A. “Over.”
Q. What advantages do engines have in running “over”?
A. The pressure of the cross-head on engines running over, is always downward upon the guides; for when the pressure is on the head end of the piston, the pressure against the connecting rod which is pointing upward, reacts by pressing the cross-head down upon the lower guide, and when the pressure is on the crank end of the cylinder, the cross-head will be dragging the crank, and as the crank is below the center line, it will pull the cross-head down upon the lower guide, while if the engine is running under, the pressure of the cross-head will be upon the top guide, both on the outward and inward strokes, and unless the cross-head is nicely adjusted to its guides and the guides are perfectly parallel, the cross-head will be lifted when subjected to thrust, and will fall on the center by its own weight, causing the engine to pound.
Q. At what point in the stroke is the pressure on the cross-head greatest with a uniform pressure in the cylinder?
A. When the crank is at right angles to the guide.
Q. How does the relative length of the connecting rod affect this pressure?
A. The longer the connecting rod as compared with the crank, the less will be the pressure on the guides.
Q. What is the usual ratio of connecting rod to crank?
A. The connecting rod is from four to six times the length of the crank.