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Parts Of An Air-Brake System: Part 3

5.1.8 – Foundation Brakes

Foundation brakes are used at each wheel. The most common type is the S-cam drum brake. The parts of the brake are discussed below.

Brake Drums, Shoes and Linings. Brake drums are located on each end of the vehicle's axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working.

S-cam Brakes. When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the S-cam (so-called because it is shaped like the letter “S”). The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.

Wedge Brakes. In this type of brake, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge-type brakes may be self-adjusting or may require manual adjustment.

Disc Brakes. In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a “power screw” is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large C-Clamp.

Wedge brakes and disc brakes are less common than S-cam brakes.

Multiple-Choice Questions:

Question #244 (1 of 6)

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Describe the basics of drum brakes:

  • To stop, the drum forces the brake shoes and linings in against the inside of the hub. This causes friction, which slows the vehicle (and creates heat).
  • To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat).
  • To stop, the brake lining wedges the drum against the inside of the hub. This causes friction, which slows the vehicle (and creates heat).
  • To stop, the brake chamber twists around the S-cam, forcing the linings against the outside of the drum. This causes friction, which slows the vehicle (and creates heat).

Brake Drums, Shoes and Linings. Brake drums are located on each end of the vehicle's axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working.

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Question #248 (2 of 6)

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Where are brake drums located?

  • On each end of the vehicle's axles.
  • On each end of the vehicle's steering axle
  • On one end of each end of the vehicle's drive axles.
  • On one end of each of the vehicle's trailer axles
Brake drums are located on each end of the vehicle's axles.
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Question #247 (3 of 6)

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Which statement is correct about disc brakes?

  • In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a “torque nut” is used. The pressure of the brake chamber on the slack adjuster turns the torque nut.
  • In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a “power screw” is used. The pressure of the brake chamber on the slack adjuster turns the power screw.
  • All of these are true
  • In air-operated disc brakes, air pressure acts on a brake shoe and wedge, like S-cam brakes. But instead of the S-cam, a “power assist” mechanism is used. The pressure of the brake chamber on the slack adjuster turns the brake drum.

Disc Brakes. In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a “power screw” is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large C-Clamp.

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Question #246 (4 of 6)

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Which statement is correct about wedge brakes?

  • Wedge brakes may have a single brake chamber or two brake chambers, pushing wedges in at both ends of the brake shoes
  • Wedge-type brakes may be self-adjusting or may require manual adjustment
  • The brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum.
  • All these are correct

Wedge Brakes. In this type of brake, the brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge-type brakes may be self-adjusting or may require manual adjustment.

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Question #243 (5 of 6)

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Foundation brakes are used at each wheel. What is the most common type of foundation brake?

  • None of these are the most common
  • A-cam drum brake
  • S-cam drum brake
  • Air-assist pressurized brake
Foundation brakes are used at each wheel. The most common type is the S-cam drum brake.
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Question #245 (6 of 6)

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Which statement is correct about the S-cam function in an air brake system?

  • None of these are correct
  • The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.
  • The S-cam forces the brake linings away from the hub and presses them against the outside of the brake drum. When you release the brake pedal, the S-cam rotates back and a knob pulls the brake shoes away from the drum, locking up the wheels
  • The S-cam forces the brake drums away from one another and presses the liners against the inside of the brake chamber. When you release the brake pedal, the V-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.

S-cam Brakes. When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the S-cam (so-called because it is shaped like the letter “S”). The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back, and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again.

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