DYNAMIC CLEARANCE

In the past, piston pumps and servo valves were protected by using high quality filters to guard them against the contamination that was present in the systems. The common belief was that unusually close tolerances made these components susceptible to contamination failure. Now, most hydraulic components have similarly close tolerances -- yet the cost of installing filters to protect each one would be ridiculously expensive.

The proliferation of these expensive and sensitive components in today's systems poses new challenges. Servo valves and piston pumps demand that the lubrication be ultra-clean. Failure to give them what they need results in immediate, expensive failures. However, if you examine the table to your right, you will note that the components generally not associated with ultra-cleanliness have demanding tolerances also. The main difference between the two classes of components is that the second group generally will continue to work for a while if subjected to excessive contamination. Although they may continue to work, they are suffering degradation failure which will not show up immediately. Instead, they slowly lose efficiency and generate contamination in the system. The particles then continue downstream to damage additional components. As this process continues, the level of contamination rises further, with each component adding more debris. A chain reaction of debris has now started. Its logical conclusion will be system failure.

 

TYPICAL OPERATING DYNAMIC FILM THICKNESS
COMPONENT Roller Element Bearings Journal Bearings Hydrostatic Bearings Gears Dynamic Seal     Pump, Gear     to Side Plate     Tip to Case Pump, Vane     Sides     Tip Pump, Piston     to Bore     Plate to Cylinder Servo Valves     Wall     to Sleeve Actuators	THICKNESS (μm) 0.1-1 0.5-100 1-25 0.1 0.05-0.5         0.5-5 0.5-5 5-13 0.5-1 5-40 0.5-5 130 450 18-63 1-4 50-250
source: ASME 1980 Wear Control Handbook

 

As will be explained in the sections on corrosion, flushing, and filtration, it is impossible to clean the system's pipe to these close tolerances after the fact. You must engineer and build them with these tolerances in mind from the start. By specifying REDOX on your prints, specifications, and purchase orders, you will have the piping your system requires.