Fluid Perfomance - Lubricating Ability

Within an aircraft hydraulic system, moving parts generate fluid pressure and convert this pressure into useful work. The hydraulic fluid must prevent wear of these moving parts by allowing them to move in relation to each other with a minimum of friction and power loss.

Investigations of hydraulic fluid lubricating ability have been conducted using bench test devices which measure friction and wear using hydraulic test rigs in which the hydraulic pump functions as a wear test device. Although bench wear machines and the test pump rigs test lubricating ability of the fluid, final evaluation for total performance must come from operational experience in actual aircraft hydraulic systems.

Skydrol Type IV hydraulic fluids have demonstrated excellent lubricating ability in millions of flight hours in a wide range of aircraft types. This experience has verified results from bench pump tests which show Skydrol Type IV fluids to be excellent lubricants.

One commonly used bench test is the Four Ball Wear Test. Typical results with the Skydrol hydraulic fluids are given in the table below. Test conditions were 600 rpm, 167°F (75°C) and one hour at various loads.