Foundation Brake
Front Brake |
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Rear Brake |
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Working Principle-Foundation Brake
Construction Details
The ‘S’ cam brake is mainly used on commercial vehicles such as trucks, tippers and trailers. Brake shoes are actuated by camshaft which has “S” Cam Profile at its one end. And it is operated by slack adjuster and brake chamber on the other side. ‘S’ cam shaft is mounted with a bracket on the torque plate (Also called as brake carrier plate) with bush bearing support. The “S” Cam rotated by the auto slack adjuster moves the rollers apart which in turn expands the brake shoes against the brake drum. Auto brake adjustment is possible with the auto slack adjuster, thus compensating the brake-to-drum clearance at the time of lining wear. The slack adjuster is fitted to the splined end of the cam shaft.
The foundation brakes include two shoes with brake retractor springs and the fulcrum pins (Abutment end) on the other end of the brake shoe. The brake shoes are kept intact on its roller on one side and the fulcrum pins which is on the steel bushes over the torque plate. The rollers are allowed to roll on the ‘S’ cam profile during rotation of the camshaft. Hence reducing the friction and ensuring smooth transfer of force from the camshaft to brake shoe.
A shoe return spring is equipped between the shoe and the anchor pin that is fitted to a shoe web near the camshaft. Two shoe retracting springs are installed on a shoe rim hole near an abutment end. Two semicircular shields are utilized to cover the rear side of the brake assembly, thus protecting the brake components from dust.
Working Principle
When the brake pedal is pressed, high pressure air is passed into the brake chamber which in turn actuates the slack adjuster. This movement of slack adjuster actuates and rotates the “S” cam shaft. The rotation of “S” Camshaft moves the roller apart pushing the brake shoes against the brake drum. The friction surface of the brake shoe contacts with the brake drum and slows – stops the drum rotation
When the brake pedal is released, the air from the brake chamber is removed and the slack adjuster moves back. This ensures “S” Cam shaft to rotate back to its original position. The Shoe return spring pulls back the brake shoes to its original position, thus ensuring sufficient clearance between the brake shoe and the drum.
Brake System Circuit – Working Principle
Schematic Layout – Dual Circuit Air Brake System |
1 | Front Axle |
2 | First Rear Axle |
3 | Second Rear Axle |
A | Air compressor |
B | Hand Brake valve |
C | Air Dryer with unloader valve |
D | Four circuit protection valve |
E | Auxiliary tank |
F | Accessories |
G | Purge tank |
H | Rear air tanks |
I | Dual brake valve |
J | Front air tank |
K | Relay valve |
L | Quick Release Valve (First Rear Axle) |
M | Automatic Slack Adjuster (First Rear Axle) |
N | Spring Brake Actuator (First Rear Axle) |
O | Spring Brake Actuator (Second Rear Axle) |
P | Automatic Slack Adjuster (Second Rear Axle) |
Q | Quick Release Valve (Second Rear Axle) |
R | Automatic Slack Adjuster (First Axle) |
S | Brake Chamber |
T | Rubber hose |
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Requirements of Air Brake System
Provide effective and safe assistance (pneumatic assistance) to the foundation brakes.
Consume least power from the engine in charging the circuit.
Fail safe system during emergencies and incase of failure in any of the circuit
Lesser maintenance requirement.
Charging and Regulating System
Air compressor which gets the drive from the engine draws clean air from the air intake system and compresses it to the required pressure. The compressed air which contains moisture is treated by the air dryer which separates the moisture from air. Then it collects the moisture by its desiccant cartridge and purges out to atmosphere through a purge tank
Unloader valve which is integrated with the Air Dryer unit maintains the required charging of air and regulates air pressure based on the actual system pressure.
The moisture free dry air then flows into the rear brake air tank, front brake air tank, auxiliary system tank and hand brake valve to release the parking brake in the rear wheels by activating the spring brake actuators. Air from the front and rear air tanks flows to dual brake valve, where the air pressure remains, and ready to be used during brake application.
Service Brake Circuit
When the brake pedal is depressed, plunger of the dual brake valve is operated, opening delivery ports 21 and 22 connected to rear spring brake actuators through a relay valve and front brake chambers respectively. The relay valve directs air from rear air tank to service port 11 of the spring brake actuators.
The diaphragm inside the spring brake actuator flexes due the air pressure and it applies the force on the push rod. The movement of the push rod operates the S-Cam through the Slack adjuster. The rotation of S-Cam expands the brake shoe against the rotating brake drum to stop the vehicle.
At the same time air from the dual brake valve directly flows into the brake chambers to apply the front brakes through an ABS modulator.
Service Brake Circuit |
A | Dual brake valve |
B | System protection valve |
C | Rear forward RHS spring brake actuator |
D | Rear forward LHS spring brake actuator |
E | Rear rearward RHS spring brake actuator |
F | Rear rearward LHS spring brake actuator |
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Parking Brake Circuit (Only on Rear Wheels)
When the vehicle is in motion, there exists a continuous air pressure on the diaphragm of the parking brake spring chamber against the spring force.
Parking Brake – ‘Engage’
To apply the parking brake the Graduated Hand control Valves is operated to ‘ON’ position & the release of air from the Parking brake circuit takes place. Hence the Parking brake spring expands to move the push rod and operates the S-Cam. This ensures the brake shoes to hold the rear brake drums.
Parking Brake – ‘Dis-Engage’
When the operator desires to move the vehicle and releases the hand brake valve to release position, pressurized air from the system protection valve flows to the spring brake actuators through the hand brake control valve to counter the spring force of the brake actuators thereby releasing the brakes.
1 | Yoke |
2 | Brake chamber |
3 | Parking spring |
4 | Return spring |
5 | Spring brake chamber |
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Regeneration or Purging Cycle
Regeneration or purging cycle is carried automatically during which the pressurized air returns from the purge tank to expel the condensed water and impurities from the air dryer when the system pressure exceeds 6 bars.
Auxiliary System
Auxiliary air tank is connected to the port 24 of the quadruple system protection valve. Auxiliary components and systems like exhaust brake system, steering column tilting mechanism, differential lock solenoid, clutch booster and Power Take off unit (if equipped) draws air from the auxiliary air tank for their operation.