Page 334 - Mechanical Engineer's Data Handbook

322 MECHANICAL ENGINEER’S DATA HANDBOOK

mass m about a point P is equal to mk2 where k is the 0 ring A toroidal 0 section ring of a material such as
‘radius of gyration’ from P at which the whole mass Neoprene used as a seal.
may be assumed to be concentrated as a ring.
momentum The product of mass and velocity of a
body, i.e. mu. parabola A conic section of the form y2 = 4ax.
multi-pass heat exchanger A heat exchanger in which parallel-flow heat exchanger A heat exchanger in
one of the fluids makes a series of passes in alternate which the two fluids flow parallel to one another and in
directions. the same direction.
pattern A form made in wood or other material
around which a mould is made.
natural vibrations Free vibrations in an oscillatory peak value For a waveform the maximum value of a
system. half-wave. For a sine wave it is r=fi times the r.m.s.
nitriding Introducing nitrogen into solid ferrous (root mean square) value.
alloys by heating in contact with nitrogenous material, pendulum The ‘simple pendulum’ consists of a small
e.g. ammonia, cyanide. heavy mass or ‘bob’ suspended from a fixed point by a
non-destrctive testing Inspection by methods which string of negligible weight. Its periodic time for small
do not destroy a part, to determine its suitability for oscillations is 2n-, where L=length of string,
use. g = acceleration due to gravity. The ‘compound pen-
non-Sow energy equation The equation in ther- dulum’is any body which oscillates about a fixed point
modynamics for a non-flow process such as compress- a distance h from the centre of gravity with radius of
ing a gas in a cylinder. It states that the change in gyration k. It has an equivalent simple pendulum
‘internal energy’ of a substance is equal to heat length of (hz + k2)/h.
supplied minus the work done. perfect gas A gas which obeys the ‘gas laws’. A gas
weNewtonian Suid A fluid which does not obey the behaves as a perfect gas as the pressure is reduced.
viscosity law. See: coefficient of viscosity. permanent set Plastic deformation in a material that
notch A vee or rectangular cut-out in a plate restrict- remains after the load is removed.
ing the flow of water in a channel. The height of water Perry-Robertson formula A practical formula for the
above the bottom of the cut-out gives a measure of the buckling load for a strut.
flow. p-h chart A pressuresnthalpy chart used for refrig-
nozzle A convergent or convergent-divergent tube eration calculations.
through which a fluid flows. Used to produce a pH value Negative logarithm of hydrogen ion activ-
high-velocity jet. ity denoting the degree of acidity or alkalinity of a
NusPelt number A dimensionless quantity used ex- solution. At 25°C: 7 is neutral, a lower number
tensively in the study of heat transfer. Defined as indicates acidity; a higher number indicates alkalinity.
Nu = Qd/kO, where Q = heat flow to or from a body per pitch The linear distance between similar features
unit area, 8= temperature difference between the body arranged in a pattern, e.g. turns of a screw thread,
and its surroundings, k = thermal conductivity, distance between rivets in a row.
d = characteristic dimension of the body. pitch circle An imaginary circle on gear wheels on
nut A metal (or other material) collar internally which the teeth are constructed, a circle on which bolt
screwed to fit a bolt usually of hexagonal shape but holes, etc., are pitched, etc.
sometimes round or square. plain bearing A bearing consisting of a plain bush or
sleeve, as opposed to a ball or roller bearing.
plastic deformation Deformation that remains after a
oil seal A device used to prevent leakage of oil, e.g. load is removed.
from a bearing in a gearbox. plasticity The ability of a metal to deform non-
orifice A small opening for the passage of a fluid. elastically without rupture.
Types: rounded entry, sharp edged, re-entrant. Poiseuille’s equation An expression for laminar flow
orifice plate A circular plate, with a central orifice, of a fluid through a circular pipe.
inserted in between pipe flanges or in a tank wall to Poisson distribution A statistical distribution charac-
measure fluid flow from the resulting pressure drop. terized by a small probability of a specific event