The first consists of the rod m, wrist plate y, and the rods c and c′, which operate the bell cranks r r, r′ r′ which are fast on the valve shafts s, s′. Upon the ends of bell cranks r r, r′ r′, are pivoted latch links u, u′, which have in them a recess for the latch blocks, of which one is seen at e (the rod r′ and its connection with the valve stem being shown broken away to expose e to view). During the admission the latch block abuts against the end y of the recess w and is tripped therefrom by the cam n′. The ends of arms g of the latch links abut against the hub of the arms d, d′ upon which are cams n, n′, and at a, a′ are springs for keeping the ends g of latch links u, u′ against the hubs and cams of d, d′.
Referring now to the valve mechanism at the head end only, suppose the piston to be at the head end of the cylinder, and latch block e will be seated in the recess provided in a to receive it, and as the bell crank moves, the latch block will be raised by the latch link, which is carried by a crank arm corresponding to that seen at x at the crank end of the cylinder, and as this crank arm is fast upon the valve spindle, the lifting of e will open the valve for admission. As soon, however, as the end g of the latch link meets the cam n′, the latch link will be moved so that the end y of its recess will leave contact with the latch block e and the dash pot will cause rod r′ to descend instantaneously and close the valve, thus effecting the cut off.
The period of admission, therefore, is determined by the amount of motion the latch link u′ is permitted to have before its end g meets the cam n′, which trips the latch link, and therefore frees e from the latch link recess.
The point at which the cut off will occur, therefore, is determined by the position of the cam n′, because if n′ is out of the way, the end g of the latch link will not meet it, the latch link will not disengage from the latch block e, and the cut off would be effected by the lap of the valve, and independently of the dash pot. As in [Fig. 3379] the parts are shown in the positions they occupy at the instant the cut off is to occur, therefore the cam n′ has just tripped the latch link, and the end of e has just left contact with the end y of the recess w in the latch link u′.
The point in the stroke at which the tripping of u′ from e will occur and effect the cut off is determined by the governor, because d′ is connected to the governor through the rod g′. In proportion as the governor balls rise, d′ is moved from left to right, and the end of cam n′ meets g earlier, or, vice versa, in proportion as the governor balls fall, the arm d′ is moved to the left, g will meet the end of cam n′ later, and the point of cut off will be prolonged.
We now come to the means employed to close the valve quickly and without shock when the latch block is released from the latch link. Referring then to the crank end of the cylinder, the latch block for that valve is carried upon arm x, to which is attached the rod r from the dash pot piston (the arm corresponding to x, but at the head end being shown removed to expose the latch block to view). We may now turn again to the head end of the cylinder, rod r′ corresponding to rod r at the other end, and it is seen that r′ connects to a dash pot piston p′ having a stepped diameter, the lower half fitting into bore h′, and the upper half fitting into a bore h. The piston p′ fits the bore h′ and fills it when the rod r′ is at the bottom of the stroke, hence as p′ is raised there is a vacuum in h that acts to cause p′, and therefore r′ and x, to fall quickly and close the valve the instant the latch block is released from the latch link. To prevent the descent of rod r′ and piston p′ from ending in a blow, a cushion of air is given in h by the following construction:
At s and s′ are valves, threaded to screw and unscrew, the ends forming a valve for a seat entering h.
As the rod r′ and its piston p′ descend, the air in h finds exit through a hole at h until that hole is closed by the piston p′ covering it, after which the remaining air in h can only find exit through the opening left by the end of the valve s′, and this amount of opening is so regulated by the adjustment of s′ that a certain amount of air cushion is given, which prevents p′ from coming to rest with a blow. The head of valve s′ is milled or knurled, and a spring t′ fits, at its end, into the milled indentation, thus holding it in its adjusted position. The under surface of the upper part of p′ is covered by a leather disc, while the part that fits in h′ is kept air-tight by a leather-cupped packing.
The connection of the cam arms d and d′ with the governor is shown in [Figs. 3381] and [3382], in which the parts are shown in the position they would occupy when the crank is on the dead centre and the piston at the crank end of the cylinder. The rod g′ connects the cam arm d′ with the upper end of lever a, which is connected to the governor and vibrates on its centre as the governor acts upon it.
Now suppose the speed to begin to diminish, and the governor balls to fall, and the direction in which a will move will be for its lower end to move to the right, thus moving d to the right and carrying its cam away from the end of the latch link, which will therefore continue to open the port for a longer period of admission. Or, referring to [Fig. 3381], it is plain that, if the governor balls were to lower from a reduced governor speed, g′ would move to the left and cam n′ would be moved away from contact with the end g of the catch link, which, not being tripped, the admission would continue. On the other hand, suppose the governor balls to rise from an increase of governor speed, and d′ ([Fig. 3379]) would be moved to the right, and the cam n′ meeting g earlier, correspondingly hastening the cut off.