Why does the cause matter?
Because replacing the bearing without removing the cause only repeats the failure. Different modes of damage can look similar even though their causes and long-term effects are not the same, which is why the operating conditions give clues to the root cause.
What are the damage mechanisms?
A sliding bearing can be damaged in five ways, and each comes from a different kind of duty.
- Contact between the surfaces (wear and galling). Where the lubricant film is missing, the surfaces touch and either wear gradually or weld locally and tear. Galling is the second case: material is pulled from one metal surface and left stuck on the other as the two slide under pressure, and it spreads quickly once it starts. In a greased steel-on-steel bearing it is the typical result of too little or unsuitable grease, a load that always acts in one direction, and a small swing angle, because those conditions keep the film from forming (Q1-04, Q1-08).
- Micro-movement (fretting). Tiny oscillating movements between surfaces that should be clamped make fine metallic particles that oxidise like rust, are abrasive and quickly make the damage worse. It happens mainly at the fit of the ring on the shaft or in the housing; on the sliding surface, very small swing angles under load can cause a similar fretting wear. "False brinelling" is a kind of fretting that makes dents, usually from vibration while the part is standing still.
- Overload or impact (brinelling). A permanent dent or flat from a static overload or repeated local impacts. The term comes from rolling bearings; in a spherical plain bearing it means local plastic deformation of the ball or race.
- Chemical attack (corrosion). Water, cleaners or process fluids get in, and the surfaces rust or pit.
- Failure of a layer (flaking and delamination). Pieces of a surface layer peel away: a PTFE liner delaminating from its backing, or a coating, plating or solid-lubricant film flaking from the ball or race. This is not rolling-contact fatigue, which is what the word "spalling" means in rolling bearings. Sliding spherical bearings are normally rated by wear, not fatigue (Q1-04).
What should you look for?
| What you see or feel | Likely mechanism and cause | What to check |
|---|---|---|
| Scored or torn ball and race, metal smeared from one surface to the other, rising torque and temperature (steel-on-steel) | Galling. Too little or unsuitable grease, a load in one direction only, a small angle, too high a pressure-speed product | Lubrication history and grease type (Q1-08); load direction and pV (Q1-04) |
| Even polish and slowly growing play, no transfer | Normal wear, the end of life. Fine scratches mean dirt is accelerating it | Whether the play is past your limit (Q1-14); seals |
| Reddish oxide at the fit lines, creaking | Fretting. Not enough interference or clamp at the shaft or housing; vibration while idle; temperature cycles | The fit class (Q1-06); torque on the clamping parts; transport and storage |
| Dents or flat spots on the ball, a step in the torque at one angle | Brinelling. Shock loading, a badly set stop, hammering during assembly | The load path; the stops; how it was fitted (Q1-10) |
| Growing play, torque spikes, flakes or debris at the edge of a liner (maintenance-free types) | Liner wear or delamination. Pressure-and-speed limit exceeded; edge loading from misalignment; abrasive dirt; heat; unsuitable grease on the liner | The maker's limits (Q1-04); alignment and clearance (Q1-05, Q1-07); temperature (Q1-12) |
| Pieces of coating, plating or film peeling from the ball or race, bright base metal beneath | Flaking of a layer. Overload for the layer, poor adhesion, or chemical attack on it | The load against the rating (Q1-03); the cleaner and fluid; whether the layer was damaged at assembly |
| Brown or red staining, rising torque, a rough feel after washdown | Corrosion. Water or chemicals in; unsuitable cleaner; poor boots or wipers | Seal and boot condition; how the jets are aimed (Q1-11) |
| Jerky starts or squeaking | Stick-slip, a lubrication and friction effect rather than damage | Q1-13 |
How do you work out the cause?
Look at the whole joint before the ball: seals, boots, grease, the pin and the housing. Find the pattern, because damage on the load side, at an edge, at the fit lines or all around each points to a different mechanism, and match it to the table. Then ask what the bearing was doing: the load and its direction, the swing and its speed, the environment, how it was fitted. Write these down, since the same damage can have different causes at a different duty. A single photograph is not enough. Fix the cause, then fit a new bearing: add a boot, change the fit, change the grease or choose a different type (Q1-02). The measurements and the decision to replace are in Q1-14.
Which failure modes show up on off-road and race vehicles?
The same five mechanisms, but some are far more likely, and the position of the damage tells you more than on a machine.
- Abrasive wear from sand and mud that got past the seal: fine scratches all over the ball and race, and play that grows steadily (Q1-11).
- Fretting at the fits, from vibration, a bolt that lost its clamp, or an aluminium housing that loosened when hot (Q1-06).
- Brinelling, dents or flats on the ball from bottoming out, a hard landing or an impact.
- Corrosion from water, road salt or cleaners.
- Binding or scuffing at one end of travel, when an arm, spacer or bracket runs out of angle (Q1-05).
Read where the wear sits. A polished patch in the load direction at one spot on the ball matches the vehicle's weight at ride height. Marks at the ball's edge or on the inner ring's end faces mean the angle limit was reached in articulation.
When the ball is dented, flaked or the play has grown, replace the bearing, and look for what delivered the load: a bent link, a cracked bracket or weld, or an elongated bore can follow an impact. Do not regrease it and send it back out.