Final answer:
A 'bell-mouthed' brake drum is often caused by excessive heat due to friction between the brake shoes and drum, resulting in the drum becoming distorted and flaring out at the ends.
Step-by-step explanation:
The phenomenon of a brake drum appearing 'bell-mouthed' is typically a result of excessive heating during braking operations. As brakes are used, the friction between the brake shoes and the drum generates heat, which can cause the drum to distort if the temperature becomes too high. This distortion is what leads to the 'bell-mouthed' condition, where the open ends of the drum flare out.
This condition can be aggravated by several factors, including overloading of the vehicle, continuous braking down long hills, or failure to maintain the braking system properly. The smoking brakes mentioned as evidence of the mechanical equivalent of heat underline the relationship between frictional force, heat production, and material deformation.
It is critical to address bell-mouthed brake drums promptly, as they can cause reduced braking efficiency, uneven wear on the brake linings, and may ultimately lead to brake failure if not corrected.