What is stick-slip?
A heavy carton on a smooth floor shows it. You push hard to overcome the static friction. The carton starts to slide, the friction drops, the carton speeds up, you can no longer give it enough force, and it stops. Then the cycle repeats: grip, release, slide, brake, grip. In a machine it is much faster and shows up as a stutter. The vibration passes into the structure as a squeak or creak, makes the movement irregular, and increases wear. The phenomenon needs a difference between the friction before the movement starts and during the movement.
Stick-slip mainly occurs in the boundary and mixed lubrication conditions, where the surfaces are not fully separated by a film. When the film builds, the movement tends to become smoother. For a spherical bearing that swings slowly through a small angle, that film is hard to build (Q1-04).
What are the usual causes, and what do you try?
This table covers what is particular to noise, sticking and binding. Damage you can see, such as corrosion or fretting dust at the fit, is in Q1-09.
| What you notice | Likely cause | What to try |
|---|---|---|
| Squeak or jerky starts, positional overshoot | Stick-slip with a big difference between starting and moving friction; a grease that does not release oil; rough pins | Change the grease; polish the pin; check the temperature and liner type (Q1-08) |
| Dry squeal from a steel-on-steel bearing | The grease film has gone and the metal surfaces are rubbing. It can be the first sign of galling | Grease it now, within what the maker allows, and check the interval and the grease path before it scores (Q1-08, Q1-09) |
| Squeak on a new maintenance-free bearing | Surfaces are still running in. On polymer plain bearings the squeak often decreases after the run-in phase, as the solid lubricant transfers to the shaft | Allow time to run in. Add only an initial lubrication if the maker allows it (Q1-02) |
| Rattle and squeak | Too much clearance, or a loose fit | Check the clearance group and the fits (Q1-06, Q1-07) |
| Squeak, with misalignment | Poor alignment of the bearing or shaft; the ball at the edge of its tilt | Check the angles through the full travel (Q1-05) |
| Stiffness or binding after fitting | Ball clamped too hard: excessive clamping force expands the ball and binds it in the race. Or too tight a fit that closes the clearance | Ease the clamp; check the fit and the clearance (Q1-06, Q1-10) |
What can you do about stick-slip itself?
There are four ways to counter it: reduce the difference between sliding and static friction, use materials that damp vibration, make the structure stiffer, or separate the two surfaces, for example by lubrication. For a greased spherical bearing, the practical steps are the grease and the pin. Change the grease first, then use a smoother pin, then a liner with a lower friction coefficient or control of the preload.
For the shaft or pin finish, follow the bearing maker's recommended roughness rather than making it as smooth as possible. A surface that is too smooth is one of the causes of squeaking.
Why does a suspension joint squeak, and when is it not the bearing?
A creak or squeak as the suspension moves is often blamed on the spherical bearing, but several other parts make the same noise: rubber or polyurethane bushings, spring seats, anti-roll bar bushings and shock mounts. Find the source before you change anything. With the load off, move each joint by hand through its travel; the one that sticks or squeaks at a certain angle is the suspect. A noise that comes only at the end of travel points to a joint that has run out of angle (Q1-05) or been over-clamped (Q1-10), not to stick-slip in the bearing.
If it is the bearing, the table above applies. A dry squeal from a steel-on-steel bearing is a warning to grease it before it scores (Q1-08), and a new liner type may be running in. Noise from polyurethane bushings and other car-part joints is a separate subject, covered on our sister site syzmachine.com.