Downloaded from https://www.everyspec.com FM 1-509 in degree of fluidity. Liquids cannot expand indefinitely to fill containers as gases do. Liquids have three physical qualities: Incompressibility. For practical purposes, liquids are incompressible. Even under extremely high pressure, a liquid cannot be made smaller. The brake system in a car takes advantage of this physical law. When you push the pedal, the brakes are applied instantly because no time is lost in compressing the liquid. Consider how much longer the brake stroke would be if the master cylinder contained air. Expansion and contraction. Liquids expand (become larger) and contract (become smaller) with changes in temperature.
When a liquid in a closed container is heated, it expands and puts pressure on the walls of the container. As the liquid cools, the pressure decreases. Pressure transmission. Pressure applied to a confined liquid is transmitted equally. Again, a car’s brake system shows how this principle works. When a force (a foot on the brake pedal) is applied to a liquid in a closed container (the car’s master cylinder), the resulting pressure is sent out equally in all directions; that is, the same amount of pressure is exerted on each wheel of the car. Types. Hydraulic fluids are classified generally as petroleum base, synthetic base, and vegetable base.
Vegetable-base fluid is no longer authorized in Army aircraft. For most operations, the Army is converting from petroleum-base fluid MIL-H-5606 to MIL-H-83282, which has a synthetic hydrocarbon base. There are several reasons for this change. MIL-H-83282 contains additives that provide better antiwear characteristics and help stop oxidation and corrosion. It also has an operational high-temperature limit of 400 o F as compared to 275°F for MIL-H-5606. Flash point, fire point, and spontaneous ignition temperatures of MIL-H-83282 exceed those of MIL-H-5606 by more than 200°F. Also, tests show that MIL-H-83282 stops burning when the outside source of flame or heat is removed.
MIL-H-83282 is compatible with all materials used in systems presently employing hydraulic fluid MIL-H-5606. It may be combined with the latter fluid with no bad effects except that it may reduce the fire-resistant properties of MIL-H-5606. However, amounts of MIL-H-5606 exceeding 3 percent by volume will compromise the fire-resistant performance of MIL-H-83282. Although MIL-H-83282 exceeds the performance of MIL-H-5606 at normal temperatures, the viscosity of MIL-H-83282 increases at low temperatures. For this reason, MIL-H-5606 is still used for cold weather operations. Petroleum-base fluid MIL-H-6083 is also authorized in Army aircraft. This fluid is supplied in only one grade.
It should be used as a preservative oil in shock struts, hydraulic equipment, and spare parts, and as a testing and flushing oil for some hydraulic components. MIL-H-6083 should not be used in composite aircraft hydraulic systems. NOTE: Use only the fluid authorized for a particular component or system. To determine the correct fluid, consult the applicable 1-2