The universal joint (also called a U-joint or Cardan joint) on an automobile's propeller shaft (driveshaft) allows the driveshaft to transmit power through an angle between the transmission and the differential. Since the transmission and differential are not always perfectly aligned (especially when the vehicle is driving over uneven terrain and the suspension moves), a rigid connection would quickly break or bind.
The U-joint's function is to accommodate this angular misalignment, allowing the driveshaft to rotate smoothly even when the angle between the input and output shafts changes. It effectively converts rotational motion from one shaft to another, even when those shafts are not collinear.
