Automotive engineers have copied parts from the human body for years. The automotive ball joint is such a design. Hip joints are a form of ball joint. On our vehicles, the ball joint acts much as it does in our body. Other similarities are, like our hip, wear and breakage are devastating.
Parts of a ball joint
Only a few parts make up the ball joint. They include the stud, the socket or housing and the rubber boot. Manufacturers use bolts, rivets and press fits to mount ball joints. The stud attaches to the other component in the suspension and movement is within the joint.
A protective rubber boot holds the lubricant in. This boot also keeps water and debris from entering the joint. Once the boot tears, the joint will fail. Torn boots can also allow corrosion to weaken the stud. If the stud breaks, we may lose control of the vehicle.
Sealed or serviceable
They design some ball joints with a grease fitting for lubrication. They seal others and they require no service. We may refer to a ball joint with a grease fitting as serviceable. The advantage is the ability to add additional lubrication. The disadvantages are, debris may enter the joint if improperly lubricated and lack of lubrication may cause the joint to fail.
Ball joint types
They use different types of ball joints on the various vehicles they build. With a load-bearing ball joint, the normal load pushes the stud into the ball joint housing.
With the other style of ball joint the load pulls the stud through the housing and into the base. For instance the stud is pulled away from the base rather than pushed into it. This is a load-suspending ball joint. Designers use both type ball joints widely and both give good service. For the technician, the main difference is in the way we check them. We need a different procedure to check each type.
When are ball joints worn?
Eventually ball joints wear out. A ball joint is considered worn out when the socket no longer holds the stud tightly. This allows side to side movement and causes wheel alignment to change. Tire wear is the most common symptom, but squeaks, pops, binding and shimmy can also result.
Checking ball joints
To check a ball joint, we use a jack to remove the load. On a load-suspending joint we lift under the arm. With a load-bearing joint we support the chassis, allowing the arm to hang free. Pulling hard on the tire, at 12:00 and 6:00 o'clock, should show no side to side movement.
On many ball joint designs the stud will move in and out when we unload the joint. In use, the weight of the vehicle holds the ball firmly in the socket. With the load removed the ball may move up and down. This is NOT the sign of a problem if no side to side movement exists.
Things that lead to premature failure:
Over greasing the joint causing boot failure.
Forcing debris into the joint, by not cleaning the fitting prior to greasing.
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Over greasing the joint causing boot failure.