A drive shaft transfers the torque produced by the engine to the rear axle in rear-wheel-drive cars. The drive shaft accomplishes this by mating the output shaft of the transmission to the differential at the rear axle. Four-wheel-drive vehicles also incorporate a second drive shaft that delivers torque to the front axle.
The most common style of drive shaft is composed of five main elements: the front yoke, the shaft, the rear flange and two U-joints. The rear flange is typically bolted to the differential, while the front of the shaft usually incorporates a slip yoke that meshes with teeth on the transmission's output shaft. The U-joints allow for flexibility to prevent binding and shaft damage under most normal driving conditions. Drive shafts are commonly made from mild steel and must be balanced to prevent wobble that could lead to premature U-joint failure, damage to the yoke or flange, or damage to the output shaft of the transmission.
The most common maintenance needed is greasing or replacing the U-joints. Older vehicles incorporate U-joints with a grease nipple that needs to be refilled periodically, but as of 2014 most vehicles use sealed, permanently greased U-joints. U-joint failure can be easily identified by a characteristic clunking sound typically heard at low speeds. U-joints should be replaced if they are making noise or if significant play is felt when the drive shaft is lifted or moved side to side by hand.