1122 Retarding wheel and brake drum torques

The maximum retarding wheel torque is limited by wheel slip and is given by where

Tw = Ma WR (Nm) Tw = wheel retarding torque (Nm) Ma = adhesion factor W = vertical load on wheel (N)

wheel rolling radius (m)

Likewise the torque produced at this brake drum caused by the frictional force between the lining and drum necessary to bring the wheel to a standstill is given by

M = coefficient of friction between lining and drum N = radial force between lining and drum (N) r = drum radius (m)

Both wheel and drum torques must be equal up to the point of wheel slip but they act in the opposite direction to each other. Therefore they may be equated.

.'. Force between TJ,n . , , Ma WR lining and drum N =-

Example A road wheel has a rolling radius of 0.2 m and supports a load of 5000 N and has an adhesion factor of 0.8 on a particular road surface. If the drum radius is 0.1m and the coefficient of friction between the lining and drum is 0.4, determine the radial force between the lining and drum.

Ma WR

0.8 x 5000

11.2.3 Shoe and brake factors (Fig. 11.2) If the brake is designed so that a low operating force generates a high braking effort, it is said to have a high self-energizing or servo action. This desirable property is obtained at the expense of stability because any frictional changes disproportionately affect torque output. A brake with little self-energization, while requiring a higher operating force in relation to brake effort, is more stable in operation and is less affected by frictional changes.

The multiplication of effort or self-energizing property for each shoe is known as the shoe factor.

The shoe factor S is defined as the ratio of the tangential drum drag at the shoe periphery Ft to the force applied by the expander at the shoe tip Fe.

i.e. Shoe factor

Tangential drum force

Shoe tip force

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