In an automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

, ball joints are spherical bearing

Spherical bearing

A spherical bearing is a bearing that permits angular rotation about a central point in two orthogonal directions...

s that connect the control arm

Control arm

thumb|right|300px|Double Wishbone SuspensionIn automotive suspension, an automobile's control arm or wishbone is a nearly flat and roughly triangular suspension member , that pivots in two places. The broad end of the triangle attaches at the frame and pivots on a bushing...

s to the steering knuckle

Steering knuckle

-Introduction:In automotive suspension, a steering knuckle is that part which contains the wheel hub or spindle, and attaches to the suspension components...

s.

More specifically, a ball joint is a steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 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

Vibration

Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...

 problems in the linkage. Commonly found in automotive throttle

Throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

 linkages, throttle body set ups, these are also widely used on construction equipment

Construction Equipment

Construction Equipment is a trade publication and web site serving the information needs of construction contractors, materials producers, and other owners and operators of construction equipment....

, the end of gas spring

Gas spring

A gas spring is a type of spring that, unlike a typical metal spring, uses a compressed gas, contained in a cylinder and compressed by a piston, to exert a force. Gas springs are used in automobiles, where they are incorporated into the design of struts that support the weight of hatchback doors...

s and in children's toys.

Theory

A ball joint is used for allowing three rotations. It fixes the three possible translations (X,Y,Z).

Purpose

Ball joints are the pivot between the wheels and the suspension

Suspension (vehicle)

Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...

 of an automobile. Ball joints play a critical role in the safe operation of an automobile's steering

Steering

Steering is the term applied to the collection of components, linkages, etc. which will allow a vessel or vehicle to follow the desired course...

 and suspension. Ball joints can also be found in most linkage systems for motion control applications, and should not be confused with spherical rod end bearings, which are a different design.

Maintenance

Sealed ball joints do not require lubrication

Lubrication

Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

 as they are "lubed for life" but standard ball joints must be lubed from time to time. It's best to inspect standard ball joints once a year. Generally speaking, standard ball joints will outlive sealed ones because eventually the seal will break, causing the joint to dry out and rust

Rust

Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...

. While there is no exact lifespan that can be put on a sealed ball joint, they can fail as early as 80,000 miles. Signs of a failing ball joint start with a clicking or snapping sound when the wheel is turned and eventually turns 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.

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.