NEW OIL STRAIGHT FROM THE DRUM	FILTERED OIL
Photographs of patch test samples - courtesy of Pall.

Even superclean fluids may be contaminated causing wear of components. For example a high humidity environment can damage an unprotected oil/fluid.

TYPES OF WEAR

The types of wear illustrated below are caused by metallic particles, sand, dirt and other wear debris. All these sources of particulate contamination add to the chain reaction increasing the rate of wear further.

• Abrasive wear occurs when particles enter the clearance space between components, bury themselves in one surface and remove material from the opposing surfaces by cutting into them. Particles generated by abrasive wear become harder than the material they originated from due to work hardening and add to the chain reaction of wear.

Particles too large to enter clearance
		Dynamic Clearance
		Clearance size particles cause abrasive wear on surface

• Fatigue wear is the result of stressing by particles trapped between bearing surfaces. Initial dents and cracks spread due to repeated stressing, eventually leading to surface failure and further metallic particles being released into the lubricating fluid.

1. Particle caught		2. Surface dented, cracking initialised
3. After 'N' cycles, cracking spreads		4. Particles released when surface fails

• Adhesive wear occurs when reduced viscosity of lubricating fluid allows metal to metal contact. Surface asperities become cold welded together and shear from the surfaces as they move apart.

Wear generated particles

• Erosive wear is caused by particles suspended in high velocity fluids which erode the surfaces they impinge upon. This type of wear is common in servo and proportional valves.

• Corrosive wear is caused by water or chemical contamination. This type of wear is accelerated by the breakdown of lubrication fluids, for example precipitation of additives and oil oxidation. Removing free water will contribute towards extending the effective life of the fluid.