Because of changes in the angle between the driveshaft or halfshaft and the axle housing or driven wheel, U-joints and CV-joints are used to provide flexibility. The engine is mounted rigidly to the vehicle frame (or sub-frame), while the driven wheels are free to move up and down in relation to the vehicle frame. The angle between the driveshaft or halfshaft and the axle housing or driven wheels changes constantly as the vehicle responds to various road conditions.
|Fig. 3: U-joints are necessary to compensate for changes in the angle between the driveshaft and the drive axle|
To give flexibility and still transmit power as smoothly as possible, several types of U-joints or CV-joints are used.
The most common type of universal joint is the cross and yoke type. Yokes are used on the ends of the driveshaft with the yoke arms opposite each other. Another yoke is used opposite the driveshaft and when placed together, both yokes engage a center member, or cross, with four arms spaced 90°apart. A bearing cup is used on each arm of the cross to accommodate movement as the driveshaft rotates.
|Fig. 4: Exploded view of a typical cross and yoke universal assembly|
The second type is the ball and trunnion universal, a T-shaped shaft that is enclosed in the body of the joint. The trunnion ends are each equipped with a ball mounted in needle bearings and move freely in grooves in the outer body of the joint, in effect creating a slip-joint. This type of joint is always enclosed.
|Fig. 5: Cut-away view of a typical enclosed ball and trunnion type U-joint|
A conventional universal joint will cause the driveshaft to speed up or slow through each revolution and cause a corresponding change in the velocity of the driven shaft. This change in speed causes natural vibrations to occur through the driveline necessitating a third type of universal joint-the double cardan joint. A rolling ball moves in a curved groove, located between two yoke-and-cross universal joints, connected to each other by a coupling yoke. The result is uniform motion as the driveshaft rotates, avoiding the fluctuations in driveshaft speeds.
|Fig. 6: Exploded view of a typical double cardan U-joint assembly|
The CV-joints, which are most commonly associated with front wheel drive vehicles, include the Rzeppa, the double offset, Tri-pod and Birfield joint.
The Rzeppa and double offset are similar in construction. They use a multi-grooved cross which is attached to the shaft. Balls ride in the cross grooves and are retained to the cross by a cage. The entire assembly then slides into an outer housing which has matching grooves for the balls to ride in.
|Fig. 7: Exploded view of a CV-joint equipped halfshaft. CV-joints shown are the Rzeppa/double offset style and the Tri-pod|
The Tri-pod design is similar to the ball and trunnion design, except it has three needle bearing mounted balls inside the housing space evenly apart (thus its name).
The newest of the CV-joints is called the Birfield. This joint is primarily found on import vehicles although some domestic vehicles are starting to use it as well. This joint is not serviceable and the manufacturers give no pictures or descriptions of its construction.