Pis′ton.
A device so fitted as to occupy the sectional area of a tube and be capable of reciprocation by pressure on either of its sides.
It may be of any shape corresponding accurately to the bore of the tube; but the cylindrical form is almost exclusively employed for both, as in the common pump and the steam-engine.
One of its sides is fitted to a rod, to which it either imparts reciprocatory motion, as in the steam-engine, or by which it is itself reciprocated, as in the pump.
In the former case, it has no opening leading from one side to the other, and is termed
solid, though generally not really so; but in the latter, an aperture controlled by a valve permits the
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passage of the fluid from one side to the other during its downward movement.
A distinction is, however, made in pumps; the
solid piston being known as a
plunger, the
hollow piston as a
bucket. The piston usually requires
packing to cause it to fit closely within its cylinder, and at the same time allow its free backward and forward motion.
For this purpose, its ends are usually formed by two connected disks, or have a deep annular groove between them for receiving the packing material, which may be hempen cord wound around it, or other somewhat expansible substance, which will not wear too rapidly nor cause excessive friction.
In modern practice, metallic rings, cut through at one side, so that their expansion may compensate for any wear, are largely employed in the steam-engine.
The simple form of piston required for the pump will be more fully explained under the head pump, this article being more particularly descriptive of the various kinds used in the steam-engine.
The earliest known use of a piston in a cylinder is found in some of the ancient blowing-machines of the native metallurgists of
Asia and
Africa.
This was much earlier than the air-pumps of Ctesibus, 150 B. C. See “Spiritalia Heronis.”
It is believed that
Papin's pistons were of wood, and that
Cartwright was the first to use a metallic piston.
One of
Cartwright's valves was in the piston, and was tripped by the contact of the valve-stem with the end of the cylinder.
James Watt found the piston imperfectly fitted in a roughly bored cylinder.
The principal packing was a body of water on the upper surface.
He patented the lubricant, 1769, preferring mutton tallow, but hemp and tallow had long been used.
To this be sometimes added plumbago-dust.
A common piston till a comparatively late date was a double cone of wood banded with leather strips.
This was superseded by a metallic piston, turned to fit neatly in the cylinder, and having a circular plate on each side clamping two leathers, cupped and chamfered off to an angle of 45°.
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Steam-engine pistons. |
The piston for the atmospheric engine was a plate of castiron, about 1 inch less in diameter than the cylinder, and 1 1/3 inch in thickness, with a rim about 4 inches from the edge.
Beyond this a flat ring is fitted and the parts are screwed together, after hemp and tallow have been inserted between them to form the packing.
The hemp-packed piston (
a, Fig 3757) is in common use. The bottom is accurately fitted to the cylinder, and a portion of the periphery is cut away to admit of a gasket of unspun hemp or soft rope to be wound evenly around it, to form the packing.
This is compressed by means of a plate and screws, so as to expand it against the inside surface of the cylinder, making a steam-tight joint.
The piston-rod is attached by an enlarged head, screw-nut, and key to the bottom of the cylinder, which has a central boss to strengthen it at that point.
The piston is lubricated by melted tallow or oil introduced into the cylinder by a funnel and a pipe with a double stop-cock.
Wolf's piston (
b) was so arranged as to be tightened without removing the cylinder cover.
A screw-thread is cut on the cylinder-rod, which is also surrounded by a toothed wheel engaging a pinion on a vertical rod passing through the cylinder cover.
By turning this, the screw-thread is caused to draw the two piston-heads together, tightening the packing.
Cartwright's piston (
a) resembled the wedge metallic piston in construction.
The pieces of which it is formed having a determined curvature, and being too stiff to yield readily, it did not well adapt itself to inequalities in the cylinder, if the latter had not been accurately bored.
Brass rings as a substitute for the hempen packing in pistons was invented by
Murdock and
Aiken of
Glasgow, in 1813.
The wedge metallic piston (
e) is formed by rings cut into a number of parts, pressed upon the cylinder by wedges, which are kept in their places by springs.
Jessop's piston (
d) has an expanding coil of metal of spiral form surrounding the piston-body between the two pistonheads, filled in with hemp packing, which assists the outward pressure of the coil and helps to prevent the passage of steam.
The spring metallic piston (
f) has wedges inserted behind the metallic rings, forming its perimeter, which bear against springs that force the rings outward into contact with the surface of the cylinder.
In
g the wedges are dispensed with, the springs bearing directly against the metallic rings.
In Askwith's packing arrangement (
A,
Fig. 3758), a divided ring is inclosed between the piston-heads
d e; a recess is formed at the point where its two ends nearly meet, into which a pivoted partition piece
a shuts, so a to prevent the passage of steam through the cut
f: on the interior is a toothed rack
a, in which a spring engages, to maintain the expansion of the ring, which is regulated by set screws
c e, working through the flange g on the piston-head: a small quantity of steam is admitted between this flange and the ring, so that the piston is both steam and metal packed.
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Steam-engine pistons. |
In Jacob's (
B), the packing is effected by steam entering through the apertures
i j, alternately above and below the plug
h, which acts as a valve, and passes to the back of the ring
f, the wedge-shaped face of which, under the influence of the steampressure, serves to force outward and apart the outer segmental rings
g g, causing them to act equally against the interior surface of the cylinder, forming an effectual packing.
The following may be cited as the distinguishing features of pistons.
They principally concern the lapping of rings, which are forced out by various means.
Screws alone.
Springs alone, supported.
Springs alone, unsupported.
Springs and wedges.
Springs and screws.
Wedges and screws.
Wedge, rack, and spring.
Cone and axial screw.
Springs on outside with wedges.
Steam admitted through apertures beneath rings.
Steam admitted through face of packing.
Steam admitted through valves.
Flexible outside disks.
Pistons with grooves for wire or jute.
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Fig. 3759 shows a number of steam-pistons with peculiar modes of packing.
a a′ is Askwith's, 1867, in which the divided portion is covered by a break-joint plate held to its seat by a spring.
The outer ring is set out by screws projecting from the inner ring.
a′ shows a dowel-plate.
b is
Brown's, 1869, which has metallic springs, thrusting outwardly the elastic break-joint rings, and seated upon a supporting boss.
c is
Fairbanks's, 1872, having a corrugated expanding spring, screwed fast to the packing-ring at its midlength.
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Steam-pistons, with different modes of packing. |
d d′ is
Lowe's, 1866, which has a beveled spiral spring, inclosed between a head and follower, and expanding the rings.
d′ shows the detached ring expanded laterally and vertically.
e,
Bower and Qualter's piston, 1866, has springs driven against the rings by the force of wedges at the back.
The wedges are set up by screws.
f,
Hoagland's, 1857.
By turning the axial screw the conical shaft is driven against the radial bars, which drive against the springs and expand the circumferential ring.
g is
Blake's piston, 1871, a part of which is shown with the cover removed.
It has screws which force a body of soft vulcanite or metallic springs against the outer rings of the piston.
The screws are turned by bars.
h is the Huss piston, 1871.
combining wedges and screws.
By turning the screws, the radial bars projecting from the central boss are forced against the wedges on the inside of the expanding ring.
i is
Allen's piston, 1873.
It has wedges and springs, and a rack to hold it to its set. The portion of the wedge-plate at
z is broken away to expose the rod and the spring which bears against the segment which distends the packing-ring.
The rotation of the wedge-plate acts upon each of the three.
j is
Carey's piston, 1873, which has an axial screw and a conical nut, the latter pushing against the stems of the followers to force outwardly the peripheral piston-ring.
k k′ are
Adams's piston, 1868, in which the piston-rings are wedge-shaped at their overlapping portions, and are driven outwardly by the lateral pressure of a wedge actuated by springs on the face of the piston.
l is
Buchanan's piston, 1865, in which the piston-rings are expanded by steam admitted behind them, through apertures on each face leading to the groove in which the rings work.
These openings are at three points of the circle, and show at
y y in the figure.
m is the
Dunbar piston, 1860; and is somewhat similar to the last stated.
n, the Hunt piston, 1867, has a T-shaped packing-ring, whose
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stem is packed in its groove, and which is expanded by steam below its extended flanges.
o is the
Smith piston, 1867.
Steam is admitted through a central groove in the packing, and passes behind the rings.
p, Witty's valve, 1869, in which steam is admitted through valve-ways to the space behind the ring.
The contact of the valve-stem with the end of the cylinder actuates the valve.
q is
Hedden's valve, 1863, in which steam is admitted by valves in the piston into the space behind the expansible ring.
r is
Cabell's piston, 1866.
It has flexible outside disks, whose peripheries move in contact with the surface of the cylinder.
s is
Cameron's, 1866.
It has a coil of wire in a spiral groove around the piston.
t is
Clark's, 1865.
It has a grooved periphery for wire or jute.
Gale's patent, July 21, 1857, has a piston with circumferential grooves occupied by steam and water of condensation, which act as a packing.
The same feature is found also in the stuffingbox, which is grooved interiorly.
The dimensions and evaporative capacity of the steam-generator for a given duty are determined by the number of cubic feet of water required to propel a piston of a given area a given number of feet per hour.
Say, for instance, that to overcome the resistance of the machinery to be moved a steam pressure of 20 pounds per square inch on a piston having an area of 5 square feet will be required.
Assuming the required speed of the piston to be 200 feet per minute to give the necessary velocity to the machinery, the calculation stands thus: The speed of 200 feet per minute is 12,000 feet per hour; the quantity of steam at 20 pounds' pressure to the square inch is, therefore, 12,000 × 5 (the area of the piston) = 60,000 cubic feet of steam per hour.
Steam at 20 pounds' pressure to the square inch bears the proportion to water of 1,281 to 1.
Therefore, 60,000 ÷ 1,281 = 47, nearly, which represents 47 cubic feet of water evaporated per hour; say, equal to 47-horse power.
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Piston-bellows. |
