A key element in fluid systems is the means of transmitting power from one location to another.
Constructing a hydraulic or pneumatic system involves the design or selection of numerous components, and determining how they will all interact. A point that is often overlooked is that the fluid, and the means of moving it from one location to another, are critical in any fluid-power system.
Because the functions of hydraulic fluid are rather basic -- to transmit power efficiently and lubricate moving parts, with low maintenance -- it is often taken for granted. But using the wrong fluid, or not maintaining it properly, means less than optimum performance, and can even destroy a system. Likewise, dirty air can foul compressors, jam valves, and ruin instrumentation.
In addition to fluids, the various lines that transmit fluid -- tubing, hose, and connectors -- and the components that keep the fluid in good working order -- heat exchangers, filters, lubricators, and dryers -- are essential ingredients to fluid-power systems.
Petroleum-based fluids (hydrocarbons) traditionally have been the principal choice for hydraulic work. This choice remains popular where there is no danger of fire, no possibility of leakage into spoilable industrial products, no wide temperature variations, and no environmental considerations.
Fire protection, however, may dictate selection of a nonpetroleum fluid, especially when a broken hydraulic line could spray fluid into an ignition source. Work environment may suggest a nonhydrocarbon fluid where a hydrocarbon fluid could spoil food-related products or pollute a river.
Hydraulic fluid must also be evaluated for general compatibility with the system. Fluids are divided into five classifications for purposes of evaluation: premium antiwear, standard antiwear, rust and oxidation-inhibited nonantiwear, water-based, and phosphate esters. Also, standard antiwear and nonwear fluids are ranked according to their stability properties.
Premium antiwear fluids are recommended for systems using both piston and vane equipment. The fluids permit operation at full catalog rating with maximum life expectancy. Seal materials usually considered for these fluids are Buna-N or Viton-A; temperature range is 0 to 160°F.
Standard antiwear fluids are somewhat less stable than the premium fluids because they contain more additives. The fluids can be used with piston pumps, but speed must be limited to 1,800 rpm, temperature to 150°F, and pressure to 3,000 psi.
Vane equipment, on the other hand, can be used at maximum catalog ratings with maximum life expectancy. Recommended seal materials are Buna-N or Viton-A ; temperature range is 0 to 160°F.
This class of fluids includes a less-stable group. The group contains fluids such as crankcase oils for gasoline and diesel engines. The same operating restrictions apply, but no stability criteria are given. Hence, systems using these fluids should have special controls for filtration, water contamination, foaming, and temperature.
Nonantiwear, rust and oxidation-inhibited fluids are also divided into stable and less-stable types. The stable fluids, such as turbine oils, are the preferred fluids for piston equipment. The fluids permit operation at full catalog ratings with maximum life expectancy.
These fluids can be used with specially designed severe-duty vane equipment but at reduced ratings and life. Recommended seal materials are Buna-N or Viton-A; temperature limits are 0 to 160°F.
The less-stable class of these fluids includes tractor and transmission fluids. They are suitable for use in piston equipment at full catalog rating, but life expectancy is difficult to predict. Again, the fluids should be used only with specially designed vane equipment at reduced ratings and life expectancy.
Water-based fluids include invert emulsions and water glycols. Piston pumps operate satisfactorily on these fluids but have certain limitations. For instance, absolute inlet pressure should be about 25% higher than with petroleum-based fluids, and minimum inlet pressure is 13 psia. Because these fluids have lower lubricity, operating pressure is limited to about 3,500 psi, and speed to 1,800 rpm.
Only specially designed severe-duty vane pumps can be used with these fluids. Again, absolute inlet pressure should be 25% higher than with petroleum-based fluids, and minimum inlet pressure is 13 psia. Recommended seal materials are Buna-N or Viton-A. Operating temperature range is 50 to 120°F.
Phosphate esters can be used with piston and vane equipment at full catalog ratings. However, absolute inlet pressure should be about 35% higher than for petroleum-based fluids. Viton-A seals are compatible with most phosphate esters; however, some of these fluids require EPR seals. Temperature range is 0 to 160°F.
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