In several years- (or even decades-) old devices, bearings are not rotating freely anymore. The problem in most cases is caused by original lubricants: decomposed, dried out, contaminated etc.
It's possible to just replace them, but high-grade bearings are expensive, and some sizes difficult to find. Simple cleaning & lubrication is enough to revive them, though. Below is a brief guide for R2 bearings widely used in many trackballs:
1. Carefully remove them from device, and put aside any parts attached to them (shafts, washers, retaining rings, encoder wheels, etc). Do not use excessive force, to avoid bending of shafts or any other damage.
2. Take off side shields. To do that, pry off the C-ring with sharp needle (it's convenient to mount it in micro-drill handle). Bear in mind: it will jump away if you don't hold it properly!
3. Wipe off as much of old grease as possible, using cloth and wooden toothpicks. Separated parts could be cleaned really easily, while bare bearings will require further steps.
4. Put the bearings into some solvent overnight. Depending on grease chemical composition, it may require petrol, citric degreaser, acetone, carburetor cleaner, xylene, isopropyl alcohol, WD-40, paint thinner, and so on. Use old toothbrush to remove softened particles of grease.
5. In some cases, rotation is needed to speed up separation of dirt. Simpe method is, to put long screw of appropriate diameter into inner ring, tighten with nut, and insert it into rotary tool like Dremel. Hold outer ring with your fingers and keep it spinning at low speed while submerged in solvent, applying light force in different directions.
6. Clean bearings in ultrasonic bath filled with warm water & dish washing detergent for several minutes. Waves and bubbles effectively remove contamination from hard to access places in separator rings.
7. Blow them with compressed air.
8. Inspect the internals with microscope or magnifying glass to make sure no any foreign particles are present.
9. Apply small amount of light lubrication oil (not grease!).
10. Install side shields back.
Clean lubricated bearings now spinning like new, or even better!
P.S. Sometimes, non-serviceable bearings are used by manufacturers, with side shields permanently pressed in. In such case, the shields could be removed by destructive way, by simple prying with sharp tool (e.g. awl) inserted into a gap between shield and inner ring. This will convert the bearings into the open variant, and make them susceptible to contamination by dust - but it's better to have perfectly rotating open bearing than non-working sealed one, anyway.
Ball bearing service
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