A sealed ball bearing allows the drive shaft to spin freely. The outside of the ball bearing is held by a thick, rubber, doughnut-shaped mount. The rubber mount prevents vibration and noise from transferring into the operator's compartment.

A bearing similar to the center support bearing is often used with long drive lines, containing a single drive shaft. This bearing is called a PILLOW BLOCK BEARING. It is commonly used in drive lines that power auxiliary equipment. Its purpose is to provide support for the drive shaft and maintain alignment. When used at or near the center of the shaft, it reduces the whipping tendency of the shaft at high speed or when under heavy loads. The construction of pillow blocks varies. The simplest form is used on solid power takeoff drive shafts, which is no more than a steel sleeve with a bronze bushing.

DRIVE LINE MAINTENANCE
A drive line is subjected to very high loads and rotating speeds. When a vehicle is cruising down the road, the drive shaft and universal joints or constant velocity joints may be spinning at full engine rpm. They are also sending engine power to either the front or rear axle assemblies. This makes drive line maintenance very important.

The drive shafts must be perfectly straight and the joints must be unworn to function properly. If any component allows the drive shafts to wobble, severe vibration, abnormal noises, or even major damage can result.

Figure 5-10.- Center support bearing.

Drive Shaft Noises
When operating a vehicle to verify a complaint. keep in mind that other components could be at fault. A worn wheel bearing, squeaking spring, defective tire. transmission, or differential troubles could be at fault. You must use your knowledge of each system to detect which component is causing the trouble.

Drive shaft noises are usually caused by worn U-joints, slip joint wear, or a faulty center support bearing. Drive shaft noises and possible causes are as follows:

Grinding and squeaking from the drive shaft is frequently caused by worn universal joints. The joints become dry, causing the rollers to wear. The unlubricated, damaged rollers then produce a grinding or squeaking sound, as they operate on the scored cap and cross surfaces.

A clunking sound, when going from acceleration to deceleration or deceleration to acceleration, may be caused by slip yoke problems. The splines may be worn. The yoke transmission extension housing bushing may also be worn. This will let the yoke move up and down with changes in drive line torque. An excessively worn U-joint or differential problem can also cause a similar noise.

A whining sound from the drive shaft is sometimes caused by a dry, worn center support bearing. Since this bearing makes complete revolutions. it will make a different sound than a bad universal joint. A high pitched. more constant, whine will usually come from a faulty center support bearing.

Any other abnormal sound should be traced using your knowledge of mechanics. a stethoscope, and the vehicles service manual troubleshooting chart.

Drive Shaft Inspection
To inspect the drive shaft for wear or damage, raise the vehicle and place it on jack stands. Look for undercoating or mud on the drive shaft. Check for missing balance weights, cracked welds, and other drive shaft problems.

To check for working U-joints, wiggle and rotate each U-joint back and forth. Watch the universal joint carefully. Try to detect any play between the cross and the yoke. If the cross moves inside the yoke, the U-joint is worn and needs to be replaced.

Also, wiggle the slip yoke up and down. If it moves in the transmission bushing excessively, either the yoke or the bushing is worn. Inspect the rear yoke bolts for tightness. Make sure the rear motor mount is NOT broken. Look at any condition that can upset the operation of the drive shaft.

If after a thorough check of the drive shaft you fail to determine the problem, notify the shop supervisor. The drive shaft may require detailed measuring (drive shaft runout and drive shaft angle) or have its balance checked.