Truck Brake Systems Objectives Identify the components of

Truck Brake Systems

Objectives • Identify the components of a truck air brake system. • Explain the basic operation of an air brake system. • Understand what is meant by pneumatic and torque imbalance. • Describe the role played by the Federal Motor Vehicle Safety Standard No. 121 (FMVSS No. 121) on present-day air brake systems.

Objectives (cont. ) • Identify the major components and systems of an air brake system. • Explain the operation of an air brake chamber. • Outline the functions of the hold-off and service circuits in truck and trailer brake systems.

Objectives (cont. ) • Describe the operation of S-cam actuated drum brakes. • Describe the operating principles of slack adjusters. • List the components and describe the operating principles of an air disc brake system. • Outline the factors that determine brake balance in truck rigs.

INTRODUCTION The brakes on a truck are probably its most vital safety component. It takes a truck with a 450 horsepower engine and an 80, 000 lb load around 90 seconds to accelerate to 50 mph. From that speed, the same truck must be brought safely to a stop in less than 5 seconds. If brakes were rated in horsepower, trucks would be required to have brakes rated between 10 and 20 times the horsepower rating of the engine.

EVOLUTION OF BRAKES Today, truck brakes are either hydraulically actuated or air actuated. Hydraulic brake systems tend to be used on light-duty trucks that usually do not have to be coupled to trailers. Medium- and heavy-duty trucks use air-actuated brakes almost exclusively.

Advantages of air brakes over hydraulic brakes in heavy-duty highway vehicles: • Air is limitless in supply. It must be cleaned, compressed, stored, and distributed properly in an air brake system. Minor leaks do not result in brake failures. • The air brake circuit can be expanded easily so that trailers can be coupled and uncoupled from the tractor circuit by a person with no mechanical knowledge.

• IAir brakes are effective even when leakage has reduced their capacity. This means that, in the event of air leakage, the system can be designed with sufficient fail-safe devices to bring the vehicle safely to a standstill.

FMVSS 121 Legislation introduced in 1975 called Federal Motor Vehicle Safety Standard No. 121 (known as FMVSS 121 in the United States, CMVSS 121 in Canada) governs all air brake system requirements. It has been modified in small ways to keep up to date with technology, but from the beginning it required all highway vehicles using air brakes to use a dual-circuit application circuit.

FMVSS 121 It also is now a requirement of FMVSS 121 that all highway tractors manufactured after March 1, 1997, and trailers manufactured after March 1, 1998, be equipped with antilock braking systems (ABS).

This schematic shows the pneumatic circuit of a heavily optioned highway tractor.

Truck air supply circuit components

Compressor-mounted air governor

Typical air dryer installation

Safety pressure relief valve, also known as a pop-off valve

LOW-PRESSURE INDICATOR In an air brake system, if the system air pressure drops too low, the brakes will cease to function properly. A low-pressure indicator is a pressuresensing electrical switch that closes at any time the pressure drops below its preset value. The lowpressure indicator should be specified to close at 50 percent governor cut-out pressure. In a typical system, that would be at approximately 60 psi. When the switch closes, a light and a buzzer are actuated.

Color- and shape-coded dash control valves

Dash-mounted trailer service brake valve

Color-coded trailer air hoses on a tractor. The trailer air supply line is red and the brake signal line is blue. The electrical cord is color-coded green and the three lines are mounted to a pogo stick.

Foundation brake types: (A) Scam brake; (B) Wedge brake; (C) Disc brake

S-cam brake operation: rotation of the S-cam is clockwise to actuate the brake in this foundation brake.

Cam geometry: (A) S-cam and rollers; (B) flat cam and roller

CAUTION S-cams are oriented with left and right cam profiles and must never be interchanged.

Brake shoe rollers

An S-cam foundation brake assembly

WARNING All truck technicians should be aware of the hazards of inhaling brake dust. Brake dust may contain asbestos, a known carcinogen, and other microparticulates that are potentially harmful. Wear a dust particulate mask, eye protection, and never clean up foundation brakes using compressed air. If a specialty brake solvent is used, be sure not to inhale the fumes nor expose it to heat.

Air disc brake foundation assembly

Air disc brake assembly cutaway. As the actuating beam moves the inner brake pad into contact with the rotor, the caliper slides away from the rotor pulling the outer brake pad into the rotor. This creates twoway, balanced clamping force on the rotor.

Cutaway view of a typical dual-circuit service brake valve

Trailer Control Brake Valves

RELAY VALVES A relay valve is a simple remote-controlled brake valve. It can be described as a proportioning pilot valve.

TRACTOR PROTECTION VALVES The tractor protection (TP) valve has two main functions. First, it acts as the means of routing and switching the trailer air supply and trailer service signal from the tractor. Second, it functions to protect the tractor air supply in the event of a trailer breakaway or total air loss.

One-Way Check Valve

Examples of single and double air brake chambers. The double units combine service and park/emergency brake functions.

Linear force = 30 sq. in. × 100 psi = 3, 000 lb Compressed air used as potential energy.

Brake Chamber Stroke The distance between the pressure and nonpressure plates of the brake chamber defines the limits of its stroke.

Cutaway view of a typical air brake chamber

Brake Chamber Size Brake chambers are sized by the effective sectional area of the diaphragm. As of now, this is always measured in square inches. For instance, a brake chamber with a 30 -squareinch diaphragm is known as a series 30. Most tractor drive and trailer axles use either series 30 or series 24 diaphragms.

Potential energy of a compressed spring compared with compressed air.

Converting the potential energy of compressed air into stopping power

Hold-Off Chamber The rear section of the spring brake chamber houses a large compressed spring capable of exerting a force of up to 1, 800 lb. This spring is sometimes known as a maxi spring, named for the manufacturer of the first spring brakes. The spring brake section of the spring brake chamber has a diaphragm that functions to “hold off” the spring.

Shop Talk The most common spring brake chamber is known as a 30/30. A 30/30 spring brake area has a service diaphragm of 30 square inches and a hold-off diaphragm of 30 square inches. A standard 30/30 spring brake chamber provides a specified maximum of 2 inches of stroke. Brake stroke should be reset when it exceeds 1. 75 inches of stroke.

Caging Spring Brakes All spring brakes have a means of mechanically caging them. Caging a spring brake compresses the main or power spring. A spring brake should be mechanically caged before it is either removed or replaced. Most spring brakes are sold with a cage bolt assembly. Cage bolt lugs fit to an internal cage plate to compress the power spring.

SLACK ADJUSTERS The linear force produced by the brake chamber must be converted into brake torque (twisting force). Rotation of the S-cam forces the shoes into the drum to effect braking. Slack adjusters are used to connect the brake chamber with the S-cam shaft.

(A) Manual and (B) automatic slack adjusters

Manual slack adjuster

Automatic slack adjuster

Summary • Air compressors are piston air pumps that are either gear- or belt-driven. • Air dryers are used to help eliminate moisture and contaminants from the truck’s air system.

Summary • The potential energy of compressed air is changed into mechanical force in an air brake system by slack adjusters and brake chambers. • Brake chambers are specified by sectional area in square inches. The most common spring brake chamber is the 30/30. • The most common type of foundation brake assembly used on air brake-equipped trucks is the S-cam type.

Summary • Slack adjusters multiply the force applied to them by the brake chamber into brake torque. • Brake torque applied to the S-camshafts results in the shoes being forced against the drum.