A core barrel with an inner tube to hold the core. The inner tube does not
rotate during drilling, thereby giving a better core recovery.
Syn:double-tube core barrel
Eng. Two crib sets are placed back to back to form a two-compartment
crib-lined raise. This technique is employed in weak ground in place of a
double compartment separated by only a single dividing member.
For double-pitch roller chains, a sprocket having two sets of effective
teeth. Tooth spaces for the second set are located midway between those of
the first set. Jackson, 1
A method of silling or working out 10 ft (3 m) or so above the haulage
level and forming a double-deck gangway. Chutes are constructed at
intervals for ore transfer into mine cars. Nelson
A longwall conveyor layout in which the center or main gate serves two
double units, one on each side. The gate belts from each double unit
deliver the coal onto cross-gate belts, which in turn deliver to the main
gate conveyor and then by trunk conveyor or cars to the pit bottom.
Nelson
Hoisting device having two cable spools or drums rotating in opposite
directions. Long
A hoist with two drums that can be driven separately or together by a
clutch. See also:main-and-tail haulage
Has two pairs of rolls, mounted in one stand, one pair of rolls being
higher than, and in advance of the other. Osborne
A term applied to any cutter loader that can cut both ways on a longwall
face without turning at each end. This requires cutting units at both ends
of the machine and duplication of other essential parts. Nelson
A diamond-shaped coal-cutter pick that is held in a special holder and
chain. Both ends of the pick are used and then discarded. The type is used
widely in the United States. See also:coal-cutter pick
Loads are raised or lowered on a slope by a stationary engine and wire
rope, as in an inclined shaft. There is a double track, or three rails and
turnout; the descending trip assists the engine to raise the ascending
trip, thus eliminating dead load, except rope. Peele
a. A pair of entries in flat or gently dipping coal so laid out that rooms
can be driven from both entries; twin entries. See also:entry
Fay
b. A system of ventilation by which the air current is brought into the
rooms through one entry and out through a parallel entry or air course.
Fay
c. See:main entry
double-entry room-and-pillar mining
See:room-and-pillar
A test in which coal dust is placed in each of two connected parallel
entries. Rice, 2
Applied to quarry equipment consisting of two independent channeling
machines on a single truck, operated by one person.
The driving of two coal headings, parallel and side by side, for
development purposes. Usually a pillar 10 to 20 yd (9.14 to 18.29 m) wide
is left between them. Formerly it was the practice at many coal mines to
drive only one heading from which the stalls were turned off right and
left. Two headings simplify ventilation and provide a second egress in an
emergency. Nelson
Closely spaced parallel tubes with right- and left-hand rounded helical
threads rotating in opposite directions, on which bags or other objects
are carried while being conveyed. Syn:helical bag conveyor
A centrifugal fan in which air enters the impeller on both sides. Also
called double-width fan. BS, 8
a. A two-hand heavy hammer, usually weighing about 10 lb (4.54 kg).
CF:single jack
b. A double or twin-screw drill column. Long
An aerial ropeway in which two parallel track ropes are used, each
carrying a carriage. Nelson
A type of bucket elevator having the carrying and return runs enclosed in
separate casings between the head and boot. See also:bucket elevator