Frenos

DRUM BRAKE: FUNCTION, COMPONENTS, WORKING PRINCIPLE AND TYPES

A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum.

The term drum brake usually means a braking system in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or «band» wrapping around the outside of a drum.

Drum brakes are primarily used at the rear axle of small and compact class vehicles.

FUNCTION

Drum brakes have been around for almost as long as the automobile itself and are still fitted today in a modified and more sophisticated form in modern cars. The term drum brake describes the design principle: namely, an enclosed cylindrical structure.

DRUM BRAKE COMPONENTS

A drum brake comprises the following components:

1. Backing plate:
Provides a solid base for other components in the drum brake attached to the axle sleeve.

2. Brake drum:
Bolted to the wheel hub and spins with the wheel. Often made of cast iron, and is resistant to heat and wear. This is what you see when you look at an assembled drum brake and is the component upon which braking force is applied to slow or stop the car.

3. Wheel cylinder:
Contains two pistons, one at each end of the cylinder, to operate the brake shoes. The cylinder applies pressure to the pistons, which pushes the brake shoes towards the drum, slowing or stopping the car. One cylinder is needed per wheel.

4. Brake shoe:
Pushes into the drum to create the friction necessary to slow or stop the car. Secured to the backing, but able to slide when pressure from the wheel cylinder is applied. It has a lining attached to it, made up of organic or metallic compounds. The lining is what actually comes in contact with the drum and wears away with use. Each brake contains two shoes. The primary shoe is closer to the front of the vehicle, while the secondary shoe is closer to the rear. Depending on the type and brand, the brake shoes may be interchangeable.

5. Automatic adjuster:
Keeps the brake shoes at a consistent distance away from the drum, even as the lining wears away.

6. Return springs:
Pulls the brake shoes back away from the drum when the driver lets off the brake pedal.

The brake drum is fixed to the wheel and turns with it. On braking, the wheel cylinder forces the fixed brake shoes apart and presses them against the brake drum, thus slowing it down. When the brake is released, the return springs move the brake shoes back to their original position.

WORKING PRINCIPLE

When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system) and changed into hydraulic pressure (oil-pressure) by the master cylinder. The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid). The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons press the brake linings, which are friction materials, against the inside surfaces of the brake drums which rotate with the wheels. The linings are pressed on the rotating drums, which in turn decelerate the wheels, thereby slowing down and stopping the vehicle.

TYPES OF DRUM BRAKES

There are mainly three types – mechanical, hydraulic & pneumatic assisted Drum Brakes.

1. Mechanical:

In the mechanical Drum brake system such as in two-wheeler & auto-rickshaw, the brake shoes are actuated by a cam, which is attached to the brake linkage & pedal. When you press the brake pedal, the cam turns. Thus, it causes the brake shoes to expand outwards and rub against the drum.
The friction between the brake linings and the drum causes the drum to stop rotating, thereby the wheel to stop. When you release the brake pedal, the retracting springs bring the brake shoes back to their original position. This results in a gap between them and the drum and to again spin it freely.

2. Hydraulic:

The hydraulic Drum brake system such as in cars is a bit superior to a mechanical one. In this design, the hydraulic wheel cylinder replaces the cam. In the hydraulic system, instead of a cam, the wheel cylinder’s pistons push the brake shoes outward. The brake shoes fit on the anchor plate or braking plate. It holds the brakes system parts together and on to the car’s axle. When you press the brake pedal, the oil in the brake master cylinder multiplies the hydraulic force sent to the wheel cylinders. Thus, it causes its pistons to push outwards. The pistons, in turn, cause the brake shoes to expand and rub against the drum. The friction between the brake linings and the drum causes the drum to stop rotating, thereby the wheel to stop.

3. Pneumatic assisted:

The third type – pneumatic assisted Drum-brake system; actuated by air-pressure, which works on the same principle of that of the mechanical Drum brake system. It is also operated by a bigger size cam or the ‘S’ shaped cam and is popularly known as the “S-Cam” brake system. However, high-pressure compressed air actuates a pneumatic piston which turns the cam. Mostly the medium to heavy commercial vehicles use this type of drum brake system.

BASED ON PRINCIPLE

1. Leading/trailing shoe type drum brake

«Leading (or primary) shoe» is a term referring to the shoe that moves in the direction of rotation when it is being pressed against the drum. The other shoe is called the “trailing (secondary) shoe.” The leading shoe is pressed in the same direction as the rotation of the drums, and this rotation helps to press the shoes against the drum with greater pressure for stronger braking force. This is called the servo effect (self-boosting effect) which realizes the powerful braking forces of drum brakes.

Structurally, it has a wheel cylinder housing a piston with which hydraulic pressure is generated to push the two shoes against the drum’s inner surface.

The two shoe function in a way they both become either the trailing shoe or leading shoe depending on whether the vehicle is traveling forward or backward. Drum brakes generate consistent braking force whether the vehicle travels forward or backward. This is because drum brakes generate the same braking force in either direction. Generally, this type is used for the rear brakes of passenger cars.

2. Twin leading shoe type drum brake

This type of drum brake has two-wheel cylinders and two leading shoes. Each wheel cylinder presses on one shoe so that both shoes act as leading ones when the vehicle moves forward, providing superior braking force.
Each of the pistons housed in the wheel cylinders displaces in one direction, so when the vehicle is in reverse both shoes act as trailing ones. This type is used mainly for the front brakes of small-to-mid-sized trucks.
The dual twin leading shoe type has pistons that displace in both directions, making it possible for both shoes to act as leading ones, regardless of the direction of travel. This type is mainly used for the rear brakes of small-to-mid-sized trucks.

3. Duo-servo type drum brake

The duo-servo type features a structure where two brake shoes, called the primary shoe and secondary shoe, are linked via an adjuster. Strong pressure from the servo effect (self-boosting effect) of the primary shoe is transmitted to the linked secondary shoe, thus generated a very large braking force.
This type is mainly used for parking brakes on passenger cars, the center brakes on trucks, and brakes on forklifts.

ADVANTAGES AND DISADVANTAGES

Advantages of Drum brake system:

1. Simple design and parts
2. Easy & cheaper to manufacture
3. Low maintenance cost
4. Comparatively longer life

Disadvantages of Drum Brake system:

1. Low braking force compared to Discs
2. Brakes ‘fade’ when applied for a prolonged time
3. The brake shoe lining made of asbestos is harmful to humans
4. When wet, the braking grip reduces considerably
5. Non-asbestos linings catch moisture, causing Drum brakes to grab suddenly

SAFETY

The brake drum is one of the most important vehicle systems when it comes to safety. It is relatively low-wear and has a long service life. A specialist workshop should be consulted immediately if the deterioration in the braking action of a drum brake becomes noticeable. Drum brakes are only to be replaced by qualified personnel. The manufacturer’s installation instructions must be observed when doing so.

Brake rotors of disc brakes rotate with the wheels, and brake pads, which are fitted to the brake calipers, clamp-on these rotors to stop or decelerate the wheels. The brake pads pushing against the rotors generate friction, which transforms kinetic energy into thermal energy.

This thermal energy generates heat, but since the main components are exposed to the atmosphere, this heat can be diffused efficiently. This heat-dissipating property reduces brake fade, which is the phenomenon where braking performance is influenced by the heat. Another advantage of disc brake is its resistance to water fade, which occurs when the water on the brakes significantly reduces braking force. When the vehicle is in motion, the rotor spins at high speeds and this rotational motion discharges the water from the rotors themselves, resulting in stable braking force.

CONSTRUCTION

The brake rotor (disc) which rotates with the wheel, is clamped by brake pads (friction material) fitted to the caliper from both sides with pressure from the piston(s) (pressure mechanism) and decelerates the disc rotation, thereby slowing down and stopping the vehicle.

1. Rotor: 
Circular disc bolted to the wheel hub that spins with the wheel. Rotors are most commonly made of cast iron or steel; however, some very high-end cars use a carbon-ceramic rotor. Rotors can be slotted or drilled for better heat dissipation.

2. Brake pads: 
The component that pushes into the rotor, creating the friction that slows and stops a car. They feature a metal portion called a shoe and a lining that is attached to the shoe. The lining is what actually comes in contact with the rotor and wears away with use. Linings are made of different materials and fall into three categories: organic, semi-metallic and ceramic. The lining material chosen will impact the length of brake life, the amount of noise heard when the brakes are applied, and how quickly the brakes bring a car to a halt.

3. Piston: 
Cylinder connected to the brake system hydraulics. The piston is what moves the brake pads into the rotor when the driver presses the brake pedal. Some brake systems have a single piston that moves both pads, while others have two pistons that push the brake pads from each side of the rotor. Others still have four, six, or even eight pistons for higher braking power, at the expense of added cost and complexity.

4. Caliper: 
Housing that fits over the rotor and holds the brake pads and pistons, as well as contains ducting for brake fluid. There are two types of brake calipers: floating (or sliding) and fixed. Floating calipers “float” over the rotor, and only have pistons on a single side. When the driver presses the brakes, the pistons press the brake pads on one side into the rotor, which causes the caliper to slide over so that the pads on the non-piston side of the caliper also contact the rotor. Fixed calipers are bolted in place, and instead, have pistons on both sides of the rotor that move when the driver applies the brakes. Fixed calipers apply brake pressure more evenly and clamp more firmly on the rotor, however floating calipers are found on most cars and are perfectly adequate for everyday driving.

5. Sensors: 
Some vehicles have brakes that contain sensors embedded in the brake pads which work to tell the driver when the pads are worn out. Other brake sensors play a part in the vehicle’s ABS system.
Disc brakes are generally used in passenger cars, but due to their stable performance at higher speeds and resistance to brake fade, they are gradually spreading into the commercial vehicle segment, where drum brakes were traditionally chosen for their longer service life. There are two types of disc brakes.
The «opposed piston type disc brake» has pistons on both sides of the disc rotor, while the «floating type disc brake» has a piston on only one side. Floating caliper type disc brakes are also called sliding pin type disc brakes.

WORKING PRINCIPLE

When the driver steps on the brake pedal, the power is amplified by the brake booster (servo system) and changed into a hydraulic pressure (oil-pressure) by the master cylinder. The pressure reaches the brakes on the wheels via tubing filled with brake oil (brake fluid). The delivered pressure pushes the pistons on the brakes of the four wheels. The pistons in turn press the brake pads, which are friction material, against the brake rotors which rotate with the wheels. The pads clamp on the rotors from both sides and decelerate the wheels, thereby slowing down and stopping the vehicle.

• When the brake pedal is pressed, the high-pressure fluid from the master cylinder pushes the piston outward.
• The piston pushes the brake pad against the rotating disc.
• As the inner brake pad touches the rotor, the fluid pressure exerts further force and the caliper moves inward and pulls the outward brake pad towards the rotating disc and it touches the disc.
• Now both the brake pads are pushing the rotating disc, a large amount of friction is generated in between the pads and rotating disc and slows down the vehicle and finally, let it stop.
• When a brake pad is released, the piston moves inward, the brake pad away from the rotating disc. And the vehicle again starts to move.

TYPES OF DISC BRAKES

There are two types of disc brakes. One is called the «opposed piston type disc brake» which has pistons on both sides of the disc rotor, and the other is the «floating type disc brake» which has a piston on only one side. The floating type disc brakes are also called the sliding pin type disc brakes.

1. Opposed Piston Type Disc Brakes

The opposed piston type is a disc brake which has pistons on both sides of the disc rotors.
The opposed piston type disc brake features stable braking force as well as a high level of controllability.
The swept areas of the brake pads are enlarged to increase braking force, and here opposed piston types are favored. This is because of its advantage where the number of pistons can be increased to realize even distribution of pressure on the rotors from both sides. Depending on the size of the brake pads, there are several types, including the 4-pot type which has two pistons on each side for a total of four, and the 6-pot type which has three pistons on each side for a total of six.

2. Floating Type Disc Brakes

Floating type is a disc brake which has a piston on only one side and is also called the sliding type disc brake.
On the floating type disc brakes, the piston pushes the inner brake pad against the rotor when the brakes are engaged. This generates a reaction force that moves the caliper itself along with the slide pin, pushing the outer pad against the rotor to clamp it from both sides.

Many passenger car disc brakes are of the floating caliper type since this type has a relatively simple and lightweight construction, which allows for lower manufacturing costs.
Floating type disc brakes for commercial vehicles
Disc brakes are used mainly for passenger cars, but due to their consistent performance at higher speeds and resistance to brake fade, they are gradually spreading into the commercial vehicle segment, where drum brakes were traditionally chosen for their resistance against wear.

TYPES OF ROTOR

1. Smooth Rotors
Smooth rotors are identified by their flat, smooth surface. For most cars and trucks on the road, smooth rotors are original equipment (OE) because of their versatility for many driving conditions. The main benefit of smooth rotors is that they tend to wear evenly, helping your brake pads last longer. If you want to keep the smooth rotor but still go for the upgrade, look for premium metal that absorbs more heat.

2. Drilled or Dimpled Rotors
Drilled rotors are identified by the pattern of holes that have been drilled all the way through the rotor disc. Dimpled rotors are similar, though instead of holes there are dimples that have been drilled to the rotor’s minimum thickness level, retaining more structural integrity than a fully drilled rotor. These rotor types help the brake pads to better grip the rotor, giving it more initial bite and increasing stopping power.
*Note that drilled or dimpled rotors are typically found in combination with slotted rotors.

3. Slotted Rotors
Slotted rotors are recognized by carved lines found on the rotor. These carved slots help to cool the rotor during high-performance use. They also help to remove dirt and other debris from the disc and brake pad, helping to maintain consistent contact for more efficient braking. Slotted rotors are perfect for vehicles that see frequent, heavy towing.

4. Drilled/Dimpled and Slotted Rotors
Rotors that are both drilled (or dimpled) and slotted, while effective, are best for trucks that want the added aesthetic, such as those with wheels that have a more open design. Not only will they look great through an open-wheel, but the drilled holes assist with an initial bite while the slots are designed to remove dust and debris from between the rotor and brake pad.

ROTOR MATERIALS

Brake rotors can be made of six different materials, each with its own advantages. Let’s take a look at each.

1. Cast Iron
This is the very definition of old school when it comes to a brake rotor. It’s one or two pieces and gets the job done. In fact, it’s the most common material for brake rotors. The right design (usually two-piece) can even work well in a performance vehicle. However, it’s also the heaviest option, which affects the overall weight of your car and its handling, since that weight is right up there with your front wheels.

2. Steel
Steel has been the racer’s choice for years because a steel brake rotor is thinner, weighs less and handles heat better. The downside: Steel rotors aren’t as durable as some others, and warped rotors can cause noise and a pulsating pedal when you brake.

3. Layered Steel
Layering sheets of steel together and laminating them makes them resistant to the warping you might find in a straight steel brake rotor. It’s a favorite of racers who don’t want frequent brake rotor replacement and repair, but manufacturers are currently only targeting professional racers and production is limited, so it’s not terribly common in passenger vehicle applications.

4. Aluminum
Aluminum brake rotors dissipate heat quickly, but they also melt at a lower temperature than other options. Aluminum is a favorite for motorcycles, which weigh less and are easier on the rotors when braking than a heavy car, truck or SUV.

5. High Carbon
These are iron, but with a lot of carbon mixed in. They can take a lot of heat and dissipate it quickly. The metallic content helps the rotor avoid cracking under high stress, and brake noise and vibration are reduced as well. The only downside is the price, which is significantly higher than straight iron or aluminum.

6. Ceramic
What’s your favorite supercar? Ferrari? Porsche? Lamborghini? Odds are it’s packing ceramic brake rotors. They offer the highest heat capacity (85 percent higher than cast iron) and superior dissipation, and they maintain a more consistent force and pressure as the temperature of the rotors rises. Ceramic is the highest-performance brake rotor available today.

ADVANTAGES AND DISADVANTAGES

Advantages

1. It is lighter than drum brakes.
2. It has better cooling ( because the braking surface is directly exposed to the air)
3. It offers better resistance to fade.
4. It provides uniform pressure distribution
5. Replacement of brake pads is easy.
6. By design, they are self-adjusting brakes.

Disadvantages

1. It is costlier than drum brakes.
2. Higher pedal pressure is required for stopping the vehicle. This brake system is installed with vacuum booster.
3. No servo action is present.
4. It is difficult to attach a suitable parking attachment.