SAFARI

BRAKES

5.5 Brakes on without vacuum (Fig. 48) :

In this condition, all chambers are in

atmospheric pressure. When the pedal is

depressed, the piston moves forward hitting

the rubber reaction disc. In this condition the

vacuum brake valve has no boosting action

and the output force on the push rod is given

as below

Output force = Input force on the operating rod

– Mechanical loss inside booster

The mechanical loss inside booster is mainly

due to spring loads and friction due to seals in

cover and rear housing.

BALANCINGARRANGEMENT :

The balancing action of vacuum brake valve is

by means of rubber reaction disc arrangement

Fig. 43. The input force on the operating rod

moves the operating rod assembly, which

further exerts force on the rubber reaction disc.

The rubber reaction disc acts as a compressed

Fig. 48

fluid transmitting the force from the operating rod

via. the piston to the push rod. The reaction force

from the master cylinder compresses the rubber

disc, which further transmits this force to the

valve body.The valve body balances this reaction

force by moving backwards, closing the air valve.

The boost ratio is given by the ratio of the reacting

areas on the push rod and the piston i.e

Boost ratio

= Area of inner diameter of reaction area of

push rod / Area of Piston

= D

Where D = Inner diameter of reaction area in

push rod

d = outer diameter of piston

By changing the diameter of piston and the

corresponding diameter of valve body, various

configurations of boost ratios can be obtained.