Page 330 - Mechanical Engineer's Data Handbook

318 MECHANICAL ENGINEER’S DATA HANDBOOK

water flows radially inwards through guide vanes and Additional components are intercoolers between com-
a runner which it leaves axially. pressors, reheat between turbines and a heat ex-
frequency The rate of repetition of a periodicdisturb- changer.
ance. Units: hertz (Hz) or cycles per second. Also gas welding Welding using the heat of an oxygen-gas
called ‘periodicity’. flame.
fretting corrosion Surface damage between surfaces gauge Mocks (slip gauges) Accurate rectangular hard
in contact under pressure due to slight relative motion, steel blocks used singly or in combination with others,
especially in a corrosive environment. the distance between them forming a gauging length.
friction The resistance to motion which takes place gear ratio The speed ratio for a pair or train of gears
when attempting to move one surface over another determined by the number of teeth on each gear.
with contact pressure. gear wheel A toothed rotating wheel used in conjunc-
friction coefkiint The ratio of the friction force to tion with another wheel of the same or different
the normal force at the point of slipping. The ‘static diameter, to transmit motion to another shaft. The
coefficient of friction’ is the value just before slipping main types are spur, bevel, worm and epicyclic.
takes place, the ‘dynamic coefficient of friction’ being geometric factor A factor dependent on the shapes of
the value just after. bodies between which heat or light is radiated. This
friction factor in pipes A dimensionless quantity from factor affects the heat-transfer coefficient.
which the pressure loss due to pipe-wall friction can be geomehic Progression A series of numbers in which
calculated. It is usually plotted against the Reynold’s each number is derived by multiplying the previous
number for various degrees of relative pipe roughness. number by a constant multiplier called the ‘ratio’.
friction laws These state that the coefficient of fric- governor A speed regulator on variable-speed elec-
tion is independent of surface area of contact and tric motors and prime movers, etc.
pressure between surfaces. These laws are not strictly gravitation The attractive force between two masses.
true. The force is proportional to the product of the masses
Froude number A dimensionless number used in the and inversely proportional to the square of the dis-
study of the motion of ships through water. It is the tance between their centres of mass.
ratio of velocity to the square root of the product of gravitational comtant The gravitational force be-
V tween two masses m1 and m,, their centres of mass a
.
length and acceleration due to gravity, - distance d apart, is given by F=Gmlm,/d2 where
& G=gravitation constant=6.67 x lo-” Nrn’kg-,.
grinding The removal of metal, etc., using an abras-
ive ‘grinding wheel’.
gas constant For a ‘perfect gas’, gas constant
R=pV/mT, wherep=pressure, V=volume,m=mass,
T= temperature. bark The resistance of metals to plastic deforma-
gasket A layer of usually soft material between two tion, usually by indentation. Measured by tests such as
mating surfaces which prevents leakage of fluids. Brinell, Rockwell, and Vickers pyramid.
gas processes Changes in the properties of a sub- bead The height of a liquid above a datum in a
stance, e.g. isothermal, isentropic, constant volume, gravity field.
etc. beat engine A system operating on a complete cycle
gas refrigeration cycle A cycle using a reversed developing net work from a supply of heat.
constant pressure cycle in which the working sub- heat Bow rate Heat flow per unit time in a process.
stance is always a gas. Unit: watt (W).
gas shielded arc welding Arc welding with a shield of heat transfer The study of heat flow by conduction,
inert gas, e.g. argon, helium, to prevent oxidation. convection and radiation.
gas turbine set A prime mover consisting of one or beat transfer coeflbieint A coefficient h relating, heat
more axial or centrifugal compressors, combustion flow q, area of flow path A and temperature difference
chamber(s) (or gas heater), and one or more axial or AT for heat transfer between two phases: q=hAAT.
radial flow turbines. The compressor(s) are driven by beat treatment Heating and cooling of solid metals to
one turbine and a turbine delivers useful power. obtain the desired properties.