The Basics of Ball Joints
The ball joint is a device that attaches the steering knuckle to the control arm. There are other parts that use an identical type of fitting but serve other purposes, such as the tie rod end link and certain styles of sway bar end links.
The ball joint is a steel bearing with a stud attached to it, and it is housed in a steel casing. The casing is fixed to the control arm allowing the stud a full range of motion available within the opening of the bearing’s housing. With the housing affixed to the control arm, the steering knuckle enables the wheels to turn left and right and travel up and down with the suspension.
Understanding that there are many different types of ball joints is recognizing that all joints are the same but that they use different methods for being bolted to the control arm and the knuckle. Nevertheless, they all use a spherical ball that is housed by a steel cup/housing.
Some ball joints are pressed into the control arm, and use a tapered stud that fits into a tapered hole in the steering knuckle. Other ball joints are bolted onto the control arm and use a shaft that is inserted into a pinch bolt style steering knuckle.
Sometimes the ball joint is part of the entire control arm. In this case, the entire control arm must be replaced in the event that the ball joint fails. The advantage of doing this is that you are getting new control arm bushings when this new part is installed, however, the part might be more expensive.
The reason for ball joint failure is relatively simple to understand. Movement of the ball joint creates friction between the metal ball and the metal housing and requires a lubricant to prevent wear of it. This lubricant is a grease and is held in with a little rubber boot. In the event that this boot rips or is torn, the grease can escape the ball joint. The grease will eventually be contaminated with dirt and reduce the ability of the grease to properly lubricate. Regular wear and tear on the part will also occur, and just like all auto parts, they won’t last forever and will eventually require replacing.