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How to inspect the ball joints?

For all of the car steering and suspension importers, it is very important to inspect the quality of the ball joint. It is vital to the safe of every user. How to check the quality then? Below is the guides. 

1. What is a ball joint?

A ball joint is a ball-and-socket joint, and they are similar in internal construction. The shell and bearings hold the ball in the unit. Usually, a spring preloads the ball firmly into the bearings to prevent backlash and looseness, which cause wear and wheel misalignment. The tapered ball stud fits into a tapered hole in a steering knuckle or other component, and when the castellated nut or locknut is tightened, it draws the stud into a tight fit. Inner tie rod ends on rack-and-pinion units are normally threaded onto the end of the rack and locked in some fashion.

Ball joint structure (Top and side view, and cutaway)

2. How many types of the ball joints?

Depending on the suspension design, a ball joint can function in either way: 1. as a weight-load carrier, 2. as a follower ball joint. On SLA suspensions with two control arms, the ball joint in the control arm that has the spring mounted on it is the load carrier. The lower ball joint is most often. On MacPherson strut suspensions, the ball joint is a follower.The load is carried by the body through a bearing at the upper strut mount. Follower ball joints provide the necessary pivot points and keep parts in their proper position.

Loaded ball joints structure
Follower ball joint structure

3. Where is the ball joint installed in the vehicles?

For Short-Long-Arm front suspension, both upper and lower control arms need the ball joints to connect with the knuckle, and for MacPherson Strut suspensions, only the lower control arms need it to mount on the knuckle.

Applications of the ball joints

4. How to check the wear of the ball joint?

What cause the ball joint failure on function?

Ball joints normally have a long service life, but they can fail due to a loss of lubricant, or from contamination if the seal (boot) fails. A worn ball joint allows looseness in the suspension between a control arm and the steering knuckle. This looseness causes poor handling and misalignment, and it doesn’t take much looseness to cause problems.

How to inspect the ball joints?

Some manufacturers make ball joint inspection easy by providing wear indicators on their ball joints. These are checked with the full vehicle weight on the suspension. On the most common type, the grease fitting is threaded into a round boss that protrudes from the base of the ball joint. As the joint wears, the boss recedes into the joint. When the shoulder of the boss is flush with the base of the ball joint, the joint should be replaced. A small screwdriver may be used to scrape away any debris and help determine the position of the boss. The service information will identify wear indicator ball joints and how to read them.

Check the wear of the ball joint

Manufacturers provide specifications for allowable movement in ball joints. The vertical (up-and-down) spec is listed as axial movement, and the horizontal (side-to-side spec), if provided, is listed as radial movement. Typically, load-carrying ball joints have an axial movement limit specification of around 0.060 of an inch, though some may allow up to 0.200 of an inch. The spec for many follower ball joints is “no perceptible movement.”

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