Mechanical Brakes

1.7. Mechanical Brakes:

The brakes which are operated mechanically by means of levers, linkages, pedals, cams, bell cranks, etc. are known as mechanical brakes.

The external contracting brake which is usually hand brake in automobiles is mechanical brake. Automobiles contain service brakes operated mechanically.

Mechanical brakes were employed in olden days but now hydraulic and other types of braking system have taken its place.

The major drawback of a "mechanical brake system" is that it is very difficult to get simultaneous brake action on all the four wheels. 

Also lengths of various rods and cables are vary and this causes unequal braking action.

Mechanical brakes are absolute now as service brakes. However, these are still used on rear wheels in many cars, as parking or emergency brakes.

Application / Types of mechanical brakes:

1. Drum brake (mechanically actuated).

2. Mechanical Hand Brake / Parking brake/ Emergency brake.

1.7.1. Drum Brakes (Mechanical Actuated):

Four wheel cars have a separate drum integrated into the hub of the rear wheels as Shown in Fig 1.13 (mechanically actuated). 

The drum brake usually fitted to rear wheels is only for mechanical brake system, and is actuated only by the cable.

Earlier drum brakes are mechanical type which is operated (internal expanding) by means of levers, linkages, pedals, cams, bell cranks, etc. 

Basically automobile brakes are internal expanding brake.

Types of drum brakes (mechanical actuated) - fixed expander type, floating expander type, floating anchor type, two leading shoe type and two trailing shoe type.

1.7.1.1. Construction of drum brake:

In this type of brakes, a brake drum is attached concentric to the axle hub whereas on the axle casing is mounted a back plate. 

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’.

Fig 1.13: Drum brake (mechanical actuated).

Two brake shoes are anchored on the back plate as shown in Fig. 1.13. Friction brake linings 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.

Fig 1.13 shows, it consists of two shoes S1 and S2. The outer surfaces of the shoes are lined with some friction material, to increase the-frictional coefficients and to prevent wearing away of the metal. 

Each shoe is pivoted at one end about a fixed fulcrum (O1 and O2) and made to contact a cam at the other end. 

When the cam rotates, the shoes are pushed outwards against the rim of the drum. The friction between the shoes and the drum produces the braking torque and consequently speed of the drum reduces. 

Brake shoe retracting spring which connects both the brake shoes at their loose end helps them in contracting after the brakes are released.

The relative braking torque obtained at the shoes for the same force 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.

1.7.1.2. Working of mechanically actuated drum brake:

When the brake pedal is pressed down, its motion is transmitted to the cam through various mechanical linkages. The motion of the cam tends to expand out the brake shoes. 

This inward motion of the brake shoes will try to slow down the motion of the rotating brake drum. Because the wheel is fixed to the brake drum, so automatically it will be held to move further.

Force F is applied by means of some brake actuating mechanism, which forces the brake shoe against the revolving drum, thereby applying the brakes.

When brake pedal is released, the pedal will move up because of the tension of the return spring. 

A retracting spring draws the shoes away from the drum when the cam is moved to its initial position and hence the brake shoes are no longer in contact with the drum, which is now free to rotate.

1.7.2.  Mechnical Hand Brake:

Most of cars use a mechanical cable linkages, hand lever to actuate the emergency brakes. It is type of a secondary brake. And this brake is second choice of the driver to applied the brakes.

It has no hydraulics. These brakes are used for parking on slopes and during emergency i.e. also known as Parking or Emergency Brake, which are actuated by hand lever via mechanical linkage. Therefore it is also called Mechnical Hand Brake.

In cars, an emergency brake has to be actuated by this separate mechanism, see Fig 1.14. When the primary (or service) brakes in case of a total brake failure, then emergency (or secondary) brake are come into play.

Fig 1.14: Hand brake (Parking brake)

Automobile hand brakes usually consist of a cable directly connected to the brake mechanism on one end and to a lever or foot pedal at the driver's position.

Hand brakes are usually the mechanical brakes as shown in Fig 1.14. These brakes operate independently of the foot brakes. Hand brake is generally located on the side of the driver's seat. On most of the vehicles hand brake applies only the rear brakes.

In order to apply the brakes the ratchet is released first by pressing the ratchet release handle, which causes the pawl to move up, disengaging the ratchet. A Fig 1.15 shows the hand brake lever.

Fig 1.15: Hand brake lever

Fig 1.16: Strut and lever for hand brake arrangement

Then the brake lever is pulled up, while further pulls the cable which in them operates the rear brakes mechanically through a linkage operating on the piston of the rear wheel cylinder, which is two valves.

The ratchet release handle which had been pressed so far is released now, so that the pawl moves down the spring action and engages with the ratchet thus keeping the brakes applied.