112 Brake shoe and pad fundamentals

11.2.1 Brake shoe self-energization (Fig. 11.2) The drum type brake has two internal semicircular shoes lined with friction material which matches up to the internal rubbing face of the drum. The shoes are mounted on a back plate, sometimes known as a torque plate, between a pivot anchor or wedge type abutment at the lower shoe ends, and at the upper shoe top end by either a cam or hydraulic piston type expander. For simplicity the expander in Fig. 11.2 is represented by two opposing arrows and the shoe linings by two small segmental blocks in the mid-region of the shoes.

When the drum is rotating clockwise, and the upper tips of the shoes are pushed apart by the expander force Fe, a normal inward reaction force N will be provided by the drum which resists any shoe expansion.

As a result of the drum sliding over the shoe linings, a tangential frictional force Ft = uN will be generated between each pair of rubbing surfaces.

The friction force or drag on the right hand shoe (Fig. 11.2) tends to move in the same direction as its shoe tip force Fe producing it and accordingly helps to drag the shoe onto the drum, thereby effectively raising the shoe tip force above that of the original expander force. The increase in shoe tip force above that of the input expander force is described as positive servo, and shoes which provide this self-energizing or servo action are known as leading shoes.

i.e. Fl = Fe + Ft where Fl = leading shoe tip resultant force

Likewise considering the left hand shoe (Fig. 11.2) the frictional force or drag Ft tends to oppose and cancel out some of the shoe tip force Fe producing it. This causes the effective shoe tip force to be less than the expander input force. The resultant reduction in shoe tip force below that of the initial

Fig. 11.2 Drum and shoe layout input tip force is described as negative servo, and shoe arrangements which have this de-energizing property are known as trailing shoes.

i.e. Ft = Fe - Ft where FT = trailing shoe tip resultant force

The magnitude of the self-energizing action is greatly influenced by the rubbing surface temperature, dampness, wetness, coefficient of friction and speed of drum rotation.

Changing the direction of rotation of the drum causes the original leading and trailing shoes to reverse their energizing properties, so that the leading and trailing shoes now become trailing and leading shoes respectively.

The shoe arrangement shown in Fig. 11.2 is described as a leading-trailing shoe drum brake.

Slightly more self-energizing is obtained if the shoe lining is heavily loaded at the outer ends as opposed to heavy mid-shoe loading.

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