Open 274 illustrates the frame of the De La Vergne four-stroke-cycle low-pressure engine. Since no scaveng ing air is used, the frame does not embody the compressor feature. This frame in many respects follows closely the design of horizontal Diesel engines, though of much lighter weight. The cylinder jacket is part of the frame, and support is given the cylinder over its entire length. This extensive foundation bear ing surface makes this frame very rigid.
Figure 275, Buckeye-Barrett engine, is a typical design of the two-stroke-cycle engine using cylinder air-compression. Since this type of air compression necessitates the use of a piston rod, a crosshead is used. The adjustments of the crosshead are iden tical with those occurring in steam engine practice. Since most oil engine operators are familiar only with the trunk piston type engine, frequently the crosshead is ignored, and the wear on the lower shoe is not taken up at the proper time. It is apparent that, if the lower crosshead shoe wears, the piston rod will ride on the bottom part of the stuffing-box. The piston is usually about 40 per cent. as long as the piston rod. Then if the crosshead shoe wears j16 inch, the rod will rest on the stuffing-box, the piston rod fulcruming at this point. When the fuel charge explodes, the piston, in forcing the crosshead to the front end, will tend to tilt an amount corresponding to the crosshead shoe wear. Being of considerable length, cylinder scoring may be caused by either the front or rear end of the piston. This has happened in a number of instances where the engineer was careless about taking up crosshead shoe wear. While the lower shoe wears more rapidly, due to the higher pressure exerted on the piston during the explosion stroke, there is some wear on the top shoe. Unlike the steam engine, there is a reversal of direction of the pressure on the crosshead; during the compression stroke the flywheel forces the piston to the rear, and the vertical component of this pressure on the crosshead is upward. Since the pressure is fairly low, the wear is slight, and usually no ad justing wedge is incorporated in the design. Even though this be true, it is advisable for the engineer to examine the upper shoe for wear. It is extremely difficult, after the engine is once in operation, for the engineer to detect on which shoe the wear has occurred. The one correct method is to remove the piston and run the center line. This involves too much time and labor to be done often. A good many operators depend on the stuffing
box to serve' as a gage in adjusting the crosshead, raising the lower shoe until the piston rod apparently centers the stuffing-box. The objection to this lies in the wear in the stuffing-box, destroy ing its usefulness as an indicator. In the data book, covering dimensions and adjustments of the engine, which every engineer should have in his possession, when the engine is first installed, the crosshead measurements should be entered. Using a mi crometer, the distances between outside surface of the shoes and the inside edges of the crosshead should be noted. The crosshead can be turned enough to cause the shoes to be horizontal and clear of the guides. In taking up any wear, the wedges should be moved to bring the shoes back to the original 'measurements. Where the upper shoe is babbitted directly onto the crosshead, the wear can only be taken up by rebabbitting. Consequently, it is necessary to allow the wear to go awhile before this rebab bitting is done. Special care should be used in keeping the oil grooves on the shoes clean of grit.
Another detail that should not escape the engineer's attention is the piston-rod lock-nut. This lock-nut serves to jamb the rod into the crosshead. If the nut works loose, the rod will likely unscrew a thread or so. This will raise the compression in the cylinder by reducing the clearance. In cases where the rod has worked entirely free from the crosshead, the cylinder head has been broken by the piston striking it. If the nut is jambed up hard, and the locking set-screw or cotter key properly adjusted, there should be but little danger. Nevertheless, the rod should be examined each time the crankcase door is opened.
Air Suction Valves.—The air suction valve is a frequent source of trouble. This valve, when made of leather and after being in use for some time, will have a tendency to cup and partly pull through the valve seat. In the inclosed frame, when the crank case is not kept drained of oil, the breathing action of the valve causes the leather to become saturated with oil, rapidly destroy ing the valve. The best leather to use in replacing the worn valves is an acid- and oil-proof leather-belting stock; oak-tanned leather will not stand up very long. Figure 276 illustrates the leather valve used on the Fairbanks-Morse horizontal engine, while Fig. 277 is the valve used on their vertical engine.