The press shown in fig. 46 has been selected by the author as an example of poor design, and the results obtained in actual operation practically justify the statement that they are complete failures. The press is belt driven; a flywheel F W is fitted to the pulley shaft, and the crank shaft is driven by means of the pinion P and spur wheel S W, the ratio of the gear reducing the speed of the pulley shaft down to 25 revolutions per minute at the crank shaft. Four rams may be operated simultaneously, or by means of the clutch D; the two presses on the right may be disconnected. The press is one of great power, the crank shaft having a diameter of 6 in., the stroke of the rams being

1 1/2 in., and the ratio of the gears 5 to 1. At the extreme right will be seen a pair of shears L, driven by means of a connecting rod (not shown) from the end E of the crank shaft.

The reason for adopting this design of press was economy of first cost, as compared with the outlay necessary to secure four independent presses. This leads at once to one great disadvantage of the design-i.e., impossibility of independent operation of the several presses. It is impossible to operate the two right-hand presses without running the other two at the same time, and it is not necessary to remind the practical mechanic that it is very difficult, if not quite impossible, to secure the conditions that will allow of all four presses remaining continuously at work. Each time that either of the presses have to be stopped, the remainder must also stand; thus the total time wasted will be at least four times that which would occur had some independent system of driving l>een adopted in the design. To secure this independence would not have involved any serious outlay when the value of continuous working is allowed for. It could have been secured by any of the familiar devices used in punching and shearing presses that are belt driven and run continuously during working hours.

An examination of the construction of the frame will reveal the multiplicity of joints between the floor and the crank shaft, each of which increases the chance of something working loose, or irregular settlement, and so forcing the shaft out of correct alignment. In the design (fig. 46) it will be noted that a brick pier supports one end of the pulley shaft, whilst the outer bearing is carried on a cast iron pedestal resting on a wooden block, which may shrink, etc., whilst the presses are fixed to a wrought-iron angle iron resting upon the brick pier at the extreme left end, and four cast- iron pedestals, supported as before upon a wooden block. If the various ways in which this frame can give trouble are counted up, considering both errors of workmanship in construction and erection, as well as carelessness in supervision during its operation, the advantage of a perfectly solid and rigid frame as free as possible from joints will be fully realised.

Fig. 46.

The end view, fig. 47, together with fig. 46, will show the construction and method of erection very clearly,;and will not require any further explanation.

Fig. 48.

In order that either of the four presses may be stopped for tool setting, etc, the mechanism shown in fig. 48 was devised.

The shaft is rotating continuously in the direction shown by the arrow, and the lower part of the eccentric strap is shaped so that there is a solid mass M to receive the stress due to the thrust exerted upon the ram on the down or working stroke, and a projection P which engages in a recess E formed on the upper part of the ram; by this means the ram is lifted for a second stroke. The heavy balance weight W is to prevent any possibility of M being forced out of gear when the load comes on, whilst the handle H is provided for lifting it out of gear when the press has to be stopped, a suitable catch being fitted to hold the handle out of gear until the press is ready to commence running again.

Fig. 49.

An important matter in press construction is to arrange the direction of rotation of the shaft so that the load due to' the reaction at its point of connection to the ram, due to its obliquity, shall come on the frame of the press continuously. The diagram in fig. 49 will make this clear. In the fig. 0 is the centre of rotation, and C the centre of the crank pin rotating about 0 in the direction shown by the arrow. The ram R is being raised between the frame F 5WP of the press and the cover plate I', and obviously the connecting rod K must bee in tension, since a chain might replace the rod and the ram lifted. But this pull in the rod K is exerted against the weight of It, which is acting downwards in the direction of a line drawn vertically through the centre of gravity of the ram. Thus there are two forces acting at the point of attachment A of the connecting rod K and the ram R, one k upwards in the direction of K, and the other W vertically downwards, as indicated in fig. 50. Under the action of these two forces the tendency of A is to set itself in the line joining the centre of the crank pin C and the centre of gravity of R; but since the face of the frame F prevents this movement, there must be a pressure exerted against F. The amount of this pressure can be ascertained readily by making the length A W, in fig. 51, to represent the magnitude of the weight W of the ram to some convenient scale; then through A draw a line A Q parallel to the direction of the connecting rod K, and, finally, through W draw W X at right angles to A W, meeting A Q at F; then:

and

Since the angle between the centre line of the connecting rod and the vertical is continuously varying during the stroke from 0 through some maximum value back to zero again, it is obvious that the line A Q will swing from A W to some position A Q and back again to A \V, so that the length of the intercept W F will vary from O when the ram is at the bottom of its stroke and the crank at C1, to a maximum value W F when the crank is at C, when the angle 0 C A is 90 deg., back to zero when the ram is at the top of its stroke-i.e., when the crank has reached the point C,.

By similar reasoning it will be seen that on the down stroke-that is, whilst the crank is moving from C2 to C and thence to C1-the crank is pushing the ram down, the stress in the connecting rod is changed from tension to compression, and the resistance due to act of cutting or stamping, the ram's motion downwards is resisted-that is, at A we have, as before, two forces acting again, but their directions are reversed, and are shown in fig. 52. The result of these two thrusts at A is to cause a movement of A to the left, which, as before, is resisted by the frame F. The magnitudes of these forces can be found as before, the exception being that the line A W must be of such a length as to represent the resistance offered to the die by the material being operated upon, less the weight of the ram, die, etc., which, of course, assists the press to do its work. The direction of rotation should be so arranged that the pressure should always come on the machine frame, and not upon the cover holding the ram in place. This is a small point, but an important one, to be considered, for the smaller the strain that comes upon the bolts securing the cover can be made, so much are the risks reduced of having difficulties with loose cover plates, etc. Another point that must be paid attention to in multiple presses-such as that shown in fig. 46-is the sequence of the cranks, or trouble will be experienced in the regular working of the machine. The action of a press is intermittent-that is, during a very small portion only of a revolution the whole of the work has to be done, and thus for a short period the pressure exerted is very great. The object of the flywheel is to store up energy daring the idle period of a revolution of the crank, in order to overcome the resistance offered to the movement of the die without throwing off the belt. In the case of a multiple press the cranks must therefore be spaced at intervals of equal angles, so that the working loads come regularly. In the machine shown in fig. 46 there are four presses; hence during a single revolution of the crank shaft there are four working strokes, and the cranks are spaced at angles of 90 deg. to each other. But in this machine, since the coupling D allows two presses to be disconnected, it may so happen that only two presses may be at work, and to meet this each of the two crank shafts have their cranks at 180 deg., or opposite to each other, and when they are coupled the second shaft is set with its cranks at right angles to the first shaft. Thus, if the presses be called No. 1, No. 2, No. 3, and No. 4, counting from left to right, then the sequence of events are shown by the following table:-

FIg. 50

Fig. 51.

Fig. 52.

Or if the presses Nos. 3 and 4 are disconnected, then the sequence is-

In this way uniformity of speed is obtained. The flywheel, however, must be designed so as to meet the worst case-i.e., the conditions of working when the idle period is least; and will l>e dealt with presently.