The governor is driven by a belt from a pulley on the crank shaft to the pulley w, [Fig. 3381], whose shaft conveys motion to the governor spindle through the medium of a pair of bevel pinions in which v represents (referring again to [Fig. 3378]) the steam or admission valve for the crank end port, and v1 that for the head end port, while v2 is the exhaust valve for the crank end, and v2 that for the head end of the cylinder. All four valves are shown in the positions they would occupy when the crank was on the dead centre and the piston at the crank end of the cylinder, hence the valve positions shown correspond to the positions the parts of the valve motion occupy in [Fig. 3381].
The faces of the valves are obviously arcs of circles of which the axes of the shafts s, s′ are the respective centres. Valve v has opened its port to the amount of the lead, which in this class of engine varies usually from 1⁄32 to about 1⁄16 inch. As separate exhaust valves are employed, the point of release, and (as the same valve edge that effects the release also effects the compression) therefore that of the compression, may be regulated at will by adjusting the lengths of the rods f, f′, [Fig. 3379], which have at one end a right and at the other a left hand screw, so that by turning back the check nuts and then revolving the rods their lengths will be altered.
Similarly the amount of admission lead may be adjusted by an adjustment of the lengths of rods c, c′, which also have right and left hand screws. Referring now to the admission valve v, it is seen that its operating rod c is at a right angle to bell crank r, r, hence the amount of valve motion will not be diminished to any appreciable extent by reason of the wrist plate end of rod c moving in an arc of a circle, and the point of attachment of rod c to the wrist plate is such that, during the admission, the valve practically gives as quick an opening as though rod c continued at a right angle to r. But, if we turn to valve v′, which has closed its port and covers it to the amount of the lap, we find that bell crank r′ and its operating rod c′ are in such positions with relation to the wrist plate, that the motion of the latter will have but little effect in moving the bell crank r′. This is an especial feature of the Corliss valve motion and is of importance for the following reasons:
The lap of the valve (which corresponds to the lap of a plain D slide valve) is usually, in this class of engine, such as to cut off the steam at about 7⁄8 stroke, but the adjustment of the cam position is usually so made that, from the action of the governor, the latest point of cut off will occur when the piston has made 5⁄8 of its stroke, the range of cut off being from this to an admission equal to the amount of the lead.
As the eccentric is fixed upon the shaft, the speed at which the valve opens the port for the admission is the same for all corresponding piston positions. Thus suppose the piston has moved an inch from the end of the stroke, and the valve speed will be the same, whether the cut off in that stroke is to occur at quarter stroke or half stroke, and as the valve continues to open the port until it is tripped, therefore, at the moment it is tripped, the direction of valve motion must be suddenly reversed.
As the duty of its reversal falls upon the dash pot, it is desirable to make this duty as light as possible, which is accomplished by the wrist motion, which acts to reduce the valve motion after the port is opened a certain amount for the admission.
We have, therefore, that during the earlier part of the admission, the port opening is quick because of the eccentric throw being a maximum, while during the later part of the port opening, this rapid motion is offset or modified by the wrist motion, thus lessening the duty of the dash pot and enabling it to promptly close the valve.
The range of governor action, so far as the governor itself is concerned, is obviously a constant amount, because a certain amount of rise and fall of the governor balls will move the cams a given amount. But the range of cut off may be varied as follows: At z, z′, are adjustment nuts, by means of which the lengths of rods g, g′ may be varied.
Lengthening rod g obviously moves arm d and its cam n further from the end of the latch link u, and therefore prolongs the admission period.
Shortening the rod g′ causes cam n′ to move around and away from the leg g of the latch link, and prolongs the admission.