ADJUSTMENT OF SIDE RODS.
To connect driving-wheels so that they will run together in perfect harmony, after ascertaining that they are the same size, the next point is to secure the crank-pins at an equal distance from the centers of the wheels. When this is done, and the wheels are trammed parallel to the line of motion, the rods will move on a plane with the centers of the crank-pins exactly the same distance apart as are the centers of the driving-axles. The rods can be adjusted to the greatest advantage with the steam raised, so that the heat of the boiler will make the frames about the same length as when the engine is at work. The expansion due to the heat of the boiler is short when measured by a foot-rule, but it affects the smooth action of the side rods to a remarkable extent.
Before tramming for the side rods, it is necessary to have the driving-box wedges set up just tight enough to let the driving-boxes move vertically in the jaws without sticking. The distance between the centers of the driving-axles and the centers of the crank-pins having now been found equal, the rods are fitted up; each connection being secured a close fit to the pin, with the brasses held brass to brass. With the brasses bored out exactly to the size of the crank-pins, and the rods accurately fitted, a connection could be made which would bind the two sets of drivers to move as an unbroken unit, were it not for the disturbing element which appears in the shape of rough track. With uneven track and worn wheel-tires, a tremendous tension is put on the rods where the connections are closely fitted. Provision is made for this source of danger by leaving the brasses of the back pins loosely fitted. A yielding space is left between the brass and the pin, not between the brass and the key or strap. The latter connections must be perfectly snug, or the strap will soon be pounded out of shape.
In the case of ten-wheel and consolidation engines, the brasses of all wheels behind the leading pair should be bored out one-sixty-fourth larger than the pins, which will generally be sufficient. In case a pin is sprung,—which is no rare circumstance,—room enough must be left in the brass to let the pin pass over its tightest point without pinching. The center is the proper position to put up side rods on. Some men like to fit side rods with the cranks on the eighths position; holding that there the greatest strain comes on, and, consequently, that there fitting up should be done. That is a mistaken idea; for rods may be put together on the eighths, and yet bind the pins badly in passing the centers. On the other hand, if they pass the centers easily, they will go round the remainder of the circle without danger.