A _piston_ is generally a cylindrical piece which slides backwards and
forwards inside a hollow cylinder. The piston may be moved by the action
of fluid pressure upon it as in a steam-engine, or it may be used to
give motion to a fluid as in a pump.
A piston is usually attached to a rod, called a _piston rod_, which
passes through the end of the cylinder inside which the piston works,
and which serves to transmit the motion of the piston to some piece
outside the cylinder, or _vice versa_.
[Illustration: FIG. 46.]
A _plunger_ is a piston made in one piece with its piston rod, the
piston and the rod being of the same diameter.
A piston which is provided with one or more valves which allow the
fluid to pass through it from one side to the other is called a
_bucket_.
_Simple Piston._--The simplest form of piston is a plain cylinder
fitting accurately another, inside which it moves. Such a piston works
with very little friction, but as there is no adjustment for wear, such
a piston is not suitable for a high fluid pressure if it has to work
constantly. This simple form of piston is used in the steam-engine
indicator, and also in pumps.
Fig. 46 shows the piston of the circulation pump of a marine engine.
A is the cast-iron casing or barrel of the pump; B is a brass liner
fitting tightly into the former at its ends, and secured by eight
screwed Muntz metal pins C, four at each end; D is the piston, which is
made of brass, and is attached to a Muntz metal piston rod E. The liner
is bored out smooth and true from end to end, and the piston is turned
so as to be a sliding fit to the liner. The wear in this form of piston
is diminished by making the rubbing surface large.
EXERCISE 46: _Piston for Circulating Pump._--Draw the vertical
sectional elevation of the piston, &c., shown in fig. 46, also a
half plan and half horizontal section through the centre. Scale 4
inches to a foot.
_Pump Bucket._--The next form of piston which we illustrate is shown in
fig. 47. This represents the air-pump bucket of a marine engine. The
bucket is made of brass, and is provided with six india-rubber disc
valves. The rod is in this case made of Muntz metal. Air-pump rods for
marine engines are very often made of wrought iron cased with brass. It
will be observed that there is a wide groove around the bucket, which is
filled with hempen rope or gasket. This gasket forms an elastic packing
which prevents leakage. This is an old-fashioned form of packing, and is
now only used for pump buckets.
[Illustration: FIG. 47.]
EXERCISE 47: _Air-pump Bucket._--Draw the sectional elevation of
the air-pump bucket shown in fig. 47. Also draw a half plan looking
downwards and a half plan looking upwards. Scale 4 inches to a
foot.
_Ramsbottom's Packing._--The form of packing used in the air-pump
bucket, fig. 47, is not suitable for steam pistons. For the latter the
packing is now always metallic. The simplest form of metallic packing is
that known as Ramsbottom's. This form is very largely used for
locomotive pistons, and for small pistons in many kinds of engines
besides. A locomotive piston for an 18-inch cylinder with Ramsbottom's
packing is shown in fig. 48. The particular piston there illustrated is
made of brass, and is secured to a wrought-iron piston rod by a brass
nut. Two circumferential grooves of rectangular section are turned out
of the piston, and into these fit two corresponding rings, which may be
of brass, cast iron, or steel. In this example the rings are of cast
iron. These rings are first turned a little larger in diameter than the
bore of the cylinder (in this example 1/2 inch), and then sprung over
the piston into the groves prepared for them. Their own elasticity
causes the rings to press outwards on the cylinder. At the point where a
ring is split a leakage of steam will take place, but with quick-running
pistons this leakage is unimportant. The points where the rings are cut
should be placed diametrically opposite, so as to diminish the leakage
of steam.
[Illustration: FIG. 48.]
EXERCISE 48: _Locomotive Piston._--A part elevation and part
section of a locomotive piston, for a cylinder having a bore 18
inches in diameter, is shown in fig. 48. Draw this, and also a view
looking on the nut in the direction of the axis of the piston rod.
Scale 6 inches to a foot.
_Note._--The reason why the part of the piston rod within the
piston has such a quick taper is that the piston has to be taken
off the rod while it is in the cylinder. The cross-head being
forged on the end of the piston rod prevents the piston and piston
rod being withdrawn together.
_Large Pistons._--Pistons of large diameter are generally provided with
two cast-iron packing rings placed within the same groove. These rings
are pressed outwards against the cylinder, and also against the sides of
the groove by one or more springs. One form of this packing
(Lancaster's) is shown in fig. 49. Here one spring only is used, and it
is first made a straight spiral spring, and then bent round and its ends
united. The action of the spring will be clearly understood from the
illustration. For the purpose of admitting the packing rings the piston
is divided into two parts, one the piston proper, and the other the
_junk ring_. In fig. 49, A is the junk ring, which is secured to the
piston by means of bolts as shown.
[Illustration: FIG. 49.]
EXERCISE 49: _Marine Engine Piston._--The piston illustrated by
fig. 49 is for the high-pressure cylinder of a marine engine. The
piston, junk ring, and packing rings are of cast iron. The piston
rod and nut are of wrought iron, so also are the junk ring bolts.
The nuts for the latter are of brass. The spiral spring is made
from steel wire 3/8 inch diameter. An enlarged section of one of
the packing rings is shown at (_a_). A front elevation of the
locking arrangement for the piston rod nut is shown at (_b_). A
sectional plan of one of the nuts for the junk ring bolts is shown
at (_c_).
First draw the vertical section of this piston, next draw a plan,
one-third of which is to show the piston complete, one-third to
show the junk ring removed, and the remaining third to be a
horizontal section through between the packing rings. The details
(_a_) and (_c_) need not be drawn separately. Scale 3 inches to a
foot.
_Proportions of Marine Engine Pistons._--Mr. Seaton, in his 'Manual of
Marine Engineering,' gives the following rules for designing marine
engine pistons:--
D = diameter of piston in inches.
_p_ = effective pressure in lbs. per square inch.
_x_ = D/50 x [sqrt (_p_)] + 1.
Thickness of front of piston near boss 0.2 x _x_.
" " " rim 0.17 x _x_.
" back of piston 0.18 x _x_.
" boss around rod 0.3 x _x_.
" flange inside packing ring 0.23 x _x_.
" " at edge 0.25 x _x_.
" junk ring at edge 0.23 x _x_.
" " inside packing ring. 0.21 x _x_.
" " at bolt-holes 0.35 x _x_.
" metal around piston edge 0.25 x _x_.
Breadth of packing ring 0.63 x _x_.
Depth of piston at centre 1.4 x _x_.
Lap of junk ring on piston 0.45 x _x_.
Space between piston body and packing ring 0.3 x _x_.
Diameter of junk-ring bolts 0.1 x _x_ + .25 inch.
Pitch of junk-ring bolts 10 diameters.
Number of webs in piston (D + 20)/12.
Thickness " 0.18 x _x_.
EXERCISE 50: _Design for Marine Engine Piston._--Calculate by
Seaton's rules the dimensions for a marine engine piston 40 inches
in diameter, and subjected to an effective pressure of 36 lbs. per
square inch. Then make the necessary working drawings for this
piston to a scale of, say, 3 inches to a foot.
_Note._--Take the dimensions got by calculation to the nearest
1-16th of an inch.