Dedendum (gears): the distance between the pitch circle and the lower working,
or flank, half of the tooth that still has the involute tooth form.
Degras (pronounced “de-grah”): animal oil extracted from the skin or wool of
sheep, typically used to control corrosion.
Degreasing: the cleaning of grease and oil from metal parts in a machine
designed to expose the metal parts to a liquid, a vaporized solvent or a special
cleaning detergent (see vapor degreasing).
Demulsibility (typically measured using ASTM D-1401 or D-2711): the ability
of a fluid insoluble in water to separate from water after thorough mechanical
Demulsifiers: additives that promote separation of oil from water.
Density: mass per unit volume.
Detergent (oils): a metallic salt additive used in engine oils to keep insoluble
particles in colloidal suspension and prevent the formation of deposits and rust.
With dispersants, detergents also remove existing surface deposits.
Detergent (cleansers): detergents in cleansers are surface-active compounds that
lower the surface tension of water or water solutions and impart emulsifying and
dispersing properties to them.
Dewaxing: a refinery process that removes paraffin wax from lubricating oils to
lower their pour points.
Diagonal passages (lube systems): passages connecting the inlet and discharge
Diametral pitch (gears): sometimes simply called “pitch”; the measure of tooth
size, equal to the number of teeth divided by the pitch diameter. Mating gears
have the same diametral pitch.
Dielectric strength (ASTM D-877): measures the capacity of an insulating
material to withstand electric stress (voltage) without failure. Fluids with high
dielectric strength (usually expressed in kV) are good electrical insulators.
Diester (dibasic acid ester): a synthetic lubricant formed by reacting
dicarboxylic acid with an alcohol, having high viscosity index and low volatility.
With additives, it finds service in compressors, internal combustion engines and
fluid power systems.
Differential (chassis): a set or train of gears that change the direction of the
vehicle propeller shaft to that of the axle shafts; also adjusts the amount of
rotation between the right and left wheels on a particular driven axle to prevent
wheel skidding when turning a corner.
Differential pressure: in an orifice meter, the difference between the pressures
on the upstream and downstream sides of the orifice; also describes the pressure
drop across a filter that increases as the filter clogs.
Diluent (“cutback solvent”): instead of heating, a solvent added to viscous
lubricants or compounds to permit application in cold weather. The solvent
evaporates after application, leaving the lubricant in place (see cutback solvent).
DIN (Deutsche Institute fur Normung): the German equivalent of ASTM.
Dip feed lubrication: a method that lubricates rubbing surfaces by dipping or
partially submerging them in lubricant.
Direction valves (hydraulics): devices that channel the fluid in a hydraulic
system to the proper location and/or prevent it from going to the wrong
Discharge passage (lube systems): passage leading from between the lands of
the inlet piston bore.
Disk filter/perforated disk: a system utilizing metal disks as the filtering
medium; frequently termed “metal disk filter”.
Dispersant: a non-metallic engine oil additive that helps to prevent sludge,
varnish, etc., by keeping particles suspended in a colloidal state. Similar to and
ordinarily used with detergents, dispersants are capable of keeping large
quantities of particles in suspension, and they are ashless when burned.
Distillation (fractionation): the first step in separating crude oil into its various
components uses a distillation tower, or pipe still, through which heated crude
oil vapors rise to progressively cooler levels, so that the various hydrocarbons
condense at different levels all the way to the top of the column. The lower
boiling point and lighter weight fractions rise to the top, the highest boiling point
and heaviest condense near the bottom, all others in between. Gases, light oils
and fuels are drawn off at the top, while heavy products like heavy fuel oil and
asphalt are drawn from the bottom, with other products in between. This step is
conducted at atmospheric pressure. Vacuum distillation, mainly for lube stocks,
is performed with heavy crudes or bottoms at sub- atmospheric pressure,
permitting fractionation at lower temperatures. The still is called a vacuum
Distillates: the lubricant and oil fractions produced in a distillation column,
except for bottoms and the natural gas liquids at the top of the column-dividing
Divider valve (lube systems): a series-progressive lubricant-metering and
distribution assembly containing an inlet section, at least three valve sections and
an end section.
DN factor: used as a guide to lubricant selection for rolling contact bearings, it is
also called a speed factor, the product of the bore of a rolling contact bearing,
expressed in mm (D), and the speed in rpm. Values up to 300,000 permit use of
normal NLGI 2 grease; higher values indicate fluid oil or specially formulated
greases, and values in the 1,000,000 range require oil-mist or air-oil lubrication or
specially formulated greases.
Double helical gears/herringbone gears (gears): have both right-hand and left-
hand helical teeth, and operate on parallel axis; are used on all mill pinions.
Drawing compound: a compound, usually containing EP additives, used during
metal forming at the surface of the die to improve die life and metal finish; also
used in dies in wire mills.
Drop-feed lubrication/drip oiler: a system of lubrication that supplies lubricant
to the bearing surfaces in the form of drops at regular intervals.
Dropping point (grease) (ASTM D-255 and D-2665): the temperature at which a
portion of grease releases liquid or passes from a semisolid to a liquid state
under specified test conditions. Though this test is a good high-temperature
screening tool, it is not, by itself, an indicator of high-temperature performance;
continuous permissible operating temperatures may be as much as 100°F below
the dropping point.
Drying oils: oils that absorb oxygen (reacting with it) to form relatively hard,
tough, elastic films when exposed in thin layers to the atmosphere; generally
added to paint to promote drying (e.g., linseed oil).
Drying film lubricant: a solid material, such as graphite, molybdenum
disulfide, boron nitride, or a plastic like a poly-tetra-fluorine resin, used with
loads in the boundary region of lubrication. These materials may be applied as
pastes, by spraying, dipping, brushing in an air-drying carrier, burnishing or
DSC/DTA: differential scanning calorimetry (DSC) and differential thermal
analysis (DTA) measure actual caloric heat changes to characterize physical
changes (phase changes in waxes, gels, grease or asphalt) and chemical reactions
(usually oxidation) in lubricants.
Dynamic viscosity: see absolute viscosity.
Dynamic demulsibility: refers to a test procedure that simulates temperatures
and circulating conditions in a rolling mill to determine the water separation
properties of an oil (see demulsibility).
Dynamic load (gears): load computed at the pitch line, including both static
transmitted load and loads superimposed by inertia of the rotating masses, tooth
form inaccuracies, spacing inaccuracies and misalignment. Buckingham’s
empirical equations take these forces into account: AGMA uses service factors for
the same purpose.
Dyne: standard c-g-s unit of force, equal to the force that produces an
acceleration of one centi-meter per second per second on a mass of one gram.