| September/October 1980


106 South Elm Street, Newkirk, Oklahoma 74647

Most all of the men who play with steam engines know something about the 'Angularity of the Connecting Rod;' but you seldom hear the angularity of the eccentric rod mentioned.

Any connecting rod on an engine of any kind; a pitman on a mowing machine; a pump; any device on which one end of a rod is carried in a circle while the other is attached to a piston that moves in a straight line, will have this same peculiarity of movement. An eccentric, which is a modified crank, will cause a valve to move in the same manner. A piston will at the instant of 'dead center' have no movement at all; but will be moving at about 3-1/7 times the average speed at a point near, but not exactly, at the center of the stroke. The writer owns an engine with a 9-inch diameter piston and 10-inch stroke with connecting rod 32' long. When the crank pin is at the 90° point or, as some call it, 'the quarter,' top or bottom, the piston will be about 7/16' past the mid-point of the stroke on the end nearest to the crank. This means that there is about 7/8' more steam in the 'head-end' of the cylinder. This would result in having about 10% more power in the 'head-end' of the cylinder if some means of compensation is not provided.

In the most simple type of stationary engine, the angularity of the eccentric rod serves to increase the time that the steam is on the 'head' end; however, millions of feet of lumber were sawn and millions of bushels of grain were threshed with valve gears of this type. The exhaust from the head end was louder than from the crank end; but that did not worry the operators much. These engines were simple to adjust and repair, so any smart farm hand could run them.

On the Stephenson reversing gear which was used on the first successful locomotives, the later cut-off on the head end could be corrected by using the indirect motion rocker arm to move the valve. This causes the angularity of the eccentric rods to work in opposition to that of the connecting rod, and serves to equalize the power in the cylinder. Most of the earlier traction engines used the Stephenson or, as it is sometimes called 'Link Reverse', as it was difficult to design a traction engine with an indirect motion rocker arm. They had more power in the head end. Some of them had inside admission piston valves which equalizes the cut-off.

A more important fault in the action of these eccentric gears is that the opening and closing of the valve occurs in the slow part of the valve motion. Early in the steam engine era engineers learned they could get more power and efficiency if they had full pressure at the beginning of the stroke.