The direction of the variation is to cause the point of cut-off to occur later on the stroke when the piston is moving from the head end of the cylinder towards the crank.
The amount of variation caused in the two points of cut off by the connecting rod depends upon the proportion that exists between the length of the crank and that of the connecting rod, and is less in proportion as the length of the connecting rod is greater than that of the crank.
An ordinary length of connecting rod is six times the length of the crank, or six cranks, as it is commonly termed.
Fig. 3296.
[Fig. 3296] represents a cylinder, piston and rod, cross head, connecting rod, and crank.
The piston b is shown in the middle of the cylinder, the cross head at e, and the crank pin at b, instead of being at g′, as it would but for the connecting rod, or if the connecting rod was infinitely long.
Now take a pair of compasses and set it from b to e, and then try it from a to d, and from c to f, and it will be seen that the three cross head positions d, e, and f correspond correctly to the three piston positions a, b, c. Then take a pair of compasses and set them to the length of the connecting rod (from e to b) and try them from d to a, from b to e, and from c to f, and it will be seen that crank pin positions a, b, and c correspond to cross head positions d, e and f, and therefore that the crank is not at half stroke when the piston is in the middle of the cylinder. Take these same compasses, and resting one point at (g′) mark the arc h, and that is where the cross head would be when the crank was at (g′). Now then we see that the connecting rod causes the piston to move slower while running from a to b than it does while running from b to c.