Fig. 3051.
[Fig. 3050] represents a drop hammer, and [Fig. 3051] is a sectional view of the lifting mechanism.
This machine consists of a base or anvil, a hammer which moves up and down between two uprights, and a lifting device, which is secured to the top of the uprights.
A board secured to the hammer passes up between two friction rolls, which revolve in opposite directions. When the two rolls are moved towards each other, the friction on the board causes the hammer to rise; and when again separated the hammer will fall. The back roll is keyed to a shaft, on each end of which is a driving-pulley; and thus by the use of two pulleys on the same shaft, equal wear comes on the bearings in which it revolves. The front roll turns freely on its shaft, and is driven by the back roll being geared to it. To secure to the gears both strength and durability, they are made with wide faces, are geared at both ends, and the teeth are of peculiar shape.
The bearings to the shaft, on which the front roll revolves freely, are eccentric to the roll, and a partial revolution of the shaft moves the front towards the back roll, pinching the board. To an arm which is secured to the front shaft is fastened the upright rod, the upward movement of which opens the rolls, and whose downward movement closes the same; the weight of the rod being sufficient to cause the hammer to rise. This arrangement, simple and yet substantial, dispenses with the two eccentric-armed bushings, and the spreading of the upright rod at the top to reach both bushings, which caused so much trouble in the old way. In place of the dog which is usually used to hold up the hammer, (which is limited in adjustment to holes located at fixed distances in one of the uprights, necessitating not only the removal of the dog to another hole, and connecting and disconnecting the same to the treadle, but also the most accurate adjustment of the collar on the upright rod to the dog holding the hammer), we use a pair of clamps, located on the lifter, under the rolls. These clamps, holding the hammer centrally, prevent the side blow against the upright, the inevitable result of the contact of hammer and dog, when the former is only held on one side, as it must be, by the use of the dog. The opening of the clamps by the foot-treadle allows the hammer to fall; and the clamps are so made that the hammer will ascend freely, whether the foot is on the treadle or not, and if the foot is off the treadle, will hold up the hammer at any point where it may be arrested in its upward movement. It will be readily seen that the only adjustment required is that of the collar on the upright rod, to any height of blow desired.
This machine has two treadles, one connected to the clamps, and the other to a lever which operates the upright rod.
To obtain repeated blows with one motion of the foot, place the foot upon the treadle connected to the clamps. If variable blows are wanted, place the foot upon the other treadle, and the hammer will follow the motion of the foot. This extra treadle is a late improvement, and is not shown in the cut. The operation required to obtain automatically any number of blows of the same height is described as follows:—
Pressure upon the treadle opens the clamps and allows the hammer to fall; just before the dies come together, the trip at the bottom which holds up the upright rod is released, and allows the rod to drop; this closes the rolls, causing the hammer to ascend. The hammer continues to rise until it strikes the collar on the upright rod, and, lifting the rod, opens the rolls, removing the pressure upon the board, and allows the trip at the bottom to go under to hold the rod up, and the hammer remains suspended, provided the foot is off the treadle. So long as pressure is kept on the treadle, the blows of the hammer will be continuous; but upon removal of the pressure, the hammer will assume its original position.