Block Brake with Short Shoe and Block Brake with long Shoe
1. NIGUSSIE ADEM DEPT. OF MANF. TECH.
BLOCK BRAKE WITH SHORT SHOE
A block brake consists of a simple block, which is pressed
against the rotating drum by means of a lever as shown below.
The angle of contact between the block and the brake drum is
usually small. When it is less than 45°, the intensity of
pressure between the block and brake drum is uniform.
Fig. Block Brake Fig. FBD (Clockwise rotation)
3. NIGUSSIE ADEM DEPT. OF MANF. TECH.
Depending upon the magnitude of coefficient of friction (𝜇) and location of
hinge pin (c), there are three different cases.
I. Case I: a > 𝜇c
In this case, the friction force (𝜇N) helps to reduce the magnitude of the
actuating force P. Such a brake is called a partially ‘self-energizing’
brake. This is a very desirable condition.
4. NIGUSSIE ADEM DEPT. OF MANF. TECH.
II. Case II: a = 𝜇c
In this case, the actuating force P is zero. Such a brake is called a ‘self-
locking’ brake. This is not a desirable condition in normal applications.
III. Case III: a < 𝜇c
Under this condition, the actuating force P becomes negative. This is a
dangerous operating condition, resulting in uncontrolled braking and
grabbing.
In order to prevent the brake arm from grabbing, the moment of
friction force about the brake arm pivot (µ N c) should be less than the
moment of brake effort about the pivot (Pb).
5. NIGUSSIE ADEM DEPT. OF MANF. TECH.
Taking moment of forces acting on the lever about the hinge point O,
The total brake effort that results from self-energizing action depends upon the following
three factors:
i. The location of pivot for the brake lever or brake arm, namely, dimensions a and c
ii. The coefficient of friction (𝜇)
iii. The direction of rotation of brake drum
6. NIGUSSIE ADEM DEPT. OF MANF. TECH.
Example 1
A single block brake with a torque capacity of 250
N-m is shown. The brake drum rotates at 100 rpm
and the coefficient of friction is 0.35. Calculate
I. The actuating force and the hinge-pin reaction
for clockwise rotation of the drum;
II. The actuating force and hinge-pin reaction for
anticlockwise rotation of the drum;
III. The rate of heat generated during the braking
action; and
IV. The dimensions of the block, if the intensity of
pressure between the block and brake drum is 1
𝑁/𝑚𝑚2. The length of the block is twice its
width.
V. State whether the brake is self-locking.
8. NIGUSSIE ADEM DEPT. OF MANF. TECH.
Example 2
A double block brake is shown below. The brake drum rotates in a clockwise
direction and the actuating force is 500 N. The coefficient of friction between
the blocks and the drum is 0.35. Calculate the torque absorbing capacity of
the brake.
10. NIGUSSIE ADEM DEPT. OF MANF. TECH.
Considering the forces acting on the link
DAB and taking moments about the pin A,
11. NIGUSSIE ADEM DEPT. OF MANF. TECH.
INTERNAL EXPANDING BRAKE
Internal expanding brake consists of a shoe, which is pivoted at one end and
subjected to an actuating force P at the other end. A friction lining is fixed on
the shoe and the complete assembly of shoe, lining and pivot is placed inside
the brake drum.
Internal shoe brakes, with two symmetrical shoes, are used on all automobile
vehicles.
The actuating force is usually provided
by means of a hydraulic cylinder or a
cam mechanism
13. 13
NIGUSSIE ADEM DEPT. OF MANF. TECH.
Consider an elemental area on the friction lining located at an angle ∅ and
subtending an angle d∅. The elemental area will be (Rd∅ 𝑤) where w is the
width of the friction lining parallel to the axis of the brake drum.
If 𝑝 is the intensity of normal pressure on this elemental area, the normal
reaction dN is given by,
The normal pressure 𝑝 is proportional to the vertical distance (R sin ∅) of
the element from the pivot. Therefore,
16. NIGUSSIE ADEM DEPT. OF MANF. TECH.
If brake drum rotates in counter clockwise direction:
17. NIGUSSIE ADEM DEPT. OF MANF. TECH.
Example 3
An automotive type internal expanding double-shoe brake is shown below. The face width
of the friction lining is 40 mm and the maximum intensity of normal pressure is limited to
1𝑁/𝑚𝑚2. The coefficient of friction is 0.32. The angle 𝜃1 can be assumed to be zero.
Calculate:
A. The actuating force P; and
B. The torque-absorbing capacity of the brake.
