=31.= It is often desired to have the drum of a hoisting engine run
loosely on the engine shaft, so that it may run independently of the
engine. With such loose-running drums, the engine generally runs only
in the direction required to hoist the load, while the cage is lowered
entirely by means of the brake. In this way, one engine provided with
several drums may be used for hoisting from several shafts or from
several levels in the same shaft at the same time. Such a loose-running
drum is connected to the engine shaft when a load is to be hoisted
by means of a clutch, of which there are two forms commonly used for
hoisting machinery: _jaw_ or _piston clutches_ and _friction clutches_.
[Illustration: FIG. 17]
=32. Jaw Clutch.=--Fig. 17 shows a =jaw clutch=, one-half _a_ of which
is shown ready to be bolted to a drum or flat rope reel, which is loose
on the shaft _b_. The other half _c_ of the clutch is moved back so
that the jaws _d_ are not in contact with the jaws _e_ on the part
_a_. The half _c_ slides freely on a feather key _f_, which is driven
tightly into a deep key seat in the shaft _b_; a collar _g_, fitting
loosely in a groove in the hub of _c_, is provided with trunnions _h_
on each side; levers _i_ connect these trunnions with the lever _j_
attached to a suitable handle, by means of which the clutch is made to
slide endwise on the shaft so that the jaws _d_ engage or disengage
the jaws _e_ and thus connect or disconnect the drum or reel from the
clutch. There are generally four or six jaws _d_ that engage the same
number of jaws _e_ on the drum, and it is necessary to have little or
no play between _d_ and _e_ when the clutch is connected or there will
be too much shock. The clutch is about 2 feet in diameter, and the jaws
are 3 or 4 inches deep for the average 20" × 48" first-motion hoisting
engine. Instead of the clutch being fastened to the shaft by feather
keys, the shaft may be hexagonal where the clutch slides on it and the
clutch is machined to match. Jaw clutches are made of either cast-iron
or cast-steel, and should be in halves, for convenience of repair, and
securely bolted together.
[Illustration: FIG. 18]
=33. Band Friction Clutches.=--Fig. 18 shows a =band friction clutch=
that is attached to and revolves with the shaft _a_. The winding
drum runs loosely on the same shaft and has a driving-band ring or
seat _b_ on one end; when the ring _c_ of the clutch is tightened by
means of the mechanism shown, the clutch and driving band become
practically one piece and the drum revolves with the clutch. The clutch
is constructed as follows: The driving disk _d_ keyed to the driving
shaft _a_ is connected to one end of the ring _c_ by a fixed arm _e_,
which is bolted firmly to the disk _d_ and revolves with it; a movable
arm _f_ that connects with the other end of the band _c_ turns on the
pin _g_. When the band _c_ is loose, it can revolve about the seat _b_
without touching it, but the band can be tightened and made to clamp
_b_ either when revolving or standing still, as follows: The sliding
sleeve _h_ may be caused to slide about 6 inches along the hub of the
disk _d_ by levers (not shown) that take hold of trunnions _i_ on a
ring on the sliding sleeve; this sleeve is connected to the movable arm
_f_ by a link _j_, and when the sleeve is on the end of the hub the
link stands at an angle of about 60° with the shaft; by sliding the
sleeve toward the disk _d_, the link is made to move the arm _f_ about
1½ inches at its outer end and to thus tighten the driving-band _c_, so
that it grips the ring _b_. The adjusting nuts _k_ take up the wear of
the wooden blocks with which the ring _c_ is lined. Band lifters _l_
hold the band clear of the ring when it is loose. The clutch shown is
built to run in the direction indicated by the arrow, but such clutches
may be built to run in either direction; they should always be run in
the direction for which they are designed, so that the load may always
come on the fixed arm. If the band be tightened slowly, there will
be no sudden start or jerk on the rope, as the slip of the band will
prevent the entire force of the grip taking effect at once; and after
the drum reaches full speed, there is little or no slipping of the
driving-band. It is best to keep the band only just tight enough to do
the work, for should the car get off the track, or be overwound, or
should a cage stick in the shaft for any reason, the band will slip and
thus become a safety appliance, and not strain or break the rope, shaft
timbering, or machinery, as would be the case if a positive clutch,
Fig. 17, were used.
=34. The Beekman Friction Clutch.=--A simple friction clutch is
shown in Fig. 19, in which _a_ is a section of the drum shell. The
wooden blocks _b_ bolted to the side of the gear-wheel _c_ are made
of suitable shape to conform to the =V=-shaped groove _d_ in the side
of the drum. The steel spring _e_ between the two steel washers _f_,
_f_ disengages the clutch, as soon as the pressure is relieved, by
reversing the motion of the lever _g_ and screw _h_ from the opposite
end of the drum. When the lever _g_ is turned, the screw _h_ is forced
against the end of the pin _i_, which, in turn, presses the cross-key
_j_ against the collar _k_, forcing the drum against the blocks _b_ and
frictionally engaging the gear-wheel _c_. This drum shaft is prevented
from moving endwise by means of the collar _l_ and the grooves _m_ in
the babbitted pillow-block. The wide bearings of the drum on its shaft
are lubricated by means of the pipes _n_.
[Illustration: FIG. 19]
A clutch is often used to change the length of the hoisting rope when
hoisting from two or more lifts or levels. In this case the shaft
carries two drums, one of which is fixed to the shaft, while the other
is provided with a friction clutch. When it is desired to change the
length of the rope, the cage attached to the loose drum is brought
to, say, the upper landing. The cages both resting on the wings, the
clutch is loosened and the other cage attached to the fixed drum is now
brought to the desired level, when the clutch is again tightened and
hoisting proceeds. The change is made in 2 or 3 minutes.