Static Analysis

Self Energizing

Friction moment cFf adds to the actuating moment aFa. This is known as self-energizing. If force Fa is applied, the friction generated at the shoe acts to increase the braking torque.But, if the brake drum rotation is reversed, the sign of friction moment becomes negative and is then self-denergizing.

Self-energizing brakes advantage

This self-energizing feature of drum brakes is a potential advantage, as it reduces the required application force compared to a disk brake of the same capacity.

Drum brakes typically have two shoes, one of Drum brakes typically have two shoes, one of which can be made self energizing in each direction, or both in one direction.

The latter arrangement is commonly used in automobile brakes to aid in stopping forward motion at the expense of stopping backward motion, which is normally at lower speeds.

Self Locking

If the brake is self-energizing and force needed to actuate the brake becomes zero or negative.

The brake is then said to be self-locking. If the shoe touches the drum, it will grab and lock.

This is usually not a desired condition except in This is usually not a desired condition except in so-called backstopping applications.

In effect, self locking brake can function as a overrunning clutch to backstop a load and prevent it from running away if power is lost. Such brakes are sometimes used in hoists.

Internally Expanding Rim Clutches

Actuating force

A

Deformation

Pivot point

Element

Pressure is proportional to sin

Designer is concerned about the largest allowable pressure, not the location.

At = 0, pressure is zero. The frictional material located at the heel therefore material located at the heel therefore contributes very little to the braking action and might as well be omitted.

A good design would concentrate as much frictional material as possible in the neighborhood of the point of maximum pressure.

Frictional material begins at an angle 1, measured from the hinge pin A, and ends at an angle 2.

Any arrangement such as this will give a good distribution of the frictional material.

Normal force

Friction force

Moment of friction forces: moment arm (r-acostheta)

Moment of normal forces: moment arm asintheta

To avoid self-locking

Hinge pin reactions

Hinge pin reactions: for counter clockwise rotation