Piston-Speed.-The velocity with which steam can drive a piston is infinitely greater than can be adopted in practice. The speed of piston in feet per minute is found by multiplying twice the length of stroke in feet by the number of revolutions per minute of the crank-shaft. A piston with a given pressure upon it, will exert power in direct proportion to its speed, therefore an engine to work economically should work at as high a speed as is possible without heating and vibration. A high speed enables large measures of expansion to be used, and gives a smooth and uniform motion. A high speed engine requires wide bearings, and the momentum, or force stored up in its moving parts, should be accurately balanced, to enable it to run steadily without tremor; the piston can be balanced by compression, and the large end of the connecting rod, and the crank, should be balanced by a counterweight revolving opposite to the crank, so that both may revolve in the same plane of revolution. The piston-speed varies in different types of steam-engines from 250 to 1200 feet per minute. Stationary noncondensing steam-engines range from 250 to 300 feet per minute. The average piston-speed of modern stationary non-condensing simple steamengines is 260 feet per minute. The piston-speed of steam-engines using high rates of expansion, should not be less than 420 feet per minute, but rather 500 feet per minute.

Area of Steam-cylinder per nominal Horse-power.—It is usual to provide 9 square inches of cylinder-area for each nominal horse-power of a stationary non-condensing simple steam-engine.

The Diameter of Cylinder required to develop a given nominal Horse-power may be found by this Rule-multiply the nominal horsepower by 9, extract the square root of the product and multiply by 11283. For instance, a non-condensing simple steam-engine of 10 nominal horsepower, requires a cylinder of 10x9/9094868 x 11283 10'704, or say 10 inches diameter.

=

The Diameter of Cylinder required for the development of a given Indicated Horse-power may be found by this Rule :

-

For instance, the diameter of cylinder required for a steam-engine of 25 indicated horse-power, making 80 revolutions per minute; length of stroke 2 feet; mean pressure of steam 30 pounds per square inch is

33000 × 25 Indicated horse-power
7854 X 30 X (2 feet stroke x 2 x 80)

or say, in round numbers, 10 inches diameter.

=10'48 inches,

Steam-Cylinders should be made of best close-grained cast-iron, not less than twice cast, as hard as it can be worked, and perfectly free from honeycomb or other defects. They should be accurately and smoothly bored, and have all joints and surfaces planed or turned and scraped to a true surface, so that perfectly steam-tight joints may be obtained. They

should be bellmouthed at each end, to prevent the formation of a ridge due to wear, which would cause a knock or thump at each end of the stroke. Good mixtures of metal for cylinders are given at page 227. A cylinder of a horizontal steam-engine is shown in section in Fig. 38. Cylinders should be clothed with silicate-cotton, and lagged with wood to prevent

loss from radiation of heat and initial condensation of steam. The joint of the steam-chest-cover may be made either with copper-wire, lead-wire, or with a strand of asbestos-packing, laid twice round the joint in a mixture of white-lead and red-lead.

Thickness of Steam-Cylinder.-The following is the average thickness of cylinders of cast-iron, including allowance for reboring, for the ordinary pressures of steam used in stationary steam-engines.

Thickness of Cylinder-Ribs three-quarters the thickness of the metal

of the cylinder.

Thickness of Cylinder-Flanges thickness of cylinder x 1'25. Thickness of Metal of Steam-Passages=three-quarters the thickness of the cylinder.

Thickness of Cylinder-Covers = thickness of cylinder-flange multiplied by 83.

Thickness of Sole-Plate of Cylinder = thickness of cylinder multiplied by 125.

Area of Steam-Port-Sufficient area of steam-port should be provided to permit free ingress and egress of steam, and obtain a moderate velocity of steam. The area of the steam-ports, as shown in Fig. 39, should be proportioned to the speed of the piston, as explained on page 29. The area of each steam-port may, in a general way, be equal to the area of the cylinder in square inches divided by 12. The distance between the valve-seat and the cylinder should be as short as possible. The steampassages should be as straight and smooth as possible.

Fig. 39. Cylinder-Ports.

Length of Steam-Port.-The steam-port should be as long as possible, in order to obtain a large area of opening of the steamport for the admission of steam with a small movement of the valve. The length of the steam-port may be equal to the diameter of the cylinder in inches multiplied by 88.

Width of Steam-Port

the steam-port.

area of steam-port divided by the length of

Width of Exhaust-Port = width of steam-port multiplied by 2'25. Width of Bridge = width of steam-port divided by 1°37.

Area of Steam-Pipe.-The area of the steam-pipe may be found by the rule on page 29. It may, in a general way, be equal to the area of the cylinder in square inches divided by 16.

Area of Exhaust-Pipe.-The area of the exhaust-pipe may be found by the rule on page 29. It may, in a general way, be equal to the area of the cylinder in square inches divided by 12.

Diameter of Piston-Rod.—The diameter of a piston-rod of wroughtiron may be = diameter of cylinder in inches x 02 × square root of the initial pressure of the steam. For a piston-rod of steel, use a multiplier of 016 instead of '02. For instance, the size of a piston-rod for a cylinder of 12 inches diameter for an initial pressure of steam of 81 pounds per square inch is = 12×02 × 3/81 = 2·16 or say 2 inches, the diameter of a piston-rod of wrought iron; or 12 × 016 × 281 = 1728, or say inches, the diameter of a piston-rod of steel. For pressures of steam up to 100 lbs. per square inch, the diameter of the piston-rod may, in a general way, be diameter of cylinder divided by 6.2.

Diameter of Piston-Rod Stuffing-Box = diameter of piston-rod multiplied by 1.8.

Depth of Piston-Rod Stuffing-Box diameter of piston-rod multiplied by 16.

Depth of Gun-Metal Bush at bottom of Stuffing-Box = one-third diameter of piston-rod.

Thickness of Flange of Gland one-fourth more than the thickness of the gland.

Thickness of Metal round the Stuffing-Box = thickness of the gland multiplied by 15.

In some cases it is convenient to make the gland in halves, notched one into the other, so that it may be removed while the piston-rod is in its place. Diameter of Slide-Valve-Spindle.-The diameter of the spindle of a slide-valve, when of wrought-iron, may be diameter of cylinder in inches X0113x square root of the initial pressure of the steam. For a valvespindle of steel, use '009 as a multiplier instead of 0113. For instance, the size of a valve-spindle for the slide-valve of a cylinder of 12 inches diameter, initial pressure of steam 81 pounds per square inch, is=12 X 0113 × 2/81= 122, or say 1 inches, the diameter of a wrought-iron valve-spindle; or 12 × 009 × 81=1972, or say 1 inch the diameter of a valve-spindle of steel. For pressures of steam up to 100 pounds per square inch, the diameter of the slide-valve-spindle may, in a general way, be one-tenth the diameter of the cylinder.

Rope-Packing for the Stuffing-Boxes of Piston-Rods and ValveSpindles.-Rope formed of hemp, or cotton, and combining a mixture of soapstone and plumbago, makes an excellent packing for the stuffing-boxes of steam-engines.

Metallic-Packing for Stuffing-Boxes.-The stuffing-boxes of engines. using steam of very high pressure should be packed with metallic-packing. When it is applied in the form of

rings it may consist of one of the alloys for this purpose given further on. An efficient arrangement of metallic-packing, having the packing-rings compressed by a spiral-spring, is shown in Fig.40. Another kind of metallic-packing is formed of very fine brass-wire plaited into a square rope, and is applied in the same manner as hemp-packing. When the wire becomes warm, it expands and fills the stuffing-box. The elastic and flexible nature of the packing effects a yielding pressure on the piston-rod, but sufficient to secure a steam-tight stuffing-box.

Lubricant for Steam-Cylinders.-The wear and friction of pistonrings and valves is diminished by efficient lubrication, which should be constant and automatic. A good lubricant for cylinders consists of tallow I part; good mineral oil 2 parts. Another cylinder-oil consists of a mixture of lard oil, 1 part; plumbago, part; good mineral oil, 2 parts. An engine.

will work without cylinder-lubrication, but it is at the expense of increased friction and wear of the piston-rings and cylinder.

Outside Lap of Slide-Valve.-The lap of a slide-valve is determined by the distance the piston is required to travel before the steam is cut off, which may be found by the rule on page 25. Engines made on stock have frequently sufficient lap to cut off the steam at one-half the stroke.

Inside Lap of Slide-Valve = inch.

Stroke of Slide-Valve.-Add together the width of steam-port and the outside lap and multiply by 2, the product will be the stroke of the valve. The Face of a Slide-Valve should be planed true, but need not be scraped, because it soon becomes beded and polished by wear.

Clearance between the Piston and Cylinder-Cover at each end of the Stroke should be as small as practicable: in sinall engines it may be diameter of cylinder divided by 32.

The Length of Stroke of engines of strong construction is usually = diameter of cylinder multiplied by 2. Engines of light construction have generally a length of stroke = diameter of cylinder multiplied by 15.

Pistons are generally made of cast-iron of the same mixture of metal as that of the cylinder, but in some cases they are of gun-metal, steel, and wrought-iron. They are made in a great variety of forms of construction.

The width of the piston is generally equal to from one-fifth to onefourth the diameter of the cylinder. A simple and very efficient form of piston is shown in Fig. 41. It consists of a disc of metal fitted with two cast-iron packing-rings, inch wide, and inch thick, spaced inch apart in turned grooves. The rings are turned inch larger in diameter than the cylinder; a piece is cut out of the ring, and it is sprung over the end of the piston into its place. The rings only require about 5 lbs. of tension to keep them steam-tight. A large piston of this description has the body cast hollow, to lighten it, and the packing-rings are inserted in a cover which is bolted to the piston as shown in Fig. 42. Pistons of this form, under 18 inches diameter, may have two packing-rings; from 18 to 26 inches diameter three rings, and from 28 to 50 inches diameter four rings. Another kind of piston is shown in Fig. 43. It has packing-rings