TIRE ROTATION
Rotating tires or changing them from one wheel to another so they wear evenly is recommended by the manufacturer. Tire rotation is performed to the manufacturer's specification for each vehicle. Examples of tire rotations are shown in figure 3-37.
TIRE SAFETY
The tire shop in most commands is supervised by the maintenance supervisor. When you are assigned to the tire shop, the maintenance supervisor should ensure you are briefed on tire safety by either the shop supervisor, the tire shop foreman, or the crew leader.
People inexperienced in tire repair should only repair tires when under the direct supervision of an experienced person. Additionally, always refer to the appropriate manufacturer's manuals for directions and instructions and remember: SAFETY COMES FIRST.
Figure 3-37.-Tire and wheel rotation.
BRAKE SYSTEMS
Good brakes are a critical element for ensuring the safe operation of equipment. The brake system applies to all vehicles or equipment, such as pickup trucks, jeeps, tractor-trailers, and construction equipment. Braking systems must not only stop the unit but also must stop it in a smooth, uniform motion.
Friction is the resistance in relative motion between two surfaces in contact with each other. When a stationary surface is forced into contact with a moving surface, the rubbing action between the two surfaces slows down the moving surface. In nearly all brake systems, the brake drums provide the moving surface, and the brake shoes provide the stationary surface. The friction between the brake drum and the brake shoes slows the drum and wheel; and the friction between the tire and the road surface slows and stops the vehicle.
Part of your prestart and operator maintenance responsibilities consist of the following:
l Using the proper brake fluid
l Checking the brake fluid level
l Inflating tires properly
l Checking for loose connections or parts
l Checking for leaks in the system
l Draining air reservoirs daily
l Checking the self-contained lubricating oil system of air compressors daily
Brake troubles in vehicle operations that you may encounter and must document are as follows:
Leaky brake cylinders.
Grabbing braking action.
The brake pedal can be depressed without slowing the vehicle.
INDIVIDUAL BRAKES
On modern equipment, individual service brakes are provided for each wheel and are operated by a foot pedal. The equipment also has an emergency or parking brake. The parking brake is operated by a separate pedal or a hand lever.
Individual brakes are classified as the external contracting brake, the internal expanding brake, the disc brake, and the mechanical parking brake.
External Contracting Brakes
External contracting brakes are sometimes used for parking brakes on motor vehicles, for cranes, and for controlling the speed of auxiliary equipment drive shafts.
In operation, the brake band (or shoe) of an external contracting brake is tightened around the rotating drum by moving the brake lever. The brake band is made of comparatively thin, flexible steel, shaped to fit the drum, with a frictional lining riveted to the inner surface (fig. 3-38). his flexible band cannot withstand the high pressure required to produce the friction needed to stop a heavily loaded or fast-moving vehicle, but it works well as a parking brake or hold brake.
Figure 3-38.-External contracting brake. l l l l l l l l l
The brake pedal goes to the floorboard with no resistance.
One brake drags.
All brakes drag.
The vehicle pulls to one side when braking.
Soft or spongy pedal.
Excessive pedal effort required.
Noisy brakes.
Air in the system.
Loss of brake fluid.
The brakes heat up during driving and fail to release. l
Figure 3-39.-External contracting transmission parkingbrake.
Figure 3-39 shows an external contracting brake. The brake band is anchored opposite the point where the pressure is applied. In addition to supporting the band, the anchor allows adjustment of the brake lining clearance. Other adjusting screws and bolts are provided at the ends of the band.
Internal Expanding Brakes
Internal expanding brakes are used almost exclusively as wheel brakes, but can be found on some cranes. This type of brake permits a more compact and economical construction. The brake shoes and brake-operating mechanism are supported on a backing plate or brake shield attached to the vehicle axle, as shown in figure 3-40. The brake drum, attached to the rotating wheel, acts as a cover for the shoe and operating mechanism and furnishes a frictional surface for the brake shoes.
The brake shoe of an internal expanding brake is forced outward against the drum to produce the braking action. One end of the shoe is hinged to the backing plate by an anchor pin, while the other end is unattached and can be moved in its support by the operating mechanism. When force from the operating mechanism is applied to the unattached end of the shoe, the shoe expands and brakes the wheel. A retracting spring returns the shoe to the original position when braking action is no longer required.
Disc Brakes
The disc brake has a metal disc (rotor) and a pair of flat brake pads instead of a drum and curved brake
Figure 3-40.-Internal expanding brake.
Figure 3-41.-Sectional view of a disc brake.
shoes. Figure 3-41 shows a sectional view of a disc brake assembly. The two flat pads are on the two sides of the disc. The assembly in which the flat pads are held is the caliper assembly. In operation, the pads are forced against the two sides of the disc by the movement of the pistons in the caliper assembly. The pistons are actuated by hydraulic pressure from the master cylinder. The effect is to clamp the rotating disc between the stationary pads, as shown in figure 3-41.
Mechanical Parking Brake
In most vehicles, a hand lever or foot pedal engages the parking brake. The parking brake has its own system and can be either an external contracting brake bands on the drive shaft (fig. 3-42, view A) or a mechanical linkage that works the rear wheel brakes (fig. 3-42, view B).
HYDRAULIC BRAKE SYSTEM
A hydraulic brake system is primarily a liquid connection or coupling between the brake pedal and the individual brake shoes and drums, as shown in figure
Figure 3-42.-Parking brake configurations.
3-43. The system consists of one master cylinder connected by pipes and flexible tubing to the wheel cylinders. The wheel cylinders control the movement of the brake shoes at each wheel. When the brake pedal is depressed, the hydraulic fluid forces the pistons in the wheel cylinder against the brake shoes, forcing the shoes against the brake drum or brake discs stopping the wheels.
Hydraulic brakes are self-equalizing brakes. If the actuating pistons were all the same size, each brake in the hydraulic system would receive an identical hydraulic force when the brakes were applied, because a force exerted at any point upon a closed liquid is distributed equally through the liquid in all directions at the same time. All brake systems have larger wheel cylinders in the front than in the rear. When you stop a vehicle, more weight is automatically shifted forward due to inertia, so more front-wheel braking is required.
The master cylinder is a reservoir for the brake fluid and contains pistons and valves which change mechanical force to hydraulic pressure when the brake pedal is depressed, as shown in figure 3-43. The pressure on the brake pedal moves the piston within the master cylinder to force the brake fluid from the master cylinder through tubing and flexible hoses to the wheel cylinders. As pressure on the pedal is increased, greater hydraulic pressure is built up within the brake cylinders, and thus greater force is exerted against the ends of the brake shoes. When pressure on the pedal is released, the retracting springs on the brake shoes return the wheel cylinder pistons to their released positions. This action forces the brake fluid back through the flexible hose and tubing to the master cylinder.
Figure 3-43.-Hydraulic brake system.
The operation of a dual system master cylinder is basically the same as a single master cylinder. However, the dual system master cylinder has two separate hydraulic pressure systems. One of the hydraulic systems normally is connected to the front brakes and the other system to the rear brakes. If either the front or rear hydraulic system fails, the other system remains operational.
The master cylinder, like other parts in the brake system, is subject to wear, leaks, and deposits or corrosion on the cylinder wall and piston. Part of your prestart operation is to check the cylinder reservoir fluid level and add clean brake fluid to maintain the manufacturer's specifications.
The brake lines transmit fluid and pressure from the master cylinder to the wheel cylinders, which are mounted on the brake-backing plate, and change the hydraulic pressure into mechanical force. Inside each cylinder are two pistons that move in opposite directions by hydraulic pressure which pushes the brake shoes against the brake drum or disc. The brake shoes are made of steel that transmits force to the lining which is attached to the face of the shoe and makes contact with the brake drums or discs. During contact with one another, the lining and the drum or disc create the frictional surface that gives the braking effect.
AIR BRAKE SYSTEM
An air brake system uses compressed air to apply the brakes. Air under pressure can be conveniently stored and carried through lines or tubes. Considerable force is available for braking since operating air pressure may be as high as 100 psi. All brakes on a vehicle and on a trailer (when one is used) are operated together by a brake valve. This valve and the relative location of most of the basic assemblies of an air brake system are shown in figure 3-44.
Air Compressor
The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is driven by the engine through gears or a V-belt. The compressor may be air-cooled or may be cooled by the engine lubrication system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, the oil should be checked during your prestart operations.
Governor
The governor controls the air compressor output. When air tank pressures rise to the cutout level at about
Figure 3-44.-Typical air brake system.
125 pounds per square inch (psi), the governor stops the compressor from pumping air. When the tank pressure falls to the cut-in pressure at about 100 psi, the governor allows the compressor to start pumping again.
Air Storage Tanks
Air storage tanks (reservoirs) are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks hold enough air to allow the brakes to be used several times, even if the compressor stops working.
NOTE: Compressed air usually has some water and some compressor oil in it which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank; therefore, each air tank is equipped with a drain valve in the bottom.
The two types of drain valves areas follows:
1. The manual valve shown in figure 3-45 is operated by turning it a quarter turn or by pulling a cable. Part of your post-operational procedures is to drain all air tanks at the end of each day.
2. The automatic valve automatically expels the water and oil. This system may also be equipped with a manual drain.
Alcohol Evaporator
Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside a brake system can make the brakes stop working.
Figure 3-45.-Air tank manual drain valve.
If your vehicle has an alcohol system, the container should be checked during each prestart operation and filled up as necessary. Daily draining of the air tanks is still required to get rid of the water and oil.
Safety Valve
A safety relief valve is installed in the first tank into which the air compressor pumps air. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve has to release air pressure, something is wrong in the air brake system. This should be documented to inform the mechanic inspectors.
Brake Pedal
The brakes are applied by depressing the brake pedal (also called the foot valve, or treadle valve) that gives the operator control of the air brake system. When the brake pedal is engaged, air from the air tanks flows through the brake pedal valve through the brake lines to the brake chambers close to the wheel brakes that contain flexible diaphragms. The force of the air admitted into these chambers causes the diaphragms to operate the brake shoes through a mechanical linkage.
Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes allows some compressed air out of the system; therefore, the air pressure in the tanks is reduced and it must be recharged by the air compressor. Pressing and releasing the pedal unnecessarily may release air out faster than the compressor can replace it, and should the pressure become too low, the brakes cannot work properly and brake failure will occur.
Pressure Gauge
An air pressure gauge lets you know if you have proper air pressure within the reservoir. A low air warning device should cut on before the pressure drops to less than 60 psi in the air tank. This gauge is usually on the instrument panel of a truck or bus. If the pressure fails to buildup or exceeds the maximum limits after building up, secure the truck until the fault is corrected.
Hand Brake Valve
Independent control of brakes is necessary under bad conditions, especially if you have to put on the
Figure 3-46.-Hand brake valve.
trailer brakes without applying the truck or tractor brakes. The hand brake valve or independent trailer control valve, as shown in figures 3-44 and 3-46, provides the operator control of the trailing load at all times.
Figure 3-48.-Air-over-hydraulic power cylinder.
NOTE: More information about the air brake system is in the chapter covering tractor and trailer operations.
AIR-OVER-HYDRAULIC BRAKE SYSTEM
An air-over-hydraulic brake system is shown in figure 3-47. This system combines the use of compressed air and hydraulic pressure for brake operation. The air-over-hydraulic brake system has an air-over-hydraulic power cylinder (fig. 3-48) that contains an air cylinder and a hydraulic cylinder in tandem. Each cylinder is fitted with a piston and a common rod. The air piston is of greater diameter than the hydraulic piston. This difference in the two pistons
Figure 3-47.-Typical air-over-hydraulic brake system.
Figure 3-49.-Typical vacuum brake cylinder.
results in much greater hydraulic pressure than air pressure admitted to the air cylinder. Valve action varies with the amount of pressure applied to the brake pedal. When heavy brake pedal pressure is applied by the operator for hard braking, the hydraulic pressure in the master cylinder (which operates the valves) causes greater valve movement. As a result, the valve admits more air pressure into the air-over-hydraulic power cylinder and this higher air pressure causes a stronger braking action.
VACUUM BRAKES
In a vacuum brake system, depressing the brake pedal opens a valve between the power cylinder, which contains a piston, and the intake manifold to which the power cylinder is connected (fig. 3-49). When you apply the brakes, air is exhausted from the cylinder head of the piston. At the same time, atmospheric pressure acts on the rear side of the piston to exert a powerful pull on the rod attached to the piston.
When the brake valve is closed, the chamber ahead of the piston is shut off from the intake manifold and is opened to the atmosphere. The pressure is then the same on both sides of the piston; therefore, no pull is exerted upon the pull rod. The brakes are released and the piston returned to its original position in the power cylinder by the brake shoe return springs.
HydrovacTM is a trade name for a one-unit vacuum power-braking system. It combines a hydraulic control valve, a vacuum power cylinder, and a hydraulic slave cylinder into one assembly. This assembly (fig. 3-50) is connected to both the master cylinder and the wheel brakes and eliminates the need for mechanical connections with the brake pedal.
Pressure on the brake pedal forces fluid from the master cylinder through the check valve to the slave cylinder and to the wheel cylinders. Also, the foot pedal pressure, acting through the master cylinder, acts also against the slave cylinder piston to help the vacuum pistons and pushrods to press against the brake shoes.
Figure 3-50.-HydrovacTM power brake cylinder.
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