Page 335 - Mechanical Engineer's Data Handbook

GLOSSARY OF TERMS 323

occurring during observations over a continuous ible only by itself and 1, e.g. 2, 3, 5, 7, 11, 13, . . ., 37,
interval. A limiting form of 'binomial distribution'. . . ., 5521, etc.
PoiBson's ratio The ratio of transverse to axial strain principal stresses Normal stresses on three mutually
in a body subject to axial load. perpendicular planes on which there are no shear
polar modolus The polar second moment of area stresses.
about an axis perpendicular to the area through the probability The number of ways in which an event
centroid divided by the maximum radius. can happen divided by the total possibilities. Symbol:
polar second moment of area The second moment for P.
an axis through the centroid perpendicular to the proof stress The stress to cause a small specified
plane. It is equal to the sum of any two second permanent set in a material.
moments of area about perpendicular axes in the proportioual Limit The maximum stress at which
plane. strain is directly proportional to stress.
polymer A material built up of a series of smaller pump A machine driven by a prime mover which
units (monomers) which may be relatively simple, e.g. delivers a fluid, pumping it to a greater height,
ethane, or complex, e.g. methylmethacrylate. The increasing its pressure, or increasing its kinetic energy.
mechanical properties are determined by molecular Main types: rotodynamic, positive displacement.
size ranging from a few hundred to hundreds of punch A tool that forces metal into a die during
thousands. blanking, coining, drawing, etc. The process is called
polynomial An algebraic expression of the form 'punching'.
ax"+bx"-'+cx"-2 . . . px+q. push fit A fit similar to a 'snug' or 'slip' fit defined by
polypbase Said of a.c. power supply circuits, usually several classes of clearance in British and other
3 phase, carrying current of equal frequency with standards.
uniformly spaced phase differences. pyrometer Device for measuring temperatures above
polytropic process A gas process obeying the law the range of liquid thermometers.
PO" =constant, where p = pressure, v = volume, n =in-
dex ofexpansion not equal to 1 or ?, the ratio of specific
heat capacities. quenching The rapid cooling of heated metal to
pdtive displacement pump A pump which displaces anneal, harden, etc.
a 'positive' quantity of fluid each stroke or revolution,
e.g. piston pump, gear pump, vane pump. rack and pinion gear A device for changing linear to
powder metallurgy The production of shaped objects rotary motion, and vice versa, in which a circular gear,
by the compressing of metal powders ranging in size or pinion, engages with a straight toothed bar or rack.
from 0.1 to 1OOOpm. radial clearaoce Half the diametral clearance. The
power The rate of doing work. Unit: watt (W). difference between the radius of a circular hole and a
power cycle A thermodynamic cycle in which net rod or shaft fitting into it.
power is produced, e.g. Otto cycle. radial stress The component of stress in a radial
power factor The ratio of total power dissipation in direction in pressurized cylinders, rotating disks, etc.
an electrical circuit to the total equivalent volt- radiatioa of heat A process by which heat is transfer-
amperes applied to the circuit. red without the aid of an intervening medium.
pres A machine tool with a fixed bed and a guided radius of gyration The imaginary radius at which the
reciprocating, usually vertical, ram. mass of a rotating body is assumed to be concentrated
press fit An interference or force fit made through the when determining its moment of inertia.
use of a press. The process is called 'pressing'. rake The angle of relief given to faces of a cutting tool
pressure At a point in a fluid, pressure is the force per to obtain the most efficient cutting angle.
unit area acting in all directions. That is, it is a scalar Rankine cyck An idealized steam cycle consisting of:
quantity; e.g. in a cylinder with a piston, pressure p is pumping water to boiler pressure, evaporation,
the force on the piston divided by the cylinder area. adiabatic expansion to condenser pressure, and com-
pressure trPasdllcer A device which produces a, plete condensation to initial point.
usually electrical, signal proportional to the pressure. Rankine etfifieocy The thermal efficiency of a
prime number A natural number other than 1 divis- Rankine cycle under given steam conditions.