Automotive ball joints are a part of a vehicle's suspension system; they are spherical bearings that attach the steering arm or linkage to the steering knuckles. Basically, they are a flexible ball and socket arrangement (similar to the humerus or femur in the human body) that allows the suspension to move, while simultaneously allowing the wheels to steer in the direction that the steering wheel is turned.
Ball joint basics
The number of ball joints in a given vehicle will vary depending upon the type of suspension system with which it is equipped. Trucks and cars that have shock absorbers will have a total of four: One upper ball joint and one lower ball joint on each side in the front of the vehicle. Minivans and cars with strut suspensions will only have two (one lower ball joint on each side of the front end of the vehicle). There are some front-wheel drive vehicles that also have them as part of the rear suspension system.
Ball joint maintenance
The ball joints on older vehicles need periodic inspection and lubrication. They are covered by a rubber boot which helps retain the lubricating grease and prevents dirt and other mechanical contaminants from getting mixed in with the lubricant. Should dirt or other debris mix into the grease, it can act as an abrasive and considerably shorten the safe, useful lifespan of a ball joint.
Newer vehicles have sealed ball joints which are "lubricated for life." This does not mean that the ball joints will necessarily last for the entire life of the vehicle, it means simply that the ball joint cannot be lubricated; should the ball joint need lubrication, it will have to be replaced. Typically, standard ball joints have a longer life than sealed ones as they can be re-lubricated. Regardless of the type of ball joint in a vehicle, the standard recommendation is that they be inspected approximately every 30,000 miles.
How to check ball joints
Checking ball joints is a relatively easy process. Many vehicles with standard ball joints come with factory-equipped ball joints that have built-in wear indicators. The wear indicators are the boss that protrudes out from the Zerk or grease fitting. When the ball joint is new and fully lubricated, the boss sticks out approximately 1/2 inch from the grease fitting. As the ball joint wears, the boss gradually recedes into the housing of the fitting. If the top of the boss is level with or below the housing, the ball joint needs to be replaced.
If the vehicle does not have the built-in wear indicators, you have to first raise the vehicle up so that the wheels are off the ground to check the ball joints, and the weight of the vehicle is supported under the frame or suspension system. Place one hand on the top of the tire, and one hand under the bottom of the tire. Attempt to push and pull the tire back and forth from top to bottom. If there is a lot of play in the tire, it is time to replace the ball joints. A small amount of play is acceptable. With new ball joints, there will be no play at all. If you don't have a set of jack stands available, it is best to take the vehicle to a qualified mechanic to have it done rather than risking your safety during inspection.
Ball joints are always replaced as either an entire set or, at a minimum, matched pairs (both uppers or both lowers). If a ball joint is bad on one side, typically the corresponding ball joint on the other side is bad as well.