Thus, if it is found necessary to raise or lower the center m, to clear wheels, frames, or other gear, this may be done till the angle of mm1 is out of the parallel of the cylinder center line up or down by one in thirteen (1 in 13); it is not well to go beyond this; but the lines mm1 and bb will be parallel, and the position of the curve oo for mid-gear will be at right angles to mm1.
Again, the point e1 may be taken either above or below the center line of the connecting rod, if it be wished to avoid piercing the rod; the pin at e1 being carried in a small bush or block attached above or below the connecting rod.
Again, for locomotives, if the wheels are so small that the link e1f would come too low, it may be cut short at the point j, and this point connected by a link ll to a small return-crank p, on the crank-pin; the movement of the counter-crank being equal to that from to j4.
The diagram is drawn for an engine where the center of the crank-axle is on the center line of the cylinder; but if this be below, as is usual in American locomotives, then the base line on which to construct the diagram of the valve-gear itself will be the average center line assumed by the connecting rod for such lowering of the crank-axle center, drawn from c, the middle position, to a point, say r, representing the lowered center of the axle. The vertical zz will be at right angles to this new base line cr, all the other processes following.
While the proportions shown on the diagram give the best average results, these proportions may be varied within very wide limits, according to the requirements of the design of the engine. Thus, when the distance between the center of the cylinder and center of valve-stem is small, as with a small cylinder and a long stroke, the link e1f may be considerably lengthened: the point j will thus be dropped, and convenient angles for all the links, etc., will be maintained, the room for the various movements being got below the center line of the cylinder when it can not be had above; the reverse proportioning of the parts being made when the conditions are reversed, as with a large diameter of cylinder and a short stroke.
This system of laying out the motion, applies, with a few modifications, to the Strong motion, and to all others where the ellipse formed by the horizontal and vertical motion of the main rod is used to actuate the valves.