Fig. 3314.
If the point of cut off only is to be found, we mark from c, [Fig. 3314], an arc g representing the amount of valve lap and arc s representing the lead. A vertical p gives the eccentric position v when the crank is on the dead centre at b, and a vertical m from g gives at v′ the eccentric position at the point of cut off. Then with the compasses set to the points v v′, we may mark from b an arc, locating at h the position of the crank at the point of cut off, and from this with compasses set to represent the length of the connecting rod on the same scale as the circle represents the path of the crank, we may, from a point on the line of centres, mark an arc y giving at r the piston position at the point of cut off.
When, therefore, the lap is given, we mark it from the center c of the crank shaft, and find the other elements from it, whereas, when the lap is to be found, we mark the width of the port from the end d of the valve travel, and find the other elements from that.
A proof of all the constructions is given in [Fig. 3314], in which the letters of reference correspond to those in the previous figures, and the positions of the parts are marked in degrees of angle.
To find the piston position at the point of cut off, measured in inches, of the piston stroke it must be borne in mind that as the circle b d represents the full travel of the valve, the diagram gives all the positions of the eccentric and valve full size, but that as it represents the crank path on a reduced scale, therefore we must multiply the measurement on the diagram by that scale.
Suppose, for example, that the piston stroke is 10 inches, and the valve travel 21⁄2 inches, and the circle being 21⁄2 inches in diameter, is, when considered with relation to the eccentric motion, full size, but when considered with relation to the piston or crank motion, it is only 1⁄4 the size, hence to find the piston position at the time of cut off, we must multiply the distance from b to r by 4.
LINK MOTION FOR STATIONARY ENGINES.
The ordinary mechanism employed to enable a stationary engine to be reversed or run in either direction is the Stephenson link motion. Other forms of link motion have been devised, but the Stephenson form has become almost universal.