• Hydraulic fluids play two roles in the mechanisms you put them in. One is to help lubricate the parts they come in contact with in the pumps that circulate them and the motors they move through. The other, equally important function is to transfer energy within the system, moving the parts. One example of this is the hydraulic clutch in a car's automatic transmission. The clutch transfers power from the engine to the gearbox to move the wheels. A hydraulic clutch does it by circulating fluid instead of pressing spring-loaded parts together.
  
  

Three Classes

• Hydraulic fluids come in different types, based on what additives are put in to affect the system's performance. But they break down into three basic classes: conventional petroleum or other mineral-based and synthetic oils, fire-resistant fluids engineered for high heat applications and specially formulated aviation fluids used in military and civilian aircraft.
  
  

Basic Fluids

• The first group of hydraulic fluids come in six forms, starting with the most basic, with the designation HH. These are hydraulic mineral oils with no special characteristics from additives. The HL fluids have antirust and antioxidation additives to help preserve the metal parts they contact. An HM designation means that the fluid is formulated to improve the life of the mechanism by reducing wear in addition to its antioxidation properties. HR oils have been formulated for better viscosity (density) along with the other factors. HG oils are the HM formulas with the addition of antistick chemicals. HS is the designation for synthetic fluids.
  
  

Fire-Resistant Fluids

• Most machines that take hydraulic fluid, be they cars and trucks, agricultural machinery or industrial machines from forklifts and cranes to industrial presses, use conventional, mineral-based hydraulic fluids. The exceptions are those machines that are subject to fire hazards. They use one of three types of fire-resistant hydraulic fluids: high water fluids, water glycols and synthetics. These have three- or four-letter designations. High-water fluids have oils or synthetic chemicals and are 80 to 95 percent water. They are recommended for use in machines running at between 120 and 32 degrees F.
  
  Water-glycol fluids are made up of synthetic chemicals related to antifreeze (ethylene glycol) in a mix of 35 to 45 percent water, with a thickening agent. They can also contain additives to prevent foaming, rust and corrosion and to add to their lubricant qualities.
  
  The synthetic fire-resistant fluids can be either full synthetics or a mix of synthetic chemicals and petroleum-based oil. They are engineered to function at higher temperatures and higher pressures than the water-based fluids but they are not good for low-temperature applications.
  
  

Aviation Fluids

• Aircraft hydraulic fluids can be either petroleum-based or synthetics, made from phosphate esters. Because they operate at high altitudes, aircraft fluids have to flow at temperatures from -65 degrees F all the way up to 475 degrees F. The military uses several grades of petroleum-based fluids in their aircraft, all of which have a "mil" designation at the beginning and a four-digit number at the end. Commercial aircraft may also use synthetic fluids such as Exxon Mobil's HyJet IV-A Plus or Solutia's Skydrol brand.