The slide valve is operated by the eccentrics, which are attached to the main crank shaft of the engine and revolve with it, the object of the eccentrics being to move the slide valve back and forth upon its seat to admit the steam alternately through the steam ports to the cylinder.
The valve gear is a most important detail and one upon which the economy of fuel in a great measure depends, and any derangement in this part of an engine causes an immediate increase in the fuel consumed and decrease in the power of the engine.
In a properly constructed valve the slide upon the seat should be reduced to the smallest possible amount, and should be so designed as to give an equal cut-off and release at both ends of the cylinder, whether working full gear or notched up.
The engine should also have the same power whether working forward or backward, and the cut-off should be as sharp as possible.
The more perfect the valve gear the more the engine can be notched up, and thus allow the steam to expand in the cylinder to its utmost. The engine which can be notched up the most is the most economical in fuel and water.
=The Cross-head= is of cast iron and connects the piston rod to the connecting rod, and is that part of an engine where the motion is changed from vibrating to rotary. The piston rod is fastened securely to it, while the connecting rod is attached by wrist-pin. On a V guide or bored guide engine frame the cross-head is supplied with adjustable shoe slides that can be adjusted to take up their wear and fit the guides properly, also to keep the piston rod in line. On a bar or locomotive guide engine the cross-head is adjusted by removing the liners from between the bars.
=The Engine Frame= is the large casting which contains the bored, V shaped or locomotive guides for cross-head shoes. It also contains the pillow block for crank shaft at one end and the cylinder is bolted to the other. They are made in many different styles and shapes, but all answer the same purpose.
=The Connecting Rod= on an engine is the connection between the cross-head and crank-pin; it is generally made of wrought iron or steel, with brass boxes at each end held in position by wrought straps. These straps are attached to the connecting rod by gibs and keys. Connecting rods are sometimes made with mortised ends to receive the brass boxes, which are held in place by wedge block and adjusting screw. The latter style is used principally on the larger makes of engines. While the connecting rod with mortised ends are considered a little the safest, the straps on the ends of rods are most commonly used. The brass boxes at ends of connecting rod are adjustable to take up the wear by use of the gibs and keys, and they should be adjusted as frequently as there is any lost motion discovered at the crank-pin or cross-head, which will be indicated by a knocking or pounding as the crank passes over the centers.