Hydraulic Fluid – A fluid serving as the power transmission medium in a hydraulic system. The most commonly used fluids are petroleum oils, synthetic lubricants, oil-water emulsions, and water-glycol mixtures. The principal requirements of a premium hydraulic fluid are proper viscosity, high viscosity index, antiwear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials. Antiwear oils are frequently used in compact, high-pressure, and high-capacity pumps that require superior lubrication protection. Certain synthetic lubricants and water-containing fluids are used where fire resistance is needed. Synthetic lubricants also are used in extreme-temperature conditions.
Hydraulic System – A system designed to transmit power through a liquid medium, permitting multiplication of force in accordance with Pascal’s law, which states that “a pressure exerted on a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas.” Hydraulic systems have six basic components: (1.) a reservoir to hold the fluid supply; (2.) a fluid to transmit the power; (3.) a pump to move the fluid; (4.) a valve to regulate pressure; (5.) a directional valve to control the flow, and (6.) a working component such as a cylinder and piston or a shaft rotated by pressurized fluid to turn hydraulic power into mechanical motion. Hydraulic systems offer several advantages over mechanical systems: they eliminate complicated mechanisms such as cams, gears, and levers; are less subject to wear; are usually more easily adjusted for control of speed and force; are easily adaptable to both rotary and liner transmission of power; and can transmit power over long distances and in any direction with small losses.
Hydrogenation – In refining, the chemical addition of hydrogen to a hydrocarbon in the presence of a catalyst; a severe form of hydrogen treating. Hydrogenation may be either destructive or non-destructive. In the former case, hydrocarbon chains are ruptured (cracked) and hydrogen is added where the breaks have occurred. In the latter, hydrogen is added to a molecule that is unsaturated with respect to hydrogen. In either case, the resulting products are highly stable. Temperatures and pressures in the hydrogenation process are usually greater than in hydrofining.
Hydrolytic Stability – The ability of a lubricant to resist chemical decomposition (hydrolysis) in the presence of water.