Ball joint

[2] A ball joint consists of a bearing stud and socket enclosed in a casing; all these parts are made of steel.

Motion-control ball joints tend to be retained with an internal spring, which helps to prevent vibration problems in the linkage.

Combining two such joints with control arms enables motion in all three planes, allowing the front end of an automobile to be steered and a spring and shock (damper) suspension to make the ride comfortable.

Camber angle could generally be adjusted by moving both inner pivots of either the upper or lower control arm inwards or outwards by an exactly equal amount.

But compliance of the control arm inner pivots, typically due to the use of elastomeric bearings, would again cause the trunnions to be stressed.

The suspension designer's freedom was limited, it was necessary to have some compliance where it might not be wanted, and very little where more would have been useful in absorbing the fore and aft impact loading from bumps.

The arrangements for adjusting the toe angle are not changed by introducing ball joints in the suspension, although the steering linkage itself must use 4 or more pivots, also usually ball joints, and in almost every vehicle ever made, some of these have been adjustable by having a threaded end and locknut, to enable the toe to be set precisely.

[4] Ball joints play a critical role in the safe operation of an automobile's steering and suspension.

Unlike a kingpin, which requires an assembly in the center of the wheel in order to pivot, joints connect to the upper and lower end of the spindle (steering knuckle), to the control arms.

For this reason, almost all ball joints on modern European or Far Eastern cars are the sealed for life type.

New technology especially applied to the internal bearing design has allowed ball joints to meet these longer service intervals.

SRJs are often used in parallel robotics applications like a Stewart platform, where high rigidity and low backlash are essential.

[8] Plain spherical bearings can be used in place of SRJs at the cost of increased friction, but offer an opportunity to preload the joint further.

If a ball joint fails, the results can be dangerous as the wheel's angle becomes unconstrained, causing loss of control.