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Driveshaft Assemblies. Objectives. • Identify the components in a truck driveline. • Explain the procedures for inspecting, lubricating, and replacing a universal joint. • Describe the various types of wear a universal joint might experience.
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Objectives • Identify the components in a truck driveline. • Explain the procedures for inspecting, lubricating, and replacing a universal joint. • Describe the various types of wear a universal joint might experience. • Outline the procedure for sourcing chassis vibration.
INTRODUCTION Driveshafts, or propeller shafts, as they are sometimes called, have a simple function: to transmit drive torque from one driveline component to another. This should be accomplished in a smooth, vibration-free manner. In a heavy-duty truck, that means transmitting engine torque from the output shaft of the transmission to a rear axle or to an auxiliary transmission.
CENTER SUPPORT BEARINGS Rubber Insulator
DRIVELINE GEOMETRY Parallel-joint driveline arrangement.
DRIVELINE GEOMETRY “Broken-back”–type drive shaft; angles “A” and “B” are equal.
A driveshaft is in phase when the lugs on the tube yoke and slip yoke line up with each other. (Socket Assembly) CV Joint
Shop Talk A U-joint, operating at an angle, causes a cyclical speed variation in the driveshaft so that it speeds up and slows down twice every revolution. The non uniform output speed produces driveline pulsations any time it is operating at an angle. Working the same way but in reverse is the U-joint at the rear of the driveline that connects to the drive axle input shaft.
Shop Talk (Cont.) In order to avoid harmful torsional vibrations that result from the nonuniformity, the rear U-joint needs to cancel out the nonuniform rotation of the forward U-joint. This can be accomplished by having the transmission output shaft and drive axle input shaft set at relatively similar angles, producing equal U-joint working angles.
Shop Talk (Cont.) This is known as driveshaft U-joint angle cancellation. When U-joint angles at either end of the drive shaft are kept within at least 1 degree of each other, uncanceled, nonuniform rotation will be minimized. Gear clatter, gear seizures, synchronizer failures, and torsional vibration, which are caused by driveshaft vibration, will be eliminated.
DRIVELINE PHASING A drive shaft will increase and decrease speed twice per revolution.
Two universal joints in-phase will cancel out the speed fluctuations in the drive shaft.
DRIVESHAFT INSPECTION Check for excessive radial end play.
LUBRICATION One of the most common causes of U-joint and slip joint problems is the lack of proper lubrication. Some U-joints are lubed for life; these may have significant longevity providing the driveshafts are not frequently removed. When U-joints are properly lubricated at recommended service intervals, they will meet or exceed their projected operating life span.
Shop Talk Clean grease fittings thoroughly to remove accumulated grease and abrasives before use. Any contaminants on the zerk nozzle can be forced through the nipple into the bearing during lubing.
Shop Talk On-highway operation is generally defined as an application that operates the vehicle running less than 10 percent of total operating time on gravel, dirt, or unimproved roads. Vehicles running more than 10 percent operating time on poor road surfaces are classified as off-highway in terms of preventive maintenance.
WARNING: If grease is only seen to exit from three of the four trunnion seals, the bearing is not properly lubed and will almost certainly fail. You must take the appropriate corrective action when U-joints fail to take grease. If a driveshaft separates because of a U-joint failure, it can take out air tanks and fuel tanks and generally cause damage that greatly exceeds the cost of replacing a single U-joint.
Shop Talk When replacing a hanger bearing, make sure you look for and do not lose track of the shim pack that is usually located between the bearing mount and cross-member. The shims set the drive shaft angles and omitting them will result in a driveline vibration.
Shop Talk Before removing a driveshaft, mark the slip yoke assembly and tube shaft with a paint stick to ensure the correct phasing alignment on reassembly. If the shaft assembly is to be cleaned before reassembly, use a steel scribe to indent alignment marks.
CAUTION: Never use a sledge hammer directly on a yoke to separate a U-joint. The result will almost certainly be a damaged driveshaft.
Shop Talk If only one end of the driveshaft requires service, disconnect that end, unscrew the slip shaft seal (dust cap) from the slip yoke assembly, and then separate the driveshaft at the slip spline joint. When removing the entire driveshaft, disassemble one end at a time, laying the disconnected end on the floor carefully.
U-JOINT FAILURE ANALYSIS Cracks Galling
Spalling Brinnelling
Shop Talk Using guide bolts when installing U-joint caps can help minimize alignment problems. Two 3/8 fine thread bolts of the appropriate length should be used.
CAUTION: Once in use, bearing caps and their trunnions should remain matched. Also, never take assembly short cuts by installing only the new bearing caps on a used trunnion because this will usually result in a rapid failure. Regard a U-joint cross, its four bearing assemblies, and mounting hardware as a unit, and replace as such.
CAUTION: It makes sense to remove the grease fittings when installing a driveshaft. If they are knocked against the yoke, they tend to shear. You can easily reinstall them after the drive shaft has been installed.
CAUTION: Use small tack welds to attach weights. Larger welds can create another imbalance or, worse, distort drive shaft tubing.
