Fig. 2596.
When the direction of rotation of the cone pulley on the countershaft requires to be occasionally reversed, there are two belts, an open one and a crossed one, from the line shaft to the countershaft, and there are three pulleys on the countershaft, their arrangement being as shown in [Fig. 2596]. l l′ are two loose pulleys, one receiving the open and the other the crossed belt, both these pulleys being a little more than twice the width of the belt; f is a fast pulley. By operating the belt skipper or shifter in the requisite direction either the open or the crossed belt is brought upon the fast pulley, the other belt merely moving across the width of its loose pulley, which must be twice that of the fast one. In the position of the belt shifter shown in the cut, both belts would be upon the loose pulleys l l′, hence the countershaft would remain at rest. If the direction of rotation of one pulley is required to be quicker than the other, two fast pulleys, each slightly more than twice the width of the belt, may be placed upon the line shaft, one of them being of enlarged diameter, to give the requisite increased velocity.
Fig. 2597.
In [Fig. 2597] Pratt’s patent friction clutch is shown applied to a countershaft requiring to rotate in both directions, but quicker in one direction than in the other; hence, one of the pulleys is of smaller diameter than the other. The pulleys are free to rotate upon the countershaft unless engaged by the clutch, which is constructed as follows:—
The inside surface of the pulley rim is bored and the end surface of the shoes is turned to correspond. The shoes are in the form of a bell crank, upon the exposed end of which is provided a small lug, clearly shown in the cut. To prevent end motion of the pulley a collar is placed on one side of it and secured to the countershaft, while, on the other, the sleeve to which the shoes are pivoted is also secured to the countershaft; upon the shaft between the two pulleys there is a sleeve, having at each end a conical hub. When this sleeve is moved to the right, its right-hand coned hub passes between the lugs on the exposed ends of the shoes, forcing these lugs apart and causing the shoes to grip the bore of the large pulley, which thereupon rotates the shaft through the medium of the sleeve upon which the shoes are pivoted. Similarly, if the engaging (and disengaging) sleeve be moved to the left it will pass between the lugs of the shoes on the left-hand pulley, which will, therefore, be caused to drive the shaft. In the position shown in the cut the engaging sleeve is clear of the ends of all the shoes, hence the pulleys would be caused to rotate (by their belts), but the shafts, &c., would remain stationary.
In yet another form the inner face of the pulley rim is coned, and in place of shoes a disk, whose circumference is coned to fit the pulley rim, is fast upon the shaft. The shaft is provided with a fixed collar, and from this collar, as a fulcrum, the pulley and disk are (by means of short levers attached to a sleeve upon the countershaft) brought into contact, the thrust on the other side of the pulley being sustained by a conical surface on the sleeve, fitting to a similar cone on the hub of the pulley. Thus the pulley is gripped between two coned surfaces, one on each side, and is released by moving the sleeve laterally so as to relieve the grip, which it does noiselessly.
By this means motion to the shaft is communicated from the pulley without the sudden shock incidental to the impact of two fixed pieces, because the grip of the cones is gradual, and a certain amount of slip may occur until such time as the grip of the surfaces is sufficient to drive by friction.