|JIM’S CORNER – AUTO REPAIR HELP
CV JOINTS AND BOOTS
The efficient operating range of a universal joint is about 1-4. When this range is exceeded, the universal joint will start to vibrate excessively. This is caused by the universal joint speeding up and slowing down as it rotates. It is a normal operating characteristic of a universal joint, and is minimized by using the front universal joint to cancel out the rear universal joint. However, this will only work if the universal joint operating angle does not exceed 5 degrees. As the angle increases, the speed fluctuation of the universal joint, as it rotates, rises beyond acceptable limits. Because of this, universal joints are not used on front wheel drive axles. Considering front wheel drive vehicles use the drive wheels to steer the vehicle, the axle shaft must be able to transfer torque at a constant speed, over a wide range of angles. In addition, the axle shaft must be able extend and retract, as the suspension reacts to the road surface. CV joints are installed on the drive axle to provide torque to the drive wheels, at a constant rotational speed, as well as allow the axle to change lengths, with the movement of the suspension.
The axle shaft generally uses two CV joints. The CV joint that is placed on the wheel side of the axle shaft is referred to as the outboard CV joint. The CV joint placed on the transmission side of the axle shaft is referred to as the inboard CV joint. The outboard CV joint is designed to swivel as the wheel is turned while steering. The inboard CV joint is designed to extend or retract as the vehicle suspension moves. For this reason, the inboard CV joint is sometimes referred to as the plunge joint.
Almost all outboard CV joints are the Rzeppa type CV joint. The Rzeppa joint uses a ball and cage design. The CV joint consists of an inner race, slid over splines on the axle shaft, and six balls, placed in individual channels cut in the inner race. A cage is placed over the balls to retain them in the inner race. The assembly is slid into an outer race containing six channels cut to receive the balls. The outer race is part of the axle stub shaft, that protrudes through the front wheel hub. When assembled, the outer race is able to swivel on the ball and inner race assembly. A flexible boot is placed over axle shaft and outer race, covering and sealing the entire CV joint assembly. Special grease is contained in the boot, to lubricate the CV joint.
When the axle shaft is rotating, the inner race transfers torque through the balls, to the outer race. The inner race and balls become the drive member, while the outer race becomes the driven member. When the angle of the CV joint changes, as in when the vehicle is turning, the balls are able to slide laterally in the channels cut in the inner and outer races. This allows the CV joint to use the balls compensate for the difference in angles between the axle shaft and the stub shaft. This allows the CV joint to rotate at a constant speed at angles up to 40 degrees. The most common inboard CV joint is the Tripot type. The Tripot joint consists of three convex rollers, each mounted on three separate shafts. The three shafts are equally spaced apart, in triangular fashion, around ring containing inner splines. The shaft and ring assembly is sometimes referred to as a spider. The spider is slid over the splines of the axle shaft, and held in place by a snap ring. The rollers are placed on the shafts of the spider, with needle bearings placed between the inner surface of the rollers and the outside of the shafts. The spider and roller assembly is placed inside a housing, that is part of the stub shaft, The housing has elongated channels cut into it to receive the convex rollers. A flexible boot is placed around the housing and the axle shaft, covering and sealing the joint. Special lubricant is contained inside the boot.
Torque is transferred from the stub shaft housing, to the roller and spider assembly, using the roller channels. The joint is capable of operating at an angle, but does not have the angular range of the Rzeppa joint. However, like the Rzeppa joint, it is able to operate at a constant rotational speed. The Tripot joint is able to extend or retract, in response to the movement of the vehicle suspension, by the rollers moving in and out of the channels cut in the housing. Some axle shafts use a plunging Rzeppa joint in place of a Tripot joint. The plunging Rzeppa joint is similar to the normal Rzeppas joint, with the exception of longer channels cut into the outer race. This allows the CV joint to compress or extend as the suspension height changes.
The most common cause of CV joint failure is due to lubricant contamination from dirt and/or moisture. This occurs when the CV boots are ripped, cracked, or damaged. When a damaged CV boot is replaced, the CV joint needs to be throroughly cleaned, inspected, and lubricated, prior to reassembly. Outboard CV joint noise complaints, particularly a knocking noise when turning, is generally a result of wear in the internal comoponents of the CV joint. Generally the easiest and most economical repair is replacement of the entire drive axle, with a reconditioned unit.
(Jim is a lifelong fan of Dodger Baseball and used to race sprint cars in the 1980s.)