Worst case wear [page 3 of 'ferrous particle problem']
The most significant damage occurs when the contamination is of a similar size to the dynamic clearances [tolerances between moving parts] of the given system. Much smaller particles simply float through the gap, and much bigger particles can’t get in.
Particles around the size of the space can get in and then get stuck in the walls, being dragged up and down and acting like a scouring pad, or the boulders trapped in the bottom of a glacier.
The contamination bridges the protective fluid barrier and forces the moving parts in the system to come into direct contact - accelerating wear very dramatically [a state called errosive wear].
To sum up, the results of errosive wear are:
- Dimensional changes
- Leakage
- Reduced efficiency
- More wear
- CATASTROPHIC FAILURE
The key question then becomes - what are these tolerance values? If we know this then we know what size particles we need to be especially concerned about.
Typical Dynamic Clearances
Here are some key features and typical tolerances for moving parts within various fluid systems:
Bearings: - Roller 0.1 - 1 micron, Journal 0.5 - 100 micron
Hydrostatics: 1 - 25 micron
Gears: 0.1 - 1 micron
Dynamic Seals 0.05 - 0.5 micron
Pumps
- Gear: Tip to Cut In Track 0.5 - 5 micron, To Side Plate 0.5 - 5 micron, Matching 0.2 - 5 micron
- Vane: Tip to Cam Ring 0.5 - 1 micron, Side of Vane 5 - 13 micron
- Piston: Piston to Bore 5 - 40 micron, Timing Plate 0.5 - 5 micron
Valves Directional 2 - 20 micron, Poppet 10 - 40 micron, Servo 2 - 4 micron
To summarise these key tolerances are typically 0.2 to 5 microns, with some a bit larger and some smaller.