The Sustainable Development Goals (SDGs) currently necessitate mechanisms that achieve effective outcomes with minimal material usage. In the realm of automotive brakes, drum brakes leverage the servo effect to attain substantial braking force through self-amplification. However, excessive amplification can lead to overwhelming braking force and self-locking, known as sprag. This condition causes component deformation, frictional vibrations, and, in severe cases, part failure. R.T. Spurr et al. have studied this phenomenon using the friction force F that it generates. For simplification, the authors introduced a dimensionless mechanical friction coefficient μｍ.

We propose to employ this μｍ in the design and production of products that maximize effectiveness without μｍ becoming infinite.

Leading
μｍ=1/(1/μ－tanθ)

Trailing
μｍ=1/(1/μ+tanθ)

The friction coefficient μ of friction materials varies with factors such as temperature, humidity, and usage history. Materials should be selected and processed to minimize these variations. Additionally, the fundamental dimensions of braking mechanisms, including sliding members, change due to wear and temperature fluctuations. These dimensional changes alter the contact position and contact angle θ, thereby influencing μｍ. Hence, careful consideration is necessary to prevent μｍ from approaching infinity even under maximum dimensional changes.

Furthermore, we propose applying μｍ not only to drum brakes but also to disc brakes to mitigate sprag occurrence.

SAE Paper 2024-01-5037