How Drum Breaks Works ?

We all know that Driving a vehicle without brakes is impossible. Brakes are crucial for stopping a vehicle and drive safely on the road. In modern vehicle the drum brakes are mostly installed at rear wheels. There are two types of brake assembly commonly available – disc brakes and drum brakes. In addition, the majority of modern cars are fitted with ABS as standard.But here I will discuss only about Drum brakes and how it works.

In order to know the How does Drum Break Works ? ..First we should know about the parts of Drum Breaks and what is the mechanism of them.....So, Drum brake components include the 
  • Backing plate, 
  • Brake drum, Break shoes, 
  • Wheel cylinder, and 
  • Various springs and pins.

Backing plate :-
The backing plate provides a base for the other components. The back plate also increases the rigidity of whole set-up, supports the housing, and protects it from foreign materials like dust and other road debris. It absorbs the torque from the braking action, and that is why back plate is also called the "Torque Plate". 

Brake Drum :-

Break Drums are made of a special type of Cast iron which are better in heat-conductive and wear-resistant. This It part plays an important role in breaking system because the drum brake is bolted to the hub of the wheel which rotates along with the wheel and axle. 

(When a driver applies the brakes, the lining pushes radially against the inner surface of the drum, and the ensuing friction slows or stops rotation of the wheel and axle, and thus the vehicle. This friction generates substantial heat.).

Wheel Cylinder :-
One wheel cylinder operates the brake on each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. The leading shoe (closest to the front of the vehicle) is known as the primary shoe. The trailing shoe is known as the secondary shoe. Hydraulic pressure from the master cylinder acts on the piston cup, pushing the pistons toward the shoes, forcing them against the drum. When the driver releases the brakes, the brake shoe springs restore the shoes to their original (disengaged) position. The parts of the wheel cylinder are shown to the right.

Break Shoe's :-
Brake shoes are typically made of two pieces of steel welded together. The friction material is either riveted to the lining table or attached with adhesive. The crescent-shaped piece is called the Web and contains holes and slots in different shapes for return springs, hold-down hardware, parking brake linkage and self-adjusting components. All the application force of the wheel cylinder is applied through the web to the lining table and brake lining.Each brake assembly has two shoes, a primary and secondary.

(i) Primary Brake Shoe: The shoe having large lining material is called as primary shoe.
(ii) Secondary brake shoe: The shoe with small lining material is called secondary shoe.

The primary shoe is located toward the front of the vehicle and has the lining positioned differently from the secondary shoe. Quite often, the two shoes are interchangeable, so close inspection for any variation is important.

Various springs and Pins :-
Spring in drum breaks play an important role these are used to retract the brake shoe after brake is applied. Two return springs are there in durm brake, one for the primary shoe and other one is for secondary shoe.Where as in case Self Adjuster are used to maintain the minimum gap between the brake shoe and drum so that they do not contact each other when pedal is not pressed.

Working Process :-
As we discussed above a drum brake is one type of Breaking System which consists of hydraulic wheel cylinders, brake shoes and a brake drum.In the motor vehicle, the wheel is attached to a drum which rotates along with the wheel and axle. 

When break pedal is applied which are attached to shoes press two curved brake shoes, which have a friction material lining, are forced by hydraulic wheel cylinders against the inner surface of a rotating brake drum.The magnitude of the friction force, multiplied by the radius of the drum, gives the torque tending to stop the drum, that is, the braking torque. Due to the brake shoe lining touches the inner surface of the drum, and due to the friction generated in between the brake shoe and drum, the motion of the wheel reduces and vehicle stops.

To draw the retracting spring are held between the brake shoes. These retracting springs draw the shoes away from the drum when the cam is turned and moved to the release position.This total mechanism in which the shoes are mounted to rub against the inside surface of the brake drum, is called internal expanding brake. In this system, each part of the linkage must be free to move. The joints must be properly lubricated to reduce friction and wear. 

Advantages of Drum Breaks :-
  • It is cheaper than the disc braking system.
  • Drum brakes emit less particulate matter (PM) than disc brakes, as the wear-particles are mostly sealed in.
  • Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels use disc brakes as the main brakes.
  • Drum brakes are used in most heavy duty trucks, some medium and light duty trucks, and few cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels since most of the stopping force is generated by the front brakes of the vehicle and therefore the heat generated in the rear is significantly less.
  • Due to the fact that a drum brake's friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake.
  • Slightly lower frequency of maintenance due to better corrosion resistance compared to disks.
  • And also built-in self energizing effect requires less input force (such as hydraulic pressure).
Disadvantages Of Drum Breaks :-
  • When the drums are heated by hard braking, the diameter of the drum increases slightly due to thermal expansion, so the shoes must move farther and the driver must press the brake pedal farther.
  • The drum brakes will not work properly if it gets wet in water. This is because water takes longer time to come out from the drum. the water reduces the friction in between the brake lining and drum.
  • Excessive brake drum heating can cause the brake fluid to vaporize, which reduces the hydraulic pressure applied to the brake shoes.
  • Therefore, the brakes provide less deceleration for a given amount of pressure on the pedal.
  • Brake fluid that is old and has absorbed moisture has a lower boiling point, so brake fade occurs sooner.
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