Packing: deformable substance used for sealing between locations at which
fluids are present under different conditions, usually where relative motion
occurs at the boundary between the fluids.
Packing box: the portion of the casing or cover through which the shaft extends
and into which a seal or packing is placed to limit leakage; also known as a
Packing gland: an adjustable follower that compresses packing in a stuffing box.
Packing gland assembly (lube systems): assembly that is screwed into the
measuring valve body.
Pad lubrication: see oil pad.
Panel coker: a testing device that involves dripping cold fluid onto a hot panel
to determine the detergency and deposit-forming tendencies of the test fluid.
PAO: see polyalphaolefins.
Paraffin wax: a high-VI crystalline substance removed from paraffinic crudes
after distillation, composed of unbranched straight chain hydrocarbons that are
solid at room temperature. Waxes are primarily used for water proofing and
candles; in small quantities, they degrade the low-temperature properties of
Paraffinic base: characterizes certain petroleum products prepared from
paraffinic crudes (crudes that contain high percentages of straight chain aliphatic
or paraffin hydrocarbon molecules).
Partial bearing: see journal bearing.
Particle count: the object of various test procedures employed in condition
monitoring. The ISO Solid Contaminant Code rates the number of particles (per
volume) larger than five microns (silting condition) and the number of particles
(per volume) larger than 15 microns (presence of wear material); the two rating
numbers are separated by a slash. The ISO standard supersedes most other
methods, but some utilize ISO codes to report particles larger than two microns.
Pascal’s Law: axiom stating that the pressure on a confined fluid is transmitted
undiminished and with equal force to all equal areas of the container.
Penetration (grease): (ASTM D-217) the depth, in tenths of a millimeter, that a
standard cone penetrates a semisolid sample under specified conditions. Test
methods include undisturbed (sample tested in its container); unworked (sample
transferred to worker cup); worked 60 X (transferred to worker cup and worked
with 60 strokes); prolonged worked (worked more than 60 X) and block (sample
of block grease cut into a cube) (see consistency).
Penetrating oil: usually a solvent based oil; loosens rusty nuts or bolts by
penetrating the rust barrier, thereby facilitating disassembly without destruction.
Penetrometer: instrument for measuring the penetration of semisolid substances
Pensky-Marten Closed Cup test: closed cup test for determining the flash point
of fuel oil or open gear lubricant; sometimes used for lubricants suspected of
being contaminated with fuel or solvent.
Pentane insolubles: see insolubles.
Petrolatum: product made from the residuals of paraffinic crudes, consisting
primarily of high molecular weight amorphous waxes, with some grades
containing microcrystalline waxes. It is pale to yellow in color, with oily or
grease-like characteristics, used in some lubricants and rust preventives or a lay-
up lubricant for some kinds of wire rope.
Petrolene: a petroleum naphtha containing asphalt, used in protective coatings.
Petroleum: oily liquids or semisolids found in the earth, composed of
hydrocarbons and primarily such nonmetallic elements as sulfur, oxygen,
nitrogen, etc. Though the composition of these dark, highly complex mixtures
varies, they are often lighter than water and highly flammable. Only a small
percentage of crude petroleum can be processed for lubricants.
Petroleum spirits: solvents obtained from petroleum with boiling ranges from
300-400°F and flash points exceeding 100°F (see mineral spirits).
Phenols: a class of aromatic chemicals used chiefly as antioxidants in lubricating
oils like hydraulic fluids and circulating oils. Because of its biotoxicity, the EPA
prohibits discharge of the parent chemical (“free phenol” or C6H5OH) into
waterways; therefore, most phenols utilized in lubricants are sterically hindered.
However, some refining extraction processes still use free phenol to remove
aromatic, naphthenic and unsaturated hydrocarbons from lube base stocks.
Phospate esters: a class of synthetic esters with superior fire resistance; used
primarily as FR fluids, they are formulated with these general properties: specific
gravity greater than one, good lubricating capability, fair high-temperature
stability, poor hydrolytic stability, and poor viscosity-temperature linkage.
Though they are harmful to paints and some seal materials, one such ester,
tricresyl phosphate, has long been used as an anti-wear additive in lubricating
Pillow block: denotes bearing support on a site other than the machine itself.
Pilot-operated: in hydraulics, the technique of using a small valve to control a
much larger one.
Pinion: the smaller of two mating or meshing gears, usually the driving gear. In
the steel industry, the term “mill pinions” describes a mating pair of gears in a
one-to-one ratio, each of which is coupled to a mill roll, one above the other in
the mill stand; employed in both unidirectional and reversing mills, they are
driven by a mill motor and mill drive coupled to the pinion stand.
Piston (lube systems): sliding part contained in the cylinder of the injector,
consisting of a rod, extension and packing.
Piston stop plug (lube systems): the lower portion of the adjusting assembly.
Piston rings: used in engines to maintain a gas-tight seal between piston and
cylinder, to assist in cooling the piston and to control cylinder wall lubrication;
the three rings include a fire ring, a compression ring and an oil ring.
Pitch (gears): used in gear geometry to characterize features governing tooth
size, shape, spacing, etc.; common terminology includes pitch circle, pitch
diameter, pitch point, normal circular pitch and normal diametrical pitch (see
Pitch circle (gears): curve where the imaginary pitch cylinder and plane normal
to the axis of rotation intersect.
Pitch diameter (gears): diameter of the pitch circle of mating gears in imaginary
line contact along the centerline between the two shafts.
Pitch line: corresponds in the cross-section of a rack to the pitch circle in the
cross-section of a gear.
Pitch line velocity (gears): linear speed at the pitch line, measured in fpm or
Pitch point (gears): point of tangency of the two pitch circles of the mating
gears, lying on the common centerline between them.
Pivot bearing: axial-load, radial-load bearing that supports the end of a shaft or
pivot (as on the balance wheel of a watch).
Pivoted pad bearing: an axial or radial-load bearing with a surface consisting of
one or more pads or shoes pivoted to tip, thereby promoting the establishment of
a hydrocarbon film.
Plain bearing: any simple sliding bearing, as distinguished from fixed pad,
pivoted pad or rolling bearings. Depending on the direction of the load on the
bearing surface, plain bearings are classified as guide bearings, journal bearings
or thrust bearings.
Planetary gear: a train of internal gears consisting of a sun gear, to which input
power is applied, and planet gears that give the output power.
Plunger (lube systems): slide valve that controls the valve port.
Plunger spring (lube systems): spiral spring in the injector body cylinder.
Poise: the standard unit of absolute viscosity in the cgs system; expressed in
Polar compounds: chemical compounds whose molecules exhibit positive
electrical charges at one end and negative charges at the other. This
characteristic, known as “polarity”, endows such compounds with an affinity for
metal surfaces. As lubricant additives, they serve as “oiliness agents”; they have
good metal-wetting properties and some polar compounds promote
emulsification between water and oil.
Polyalphaolefins (PAOs): a class of synthetic lubricant bases formed by
polymerization of an olefin monomer, such as ethylene or propylene, whose
properties after polymerization include good oxidation stability at high
temperatures, good hydrolytic stability, compatibility with mineral oils and low
volatility. They have found service in turbines, gears, compressors and
Polybutene: synthetic lubricating oil, a polymer of butene (C4H8); principal uses
include insulating oils, gas compressor oils and process oils in the aluminum
Polyglycol: a polymer of ethylene glycol (C2H602) used as a synthetic base stock;
water-soluble polyglycols serve as thickeners or anti-freezes in FR fluids;
insoluble forms are used as heat transfer and hydraulic fluids or high-
temperature bearing oils.
Polyesters: synthetic resins, usually obtained from polymerization of a dibasic
acid with a dihydric alcohol, not usually used as lubricant stocks. (see diesters).
Polymers: organic compounds created by polymerization that become
progressively heavier and acquire diverse properties as the multiple linkages
increase. The original monomer may be a gas or a liquid; according to the extent
of polymerization, the final product will be a high molecular weight liquid or
solid that retains the same proportion of elements as the original monomer.
Polymerization: the chemical combination and recombination of the same
unsaturated hydrocarbon with itself to form an extensive chain; the chemical
process of combining similar molecules to form larger molecules.
Polyolesters: a class of synthetic esters formed by reacting fatty acids with a
polyol such as glycol; physical properties vary according to the polyols and acids
used. Polyolesters formulated as lubricants have low volatility and good
oxidation stability at high temperatures: they are used as base oils for turbines,
compressors, jet engines and automotive engines and as base fluids for certain
Polyureas: polymeric thickeners for grease, made from isocyanates and amines.
Greases thickened with polyureas have high oxidation resistance and high
dropping points; they work well in ball bearings for electric motors.
Porous bearing: bearing made from porous material such as compressed metal
powders; the pores serve as reservoirs or passages for lubricant.
Positive displacement oil pumps: vane, gear or piston pumps that build up
high pressure on the discharge side because the capacity output of the pump is
positive. If the discharge is not utilized, the oil pressure regulator or by-pass
prevents damage (see controlled volume pump, gear pump).
Pour point: (ASTM D-97) the lowest temperature at which a lubricant will pour
or flow under specified conditions.
Pour point depressant: an additive in lubricating oil that lowers the pour point,
by preventing any wax present from crystallizing to form a solid mass.
ppm: parts per million.
Precipitation number: (ASTM D-91) the number of milliliters of solid matter
precipitated from a mixture of oil and petroleum solvent under specified
conditions; chiefly used to determine the presence of asphalts in semi-refined or
black oils, or to examine sludge in used oils.
Precision: see tolerance.
Preloading: procedures employed during assembly and mounting to remove all
looseness or play in a bearing, usually performed on shafts or spindles in
machine tools and precision machines that must rotate without clearance in
either the axial or radial direction. Preloaded bearings are not used where
deflection is excessive.
Pre-lubed bearings: bearings lubricate by the manufacturer to preserve their
integrity during storage.
Pressure angle (gears): angle between the line of action and a line tangent to
both pitch circles. This angle remains constant with involute form teeth at any
point in the contact path. Common pressure angles are 14.5° and 20°; when
stronger teeth are needed, larger angles are used. Pressure angles increase with
Pressure, atmospheric: see normal pressure.
Pressure control valves: devices that control the pressure in a hydraulic system,
including relief, unloading, counter balance, sequence and pressure-reducing
valves and, occasionally, brake valves.
Pressure drop: loss of pressure caused by restriction in a hydraulic system,
where restriction includes valves, orifices and pipes; synonymous with “pressure
differential” or “upstream minus downstream pressure” across any device in a
Pressure, gauge, (psig): pressure differential above or below atmospheric
Pressure-reducing valve (hydraulics): device that keeps pressure in a branch of
a hydraulic circuit below the pressure in the remainder of the circuit.
Pressure viscometer/viscometer (grease): a capillary instrument used to
determine apparent viscosity.
Preventive and predictive maintenance (PM and PDM): two basic programs
that use selected features of condition monitoring procedures in managing
maintenance practices and costs to increase plant productivity; PM programs
schedule maintenance at regular intervals, while PDM programs schedule
maintenance on the basis of information obtained from sophisticated condition-
Priming: in pump operation, filling the liquid end of a pump with liquid to
remove vapors and eliminate the possibility of becoming vapor bound.
Principal reference planes (gears): pitch plane, axial plane and transverse plane,
all intersecting at a point and mutually perpendicular.
Process oil/process lubricants: in the steel industry, materials used in direct
contact with the product being produced, e.g., rolling oils in hot and cold rolling
mills, wire drawing compounds, forging compounds, slushing oils for rust
protection, stamping and drawing compounds, quenching oils, wire rope laying-
up lubricant, etc.
Profilometer: a device that profiles or measures surfaces to determine
Proportional valve (hydraulics): a hydraulic valve that produces an output
proportional to its input signal, that can be adjusted electronically, remotely; uses
proportional solenoids with constant force for a given signal.
Proximity switches (lube systems): magnetic (dry contact) switches that detect
divider valve piston movement without a cycle indicator pin attached to the
Pumpability: see mobility.
Pycnometer: a device for measuring densities of liquids.
Pyrolysis: chemical decomposition by the action of heat.
Pyrometer: a device for recording high temperatures that uses a thermocouple
or an infrared pyrometer to measure invisible light emitted by the hot object.