SINGLE BLOCK OR SHOE BRAKE
A single block or shoe brake is shown below. It consists of a block or shoe which is pressed against the rim of a revolving brake wheel drum. The block is made of a softer material than the rim of the wheel. This type of a brake is commonly used on railway trains and tram cars. The friction between the block and the wheel causes a tangential braking force to act on the wheel, which retard the rotation of the wheel. The block is pressed against the wheel by a force applied to one end of a lever to which the block is rigidly fixed. The other end of the lever is pivoted on a fixed fulcrum O.
Let
P = Force applied at the end of the lever,
RN = Normal force pressing the brake block on the wheel,
r = Radius of the wheel,
2θ = Angle of contact surface of the block,
µ = Coefficient of friction, and
Ft = Tangential braking force or the frictional force acting at the contact surface of the block and the wheel.
If the angle of contact is less than 60°, then it may be assumed that the normal pressure between the block and the wheel is uniform. In such cases, tangential braking force on the wheel,
Ft= µ.RN
and the braking torque,TB = Ft.r = µ RN. r
Let us now consider the following three cases :
CASE I : When the line of action of tangential braking force (F) passes through the fulcrum O of
the lever, and the brake wheel rotates clockwise as shown in Fig. 25.1 (a), then for equilibrium, taking
moments about the fulcrum O, we have,
P = Force applied at the end of the lever,
RN = Normal force pressing the brake block on the wheel,
r = Radius of the wheel,
2θ = Angle of contact surface of the block,
µ = Coefficient of friction, and
Ft = Tangential braking force or the frictional force acting at the contact surface of the block and the wheel.
If the angle of contact is less than 60°, then it may be assumed that the normal pressure between the block and the wheel is uniform. In such cases, tangential braking force on the wheel,
Ft= µ.RN
and the braking torque,TB = Ft.r = µ RN. r
Let us now consider the following three cases :
CASE I : When the line of action of tangential braking force (F) passes through the fulcrum O of
the lever, and the brake wheel rotates clockwise as shown in Fig. 25.1 (a), then for equilibrium, taking
moments about the fulcrum O, we have,
∴ Braking torque,
It may be noted that when the brake wheel rotates anticlockwise, then the braking torque is same, i.e.
Case 2 : When the line of action of the tangential braking force (F) passes through a distance ‘a’ below the fulcrum O, and the brake wheel rotates clockwise, then for equilibrium, taking moments about the fulcrum O,
Hence,
and braking torque,
When the brake wheel rotates anticlockwise, then for equilibrium,
and braking torque,
Case 3 : When the line of action of the tangential braking force passes through a distance ‘a’ above the fulcrum, and the brake wheel rotates clockwise then for equilibrium,taking moments about the fulcrum O, we have
Hence,
and braking torque,
When the brake wheel rotates anticlockwise, then for equilibrium, taking moments about the fulcrum O, we have
and braking torque,