Half bearing: bearing that surrounds only one-half of a journal, e.g., the AAR journal bearing, available as upper or lower arch bearing; also known as 180° arch bearing. All nonsleeve bearings are designated as partial journal bearings.

Halogenated solvents: solvents formulated with one of the halogens, usually chlorine or fluorine. Halogenated solvents are associated with the atmospheric depletion of ozone.

Hardness – resistance of metal to plastic deformation, usually by indentation. It also includes resistance to scratching, abrasion or cutting. It is the property of a metal, which gives it the ability to resist being permanently deformed when a load is applied.

Head (hydraulics): the pressure exerted by a fluid on a unit area because of the height of the surface of the fluid above the point where pressure is measured; may be expressed as psi or “feet”.

Heat exchange: a device that transfers heat from one fluid to another (see cooler, intercooler). Oil coolers are common in mill circulating oil systems that use water to cool hot oil from the return reservoir before returning it to the mill units.

Heating element: a submersible oil heater, often utilized for heavy oil storage tanks. Since excessive temperatures of such heating elements can degrade lubricants, they are sometimes derated in watts/sq in. and placed in sealed wells to avoid this problem.

Heat transfer oils: oils utilized as heat transfer media; typical applications include asphalt plants, grease plants, etc. These oils have low volatility and contain additives to inhibit cracking and sludging.

Heavy-duty engine oils: oils having oxidation stability, bearing corrosion preventive properties and the detergent dispersant characteristics necessary to make them generally suitable for use in both high-speed diesel and gasoline engines under heavy duty service conditions.

Helical gear (gears): cylindrical in form with helical teeth.

Helix angle: angle of inclination to the axial direction, equivalent to the angle between the normal plane through the “twisted tooth” and the transverse plane of rotation, symbolized by ß.

Herringbone gears (gears): two helical gears on the same shaft, one with a right- hand helix and the adjacent one with a lift-hand helix, giving the appearance of a herringbone.

Hertz contact stress: stress (psi) measured along a narrow band of contact between two gear teeth under load; based on formulas devised by Hertz. Momentary stresses exceeding 200,000 psi are not uncommon in heavily loaded gears. Also may occur at contact points in rolling element bearings.

Hexane: a straight chain paraffinic hydrocarbon containing six carbon atoms (C6H12); useful as a solvent.

High-pressure indicator(s) (lube systems): various divider valve accessories that provide visual indications of line blockage or locked pistons. Some also provide relief to atmosphere, permitting the system to continue lubricating unaffected points; others that are sealed require immediate system shutdown and repair.

High solvency naphthas: special naphthas characterized by their high solvent power (low precipitating tendency) for various resins, oils and plastics; their high aromatic content promotes solvency (see Kauri-butanol/KB value).

High water-content/high water-base fluids: (abbreviated HWBF) fire-resistant, thickened or unthickened water-based fluids; typically containing 90-99% water.

Horsepower: unit that measures the rate at which work is done and rates power output; one hp is equal to 746 watts of electrical energy, or the energy required to lift 33,000 pounds, one ft in one min.

Hot plate/crackle test: qualitative test to determine the presence of entrained water in a high flash-point oil; when oil is dripped onto a hot surface, an audible “crackle” indicates the presence of water.

Humidity cabinet test (ASTM D-1748): metal panels are installed in hot, moist controlled cabinet to test rust preventives; quality is measured by the number of hours preceding initial corrosion.

Hyatt bearing (flexible bearing): roller bearing with rollers constructed of flexible coils of strip steel that are hardened, then ground to size; designed to withstand considerable deflection in the alignment of shaft or housing.

Hydrated grease/soap: scap grease one of whose structural components is water, e.g., a water-stabilized calcium soap grease that owes its stability to hydrated calcium soap.

Hydraulic oil: low-viscosity mineral oil for hydraulic and/or fluid power systems, generally less than 500 SSU at 100°F. Hydraulic (fluid power) system: see fluid power system.

Hydrocarbon: a compound composed of carbon and hydrogen, the basic element of countless organic compounds and the principal constituents of petroleum. Hydrocarbons in petroleum fall into three basic categories: the straight-chain paraffins (aliphatic); the cyclic (ring) chain (naphthenic) and the cyclic chain (aromatic).

Hydrodynamic lubrication: often referred to as “full film lubricant”; a film of oil or other fluid completely separates moving components under load, thus maintaining low friction and eliminating wear.

Hydrometer: an instrument for determining the specific gravity or the API gravity of a liquid.

Hydrophilic: having an affinity for water; capable of uniting with or dissolving in water (see hygroscopic, hydrophobic).

Hydrophobic: lacking affinity for water: incapable of uniting or mixing with water.

Hydrostatic lubrication: system of lubrication that supplies the lubricant under high pressure, to promote a fluid film between opposing surfaces.

Hygroscopic: capable of attracting or absorbing moisture.

Hypoid gear lubricant (gears): lubricant with extreme pressure (EP) characteristics for use with hypoid gears, e.g., in the differential of an automobile.

Hysteresis (hydraulics): a condition encountered particularly when dealing with servo or proportional valves, in which a given input signal produces different amounts of spool movement when increasing and decreasing. It is caused by unevenly distributed friction.