車架、制動(dòng)系設(shè)計(jì)-5,車架,制動(dòng),設(shè)計(jì) Brake System Service and Maintenance The types of trouble that may develop in drum-type hydraulic braking systems include the following. (1) Brake pedal goes to floorboard. When this happens, it means that there is no pedal reserve, since full pedal movement does not provide adequate braking. This would be a very unlikely situation with a dual-brake system. One section might fail (front or rear) but it would be rare for both to fail at the same time. If this happens, chances are the driver has been driving for some time with one section out . (2) One brake drags. This means that the brake shoes are not moving away from the brake drum when the brakes are released. (3) All brake drag. When all brakes drag, it may be that the brake pedal does not have sufficient play, so that the piston in the master cylinder does not fully retract. (4) Car pulls to one side. If the car pulls to one side when the brakes are applied, this means that more braking pressure is being applied to one side than to the other. (5) Soft, or spongy, pedal. If the pedal action is soft, or spongy, the chances are that there is air in the system, although out-of adjustment brake shoes could cause this. (6) Poor braking action requiring excessive pedal pressure. If the brake lining is soaked with oil or brake fluid, they will not hold well, and excessive pedal pressure is required for braking action. Improper brake-shoe adjustment or the use of the wrong brake lining could cause the same trouble. (7) Brakes too sensitive or grab. When the brakes are too sensitive and brake hard or grab with slight brake –pedal pressure, it may be that the linings have become greasy , that the brake shoes are out of adjustment , that the wrong lining is being used and that drums are scored or rough . (8) Noisy brakes. Brakes will become noisy if the brake linings wear so much that rivets come into contact with the brake drum, if the shoes become warped so that pressure on the drum is not uniform ,if shoe rivets become loose so that they contact the drum , or if the drum becomes rough or worn . （9）Air in system. If air gets into the hydraulic system, poor braking and a spongy pedal will result. It is possible accidentally to plug the vent (by wrench action ) when the filler plug is removed . (10) Loss of brake fluid. Brake fluid can be lost if the master cylinder leaks, if the wheel cylinder leaks, if the wheel cylinder leaks, if the line connections are loose, or if the line is damaged. (11) Warning light comes on when braking (dual system). This is a signal that one of the two braking systems has failed. Both systems (rear and front) should be checked so that the trouble can be found and eliminated. It is dangerous to drive with this condition, even though braking can be achieved, because only half the wheels are being broken. Many of the troubles on the disk in the disk type of brake system are similar to those that may be found in the drum type. Whenever you encounter a complaint of faulty braking action, always try to analyze it and determine its cause. Sometimes, all that is necessary (in earlier drum-type brakes) is a minor brake with the self-adjuster; the brakes automatically adjust themselves to compensate for lining wear. Other brake services include addition of brake fluid, bleeding the hydraulic system to remove air, repair or replacement of master cylinder and wheel cylinders, replacement of master cylinder and wheel cylinders, replacement of brake linings, and refinishing of brake drums. Tire service and maintenance includes periodic inflation to make sure that the tire is kept at the proper pressure, periodic tire inspection so that small damages can be detected and repaired before they develop into major defects, and tire removal, repair, and replacement. Incorrect tire inflation can cause many types of steering and braking difficulty. Low pressure will cause hard steering, front wheel shimmy, steering kickback, and tire squeal on turns. Uneven tire pressure will tend to make the car pull to one side. For these reason, it is very important to maintain proper pressure in the tires. There are a few points you should remember when inflating tires: (1) Don’t inflate a tire when it is hot, as, for instance, after hard driving on the highway. (2) Always replace the cap (where used) after checking air pressure or inflating a tire. The removal and replacement of tires are not difficult on smaller vehicles, but on large, heavy-duty applications special tools are required to remove and handle them. Air must be released from the center of the rim. A tire tool or flat stock can be used to pry one part of the bead up over the rim flange (start near the valve stem). Care must be exercised to avoid damaging the tire bead or inner tube. After the bead is started over the rim flange with the tool, the remainder of the bead can be worked out over the flange with the hands. The other bead of the tire is removed from over the same side of the rim flange in a similar manner. In tire shops where special tire-changing machines are used. In these, the wheel is put into place and air pressure is used to force the tire bead away from the rim. This machine will remove a tire from a wheel rim in a few seconds. Further, it can be used to quickly install a tire on the rim. The following cautions should be carefully observed in tire service and maintenance work: (1) On tubeless tires, do not use tire irons to force the beads away from the rim flanges; this could damage the rim seals on the beads and cause an air leak. Instead, use a bead breaker tool. (2) If a tire has been deflated, never inflate it while the car weight is on the tire. Always jack up the car before inflating the tire so that the tube can distribute itself around the tire evenly. If this is not done, some parts of the tube will be stretched more than other parts, and this puts a strain on the tube that might cause it to blow out. A number of repairs can be made on tires and tubes, ranging from the patching of nail holes, punctures, or cuts to vulcanizing new tread material to the tire casing. This latter operation is known as recapping, since a new cap, or tread, is placed on the tire. Repair procedures vary according to whether the tire is or is not of the tubeless type. With the tube type, puncture repair requires removal of the tire so that the puncture in the tube can be attached. This is done with a special kit. Patches require curing, or vulcanizing, at a temperature of around 150℃. Tubeless-tire punctures can usually be repaired without taking the tire off the rim. One method uses a rubber plug, which is inserted, into the puncture hole along with special cement. After the cement dries and the plug is trimmed flush with the tread, the repair is complete. Applying a patch on the inside can repair larger holes and cuts in tires. In addition, when treads have worn down, new treads can be vulcanized onto the casing provided the casing is in good condition. Tires have two functions. First, they interpose a cushion between the road and the car wheels to absorb shocks resulting from irregularities in the road. The tires flex, or give, as bumps are encountered, thus reducing the shock effect to the passengers in the car. Second, the tires provide frictional contact between the wheels and the road so that good traction is secured. This permits the transmitting of power through the tires to the road for rapid accelerating, combats the tendency of the car to skid on turns, and allows quick stops when the brakes are applied. Tires are of two basic types, solid and pneumatic (air filled). Solid tires have very limited usage, being confined largely to specialized industrial applications. Pneumatic tires are of two types, those using an inner tube and the tubeless type. The amount of air pressure used depends on the type of tire and operation. Passenger-car tires are inflated to about 275 to 413 KPa. Air is introduced into the tire (or inner tube) through a valve that opens when the chuck on the air hoses if applied. On the tire with an inner tube, the valve is mounted on the tube. On the tubeless tire, the valve is mounted on the wheel rim. 4