A ball joint is a style of pivot joint that gives relative freedom of movement to a connected object. These joints typically consist of two parts, the ball and the case, but some ball joints use a connective piece that links the two parts together. It is possible to find a ball joint in millions of different systems. Nearly every car manufactured has several ball joints in the steering and suspension systems—they are also common in other steerable devices like industrial machines and children’s toys. The anatomical ball and socket joint, like in a human shoulder, is also a style of ball joint.
The basic design of a ball joint is very simple. The ball is a bearing connected to a stud, which is often threaded to allow it to screw into a larger machine or coupling. The ball fits loosely into a case, but an internal lip prevents the case from releasing the ball. As a result, the ball can roll around inside the casing, creating a large amount of motion relative to the two halves of the joint.
There are two main modifications commonly found in the basic ball joint. This first is a connective piece between the ball and the case. This is generally a spring, but other connection types are possible. This connective piece reduces vibration caused from the ball moving in the casing. On the other hand, the connection will occasionally limit the full mobility of the joint.
The other common alteration to the basic ball joint model is a full seal. Normal ball joints are open and require occasional lubrication to continue working properly. Without lubrication, the steel ball will often begin to rust and break down. Sealed ball joints have a liquid-tight seal around the opening of the casing and lubrication inside the joint. These joints require much less maintenance, but breaks in the seal typically require that the entire unit be replaced.
Ball joints provide multi-directional movement between two connected objects. At the extreme, a ball joint has almost 180-degree directional motion and 360-degree rotational motion. While very few ball joints are this loose, it is a possible setup. A typical ball joint hovers around 100-degree motion, just slightly larger than perpendicular.
These joints are typically designed for locations with quick, often unexpected, multi-directional movement. One of the most common locations to find a ball joint outside the anatomical ball and socket joint is in an automobile. These joints provide the rotational capabilities of the steering system, while allowing adjustments in the height of the system. In addition, they are common in suspensions where they connected the suspension system to the wheel.