CLUTCHES
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.
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.
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.
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.