Hydraulic System Components And Design
  • Hydraulic Fluid
    • The fluid must be ale to:
      • Flow through all of the lines with a minimum of opposition.
      • It must be incompressible.
      • It must have good lubricating properties to prevent wear in the pump and valves
      • Must inhibit corrosion and not chemically attack any of the seals used in the system
      • It must not foam in operation
    • Things to consider:
      • Working conditions
      • The service required
      • Temperature expected inside and outside the system
      • Viscosity
      • Chemical Stability
      • Flash Point
      • Fire Point
  • Viscosity - Is internal resistance to flow. It increases as temperature decreases.
    • A liquid that is to thick can lead to power loss and higher operating temperatures.
    • A liquid that is to thin can lead to rapid wear of parts which move or are subject to heavy loads, due to excessive friction.
    • The Viscosity of a liquid is measured with a viscosimeter or viscometer.
      • The one most often used in the US is the Saybolt universal viscosimeter.
  • Chemical Stability - The ability of the liquid to resist oxidation ad deterioration for long periods.
    • Excessive temperature greatly affects the life of a liquid.
      • Localized hot spots occur:
        • On bearings
        • Gear Teeth
        • At the point where liquid under pressure is forced through a small orifice.
      • Liquids may break down if exposed to:
        • Air
        • Water
        • Salt
        • other impurities
  • Flash Point - The temperature at which a liquid gives off vapor n sufficient quantity to ignite momentarily (flash) when a flame is applied.
    • A high flash point is desirable for hydraulic fluids because it indicates a god resistance to combustion and a low degree of evaporation at normal temperatures.
  • Fire Point - The temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame.
    • A high fire point is desirable in hydraulic fluids.
  • Types of Hydraulic Fluid - the only fluid that should be used is the type specified in the aircraft manufacturer's maintenance manual or on the instruction plate affixed to the reservoir or unit being serviced
Type Fluid MIL Spec Color Material Made of Compatible w/ Flushed w/
Vegetable-Base Fluid MIL-H-7644 Blue Caster Oil and Alcohol Natural Rubber Alcohol
Mineral-Base Fluid MIL-H-5606 Red Kerosene Petroleum Neoprene Naptha, Varsol or Stoddard solvent
Synthetic Fluid MIL-H-8446 Light Purple Phosphate Ester Butyl Rubber or ethylene-propylene elastomers Trichlorethylene
  • Basic Hydraulic Systems - Hydraulic system is much like an electrical system. It must have source of power, a means of transmitting this power, and finally some type of device to use the power, a means of transmitting this power, and finally some type of device to use the power.
    • Open Hydraulic Systems - has no practical application to airborne systems.
    • Close Hydraulic Systems - An enclosed system
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