Engine Details
[Figs. 29] to [37] show various engine details, and illustrate in a simple way some of the more important principles involved in steam engine design.
A partial cross-section of an adjustable piston is shown in [Fig. 29], and a longitudinal section of the same piston in [Fig. 30]. The principal feature to be emphasized is the method of automatic expansion employed to take up any wear and keep the piston tight. In setting up the piston a hand adjustment is made of the outer sleeve or ring R by means of the set-screws AA. Ring R is made in several sections, so that it may be expanded in the form of a true circle. Further tightness is secured without undue friction by means of the packing ring P which fits in a groove in R and is forced lightly against the walls of the cylinder by a number of coil springs, one of which is shown at S. As the cylinder and piston become worn, screws A are adjusted from time to time, and the fine adjustment for tightness is cared for by the packing ring P and the coil springs S.
Fig. 31. A Typical Cross-head
The points to be brought out in connection with the cross-head are the methods of alignment and adjustment. A typical cross-head is shown in cross and longitudinal sections in [Fig. 31]. Alignment in a straight line, longitudinally, is secured by the cylindrical form of the bearing surfaces or shoes, shown at S. These are sometimes made V-shaped in order to secure the same result. The wear on a cross-head comes on the surfaces S, and is taken up by the use of screw wedges W, shown in the longitudinal section. As the sliding surfaces become worn, the wedges are forced in slightly by screwing in the set-screws and clamping them in place by means of the check-nuts.
Figs. 32 and 33. Methods Commonly Used for Taking Up Wear in a Connecting-rod
The method commonly employed in taking up the wear in a connecting-rod is shown in [Figs. 32] and [33]. The wear at the wrist-pin is taken by the so called brasses, shown at B in the illustrations. The inner brass, in both cases, fits in a suitable groove, and is held stationary when once in place. The outer brass is adjustable, being forced toward the wrist-pin by a sliding wedge which is operated by one or more set-screws. In [Fig. 32] the wedge is held in a vertical position, and is adjusted by two screws as shown. The arrangement made use of in [Fig. 33] has the wedge passing through the rod in a horizontal position, and adjusted by means of a single screw, as shown in the lower view. With the arrangements shown, tightening up the brasses shortens the length of the rod. In practice the wedges at each end of the rod are so placed that tightening one shortens the rod, and tightening the other lengthens it, the total effect being to keep the connecting-rod at its original length.
| Fig. 34. Outboard Bearing for Corliss Type Engine | Fig. 35. Inner Bearing and Bed of Corliss Engine |
Fig. 36. Common Form of Throttling
Governor
A common form of outboard bearing for an engine of the slow-speed or Corliss type is illustrated in [Fig. 34]. The various adjustments for alignment and for taking up wear are the important points considered in this case. The plate B is fastened to the stone foundation by anchor bolts not shown. Sidewise movement is secured by loosening the bolts C, which pass through slots in the bearing, and adjusting by means of the screws S. Vertical adjustment is obtained by use of the wedge W, which is forced in by the screw A, as required. The inner bearing and bed piece of a heavy duty Corliss engine is shown in [Fig. 35]. The bearing in this case is made up of four sections, so arranged that either horizontal or vertical adjustment may be secured by the use of adjusting screws and check-nuts.
Engines of the slide-valve type are usually provided either with a fly-ball throttling governor, or a shaft governor. A common form of throttling governor is shown in [Fig. 36]. As the speed increases the balls W are thrown outward by the action of the centrifugal force, and being attached to arms hinged above them, any outward movement causes them to rise. This operates the spindle S, which, in turn, partially closes the balanced valve in body B, thus cutting down the steam supply delivered to the engine. The action of a throttling governor upon the work diagram of an engine is shown in [Fig. 38]. Let the full line represent the form of the diagram with the engine working at full load. Now, if a part of the load be thrown off, the engine will speed up slightly, causing the governor to act as described, thus bringing the admission and expansion lines into the lower positions, as shown in dotted lines.
The shaft governor is used almost universally on high-speed engines, and is shown in one form in [Fig. 37]. It consists, in this case, of two weights W, hinged to the spokes of the wheel near the circumference by means of suitable arms. Attached to the arms, as shown, are coil springs C. The ends of the arms beyond the weights are connected by means of levers L to the eccentric disk. When the engine speeds up, the weights tend to swing outward toward the rim of the wheel, the amount of the movement being regulated by the tension of the springs C. As the arms move outward, the levers at the ends turn the eccentric disk on the shaft, the effect of which is to change the angle of advance and shorten the cut-off. When the speed falls below the normal, the weights move toward the center and the cut-off is lengthened. The effect of this form of governor on the diagram is shown in [Fig. 39]. The full line represents the diagram at full load, and the dotted line when the engine is under-loaded.
Fig. 37. Shaft Governor for High-speed Engine
