A conveyor or elevator in which the carrying and return runs are operated
in separated parallel and adjacent casings.
A charge in a borehole separated by a quantity of inert material for the
purpose of distributing the effect, or for preventing part of the charge
blowing out at a seam or fissure, in which case the inert material is
placed so as to include the seam. Fay
A form of strip packing that removes the localized high roof pressure from
the vicinity of a roadway into a region in the goaf. It consists of two
parallel packs adjacent to, and on each side of, the roadway, with the
packs immediately at the roadsides built of such a width as to offer less
resistance than wider and stronger packs (called buttress packs) more
remote from the roadway. The principle of double packing was developed by
D.W. Phillips in Great Britain. See also:gate side pack; strip packing;
yield-pillar system. CF:single packing
A bypass for mine cars. See also:junction
A roller chain having double the pitch of a standard roller chain, but
otherwise having standard pins, bushings, and rollers. Jackson, 1
A conveyor in which power is transmitted to the belt by two pulleys.
NEMA, 2
See:Iceland spar
Refraction shown by certain crystals that split the incident ray into two
refracted rays, polarized in perpendicular planes.
See also:birefringence
a. A coalbreaker that relies on the impact of special teeth for the bulk
of reducing, rather than on the compression between the rolls. An
important feature is adjustment, which may be made during operation. The
machines are flexible enough to produce top size ranging from 6 to 14 in
(15.4 to 35.56 cm). Mitchell
b. See:double-roll crusher
A machine for breaking down ore, rock, or coal and to discharge the
crushed material below. See also:spring-roll crusher; roll crusher;
single-roll crusher. Nelson
A press in which pressure is applied by the mating of one or more pairs of
indented rolls of equal diameter, revolving in opposite directions.
Syn:Belgian press
See:room-and-pillar
The cross-sectional view of the cutting face portion of a coring bit when
its profile is a full half circle, the radius of which is one-half the
wall thickness or kerf of the bit face. CF:single-round nose
Long
A leveling procedure whereby observations are duplicated by resetting the
instrument to detect errors of measurement immediately. Also called dual
setting. BS, 7
At collieries where there is only one recognized coal-winning shift in 24
h, it is a general practice to have double shifts (and sometimes treble)
of workers in development headings that require a speedy advance.
Nelson
This method employs two neutron logging tools with different spacings
between the source and the detector or two detectors in the same tool at
different spacings. The spacings usually differ by 6 to 10 in (15.24 to
25.4 cm). The long-spaced log is run slowly and with a large time constant
so that its statistical variation is not excessive, for the counting rate
is much lower than that of the regular-spaced log. This technique has
proved to be a potent technique for discriminating gas sands from oil
sands in Venezuela. Wyllie
A cylindrical drill-round cut whose spiral hole pattern gives the widest
opening and permits opposite holes to be ignited successfully. This gives
the best cleaning of the opening and safety in the advance is increased,
since one section of the double spiral can give breakage irrespective of
the other. Langefors
An earlier system of working thick seams in South Wales. Two narrow stalls
are turned off the heading and after advancing some 8 to 12 yd (7.32 to
10.97 m) (so as to leave a pillar of coal next to the heading) are
connected and the coal between them worked as a single face. Double stalls
are intermediate between pillar-and-stall and longwall. Nelson
An assembled gem substitute composed of two pieces of material fused or
cemented together. If both parts are of the species being imitated, it is
a genuine doublet; if one part, it is a semigenuine doublet; if it
contains no parts of the species being imitated, it is a false doublet; or
if no part is a mineral, it is an imitation doublet. CF:triplet
A tub-changing arrangement for a tunnel face. The double-track loop is
superimposed on the tunnel track and equipped with ramps, clamps, and
spring switches so arranged that the loaded cars take one track outward
while the empties take the other track inward. Syn:portable shunt
Nelson
A system of electric traction where, instead of the running rails, a
second insulated contact wire is used for the return or negative current.
CTD