Air brake basics

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It’s impossible to fix something without understanding how it’s supposed to work. Brake system repair decisions can be difficult to make, and costly to execute, without a basic understanding of system components and their operation.

The following discussion details the operation of a typical S-cam air brake system on a single-axle tractor and trailer (see diagrams). Systems vary somewhat, depending on the manufacturer and on optional equipment and configuration, but all can be thought of as comprising three sub-systems.

The supply system
The supply system, as the name implies, supplies pressurized air – the energy source for any air brake system. The key player in this sub-system is an engine-driven air compressor (1). A governor (2), which may be integrated with the compressor, controls the compressor’s output by unloading or cycling it. Pressure is generally maintained at 100 to 120 psi, and is monitored by the driver by means of dash-mounted pressure gauges (3). A low-pressure switch (4) senses system pressure and sends an electrical signal to a dash light or buzzer to alert the driver when air pressure falls below 60 psi.

Reservoirs, three per tractor and, usually, two per trailer, store the compressed air until it is needed to actuate the brakes. Check valves (5) prevent pressurized air in the primary and secondary reservoirs from passing back through the compressor while it is not running. A safety or “pop-off” valve (6) is usually installed in the reservoir closest to the compressor. In the event of over-pressurization, the safety valve allows air to escape, preventing damage to air lines, reservoirs and other components.

Effect of brake chamber type (diaphragm area) on pushrod output force, with constant 60-psi application. Except where noted, illustrations courtesy of ArvinMeritor, Bendix and Dana.

The reservoir closest to the compressor is often referred to as a wet tank, because that is where atmospheric moisture – the No. 1 enemy of air brake systems – condenses in the greatest quantities. Reservoirs are equipped with drain valves (7) so water can periodically be eliminated. These can be manually or automatically operated. Alcohol is sometimes introduced into air systems operating in cold climates to prevent water from freezing and plugging air lines.

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An air dryer (8) is a device that condenses and eliminates most of the water from an air system. An air dryer is a canister that usually contains a bed of desiccant material. As air passes through, the material captures moisture and blow-by from the compressor.

The control system
The control system consists of a series of pneumatic valves that direct air and control pressure to appropriate components. Although discussed here individually, different valves are often combined in a common housing.

The main valve is the dual-control foot valve (9), so called because it is actually two valves that operate simultaneously, in response to input from the driver’s foot at the brake pedal.

Two valves are necessary because, after the wet tank output, the system splits into two separate brake circuits. Air downstream of the wet tank is divided between primary and secondary reservoirs. The split system ensures that, in the event of a failure, the entire system will not become inoperative, and the truck can be brought to a controlled stop.

When the brake pedal is depressed, air flows from the primary reservoir and through the primary portion of the dual-control foot valve to actuate the rear axle brakes. Meanwhile, air flows from the secondary reservoir, through the secondary portion of the dual-control foot valve, to actuate the front axle brakes. A two-way check valve (10) senses primary and secondary supply pressure, and allows the dominant pressure to actuate the trailer brakes. Primary air can also be manually supplied to the trailer by means of a hand valve, (11) usually located on or near the steering column.

Two-way check valves are also used to allow dominant pressure to activate the stop light switch (12), and to release the parking brakes.

Relay valves (13) are used on trailers and on the rear axles of long-wheel-based tractors to minimize delays of brake application due to length of plumbing. These valves are directly supplied with unmodulated air pressure, and use air from the dual-control foot valve or manual trailer valve as a signal to quickly direct air to the brakes they serve.

Relay valves come in a variety of “crack” pressures. Crack pressure is the air pressure value required at the input from the foot valve before the relay valve will send air pressure to the brakes controlled by that valve. Crack pressure is an important element of brake timing and balance, and is determined – axle by axle – by how heavily loaded the axle served by a valve is, how big its brakes are, and how aggressive the linings are on those brakes.

A valve that cracks at too low a pressure for a given axle can cause premature application, wheel lockup and trailer pushing, if the affected axle is on the tractor. Too high a crack pressure can cause delayed application, insufficient braking and trailer pushing, if the affected axle is on the trailer.

After a stop, when the driver lifts his foot from the brake pedal, a quick release valve (14) allows brake actuation air to be quickly exhausted near the brakes it serves, rather than having to travel back through the supply line, thus speeding brake release time.

Foundation brake operation. When pushrod is extended, brake adjuster, camshaft and S-cam rotate. S-cam spreads brake shoes apart and against brake drum.

Dash-mounted valves (15 and 16) control air pressure to the parking brakes. In most cases, these are spring brakes, so called because when no pressure is supplied, the brakes are applied by means of a spring. Air pressure, when applied, overcomes spring force and releases the brakes. More on that a little later.

A tractor protection valve (17) senses pressure in one or both lines that carry air to the trailer. These lines are connected to the trailer by means of quick-connect air fittings called gladhands. When there is no pressure in the line(s) – due to trailer breakaway or a gross air leak in the trailer circuit – the valve closes to maintain air pressure in the tractor circuit. In everyday use, the valve also works in conjunction with the dash-mounted trailer parking brake valve (16), to shut off air to the trailer circuit before disconnecting tractor from trailer.

The spring brake (or multi-function) valve (18) limits the air pressure used to keep the trailer parking brakes off and, by means of an integral check valve, isolates a failed reservoir, which would otherwise cause the parking brakes to be automatically applied.

The foundation brakes
Foundation brakes are where properly supplied and controlled air is used to stop a vehicle. When the brake pedal is depressed, air pressure is directed to brake chambers (19) at each wheel end. Brake chambers consist of a pressure housing, diaphragm and pushrod. As air pressure is exerted on the diaphragm, the pushrod on the other side of the diaphragm is extended. The force the pushrod exerts is the product of the amount of air pressure applied in psi, and the area of the diaphragm in square inches. For example, 60 psi, applied to a chamber with a 16-square-inch diaphragm would create a force at the pushrod of 960 pounds. A 60-psi application to a chamber with a 30-square-inch diaphragm would yield 1,800 pounds of pushrod force. Improperly matched brake chambers, therefore, can cause severe brake balance problems.

The pushrod is connected to one end of a lever called a brake adjuster – often called a slack adjuster (20). The other end of the brake adjuster is connected to a shaft that runs perpendicular to the plane formed by the pushrod and slack adjuster. As the pushrod is extended, the shaft rotates.

The shaft, in turn, is connected to an S-shaped cam between the brake shoes. As the shaft rotates, so does the cam. The brake shoes are forced apart and against the brake drum, creating the friction needed to slow the vehicle. The amount of friction produced is determined, in part, by the size of the brakes, the coefficient of friction (aggressiveness) of the brake lining material, and the mass and heat-rejection potential of the drum.

The slack adjuster is equipped with an adjusting mechanism to compensate for brake lining wear. If this were not so, the pushrod would be required to extend farther and farther as brake lining wear progressed. It wouldn’t take long before the pushrod would not be able to extend far enough to apply the brakes. Modern brake adjusters accomplish this automatically.

The brake adjuster has another function. It is essentially a lever, and a lever multiplies force in proportion to its length. A 4-inch long brake adjuster converts 1,000 pounds of force at the pushrod to 4,000 ib-in torque at the camshaft.

The brake adjuster’s length and the brake chamber’s size are two variables commonly altered to meet braking requirements. The product of these two values is expressed as the “AL factor.” This factor, when multiplied by 60 psi air pressure, is the industry standard for braking calculations.

For example, 60 psi, applied to a chamber with a 16-square-inch diaphragm (the “A” part of the AL factor) would create a 960-pound pushrod force. Multiplied by a 4-inch brake adjuster (L), the actual torque on the brake camshaft would be 3,840 lb-in.

In addition to applying the service brakes used in everyday driving, the brake chambers on the rear tractor axles and on the trailer axles apply the parking brakes. These brake chambers, (spring brakes), incorporate a second chamber, containing a second diaphragm and a powerful spring.

When the vehicle is in use, the dash-mounted parking brake valves are in the “run” (pushed-in) position. This supplies air pressure to the spring chamber, on the side of the diaphragm opposite the spring. Air pressure acting on the diaphragm compresses the spring, and the parking brakes are held off. This does not affect the operation of the service brakes.

When the vehicle is parked, the dash valves are pulled out. This exhausts spring brake hold-off air, allowing the spring to apply the parking brakes. In the event of a loss of system pressure, hold-off air pressure is overcome by the parking brake spring, and the brakes are automatically applied to provide emergency stopping.

Per federal regulations, the parking brakes must be able to hold a vehicle, loaded to its gross weight rating, stationary on a smooth, dry, concrete roadway, facing uphill or downhill, on a 20 percent grade.

More to come
We hope that this overview of air brake basics helped clarify your understanding and provide a grounding for dealing with more complex braking topics to be covered in future issues of CCJ.