Crossheads and Guides. A variety of forms of crossheads and guides are now found in use on locomotives, two of the most common of which are illustrated in Fig. 80 and Fig. 81. The form illustrated in Fig. 80 is known as the 4-bar guide and that shown in Fig. 81, as the 2-bar guide. The form used depends largely on the type of engine. The 4-bar guide now used on light engines consists of four bars A which form the guide with the crosshead B between them. The bars are usually made of steel and the crosshead of cast-steel having babbitted wearing surfaces. The 4-bars A are bolted to the guide blocks C and D which are held by the back cylinder head and the guide yoke E, respectively. The guide yoke E is made of steel, extends from one side of the locomotive to the other, is securely bolted to both frames, and serves to hold the rear end of both guides. There is usually a very strong brace connected to the guide yoke which is riveted to the boiler. The wrist pin used in the crosshead of the 4-bar type is cast solid with the crosshead.
The 2-bar guide consists of two bars, one above and one below the center line of the cylinder with the crosshead between them. In this type the parts are more accessible for making adjustments and repairs and the wrist pin is made separate from the crosshead.
In the design of the crosshead, the wearing surface must be made large enough to prevent heating. In practice it has been found that for passenger locomotives the maximum pressure between the cross-head and guides should be about 40 pounds per square inch while for freight locomotives it may be as high as 50 pounds per square inch. For crosshead pins, the allowable pressure per square inch of projected area is usually assumed at 4,800 pounds, the load on the pin to be considered as follows: For simple engines, the total pressure on the pin is taken to be equal to the area of the piston in square inches multiplied by the boiler pressure in pounds per square inch; for compound engines of the tandem and Vauclain types, the total pressure on the pin is taken to be equal to the area of the low-pressure piston in square inches multiplied by the boiler pressure in pounds per square inch, the whole being divided by the cylinder ratio plus 1. In the latter case, the cylinder ratio equals the area of the high-pressure cylinder divided by that of the low-pressure cylinder.
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