Some older DC units still operate with which type of bearing?

• A. Sleeve bearings
• B. Ball bearings
• C. Roller bearings
• D. Needle bearings

Answer: (A)

Plain bearings, or sleeve bearings, are what older DC machines typically use to support the rotor. They consist of a smooth bore in the bearing housing that the shaft spins in, with lubrication forming a film between surfaces. This simple, non-rolling-element design makes them cheap to manufacture, easy to replace, and forgiving of minor misalignment or shaft runout, which fits older equipment where maintenance practices and tolerances were less strict. With proper lubrication, sleeve bearings can run at the speeds and handle the radial loads of older DC units. As speeds and performance demands increased, designers moved toward ball or roller bearings to reduce friction and extend life, while needle bearings served in tight spaces in some designs. Therefore, some older DC units still operate with sleeve bearings.