Mechanical brakes are obsolete now as service brakes. However, these are still used on rear wheels in many cars, as parking or emergency brakes.
Brake shoe operation :- The brake shoes are operated by means of a cam or toggle lever which itself is actuated through various mechanical linkages. Earlier the drum brakes were mostly mechanically operated. These days their use is limited to parking brakes. However, some large trucks and articulated trailers also employ such type of brakes. Figure shows one method of making both shoes leading by mechanical means. This is known as girling type mechanism. When the brakes are applied, force at the expander pushes the arm of upper Bell crank Lever. This transmits its motion through the vertical strut to the lower Bell crank and thus the lower arm of the bell crank tends to move towards the left. But as the adjuster the mechanism is there, it cannot actually move to the left, it gets a reaction and thereby the whole of the brake shoe at the lower end moves towards the right, acting as a leading shoes. The other is already leading. This type has got one obvious disadvantage. On reverse, both the shoes will become trailing, thus reducing the braking effect considerable. As explained already this is not objectionable ordinarily but if desired the defect can be remedied by using the arrangement on both shoes as shown in figure. When the vehicle is in reverse, the shoes bear against the expanding mechanism and consequently become leading, thus obviating the disadvantage of the one Strut arrangement in reverse. Alternately, the two leading shoes are employed only on Front brakes, where more braking effort is required on account of transfer of weight.
Linkage :- Figure shows a simple outline for the linkage of mechanical brakes. When the pedal is pressed, the brakes at the wheels are applied. This simple linkage system, however, has the disadvantage that it is very difficult to obtain equal braking effect at all the wheels, for which the compensations have to be used. Further it contains no suitable means to take up the effects of wear during use.
Brake compensation :- Consider the brakes to be applied on the rear (or front) wheels. If the Brake Linings at the near side and offside brakes have unequal wear, the mechanism shown in figure will not be able to produce simultaneously the same braking effect at the two wheels, which is an essential requirement for smooth brake operation. The same arrangement applies if the rear brake shoe lining wear is different from front brake shoe lining. Therefore to obviate this defect some special mechanism has to be incorporated in the linkage. Such a mechanism is called the compensator or Equalizer and this process of ensuring equal braking effect at all the wheels inspite of unequal lining wear etc on them is called compensation. For complete compensation three compensators are required one between front and rear wheels and one each at the front and at the rear.
The most simple compensator is a balance beam shown in figure. The rod A is operated by means of the brake pedal and is hinged to the balance beam D at the centre. Rod B goes to operate the offside brake and is hinged to one end of the balance beam, while to the other is hinged rod C meant to operate near side brake.
To understand the operation of this compensator, considered that the wear of lining at the offside brake shoes is more so that the shoe travel for brake application there, has to be more than in case of near side brake. When rod A is pulled, the balance beam is also moved parallel to its own length till the shoe clearance at the near side break is taken up and the shoes there come in contact with the brake drum. Further the movement of the rod A would cause the balance beam to rotate slightly with near side end as the fulcrum and only offside end moving to the left till the shoes at offside brake come in contact with their drum. Thus the extra clearance in the offside brake is taken up. By now the force in the various linkage rods would be very small, just enough to overcome the friction in the linkage and also the forces in the retractor springs. It is only after this stage that the forces in the rods increase and Brakes are applied simultaneously.
It must be remembered, however, that this type of compensator will not balance the braking in case the coefficient of friction of Brake Linings are different in different brakes. The situation may arise due to the presence of oil in some of the brake drums, decreasing the friction there.
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