In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles. They are used on virtually every automobile made  and work similar to the ball and socket design of the human hip joint.
A ball joint consists of a bearing stud and socket enclosed in a casing; all these parts are made of steel. The bearing stud is tapered and threaded, and fits into a tapered hole in the steering knuckle. A protective encasing prevents dirt from getting into the joint assembly. Usually, this is a rubber-like boot that allows movement and expansion of lubricant. Motion control ball joints tend to be retained with an internal spring, which helps to prevent vibration problems in the linkage.
A ball joint is used for allowing free movement in two planes at the same time, including rotating in those planes. 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.
Unlike a kingpin suspension, this allows for adjusting the camber and caster of the wheel suspension. Both systems allow adjustment of the toe angle. This ability to fine tune the suspension allows manufacturers to make the automobile more stable and easier to steer, compared to the older kingpin style suspension. The smoother ride also increases tire tread life, since the ball-joint suspension allows the vehicle to be adjusted to track in a level fashion.
Ball joints are the pivot between the wheels and the suspension of an automobile. They are universally used in the front suspension, but can also be found in the rear suspension of a few higher performance autos. Ball joints play a critical role in the safe operation of an automobile's steering and suspension.
In automobile suspension nomenclature, the upper ball joint is called the "upper ball joint" and the lower is called the "lower ball joint". Lower ball joints are typically larger and wear out faster as the entire front end weight rests solely upon them.
Front wheel drive 
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. These leaves the center section open to allow the use of front wheel drive. Older kingpin designs can only be used in a rear wheel drive configuration.
Sealed ball joints do not require lubrication as they are "lubed for life", but most ball joints have grease fitting (sometimes called a grease zerk) and are designed for periodic addition of a lubricant. This is usually a very high viscosity lubricant. Generally speaking, standard ball joints will outlive sealed ones because eventually the seal will break, causing the joint to dry out and rust. Additionally, the act of adding new lubricant pushes out old and dry lubricant, extending the life of the joint.
SRJs (Spherical Rolling Joints) are high precision ball joints consisting of a spherical outer and inner race separated by ball bearings. The ball bearings are housed in a spherical retainer and roll along both the inner and outer surfaces. This design allows the joint to have very low friction while maintaining a large range of motion and backlash as low as 1 micron. SRJs are often used in parallel robotics applications like a Stewart platform, where high rigidity and low backlash are essential.
While there is no exact lifespan that can be put on sealed ball joints, they can fail as early as 80,000 miles in modern vehicles, and much sooner in older vehicles. Signs of a failing ball joint start with a clicking, popping or snapping sound when the wheel is turned and eventually turn into a squeaking sound at the end of a stop, when the gas pedal is used and/or also when hitting bumps. Another symptom could be 'thud' noises coming from front suspension when going over bumps. Dry ball joints have dramatically increased friction and can cause the steering to stick or be more difficult.
If a ball joint fails, the results can be dangerous as the wheel's angle becomes unconstrained, causing loss of control. Because the tire will be at an unintended angle, the vehicle will come to an abrupt halt, damaging the tires. Also, during failure, debris can damage other parts of the vehicle.
Other uses 
While in automotive parlance the term "ball joint" usually refers to the primary ball joint connections at the ends of the control arms, this type of joint is used in other parts as well, including tie rod ends. In these other applications, they are typically called ball and socket joints. These joints are also used in a number of other non-automotive applications, from the joints of dolls to other mechanical linkages for a variety of devices, or any place where a degree of rotation in movement is desired.
See also 
- Rod end bearing (heim joint)
- Bumbeck, Mike. "Ball Joints - How to Keep Your Front Suspension Together". Mobile Oil. Retrieved October 10, 2012.
- "Your Car's Ball Joints - The Pivotal Part of the System". California Dept. of Consumer Affairs, Bureau of Automotive Repair. 2010. Retrieved October 10, 2012.
- Allen, Mike (March 29, 2006). "Ball Joint Replacement". Popular Mechanics. Retrieved October 10, 2012.