Construction and types

In this type of brakes, a brake drum is attached concentric to the axle hub whereas on the axle casing is mounted a backplate. In case of front axle, the brakes plate is bolted to the steering Knuckle. The back plate is made of pressed Steel sheet and is ribbed to increase rigidity and to provide support for the expander, anchor and brake shoes. It also protects the drum and shoe assembly from mud and dust. Moreover, it absorbs the complete torque reaction of the shoes due to which reason it is sometimes also called "torque plate". Two brake shoes are anchored on the back plate as shown in figure. Friction lining are mounted on the brake shoes. One or two retractor springs are used which serve to keep the brake shoes away from the drum when the brakes are not applied. The brakes shoes are anchored at one end, whereas on the other ends force F is applied by means of some brake actuating mechanism (to be described later), which forces the brake against the revolving drum, there by applying the brakes. An adjuster is also provided to compensate for wear of friction lining with use. The relative braking torque obtained at the shoes for the same force is applied at the pedal varies depending upon whether the expander (cam or toggle lever) is fixed to the back plate or it is floating whether the anchor is fixed or floating and whether the shoes are leading or trailing. All these types are explained below

Fixed expander type

To understand the action of this type it is necessary to understand the term leading and trailing shoes. This are shown in figure.

It is seen that a leading shoe tip is dragged along the drum even when there is no breaking force, while the tip of the trailing shoe is thrown off the brake drum. Thus when the brakes are applied, the net force exerted on the leading shoe becomes more than the net force exerted on the training shoe and as such  unequal braking effect is produced at the two shoes. Moreover, with increased braking effect and consequently higher temperatures, the coefficient of friction gets reduced more with prolonged application in case of leading shoe, due to which reason, leading shoe fades quicker than the training shoes.

Floating expander type

In this type the expander is not fixed on the back plate, but is kept floating. In this way the unequal braking effects at the two shoes are automatically balanced and made equal. Even if the lining on one shoe is worn more than the other, the floating expander will move to one side so that the shoes still share equally the actuating force. However, the lining wear on two shoes is still unequal.

Floating anchor type

Figure shows this type of shoe operating mechanism. The two shoes are linked together at the floating anchor and have a common fixed anchor as shown. For the direction of rotation shown in the figure, it is seen that both the shoes become leading, the details including merits and demerits of which are given below under separate heads, since shoes can be made leading by other means also.

Two leading shoe type

We have already seen that a leading shoe experiences an added braking force or "self energization". Thus if both the shoes are made leading, it definitely increases the braking torque. The lining wear also becomes uniform on both the shoes. However, the disadvantages are that firstly when the vehicle is moving in reverse, both the shoes will become training shoes and braking effect is considerable decreased. This may not matter much because generally the vehicles in reverse are driven at comparatively slow speeds where the braking torque required is also less. Secondary, this type is sensitive to coefficient of friction changes and thus will not be best suited for brakes meant for prolonged application.

Two trailing shoe type

In this type both the shoes are training shoes, as a result of which braking effort at the wheels is decreased, for the same force applied at the brake pedal as in case of two leading shoe type or even the more common, one leading and one training shoe type. That is why this type of brake is generally used with Servo brakes or Power brakes, so that the driver is not fatigued.

Apart from the above disadvantage of decreased braking effort, this type has got definite advantage. It has better antifade properties than the two leading shoe type and thus provides more consistent braking.