Page 328 - Mechanical Engineer's Data Handbook

316 MECHANICAL ENGINEER’S DATA HANDBOOK

dynamic pressure Pressure in a moving fluid result- obtain the required shape. The process is the reverse of
ing from its instantaneous arrest equal to pv2/2, where electroplating.
p=fluid density, V=velocity. elongation In tensile testing the increase in length of a
dynamics A study of the way in which forces produce specimen at fracture as a percentage of the original
motion. length.
dynamic viscosity (coefficient of viscosity, absolute emissivity Ratio of the emissive power of a surface to
viscosity) In a fluid the ratio of shear stress to that of a ‘black body’ at the same temperature and with
velocity gradient. Units are newton seconds per square the same surroundings.
metre (N-s rn-’). end milling Machining with a rotating peripheral
dynamo An electromagnetic machine which converts and end cutting tool (see face milling).
mechanical to electrical energy. endurance limit Same as ‘fatigue limit’.
dynamometer A device for measuring the power energy The capacity of a body for doing work. Types
output from a prime mover or electric motor. are: kinetic, potential, pressure, chemical, electric, etc.
energy fluctuation coefficient The ratio of the vari-
ation in kinetic energy in a flywheel due to speed
effectiveness of a heat exchanger The ratio of the ‘heat fluctuation, to the average energy stored.
received by the cold fluid’ to the ‘maximum possible enthalpy Thermodynamic property of a working
heat available in the hot fluid’. substance equal to the sum of its ‘internal energy’ and
efficiency A non-dimensional measure of the perfec- the ‘flow work’ (pressure multiplied by volume). Used
tion of a piece of equipment, e.g. for an engine, the in the study of ‘flow processes’.
ratio of power produced to the energy rate of the fuel enthalpy-ntropy diagram (h-s or Mollier chart) A
consumed, expressed as a fraction or as a percentage. diagram used for substances on which heat and work
elastic constants The moduli of elasticity for direct are represented by the length of a line. Used extensive-
stress, shear stress and hydrostatic stress and also ly for calculations on steam cycles and refrigeration.
Poisson’s ratio. entropy In thermodynamics, entropy is concerned
elastic deformation Change of dimensions in a ma- with the probability of a given distribution of momen-
terial due to stress in the elastic range. tum among molecules. In a free system entropy will
elasticity The property of a material by virtue of tend to increase and the available energy decrease. If,
which it recovers its original size and shape after in a substance undergoing a reversible change, a
deformation. quantity of heat dQ at temperature Tis taken in, then
elastic limit The greatest stress that can be applied to its entropy S is increased by an amount dQ/T. Thus the
a material without permanent deformation. area under a curve on a T-S graph represents the heat
electrical resistance The real part of impedance transferred. Units: joules per kelvin (J K- ’).
which involves dissipation of energy. The ratio of epicyclic gear A system of gears in which one or more
voltage drop to current in a conductor. wheels travel round the outside or inside of another
electrical discharge machining (EDM) Machining wheel the axis of which is fixed.
process in which metal is removed by erosion due to an equilibrium The state of a body at rest or in uniform
electric spark in a dielectric fluid using a shaped motion. A body on which the resultant force is zero.
electrode. erosion The destruction of metals, etc., by abrasive
electric potential Potential measured by the energy of action of fluids usually accelerated by the presence of
a unit positive charge at a point expressed relative to solids.
zero potential. Euler strut formula A theoretical formula for deter-
electric strength The maximum voltage that can be mining the collapsing load for a strut.
applied to a piece of insulation before breakdown excess air The proportion of air used in excess of the
occurs. theoretical quantity for complete combustion of a fuel.
electrochemical corrosion Corrosion due to the flow expansion The increase in volume of a working fluid,
of current between anodic and cathodic areas on metal e.g. in a cylinder with moving piston. The opposite is
surfaces. ‘compression’. In mathematics the expression of a
electrochemical machining (ECM) The removal of function as an infinite series of terms.
metal by electrolytic action, masks being used to expansion coefficient (coefficient of expansion) The