Component Protection

Fig. 2-5. Effect of Oil on Contacting Surfaces

The rubbing surfaces of bearings, vanes, valve spools, piston and cylinders are required to move freely while operating under high pressure. Although these precision-made components are manufactured to very close dimensional tolerances and then assembled to very close fits, they still have surface irregularities like those in Fig. 2-5(A). If they are to function efficiently, they must be protected by the hydraulic fluid from wear. Without hydraulic fluid, the two contacting surfaces not only have a lot of surface friction, but tend the wear out. Also, if the two moving parts have enough metal-to-metal contact, they can generate so much heat that they would weld or fuse together.

Hydraulic fluids protect the rubbing surfaces in a hydraulic system from excessive wear by providing a lubricating film between the surfaces. This film should be durable and thick enough to prevent metal-to-metal contact between the surfaces either while they are moving or standing still. Also, the film must be present under light or heavy loads. Fluids must have the right viscosity to be able to penetrate into the small clearance between the closely-fitted pars and they must remain there as shown in Fig. 2-5(b). The property of a fluid that permits it to maintain a film under heavy load is called its FILM STRENGTH. They more this lubricating film succeeds in preventing metal-to-metal contact between rubbing surfaces, the more war-resistant it is 

Hydraulic fluids also reduce the friction between rubbing surfaces by substituting the comparatively low friction inherent in a lubricating film for the friction between the contacting surfaces. The film should be “slippery” so the moving parts can move easily. In addition, the fluid should wet the metallic surfaces and cling to them. The property of a fluid which gives it the clinging slipperiness to lubricate well is called its LUBRICITY. In order to protect non-rubbing surfaces from corrosion, the hydraulic fluid should cover all surfaces with a thin, durable protective film.