Page 340 - Mechanical Engineer's Data Handbook

328 MECHANICAL ENGINEER’S DATA HANDBOOK

upthrust The force on a floating body due to fluid Venturi meter A flowmeter in which the pressure
pressure. Equal to the weight of fluid displaced. drop in a Venturi is used to give an indication of flow.
U tube A simple type of pressure-measuring device, Vernier In instruments, such as the Vernier caliper
or manometer, consisting of a glass (or perspex, etc.) gauge, a small movable auxiliary scale attached to a
U-shaped tube partially filled with a liquid, e.g. water, slide in contact with a main scale. It enables readings
mercury, and provided with a scale. A pressure to be taken to, usually, a tenth of a division.
difference across the U tube causes a difference in vibration damper A device fitted to a reciprocating
liquid levels. engine crankshaft to minimize torsional oscillations.
Vickers’ hardness test A hardness test using the
indentation from a pyramidal diamond.
vacuum forming A shaping process applied to a sheet viscosity The resistance of a fluid to shear force. The
of thermoplastic which is heated and sucked into a shear force per unit area is a constant times the velocity
mould by vacuum. gradient, the constant being the coefficient of viscosity.
vacuum pump General name for a pump which Units: newton-seconds per square metre (Ns rn-’).
displaces a gas against atmospheric pressure. Symbol: p.
vane A curved metal plate used in pumps and viscous flow The same as ‘laminar flow’.
turbines for directing flow. Same as ‘blade’. volute The snail-shell-shaped casing into which the
vane anemometer A type of anemometer with a impeller of a centrifugal pump discharges, terminating
vaned rotor which rotates at a speed proportional to a in a circular pipe. A similar casing is used at the inlet of
fluid velocity passing through the rotor. A mechanical water turbines.
counter or magnetic transducer counts the revolutions vortex flow Rotational flow. In a ‘forced vortex’ the
which are expressed as velocity. fluid rotates as a solid cylinder. In a ‘free vortex’ (such
vane pump A type of positive-displacement pump as an eddy in a water surface) the velocity of rotation
with sliding radial vanes in slots in a rotor running decreases with radius.
eccentrically in a fixed casing.
vapour compression cycle A reversed Carnot cycle
used in refrigerators. washer An annular, usually flat, piece of metal, etc.,
vapour cycle A thermodynamic cycle using a vapour used under a nut to distribute the load.
as the working substance, e.g. steam. weir A dam in a water channel sometimes used in
vapour process A thermodynamic process using a flow measurement.
vapour, e.g. steam. weld A union made by welding.
vector A vector, or vector quantity, has magnitude, weld group A group of welds used to make a joint.
sense and direction, e.g. velocity, force. welding The joining of two or more pieces of material
vee belt A power-transmission belt with a truncated by applying heat and/or pressure, with or without a
vee cross-section running in a vee-groove pulley. filler material, to produce local fusion.
velocity The rate of change of position of a point with welding rod Filler in rod or wire form used in
respect to time. Unit: metres per second (m s- ’). welding.
velocity head The head equivalent of the kinetic weldment An assembly of several parts joined by
energy of a fluid equal to u2/(2g). welds.
velocity pressure Velocity head expressed as a press- wet steam A steam-water mixture such as results
ure equal to (puz)/2. The pressure realized by suddenly from partial condensation of dry saturated steam.
stopping a fluid stream. whirling speed (critical speed) The speed at which
velocity ratio In a ‘machine’ the ratio of distance excessive deflection of a shaft occurs being numerically
moved by the ‘effort’ to that moved by the ‘load’. the same as the natural frequency of transverse
Venn diagram In logic and mathematics, a diagram vibration or harmonics.
consisting of shapes, e.g. circles and rectangles, that white metal General term for low-melting-point
show by their inclusion, exclusion or intersection the alloys of lead, tin, bismuth, zinc and antimony used for
relationship between ‘classes’ and ‘sets’. plain bearings.
Venturi A convergent-divergent duct in which press- work A type of energy involving mechanical effort,
ure energy is converted to kinetic energy at the throat. e.g. the output from an engine.