of cast-iron, expanded by a V-shaped spring-ring. The packing-rings are expanded by screwing down the cover of the piston. A large piston of another form, cast hollow or box-shaped, and fitted with cast-iron packingrings, is shown in Fig. 44. Piston-Rings.-Cast-iron works the best of all metals upon cast-iron, and is a good material for piston-rings. Cast-iron rings lose their elasticity when reduced by wear to five-eighths of their original thickness. The elasticity of the rings may be increased by hammering them on the inside. An alloy has been successfully used in marine engines, consisting of copper, 15 parts; tin, 5 parts; these rings, it is said, require no lubrication, do not score the cylinder, are very durable, and cause very little wear in the cylinder, which they soon work up to a polished face. Piston-rings should be cut diagonally to prevent scoring the cylinder. The piston-rings should traverse the entire bore of the cylinder, and a little beyond into the counter-bore, to avoid the formation of a ridge. Diameter of Crank-Shaft.-This should be proportioned to the strain upon it, by the rules on pages 141-143; but in a general way, the diameter of a wrought-iron crank-shaft may be to the diameter of cylinder multiplied by 4. The diameter of a steel-crank-shaft may be diameter of cylinder multiplied by 32. 0.32 ? Diameter of neck of crank-shaft, recessed in the crank-shaft Diameter of crank-shaft multiplied by 8. The fillet or corner of the neck should be formed with a good radius, to obviate fracture. Recessing the necks weakens the shaft, and it is better to form the necks by collars on the shaft. Length of neck of crank-shaft = diameter of crank-shaft multiplied by 16 for moderately high speeds, and by at least 2 for high speeds. Crank, Cast-iron, of the form shown in Figs. 45 and 46.- Diameter of boss for crank-shaft diameter of shaft multiplied by 2. Depth of boss = diameter of shaft. Crank to be shrunk on and keyed on with a key in width to 4th the diameter of shaft. Thickness of key width of key n ultiplied by 42. = = Diameter of the boss for crank-pin diameter of the crank-pin multiplied by 2.25. Depth of boss for crank-pin = diameter of crank-pin multiplied by 15. Crank-pin to be shrunk in and riveted at the back. Thickness of web of crank = diameter of crank-pin. Crank-Pin.-Diameter of crank-pin = diameter of cylinder multiplied by 20 to 24. It is frequently diameter of the neck of crank-shaft multiplied by '66. = Length of crank-pin diameter of crank-pin multiplied by 15. = When a crank-pin becomes strained by wear, and is not perfectly parallel to the crank-shaft, it is liable to heat, or knock. Eccentric. The eccentric or eccentric-tumbler is either formed with a recess on its circumference to guide the strap, as shown in Fig. 47, or with a projection as shown in Fig. 48. The latter design occupies the least room on the shaft and favours lubrication. Throw of eccentric when it works the valve direct = the travel of the slide-valve. Width of the recess for the eccentric-strap = diameter of cylinder multiplied by 18. Depth of recess in eccentric according to size. = from inch to inch Thickness of flange on each side of the recess inch to inch according to size. Diameter of boss of eccentric = diameter of shaft multiplied by 16. Depth of boss of eccentric diameter of shaft multiplied by 7. Eccentric-Strap.-Thickness when of cast-iron to its width multiplied by 67. An eccentric-strap is shown in Fig. 49, the rod of which is formed with a butt-end for ready adjustment. Thickness when of gun-metal the width of strap multiplied by '53. When the strap is iron lined with gun-metal, the thickness of the lining should be one-fourth that of the strap. White-metal makes a good lining for eccentric-straps. The distance-piece between the lugs of the strap should admit of removal without withdrawing the bolts. Eccentric-straps when bolted together, should only be made tight enough to move round by their own weight. Eccentric-Rod.-Diameter at slide-valve spindle-end diameter of slide-valve spindle. = Fig. 49.-Eccentric and Strap. Diameter at eccentric-strap end = diameter of slide-valve spindle multiplied by 1.3. diameter Feed-Pump, shown in Fig. 50.-The diameter of the ram may in most cases be diameter of cylinder when stroke of piston; and of cylinder when stroke of piston. = The wings of the suction and delivery valves should be placed at an angle, as shown in the engraving, so that when the valve lifts, the water will cause it to partly rotate and change its position on the seat at each beat, in order to prevent attachment of grit, and induce uniform wear of the faces of the valve and seat. Rules and data for pumps are given in Section II. A Wrought-iron Cross-Head, of the form shown in Fig. 51, has jaws cut out of the solid and is fitted with four slide-bars. Diameter of recessed part of boss A = diameter of piston-rod multiplied by 1.75. Length of recessed part of boss A diameter of piston-rod multiplied by 1.2. Diameter of collar at end of boss B = diameter of piston-rod multiplied by 2. Width of collar at end of boss B = diameter of piston-rod multiplied by 42. Thickness of fork at the boss C = diameter of piston-rod multiplied by '6. Thickness of fork below the boss D = diameter of piston-rod multiplied by 42. Diameter of the boss of the fork C = diameter of cross-head pin multiplied by 2. Diameter of cross-head pin E = diameter of crank-pin multiplied by 75. Length of cotter-hole in boss diameter of piston-rod multiplied by 8. Diameter of the slideblock-pin diameter of crosshead-pin multiplied by 75. Taper of hole in crosshead for piston-rod = of an inch per foot. = Slide-Block.-Width of sliding-surface of the slide-block of the crosshead, shown in Fig. 51, diameter of piston-rod for wrought-iron slidebars; and diameter of piston-rod multiplied by 14 when the slidebars are of cast-iron. = Thickness of slideblock= diameter of slideblock-pin multiplied by 18. Length of sliding surface of the slide-block = width of sliding surface multiplied by 3 to 4. has 2 A Wrought-Iron Cross-head, of the form shown in Fig. 52, slidebars, arranged one above and one below the cross-head, the slide-blocks being adjustab'e by lock-nuts on the slide-block-pin. Width of the sliding surface of the slide-block = diameter of piston-rod multiplied by 2. Length of sliding surface of slide-block width of sliding surface multiplied by 4. The vibrating action of the connecting-rod tends to rock the cross-head on its bearings. The tendency to rock practically ceases when the length of the slide-block is equal to two-thirds the length of the stroke. From centre of the slide-block-pin to the centre of the cross-head-pin = diameter of cross-head-pin multiplied by 25. From centre of the slideblock-pin to the outside of the collar on the end of the boss of cross-head The proportions of the = diameter of cross-head-pin multiplied by 25. fork and cross-head-pin may be found by the same rules as the other crosshead given above. Strips of white-metal are in some cases let into the bearing surfaces of slideblocks in order to reduce friction. A Steel Cross-Head for two slide-bars of another design is shown in Figs. 53 and 54. The slideblocks are V-shaped, adjustable by cotters with screw-ends and nuts. Slide-Bars of the form shown in Fig. 55 admit of ready adjustment. The bars are four in number, two being placed on each side of the cross-head. Slide-bars, width to diameter Depth of rib = width of bar multiplied by 7. Thickness of ribof the depth of the rib. Fig. 55.-Slide-Bars. Diameter of bolts for slide-bar width of slide-bar multiplied by '4. Slide-Bars, 2 in number, arranged above and I below the cross-head. = Width diameter of pistonrod multiplied by 2. The slide-bars should be recessed at each end, to prevent a ridge forming, due to wear, which would cause a knock or thump at each end of the stroke. The Connecting-rod with strap-end, shown in Fig. 56, Fig. 56.-Connecting Rod with strap-end. |