In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles. More specifically, a ball joint is a steel bearing stud and socket enclosed in a steel casing. The bearing stud is tapered and threaded. It fits into a tapered hole in the steering knuckle. A protective encasing prevents dirt from getting into the joint assembly. Motion control ball joints tend to be retained with an internal spring, which helps to prevent vibration problems in the linkage. Commonly found in automotive throttle linkages, throttle body set ups, these are also widely used on construction equipment, the end of gas springs and also in children's toys.


From wikipedia