The driveshaft, sometimes called the impeller shaft, has the job of transferring engine power to the rear wheels. In the case of four-wheel-drive vehicles, shorter driveshafts transfer energy to individual wheels, while the front-wheel transaxle has one driveshaft for each wheel. The driveshaft has components that allow it to couple with the differential housing (rear end) at any angle by using joints that allow flex and off-angle positioning when the suspension moves up and down. Understanding the driveshaft parts can help you anticipate problems during routine inspections and driving.
The driveshaft consists of a long hollow tube made from high-strength steel. Its long length requires it to bridge the gap between the transmission end shaft and the differential housing (rear end). The driveshaft transfers direct torque energy from front to rear. The driveshaft's hollow design cuts down on weight, which reduces rotational drag. The stress loads on driveshafts can be tremendous, but their hollow design that incorporates an inside and outside surface structure resists the most powerful stresses and shock loads.
Every driveshaft has metal weights welded onto the tube housing at strategic locations. The weights resemble match books in size and thickness. For the driveshaft to spin true in perfect balance, weights must be added or subtracted to it. A machine shop normally provides this service. Without weight balancing, the driveshaft would radically shudder or vibrate, which if prolonged, could snap or bend the driveshaft tube.
The typical rear-wheel driveshaft has four connecting yokes--two at the front end of the driveshaft tube and two at the rear. Three of the yokes have a U-shaped design that allows them to connect to universal joint bearing caps. The heavier front-end yoke comes designed with a short, inside-spline shaft that fits into the transmission input shaft. The furthest yoke on the end of the driveshaft connects to the differential housing, and it has two C-shaped mounting straps and retainer nuts.
Universal joints have an X-shaped design that allows them to couple the driveshaft with the transmission and differential housing. Four bearing caps sit on each end of the universal joint. The caps hold precision needle bearings inside that rotate with the driveshaft. The universal joint allows the off-angle positioning of the driveshaft that moves up and down with the suspension. The bearing caps press directly into the yoke ends. Many driveshafts have factory-sealed universal joints--the bearings do not have to be lubricated for life.
Some of the older driveshafts came with "zerk" (grease nipples) fittings screwed into the universal joint body. These type of universal joints have to be manually lubricated with a grease gun at certain maintenance intervals. Car enthusiasts often desire the old style universal joints because lubrication was assured instead of waiting for the symptoms of a dry universal joint that can sometimes crack or break loose suddenly without warning.