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|Fig: Drum Brakes|
Basics of Drum Brakes:
Drum brakes is a type of mechanical braking system that uses friction caused by a set of shoes( a.k.a brake shoe) that presses outwards against a rotating brake drum. The braking action is caused inside of a brake drum thus, giving it a name ‘Drum Brake’. They need regular adjustment of brake wire, brake lever adjustment and requires frequent changing of shoes at a particular period of time after usage.
However, these brakes are generally operated with mechanical force by brake pedals but in some other applications they were also operated by hydraulic force mechanism.
However, Drum Brakes are only used in low cost bikes and scooter. But earlier, only performance bikes up to 250 cc were available with rear drum brake as an option and with disc brakes on front again for keeping the prices low.
Also when they were introduced in 1900’s and in earlier time they were used in cars and heavy vehicles. But with changing technology and availability of better capability of braking power they are not used on modern cars , trucks and performance bikes. However, we might find old age cars and trucks with drum brakes.
Main parts of Drum brakes:
|Fig: Parts of a Drum Brake|
- Brake Drum: A hollow cylinder shaped inside which the braking action occurs. It is the outer most part of the drum brake. Iot is attached to the wheel rim. It also rotates with the wheel.The brake drums are usually made of nickel-iron castings, iron and steel castings,etc
- Brake shoes: It carries the brake lining and is present inside the brake drum. When brake is applied the brake shoe comes in contact with drum which causes the lining to come in contact with drum.
- Brake linings: It is the consumable part of drum braking system., means more wear causes on the linings. They are hard material because they are rubbed against the drum. It is attached/secured to the brake shoe with the help of rivets or by cementing.
- Cam or toggle lever or expander: It is a mechanism which slides or moves the brake shoe towards the brake drum. It is the first part in braking system which executes the force and the forces the shoe towards drum. The toggle lever is connected to brake shoe.
- Retracting spring: A spring to retract the shoe in its original position to keep no contact between drum and lining-shoe after braking action.
Working of Drum Brakes:
When the brake pedal or lever is pressed, the toggle lever/cam turns which forces the brake shoe to move away and consequently it makes surface contact with the brake drum. As a result, brake lining mounted on the brake shoe comes into contact with brake drum and friction is produced. Heat is produced. This friction causes to slow down or stop the wheel from rotating. As soon as the brake pedal is released the retracting spring forces the brake shoe to come back in its original position. Thus the surface contact between lining-shoe and drum is over. Now the wheel is free to rotate.
Generally the braking in drum brakes is caused by the brake pedal force and consequently the linkage transmits the pedal force to the brake shoe. More the pedal force within the usable range more is the braking action. However, beyond the usable range the linkage might brake off due to impact energy.
- Low cost and small cc bikes or scooters (50-125 cc) to keep the cost of product low.
- Above all, brake shoe or the replaceable parts are cheap and easily available.
- Simple design.
- Adjusting and installation of brakes is easy.
- Doesn’t dissipate heat like disc brakes.
- Less powerful and stopping distance is more.
- When water comes in contact with brakes it makes scratching noises and the braking becomes less effective after 2 or 3 times of usage.
- 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.
- Only used for low cost and light applications like small bikes(50-125 cc). They can’t be used in cars or trucks because the speed and weight of the vehicle is more and while braking any part may break.