The press just considered is intended for use in operations where only a very moderate amount of energy is required to complete the operation to be performed, and the attention of the student should be turned next to a simple form of press intended for operation by power. In figs. 37 and 38 are shown different views of a press operated by means of a belt through gearing. The frame F, fig. 37, is of cast iron, carrying at the top the bearings, in which the steel crank shaft C S is placed. The brasses of the bearings are held down by caps C, C', which form part of the cast iron bridge piece B P. On one end of the shaft is fixed the spur wheel S W, which is driven by the ,shrouded pinion S P, fixed to one end of the counter-shaft C S'. The other end of this counter-shaft carries two belt pulleys, one of which, L P, is loose, whilst the other, F P, is fixed. The ram R is moved by the crank through the connecting rod C R. The exact position of the ram is obtained by means of the adjusting screw A, which screws into the ram R, and is locked in position by the nut L N. The construction of the guide strips S are clearly shown in the figures, and B is base of the press, which is prepared to carry the lower die. On the countershaft next to the pinion is shown a cast-iron disc D, in the circumference of which are drilled four holes, so that the position of the ram may be readily moved as required whilst setting fresh tools in position, the belt being then upon the loose pulley. H L, fig. 38, is the handle operating the belt striking gear S G for controlling the position of the belt B1.

__ In single-sided power presses the ram is frequently driven by an overhung crank pin. In fig. 39 is shown an example of this form of construction. The ram is slotted S at the back, and in this a block N is fitted, so that it can move freely from side to side of ram. The block is bored to suit the crank pin (see fig. 40). The guide strips are shown at S2, S1 in the plan view, fig. 39.

Fig . 38.

Fig. 40 shows the complete arrangement The driving shaft C S has a pin let into its enlarged end to form the driving crank, the construction being clearly shown in the left-hand sectional view. The right-hand drawing shows the front view, one of the guide strips and the ram having been removed so that the block N is shown in its position on the crank pin, and the lower view shows the relative positions of the various parts.

Fig. 39.

Fig. 40.

Another method is illustrated in fig. 41, and it will be noticed that the slot extends completely through the ram; in fact, it is a rectangular hole. The depth of the slot is, however, greater than necessary to accommodate the block N, in order to accommodate the thickness strips shown, which are used in order to provide a means of adjusting the relative positions of crank pin and bottom of the ram to suit different sets of tools. This adjustment is obtained by varying the number above or below the block N, according to requirements. The six strips, a to a5, are all different thicknesses, to enable minute adjustments to be made, and are shown to a larger scale in fig. 42. They are retained in their proper position by means of a metal sliding cover held in place by the beveled edges shown at I", in fig. 41. Crank pin, fig. 41, is turned solid on the shaft.

Fig. 41.

Fig. 42.

The method of driving the ram shown in fig. 39 is open to objection, because of the excessive wear that often occurs in the_slot S, which leads, of course, to noise due to backlash. This hammering in its turn increases the damage done both to the slot S and the block N, sometimes in the case of the design shown at fig, 40 causing the crank pin to work loose. The tool-setter, in taking up the slack by the packing strips S1 S2, can put a heavy load upon the machine, thus reducing its effective capacity, straining the crank pin, and grooving of the sides of the ram. To prevent this the machine fitter, when fitting in the strips S1 S2, should make., them butt on the machine casting, metal on metal. Then, should grooving take place, it can be traced to imperfect adjustment of the side screws for setting up the strips S1 S2. Where this design is employed, the crank pin should be made of large diameter, and as short as possible, in order to obtain stiffness. In many instances, after a crank pin has given trouble by tending, cutting off one-third of its length has removed the difficulty.

Fig. 43.

The power press is sometimes driven by means of a clutch instead of using fast and loose pulleys as shown in fig. 37. One form of clutch is shown in fig. 43. The wheel W is a flywheel pulley fitted with a brass bush and running freely on the shaft. The boss is continued so as to form the jaws of the clutch, as shoan at WC, a perspective view being shown in 6g. 44. On the end of the shaft the other half of clutch C is fitted so that it may slide freely along the axis of the shaft, but it cannot rotate upon the shaft, the two keys K being fitted for driving the shaft. This part of the clutch C is moved to and fro by a lever and rollers operating in the usual manner, the groove 0 being provided for this purpose. The mode of action is easily understood by refereuce to fig. 44, which shows the relative positions of C and W C when the clutch is engaged. This clutch was intended for intermittent working, and is only suitable when the press is running continuously for long periods without being stopped. It is obvious that the shock at starting is only tolerable when the speed is low, and in consequence this form of clutch is very liable to damage, but in certain classes of press work it can be used with advantage. Any damage to the clutch is a serious matter, for castings cannot always be easily replaced, so that the jaws may have to be put into working order by means of hand labour, always a costly business.

Fig. 44.

Fig. 45.

In another form of clutch, fig. 45, a wrought-iron or steel pin V is fitted to the sliding collar 0, and passes through the boss of the flywheel pulley F W P, engaging with the pin L, which is fixed into the collar M, which is keyed fast to the shaft. In the position shown in the figure the clutch is disengaged, the pin V, as it rotates with the clutch and flywheel, passing clear of the pin L, and the coupling is effected by sliding 0 to the left when the pin V engages with the fixed pin L. This form of clutch is open to the same objections as the preceding, but has the advantage that the pins V and L can be more easily replaced in case of damage, since it can be made from a wrought-iron bar of suitable diameter very quickly.