The driveshaft (propeller shaft) is a long steel tube that transmits engine power from the transmission to the rear axle assembly. It is connected to, and revolves with, the transmission output shaft (remember, the transmission shaft is connected to and revolves with the engine crankshaft) whenever the transmission is put into gear. With the transmission in neutral, the driveshaft does not move. Located at each end of the driveshaft is a flexible joint that rotates with the shaft. These flexible joints, known as U-joints (universal joints) perform an important function. The rear axle assembly moves with the car. It moves up and down with every bump or dip in the road. The driveshaft by itself is a rigid tube incapable of bending. When combined with the flexing capabilities of the U-joints, however, it can do so.
A slip joint is coupled to the front of the driveshaft by a universal joint. This U-joint allows the yoke (slip joint) to move up and down with the car. The yoke is a cylinder containing splines that slide over the meshes with splines on the transmission output shaft. When the rear axle moves up and down, the yoke slides back and forth a small amount on the transmission shaft. Therefore, it combines with the U-joints in allowing the driveshaft to move with the movements of the car. The rear universal joint is secured to a companion flange which is attached to, and revolves with, the rear axle drive pinion.
A U-joint consists of a cross piece (trunnion) and, on each of the four ends, a dust seal and a series of needle bearings that fit into a bearing cup. Each U-joint connects one yoke with another and the bearings allow the joints to revolve within each yoke.
The U-joint is secured to the yoke in one of two ways. Dana and Cleveland shafts use a conventional snapring to hold each bearing cup in the yoke. The snapring fits into a groove located in each yoke end just on top of each bearing cup. The Saginaw design shaft secures its U-joints in another way. Nylon material is injected through a small hole in the yoke and flows along a circular groove between the U-joint and the yoke, creating a synthetic snapring. Disassembly of the Saginaw U-joint requires the joint to be pressed from the yoke. This results in damage to the bearing cups and destruction of the nylon rings.
Replacement kits include new bearing cups and conventional snaprings to replace the original nylon rings. These replacement rings must go inboard of the yoke in contrast to outboard mounting of the Dana and Cleveland designs. Previous service to the Saginaw U-joints can be recognized by the presence of snaprings inboard of the yoke.
Bad U-joints, requiring replacement, will produce a clunking sound when the car is put into gear. This is due to worn needle bearings or a scored trunnion end possibly caused by improper lubrication during assembly. U-joints require no periodic maintenance and therefore have no lubrication fittings. The replacement U-joints generally have lubrication fitting.