The Connecting Rod

There are two principal kinds of connecting rods, the “strap ended” and the “solid ended.” The solid ended wear the best, but are more difficult to get on and off the engine.

Connecting rod straps are secured to the stub ends (as the ends of the rod are called), either by bolts or by one or two gibs, and the brasses are set up by a taper key or wedge.

The taper for connecting rod keys is about an inch per foot.

The angularity of a connecting rod is a term that applies to its path of motion, which is (during all parts of the stroke except on the dead centre) at an angle to the line of engine centres. The effect of this angularity is to cause the piston motion to be accelerated at one part of the stroke and retarded at another, thus causing the point of cut-off to occur at different points of the two strokes.

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 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.