The simpler of the two traditional bearing designs is the sleeve bearing, so called because the central shaft rotates inside a cylindrical sleeve-like structure. Oil gets added to lubricate the bearing and enables the shaft to turn, while the bearing sleeve holds the rotor in the correct position relative to the motor stator, and ensures there is a sufficient gap between the two. Because of these characteristics, sleeve bearings are highly impact-resistant and are significantly less expensive than ball bearings, but they also have their disadvantages.