|KYLE’S CORNER – AUTO REPAIR HELP
DRIVESHAFT AND U-JOINT
Both the slip yoke and the universal joints allow the driveshaft to counteract the normal movement of the rear suspension, as it reacts to changes in the road surface. When the rear suspension extends, or moves down, the driveshaft must become longer. When the rear suspension compresses, or moves upward, the driveshaft must become shorter. The slip yoke is designed to slide back and forth, on the transmission output shaft, to compensate for the change in distance, between the differential and the transmission. In addition, as the rear suspension moves, the angular relationship of the rear differential and the transmission output shaft also changes. The universal joints allow the driveshaft to react to the change in angles, between the differential and the transmission.
Universal joints can only efficiently operate over a small angle. Because of this, the relationship between the rear differential and the transmission output shaft, is carefully designed. The rear differential is generally set, at such an angle, to align with the transmission output shaft. In short, the rear differential is aimed at the transmission output shaft, to provide a good average working angle. When this relationship is altered by excessive weight, or suspension height change, the driveshaft angle can exceed working design. The usual result is driveshaft vibration, as the universal joint must speed up and slow down, each rotation, due to excessive driveline angle. Also, when the suspension height of a vehicle is raised beyond normal limits, the driveshaft length may become too short, due to the excessive distance between the rear differential and the transmission. The driveshaft may become easily disconnected, due to the distance the slip yoke must extend to compensate for the suspension height.
Some vehicles use a double universal joint assembly, at the rear of the driveshaft. This type of universal joint assembly is known as a constant velocity joint. By using two universal joints, the driveshaft can operate at a greater angular range, than a single universal joint design. These are mostly equipped on larger passenger cars such as station wagons, or luxury cars, to provide a smoother ride, especially under heavy loads. Light duty trucks and large passenger cars can be equipped with two piece drivelines. This type of driveshaft will use a center support bearing, mounted on a frame crosspiece, to hold up the front half of the driveshaft. With the two piece driveshaft, the slip yoke is not used at the transmission. Instead a slip joint is placed between the front and rear halves of the driveshaft, at the center support bearing. The slip joint works in the same manner as the slip yoke, sliding in and out, to compensate for suspension height changes.
Most universal joint failure is a result of lack of lubrication. When the universal joint lubricating grease breaks down, or is contaminated by dirt or moisture, the needle bearings rapidly wear. This can result in a squeaking sound, as the vehicle slowly accelerates, and may result in driveshaft vibration. Most vehicles today do not have lubrication fittings on the factory equipped universal joints. However, almost all replacement universal joints are equipped with lubrication fittings.
(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)