In an automobile, a ball joint is  spherical bearings, connecting the control arms to the steering knuckles. More specifically, a ball joint is a steel bearing stud(also called Ball Pin) and socket enclosed in a steel casing(also called ball housing). The bearing stud is tapered and threaded. It fits into a tapered hole in the steering knuckle. A protective encasing(Dust Cover) 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 and throttle body set ups, these are also widely used on construction equipment and on the end of gas springs. Ball joints are used on the front end of virtually every car and light truck. Vehicles with conventional suspension systems use two ball joints per wheel: an upper and a lower ball joint. Cars with MacPherson strut suspension systems use a lower ball joint and an upper strut bearing rather than an upper ball joint. 

 

Purpose

Ball joints are the pivot between the wheels and the suspension of an automobile. Ball joints play a critical role in the safe operation of an automobile's steering 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 Tips

Sealed ball joints do not require lubrication 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. 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.

 

If a ball joint fails, the results can be dangerous as the wheel's angle will be unconstrained, causing loss of control. Because the tire will be at an unintended angle, the vehicle will come to a screeching halt damaging the tires. Also, during failure, debris can damage other parts of the vehicle.