this, but they would then be open to the same objection as raised against the Ramsbottom rings. The old method of pressing the ring out by means of dished springs or coach-springs, as shown in Fig. 27, is now seldom used in new engines; the objections to it are the uneven and unknown pressure exerted, and the reaction of the piston itself, from the fact of the springs pressing on it. It was a very difficult thing to so set every spring that the pressure on the ring was uniform; and the range of action of this form of spring is very limited, so that although the ring might be very tight when first fitted, after a few days' running it would be passing steam. The surface exposed to pressure too was small, and the springs were apt to bed themselves into the ring, and in doing so wear through their curved ends. These defects were partially remedied by adding to each one or more subsidiary springs on the principle of coach springs; but that only tended to aggravate the other evil spoken of-viz., the reaction of the piston itself. When a piston is moving through its course, and guided therein by the rod at one end and the tail rod (or back guides in case of a horizontal engine) at the other, it should be quite free laterally from the packing ring, which may follow its course freely. When the bore of the cylinder is quite true, and its axis coincides with the line of motion of the piston centre, it is of no consequence if the springs do bear on the piston; but if the cylinder wears somewhat out of truth in either direction, it is important that the spring-ring shall follow the sides of the cylinder freely; it cannot do this if the springs react from the piston body. FIG. 28. Cameron's Patent.-Fig. 28 shows a piston ring pressed out with a corrugated ribbon of steel; the lateral pressure here is obtained by the resistance of the spring to being bent into a circle, and by the pressure exerted by the corrugations when the ends of the spring are pressed apart. This spring exerts an almost uniform lateral pressure on the packing ring without touching the body of the piston, and by making the packing ring comparatively thin, it will adapt itself to the shape of the cylinder when worn. The pressure on the ring can also be easily and nicely adjusted by packing pieces between the ends of the spring. One great advantage of this spring is that it can be fitted to any piston without condemning any of the parts beyond the springs. PISTON RODS. The following rules will give results sufficiently accurate for all practical purposes: Diameter of piston-rod = Diameter of Cylinder. F The following are the values of F: Naval engines, direct-acting... return connecting rod, 2 rods medium stroke, oscillating. GUIDE BLOCKS AND SLIDES. Surface of Guide-block.-The area of the guide-block, or slipper-surface, on which the thrust is taken, should in no case be less than will admit of a pressure of 400 lbs. on the square inch; and for good working those surfaces which take the thrust when going ahead should be sufficiently large to prevent the maximum pressure per square inch exceeding 100 lbs. per square inch. When the surfaces are kept well lubricated this allowance may be exceeded, but the reduction in surface should be effected by making shallow grooves and recesses in the face of the slipper, in which the lubricant can lodge and impart itself to the guide as it is carried along. A good method of carrying this into effect is to provide a surface calculated on the allowance of 100 lbs. per square inch, and by cross-planing |