10 September 2018

Drum brakes | Construction and Types | Factors

Construction and Types : 

In this types of brakes, a brake drum is attached concentrically to the axle hub whereas on the axle casing is mounted a backplate. In case of the front axle, the brakes plate is bolted to the steering knuckle. The backplate is made of pressed steel sheet and is ribbed to increase rigidity and to provide support for the expander, anchor and the brake shoes are protects the drum and shoe assembly from dust and mud. 

However, it absorbs the complete torque reaction of the shoe due to which reason it is sometimes also called a torque plate. Both brake shoes are anchored on the backplate. Friction linings are mounted on the brake shoes. Retractor springs are used to serve to keep the brake shoes away from the drum when the brakes are not applied. The brake shoes are anchored at one end whereas on the other hand force F is applied by means of some brake actuating mechanism. Which forces the brake shoe against the revolving drum, and hence applying the brakes. An adjuster is also provided to compensate for wear of friction lining with use. 

The relative braking torque obtained at the shoes for the same force applied at the pedal varies depending upon whether the expander is fixed to the brake plate or floating If the anchor is fixed or floating and then the shoes are leading or trailing. 

Drum brakeA

  • Fixed expander type : 
To understand the action of this type it is necessary to understand the terms 'leading' and 'trailing' shoes. 

It is seen that a leading shoe tip is dragged along the drum even when there is no braking force, while the tip of the trailing shoe is thrown off the brake drum. Thus when the brakes are applied, the net force exerted on the leading shoe becomes more than the net force exerted on the trailing shoe and as such unequal braking effect is produced at the two shoes. With increased braking effect and consequently higher temperatures, the coefficient of friction gets reduced more with prolonged application in case of the leading shoe, due to which reason, leading shoe fades quicker than the trailing shoe. 

Fixed expander type drum brake

  • Floating expander type :
In this type, the expander is not fixed on the backplate but is kept floating. In this way, the unequal braking effects at the two shoes are automatically balanced and made equal. Even if the lining on one shoe is worn more than on the other, the floating expander will move to one side so that the shoes still share equally the actuating force. However, the lining wear on two shoes is still unequal. 

  • Floating anchor type :  
In this type of shoe operating mechanism, the two shoes are linked together at the floating anchor and have a common fixed anchor. For the direction of rotation, it is seen that both the shoes become leading, the details including merits and demerits of which are given below under separate bread since the shoes can be made leading by other means also. 

  • Two leading shoe type : 
For this type, the leading shoe experiences an added breaking force or self energization. Thus if both the shoes are made leading, it definitely increases the braking torque. The lining wear also becomes uniform on both the shoes. However, the disadvantages are that firstly when the vehicle is moving in reverse, both the shoes will become trailing shoes and the braking effect is considerably decreased. This may not matter much because generally the vehicles in reverse are driven at comparatively slow speeds where the braking torque required is also less. Secondly, this type is sensitive to the coefficient of friction changes and thus will not be best suited for brakes meant for prolonged applications. 

  • Two trailing shoe type :
In this type, both the shoes are trailing shoes as a result of which the braking effort at the wheels is decreased for the same force applied at the brake pedal as in case of two leading shoe type. That is why this type of brake is used generally with servo brakes or power brakes so that the driver is not fatigued. 
Apart from the above disadvantage of the decreased braking effort, this type has got a definite advantage. It has better anti-fade properties than the two leading shoe type and thus provides more consistent braking. 

Factors affecting braking effect : 
  • The radius of the brake drum and the wheel 
Where Retarding force produced on a brake drum F = FB * Rb / Rw

  • The area of the brake lining and the amount of pressure applied at the brake lining increases the braking effect directly. 
  • The higher coefficients of friction between braking surfaces and between tyre and road are also useful in increasing the braking effect, but two high coefficient may cause locking of wheels, which must be avoided.