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.