Textar is now offering 16 expanding locks in its current portfolio, thus covering almost every required application. In addition to good manoeuvrability, attention must also be paid to the material quality of this safety component as imprecise production quality can result in the hand brake not holding under load, such as when parked on an incline.
The expanding lock is a drum brake component. However, vehicles with disk brakes on the rear axle may be affected in the same way if the hand brake is designed as an internal drum brake. Damage to the expanding locks is quite common in this design, as the parking brake linings barely wear and the condition of the hand brake can only be determined when the brake discs are removed. Therefore, BMW, for example, recommends replacing the expanding locks every 4 years.
“There is no better way to ensure the full effectiveness of the hand brake, because when the brake shoes are replaced, the expanding lock can usually be easily checked and replaced”, says Dalibor Stojnic, Global Category Manager at TMD Friction. “It is essential that workshops always use premium quality spare parts to ensure high safety standards.”
SOURCE: TMD Friction
