Chapter III: Appendix: A 99 (2)

_Hangers._--When a shaft is suspended from a ceiling it is carried by hangers, one form of which is shown in fig. 29, and which will be readily understood. The cap of the bearing, it will be noticed, is secured by means of a bolt, and also by a square key.

EXERCISE 29: _Shaft Hanger._--Draw the two elevations shown in
fig. 29, and also a sectional plan. The section to be taken at a
point 5 inches above the centre of the shaft. Scale 6 inches to a
foot.

_Wall Boxes._--In passing from one part of a building to another a shaft may have to pass through a wall. In that case a neat appearance is given to the opening and a suitable support obtained for a pillow block by building into the wall a _wall box_, one form of which is shown in fig. 30.

EXERCISE 30: _Wall Box._--Draw the views of the wall box shown in
fig. 30, and also a sectional plan; the plane of section to pass
through the box a little above the joggles for the pillow block.
Scale 3 inches to a foot.

VIII. PULLEYS.

_Velocity Ratio in Belt Gearing._--Let two pulleys A and B be connected by a belt, and let their diameters be D_{1} and D_{2}; and let their speeds, in revolutions per minute, be N_{1} and N_{2} respectively. If there is no slipping, the speeds of the rims of the pulleys will be the same as that of the belt, and will therefore be equal. Now the speed of the rim of A is evidently = D_{1} x 3.1416 x N_{1}; while the speed of the rim of B is = D_{2} x 3.1416 x N_{2}. Hence D_{1} x 3.1416 x N_{1} = D_{2} x 3.1416 x N_{2}, and therefore

N_{1} D_{2}
----- = -----.
N_{2} D_{1}

_Pulleys for Flat Bands._--In cross section the rim of a pulley for carrying a flat band is generally curved as shown in figs. 31 and 32, but very often the cross section is straight. The curved cross section of the rim tends to keep the band from coming off as long as the pulley is rotating. Sometimes the rim of the pulley is provided with flanges which keep the band from falling off.

Pulleys are generally made entirely of cast iron, but a great many pulleys are now made in which the centre or nave only is of cast iron, the arms being of wrought iron cast into the nave, while the rim is of wrought sheet iron.

The arms of pulleys when made of wrought iron are invariably straight, but when made of cast iron they are very often curved. In fig. 31, which shows an arrangement of two cast-iron pulleys, the arms are straight; while in fig. 32, which shows another cast-iron pulley, the arms are curved. Through unequal cooling, and therefore unequal contraction of a cast-iron, pulley in the mould, the arms are generally in a state of tension or compression; and if the arms are straight they are very unyielding, so that the result of this initial stress is often the breaking of an arm, or of the rim where it joins an arm. With the curved arm, however, its shape permits it to yield, and thus cause a diminution of the stress due to unequal contraction.

The cross section of the arms of cast-iron pulleys is generally elliptical.

EXERCISE 31: _Fast and Loose Pulleys_.--Fig. 31 shows an
arrangement of fast and loose pulleys. A is the fast pulley,
secured to the shaft C by a sunk key; B is the loose pulley, which
turns freely upon the shaft. The loose pulley is prevented from
coming off by a collar D, which is secured to the shaft by a
tapered pin as shown. The nave or boss of the loose pulley is here
fitted with a brass liner, which may be renewed when it becomes
too much worn. Draw the elevations shown, completing the left-hand
one. Scale 6 inches to a foot.

By the above arrangement of pulleys a machine may be stopped or
set in motion at pleasure. When the driving band is on the loose
pulley the machine is at rest, and when it is on the fast pulley
the machine is in motion. The driving band is shifted from the one
pulley to the other by pressing on that side of the band which is
advancing towards the pulleys.

EXERCISE 32: _Cast-iron Pulley with Curved Arms and Cone
Keys_.--Draw a complete side elevation and a complete cross
section of the pulley represented in fig. 32 to a scale of 3
inches to a foot. In drawing the side elevation of the arms first
draw the centre lines as shown; next draw three circles for each
arm, one at each end and one in the middle; the centres of these
circles being on the centre line of the arm, and their diameters
equal to the widths of the arm at the ends and at the middle
respectively. Arcs of circles are then drawn to touch these three
circles. The centres and radii of these arcs may be found by
trial. The cone keys for securing the pulley to the shaft were
described on p. 23.

_Pulleys for Ropes_.--Ropes made of hemp are now extensively used for transmitting power. These ropes vary in diameter from 1 inch to 2 inches, and are run at a speed of about 4,500 feet per minute. The pulleys for these ropes are made of cast iron, and have their rims grooved as shown in fig. 33, which is a cross section of the rim of a pulley carrying three ropes. The angle of the V is usually 45 deg., and the rope rests on the sides of the groove, and not on the bottom, so that it is wedged in, and has therefore a good hold of the pulley. The diameter of the pulley should not be less than 30 times the diameter of the rope. Two pulleys connected by ropes should not be less than thirty feet apart from centre to centre, but this distance may be as much as 100 feet.

EXERCISE 33: _Section of Rim of Rope Pulley._--Draw, half size,
the section of the rim of a rope pulley shown in fig. 33.

IX. TOOTHED WHEELS.

_Pitch Surfaces of Spur Wheels._--Let two smooth rollers be placed in contact with their axes parallel, and let one of them rotate about its axis; then if there is no slipping the other roller will rotate in the opposite direction with the same surface velocity; and if D_{1}, D_{2} be the diameters of the rollers, and N_{1}, N_{2} their speeds in revolutions per minute, it follows as in belt gearing that--

N_{1} D_{2}
----- = -----.
N_{2} D_{1}

If there be considerable resistance to the motion of the follower slipping may take place, and it may stop. To prevent this the rollers may be provided with teeth; then they become _spur wheels_; and if the teeth be so shaped that the ratio of the speeds of the toothed rollers at any instant is the same as that of the smooth rollers, the surfaces of the latter are called the _pitch surfaces_ of the former.

_Pitch Circle._--A section of the pitch surface of a toothed wheel by a plane perpendicular to its axis is a circle, and is called a _pitch circle_. We may also say that the pitch circle is the edge of the pitch surface. The pitch circle is generally traced on the side of a toothed wheel, and is rather nearer the points of the teeth than the roots.

_Pitch of Teeth._--The distance from the centre of one tooth to the centre of the next, or from the front of one to the front of the next, _measured at the pitch circle_, is called the _pitch of the teeth_. If D be the diameter of the pitch circle of a wheel, _n_ the number of teeth, and _p_ the pitch of the teeth, then D x 3.1416 = _n_ x _p_.

By the diameter of a wheel is meant the diameter of its pitch circle.

_Form and Proportions of Teeth._--The ordinary form of wheel teeth is shown in fig. 34. The curves of the teeth should be cycloidal curves, although they are generally drawn in as arcs of circles. It does not fall within the scope of this work to discuss the correct forms of wheel teeth. The student will find the theory of the teeth of wheels clearly and fully explained in Goodeve's 'Elements of Mechanism,' and in Unwin's 'Machine Design.'

The following proportions for the teeth of ordinary toothed wheels may be taken as representing average practice:--

Pitch of teeth = _p_ = arc _a b c_ (fig. 34).
Thickness of tooth = _b c_ = .48_p_.
Width of space = _a b_ = .52_p_.
Total height of tooth = _h_ = .7_p_.
Height of tooth above pitch line = _k_ = .3_p_.
Depth of tooth below pitch line = _l_ = .4_p_.
Width of tooth = 2_p_ to 3_p_.

EXERCISE 34: _Spur Wheel._--Fig. 35 shows the elevation and
sectional plan of a portion of a cast-iron spur wheel. The diameter
of the pitch circle is 23-7/8 inches, and the pitch of the teeth is
1-1/2 inches, so that there will be 50 teeth in the wheel. The
wheel has six arms. Draw a complete elevation of the wheel and a
half sectional plan, also a half-plan without any section. Draw
also a cross section of one arm. Scale 4 inches to a foot.

_Mortise Wheels._--When two wheels gearing together run at a high speed the teeth of one are made of wood. These teeth, or cogs, as they are generally called, have tenons formed on them, which fit into mortises in the rim of the wheel. This wheel with the wooden teeth is called a _mortise wheel_. An example of a mortise wheel is shown in fig. 36.

_Bevil Wheels._--In bevil wheels the pitch surfaces are parts of cones. Bevil wheels are used to connect shafts which are inclined to one another, whereas spur wheels are used to connect parallel shafts. In fig. 36 is shown a pair of bevil wheels in gear, one of them being a mortise wheel. At (_a_) is a separate drawing, to a smaller scale, of the pitch cones. The pitch cones are shown on the drawing of the complete wheels by dotted lines.

The diameters of bevil wheels are the diameters of the bases of their pitch cones.

EXERCISE 35: _Pair of Bevil Wheels._--Draw the sectional elevation
of the bevil wheels shown in gear in fig. 36. Commence by drawing
the centre lines of the shafts, which in this example are at right
angles to one another; then draw the pitch cones shown by dotted
lines. Next put in the teeth which come into the plane of the
section, then complete the sections of the wheels. The pinion or
smaller wheel has 25 teeth, and the wheel has 50 teeth, which makes
the pitch a little over 3 inches. Each tooth of the mortise wheel
is secured as shown by an iron pin 5/16 inch diameter. Scale 3
inches to a foot.

X. CRANKS AND CRANKED SHAFTS.

The most important application of the crank is in the steam-engine, where the reciprocating rectilineal motion of the piston is converted into the rotary motion of the crank-shaft by means of the crank and connecting rod.

At one time steam-engine cranks were largely made of cast iron, now they are always made of wrought iron or steel. The crank is either forged in one piece with the shaft, or it is made separately and then keyed to it.

_Overhung Crank._--Fig. 37 shows a wrought-iron overhung crank. A is the crank-shaft, B the crank arm, provided at one end with a boss C, which is bored out to fit the shaft; at the other end of the crank arm is a boss D, which is bored out to receive the crank-pin E, which works in one end of the connecting rod. The crank is secured to the shaft by the sunk key F. It is also good practice to _shrink_ the crank on to the shaft. The process of shrinking consists of boring out the crank a little smaller than the shaft, and then heating it, which causes it to expand sufficiently to go on to the shaft. As the crank cools, it shrinks and grips the shaft firmly. The crank may also be shrunk on to the crank-pin, the latter being then riveted over as shown in fig. 37.

A good plan to adopt in preference to the shrinking process is to force the parts together by hydraulic pressure. This method is adopted for placing locomotive wheels on their axles, and for putting in crank-pins. As to the amount of pressure to be used, the practice is to allow a force of 10 tons for every inch of diameter of the pin, axle, or shaft.

Instead of being riveted in, the crank pin may be prolonged and screwed, and fitted with a nut. Another plan is to put a cotter through the crank and the crank-pin.

The distance from the centre of the crank-shaft to the centre of the crank-pin is called the radius of the crank. The _throw_ of the crank is twice the radius. In a direct-acting engine the throw of the crank is equal to the stroke of the piston.

EXERCISE 36: _Wrought-iron Overhung Crank._--Draw the two
elevations shown in fig. 37, also a plan. Scale 1-1/2 inches
to a foot.

_Proportions of Overhung Cranks._

D = diameter of shaft.
_d_ = " crank-pin.
Length of large boss = .9 D.
Diameter " = 1.8 D.
Length of small boss = 1.1 _d_.
Diameter " = 1.8 _d_.
Width of crank arm at centre of shaft = 1.3 D.
" " crank-pin = 1.5 _d_.
The thickness of the crank arm may be roughly taken as = .7 D.

EXERCISE 37.--Design a wrought-iron crank for an engine having a
stroke of 4 feet. The crank-shaft is 9 inches in diameter, and
the crank-pin is 4-3/4 inches in diameter and 6-1/2 inches long.

_Locomotive Cranked Axle._--As an example of a cranked shaft we take the cranked axle for a locomotive with inside cylinders shown in fig. 38; here the crank and shaft or axle are forged in one piece. A is the wheel seat, B the journal, C the crank-pin, and D and E the crank arms. Only one half of the axle is shown in fig. 38, but the other half is exactly the same. The cranks on the two halves are, however, at right angles to one another. The ends of the crank arms are turned in the lathe, the crank-pin ends being turned at the same time as the axle, and the other ends at the same time as the crank-pin. This consideration determines the centres for the arcs shown in the end view.

EXERCISE 38.--Draw to a scale of 2 inches to a foot the side and
end elevations of the locomotive cranked axle partly shown in
fig. 38. The distance between the centre lines of the cylinders
is 2 feet.

_Built-up Cranks._--The form of cranked shaft shown in fig. 38 is largely used for marine engines, but for the very powerful engines now fitted in large ships this design of shaft is very unreliable, the built-up crank shown in fig. 39 being preferred, although it is much heavier than the other. It will be seen from the figure that the shaft, crank arms, and crank-pin are made separately. The arms are shrunk on to the pin and the shaft, and secured to the latter by sunk keys. These heavy shafts and cranks are generally made of steel.

EXERCISE 39.--Keeping to the dimensions marked in fig. 39, draw
the views there shown of a built-up crank-shaft for a marine
engine. Scale 3/4 inch to a foot.

XI. ECCENTRICS.

The _eccentric_ is a particular form of crank, being a crank in which the crank-pin is large enough to embrace the crank-shaft. In the eccentric what corresponds to the crank-pin is called the sheave or pulley. The advantage which an eccentric possesses over a crank is that the shaft does not require to be divided at the point where the eccentric is put on. The crank, however, has this advantage over the eccentric, namely, that it can be used for converting circular into reciprocating motion, or _vice versa_, while the eccentric can only be used for converting circular into reciprocating motion. This is owing to the great leverage at which the friction of the eccentric acts.

The chief application of the eccentric is in the steam-engine, where it is used for working the valve gear.

To permit of the sheave being placed on the shaft without going over the end (which could not be done at all in the case of a cranked axle, and would be a troublesome operation in most cases) it is generally made in two pieces, as shown in fig. 40, which represents one of the eccentrics of a locomotive. The two parts of the sheave are connected by two cotter bolts. The part which embraces the sheave is called the eccentric strap, and corresponds to, and is, in fact, a connecting rod end: the rod proceeding from this is called the eccentric rod.

The distance from the centre of the sheave to the centre of the shaft is called the _radius_ or _eccentricity_ of the eccentric. The _throw_ is twice the eccentricity.

The sheave is generally made of cast iron. The strap may be of brass, cast iron, or wrought iron; when the strap is made of wrought iron it is commonly lined with brass.

EXERCISE 40: _Locomotive Eccentric._--In fig. 40 D E is the
sheave, F H the strap, and K the eccentric rod. The sheave and
strap are made of cast iron, and the eccentric rod is made of
wrought iron. (_a_) is a vertical cross section through the
oil-box of the strap; (_b_) is a plan of the end of the eccentric
rod and part of the strap. All the nuts are locked by means of
cotters. Draw first the elevation, partly in section as shown.
Next draw two end elevations, one looking each way. Afterwards
draw a horizontal section through the centre, and also a plan.
Scale 4 inches to a foot.

XII. CONNECTING RODS.

The most familiar example of the use of a connecting rod is in the steam-engine, where it is used to connect the rotating crank with the reciprocating piston. The rod itself is made of wrought iron or steel, and is generally circular or rectangular in section. The ends of the rod are fitted with steps, which are held together in a variety of ways.

_Strap End._--A form of connecting rod end, which is not so common as it used to be, is shown in fig. 41. At (_a_) is shown a longitudinal section with all the parts put together, while at (_b_), (_c_), _(d)_ and (_e_) the details are shown separately. A B is the end of the rod which butts against the brass bush C D, which is in two pieces. A _strap_ E passes round the bush and on to the end of the rod as shown. The arms of the strap have rectangular holes in them, which are not quite opposite a similar hole in the rod when the parts are put together. If a wedge or _cotter_ F be driven into these three holes they will tend to come into line, and the parts of the bush will be pressed together. To prevent the cotter opening out the strap, and to increase the sliding surface, a _gib_ H is introduced. The gib is provided with horns at its ends to keep it in its place. Sometimes two gibs are used, one on each side of the cotter; this makes the sliding surface on both sides of the cotter the same. The cotter is secured by a set screw K. The unsectioned portion of fig. (_a_) to the right of the gib, or to the left of the cotter, is called the _clearance_ or _draught._

EXERCISE 41: _Connecting Rod End._--Make the following views of
the connecting rod end illustrated by fig. 41. First, a vertical
section, the same as shown at (_a_). Second, a horizontal section.
Third, side elevation. Fourth, a plan. Or the first and third
views may be combined in a half vertical section and half
elevation; and the second and fourth views may be combined in
a half horizontal section and half plan.

All the dimensions are to be taken from the detail drawings (_b_),
(_c_), (_d_), and (_e_), _but the details need not be drawn
separately_. The brass bush is shown at (_d_) by half elevation,
half vertical section, half plan, and half horizontal section.
The draught or clearance is 7-16ths of an inch.

_Box End._--At (_a_), fig. 42, is shown what is known as a box end for a connecting rod. The part which corresponds to the loose strap in the last example is here forged in one piece with the connecting rod. In this form the brass bush is provided with a flange all round on one side, but on the opposite side the flange is omitted except at one end; this is to allow of the bush being placed within the end of the rod. The construction of the bush will be understood by reference to the sketch shown at (_b_). The bush is in two parts, which are pressed tightly together by means of a cotter. This cotter is prevented from slackening back by two set screws. Each set screw is cut off square at the point, and presses on the flat bottom of a very shallow groove cut on the side of the cotter.

The top, bottom, and ends of this box end are turned in the lathe at the same time as the rod itself; this accounts for the curved sections of these parts.

It is clear from the construction of a box end that it is only suitable for an overhung crank.

EXERCISE 42: _Locomotive Connecting Rod._--In fig. 42 is shown a
connecting rod for an outside cylinder locomotive. (_a_) is the
crank-pin end, and (_c_) the cross-head end. The end (_a_) has just
been described under the head 'box end.' We may just add that in
this particular example the brass bush is lined with white metal as
shown, and that the construction of the oil-box is the same as that
on the coupling rod end shown in fig. 44. The end (_c_) is forked,
and through the prongs of the fork passes the cross-head pin, of
which a separate dimensioned drawing is shown at (_d_). Observe
that the tapered parts A and B of this pin are parts of the same
cone. The rotation of the pin is prevented by a small key as shown.
The cross-head pin need not be drawn separately, and the isometric
projection of the bush at (_b_) may be omitted, but all the other
views shown are to be drawn to a scale of 6 inches to a foot.

_Marine Connecting Rod._--The form of connecting rod shown in fig. 43 is that used in marine engines, but it is also used extensively in land engines. A B is the crank-pin end, and C the cross-head end. The end A B is forged in one piece, and after it is turned, planed, and bored it is slotted across, so as to cut off the cap A. The parts A and B are held together by two bolts as shown. This end of the rod is fitted with brass steps, which are lined with white metal. The cross-head end is forked, and through the prongs of the fork passes a pin D, which also passes through the cross-head, which is forged on to the piston rod or attached to it in some other way.

EXERCISE 43: _Marine Connecting Rod._--Draw all the views shown in
fig. 43 of one form of marine connecting rod. For detail drawings
of the locking arrangement for the nuts see fig. 19, page 21. Scale
4 inches to a foot.

_Coupling Rods._--A rod used to transmit the motion of one crank to another is called a _coupling rod_. A familiar example of the use of coupling rods will be found in the locomotive. Coupling rods are made of wrought iron or steel, and are generally of rectangular section. The ends are now generally made solid and lined with solid brass bushes, _without any adjustment for wear_. This form of coupling rod end is found to answer very well in locomotive practice where the workmanship and arrangements for lubrication are excellent. When the brass bush becomes worn it is replaced by a new one.

Fig. 44 shows an example of a locomotive coupling rod end for an outside cylinder engine. In this case it is desirable to have the crank-pin bearings for the coupling rods as short as possible, for a connecting rod and coupling rod in this kind of engine work side by side on the same crank-pin, which, being overhung, should be as short as convenient for the sake of strength. The requisite bearing surface is obtained by having a pin of large diameter. The brass bush is prevented from rotating by means of the square key shown. The oil-box is cut out of the solid, and has a wrought-iron cover slightly dovetailed at the edges. This cover fits into a check round the top inner edge of the box, which is originally parallel, but is made to close on the dovetailed edges of the cover by riveting. A hole in the centre of this cover, which gives access to the oil-box, is fitted with a screwed brass plug. The brass plug has a screwed hole in the centre, through which oil may be introduced to the box. Dust is kept out of the oil-box by screwing into the hole in the brass plug a common cork. The oil is carried slowly but regularly from the oil-box over to the bearing by a piece of cotton wick.

EXERCISE 44: _Coupling Rod End._--Draw first the side elevation and
plan, each partly in section as shown in fig. 44. Then instead of
the view to the left, which is an end elevation partly in section,
draw a complete end elevation looking to the right, and also a
complete vertical cross section through the centre of the bearing.
Scale 6 inches to a foot.

XIII. CROSS-HEADS.

An example of a steam-engine cross-head is shown in fig. 45. A is the end of the piston rod which has forged upon it the cross-head B. The cross-head pin shown at (_d_), fig. 42, and to which the connecting rod is attached, works in the bearing C. Projecting pieces D, forged on the top and bottom of the cross-head, carry the slide blocks E which work on the slide bars, and thus guide the motion of the piston rod.

EXERCISE 45: _Locomotive Cross-head._--In fig. 45 are shown side
and end elevations, partly in section, of the cross-head and slide
blocks for an outside cylinder locomotive. Draw these views half
size, showing also on the end elevation the cross-head pin and a
vertical section of the connecting rod end from fig. 42. The bush
in the cross-head which forms the bearing for the cross-head pin is
of wrought iron, case-hardened, and is prevented from rotating by
the key shown. The cross-head is of wrought iron, and the slide
blocks are of cast iron, and are fitted with white metal strips as
shown. A short brass tube leads oil from the upper slide block into
a hole in the cross-head as shown, which carries it to a slot in
the bush which distributes it over the cross-head pin.

XIV. PISTONS.

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_.

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.

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.

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.

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.

XV. STUFFING-BOXES.

In fig. 50 is shown a gland and stuffing-box for the piston rod of a vertical engine. A B is the piston rod, C D a portion of the cylinder cover, and E F the _stuffing-box_. Fitting into the bottom of the stuffing-box is a brass bush H. The space K around the rod A B is filled with _packing_, of which there is a variety of kinds, the simplest being greased hempen rope. The packing is compressed by screwing down the cast-iron gland L M, which is lined with a brass bush N. In this case the gland is screwed down by means of three stud-bolts P, which are screwed into a flange cast on the stuffing-box. Surrounding the rod on the top of the gland there is a recess R for holding the lubricant.

The object of the gland and stuffing-box is to allow the piston rod to move backwards and forwards freely without any leakage of steam.

Fig. 51 shows a gland and stuffing-box for a horizontal rod. The essential difference between this example and the last is in the mode of lubrication. The gland flange has cast within it an oil-box which is covered by a lid; this lid is kept shut or open by the action of a small spring as shown. A piece of cotton wick (not shown in the figure) has one end trailing in the oil in the oil-box, while the other is carried over and passed down the hole A B. The wick acts as a siphon, and drops the oil gradually on to the rod. In this example only two bolts are used for screwing in the gland; and the flanges of the gland and stuffing-box are not circular, but oval-shaped.

In the case of small rods the gland is made entirely of brass, and no liner is then necessary. Fig. 52 shows a form of gland and stuffing-box sometimes used for small rods. The stuffing-box is screwed externally, and carries a nut A B which moves the gland.

EXERCISE 51: _Gland and Stuffing-box for a Vertical Rod._--Draw the
views shown in fig. 50 to the dimensions given. Scale 6 inches to a
foot.

EXERCISE 52: _Gland and Stuffing-box for a Horizontal Rod._--Fig.
51 shows a plan, half in section, and an elevation half of which is
a section through the gland flange. Draw these to a scale of 6
inches to a foot, using the dimensions marked in the figure.

EXERCISE 53: _Screwed Gland and Stuffing-box._--Draw, full size,
the views shown in fig. 52 to the given dimensions.

A more elaborate form of gland and stuffing-box is shown in fig. 53. This is for a large marine engine with inverted cylinders, such as is used on board large ocean steamers. The stuffing-box is cast separate from the cylinder cover to which it is afterwards bolted. The lubricant is first introduced to the oil-boxes marked A, from which it passes to the recess B, where it comes in contact with the piston rod. To prevent the lubricant from being wasted by running down the rod, the main gland is provided with a shallow gland and stuffing-box which is filled with soft cotton packing, which soaks up the lubricant.

The main gland is screwed up by means of six bolts, and to prevent the gland from locking itself in the stuffing-box, it is necessary that the nuts should be turned together. This is done in a simple and ingenious manner. One-half of each nut is provided with teeth, and these gear with a toothed wheel which has a rim only; this rim is held up by a ring C. When one nut is turned, all the rest follow in the same direction.

EXERCISE 54: _Gland and Stuffing-box for Piston Rod of Large
Inverted Cylinder Engine._--The lower view in fig. 53 is a half
plan looking upwards, and a half section of the gland looking
downwards. The upper view is a vertical section. Complete all these
views and add an elevation. Scale 3 inches to a foot.

_Note._--The large nuts, the wheel, the supporting ring, and small
gland are made of brass.

_Dimensions of Stuffing-boxes and Glands._

_d_ = diameter of rod. _t__{1} = thickness of
_d__{1} = diameter of box (inside). stuffing-box flange.
_l_ = length of stuffing-box _t__{2} = thickness of gland
bush. flange.
_l__{1} = length of packing space. _t__{3} = thickness of bushes in
_l__{2} = length of gland. box and gland.
_t_ = thickness of metal in _d__{2} = diameter of gland bolts.
stuffing-box. _n_ = number of bolts.

+----------------------------------------------------------+
| _d_ | _d__{1} | _l_ | _l__{1} | _l__{2} | _t_ | _t__{1} |
+-----+---------+-----+---------+---------+------+---------+
|1 | 1-3/4 | 3/4| 2 | 1-1/2 | 7/16| 1/2 |
|1-1/2| 2-1/2 |1-1/4| 2-5/8 | 2 | 9/16| 11/16 |
|2 | 3-1/2 |1-3/4| 3-1/4 | 2-1/2 | 11/16| 7/8 |
|2-1/2| 4-1/8 |2-1/4| 3-7/8 | 2-7/8 | 13/16| 1-1/16 |
|3 | 4-3/4 |2-3/4| 4-1/2 | 3-1/4 | 15/16| 1-1/4 |
|3-1/2| 5-1/4 | 3 | 5-1/8 | 3-5/8 |1 | 1-3/8 |
|4 | 5-7/8 |3-1/4| 5-3/4 | 4 |1 | 1-3/8 |
|4-1/2| 6-3/8 |3-1/2| 6-3/8 | 4-3/8 |1-1/16| 1-9/16 |
|5 | 7 |3-3/4| 7 | 4-5/8 |1-1/16| 1-9/16 |
|6 | 8 |4-1/4| 8-1/4 | 5 |1-1/8 | 1-11/16 |
+----------------------------------------------------------+

+-------------------------------------------------+
| _d_ | _t__{2} | _t__{3} | _d__{2} | _n_ |
+-----+-----------------+---------+---------+-----+
|1 | _t__{2}=_t_ | 3/16 | 7/16 | 2 |
|1-1/2| when gland | 1/4 | 5/8 | 2 |
|2 | flange is | 5/16 | 3/4 | 2 |
|2-1/2| made of cast | 5/16 | 7/8 | 2 |
|3 | iron and | 3/8 | 1 | 2 |
|3-1/2| _t__{2}=_t__{1} | 3/8 | 1 | 2 |
|4 | when gland | 7/16 | 1 | 2 |
|4-1/2| flange is | 7/16 | 7/8 | 4 |
|5 | made of | 7/16 | 1 | 4 |
|6 | brass. | 1/2 | 1-1/4 | 4 |
+-------------------------------------------------+

The proportions of glands and stuffing-boxes vary considerably but the above table represents average practice.

EXERCISE 55:--Make the necessary working drawings for a gland and
stuffing-box for a locomotive engine piston rod 2-1/2 inches in
diameter, to the dimensions given in the table.

XVI. VALVES.

Professor Unwin divides valves, according to their construction into three classes as follows:--(1) flap valves, which bond or turn upon a hinge; (2) lift valves, which rise perpendicularly to the seat; (3) sliding valves, which move parallel to the seat.

Examples of flap valves are shown in figs. 54 and 55; two forms of lift valves are shown in figs. 56 and 57, and in figs. 58 and 59 are shown two forms of slide valve. The slide valve shown in fig. 58 moves in a straight line, while that shown in fig. 59 (called a cock) moves in circle.

_India-rubber Valves._--In india-rubber valves there is a grating covered by a piece of india-rubber, which may be rectangular, but is generally circular, and which is held down along one edge if rectangular, or at the centre if circular. Water or other fluid can pass freely upwards through the grating, but when it attempts to return the elasticity of the india-rubber, and the pressure of the water upon it, cause it to lie close on the grating, and thus prevent the return of the water. The india-rubber is prevented from rising too high by a perforated guard. In fig. 54 is shown an example of an india-rubber disc valve. A is the grating, B the india-rubber, C the guard secured to the grating or seat by the stud D and nut E. The grating is held in position by bolts and nuts F. The grating and guard are generally of brass.

India-rubber disc valves are also shown on the air-pump bucket, fig. 47.

EXERCISE 56: _India-rubber Disc Valve._--Fig. 54 shows a vertical
section and a plan of an india-rubber disc valve. In the plan
one-half of the guard and india-rubber are supposed to be removed
so as to show the grating or seat. Draw these views, and also an
elevation. A detail drawing of the central stud is shown in fig.
16, page 18. In fig. 54 the elevation of the guard is drawn as it
is usually drawn in practice, but if the student has a sufficient
knowledge of descriptive geometry he should draw the elevation
completely showing the perforations. Scale 6 inches to a foot.

_Kinghorn's Metallic Valve._--The action of this valve is the same as that of an india-rubber valve, but a thin sheet of metal (phosphor bronze) takes the place of the india-rubber.

This valve is now largely used in the pumps of marine engines, and is shown in fig. 55 as applied to an air-pump bucket. Three valves like the one shown are arranged round the bucket.

EXERCISE 57: _Kinghorn's Metallic Valve._--Fig. 55 shows an
elevation and plan of one form of this valve. In the plan one-half
of the guard and metal sheet are supposed to be removed, so as to
show the grating, which in this case is part of an air-pump bucket.
Draw the views shown, and also a vertical section of the guard
through the centres of the bolts. All the parts are of brass except
the valve proper, which is of phosphor bronze. Scale 6 inches to a
foot.

_Conical Disc Valves._--A very common form of valve is that shown in figs. 56 and 57. This form of valve consists of a disc, the edge of which (called the face) is conical. The conical edge of this disc fits accurately on a corresponding seat. The angle which the valve face makes with its axis is generally 45 deg.. If the disc is raised, either by the action of the fluid as in the india-rubber valve, or by other means, an opening is formed around the disc through which the fluid can pass. The valve is guided in rising and falling either by three feathers underneath it, as in fig. 56, or by a central spindle which moves freely through a hole in the centre of a bridge which stretches across the seat, as in fig. 57. The lift of the valve is limited by a stop above it, which forms part of the casing containing the valve. The lift should in no case exceed one-fourth of the diameter of the valve, and it is generally much less than this. The guiding feathers (fig. 56) are notched immediately under the disc for the purpose of making available the full circumferential opening of the valve for the passage of the fluid. These notches also prevent the feathers from interfering with the turning or scraping of the valve face.

Conical disc valves and their seats are nearly always made of brass.

EXERCISE 58: _Conical Disc Valves._--Draw, half size, the plans and
elevations shown in figs. 56 and 57. In fig. 57 the valve is shown
open in the elevation, and in the plan it is removed altogether in
order to show the seat with its guide bridge.

_Simple Slide Valve._--The form of valve shown in fig. 58, often called the _locomotive slide valve_, is very largely used in all classes of steam-engines for distributing the steam in the steam cylinders. The valve is shown separately at (_d_), (_e_), and (_f_), while at (_a_), (_b_), and (_c_) is shown its connection with the steam cylinder.

It will be observed that the valve itself is in the shape of a box with one side open, the edges of the open side being flanged. When the valve is in its middle position, as shown at (_a_), two of these flanged edges completely cover two rectangular openings S_{1} and S_{2}, called _steam ports_, while the hollow part of the valve is opposite to a third port E, called the _exhaust port_. As shown at (_a_) the piston P would be moving upwards and the valve downwards. By the time the piston has reached the top of its stroke the valve will have moved so far down as to partly uncover the steam port S_{1}, and admit steam from the valve casing C through S_{1} and the passage P_{1} to the top of the piston. The pressure of this steam on the top of the piston will force the latter down. While the above action has been going on, the port S_{2} will have become uncovered, and the hollow part of the valve will be opposite both the steam port S_{2} and the exhaust port E, so that the steam from the under side of the piston, and which forced the piston up, can now escape by the passage P_{2}, the steam port S_{2}, and the exhaust port E to the exhaust outlet O, and thence into the atmosphere, if it is a non-condensing engine, or into the condenser if it is a condensing engine, or into another cylinder if it is a compound engine. After the piston has performed, a certain part of its downward stroke, the valve, which has been moving downwards, will commence to move upwards, and when it has reached a certain point it will cover the port S_{1}, and shut off the supply of steam to the top of the piston. It is generally arranged that the steam shall be cut off before the piston reaches the end of the stroke. When the piston reaches the bottom of its stroke the valve has moved far enough up to uncover the port S_{2} and admit steam to the bottom of the piston, and to uncover the port S_{1} and allow the steam to escape from the top of the piston through the passage P_{1}, the port S_{1}, the port E, and outlet O. In this way the piston is moved up and down in the cylinder.

The valve is attached to a valve spindle S by nuts as shown, the hole in the valve through which the spindle passes being oval-shaped to permit of the valve adjusting itself so as to always press on its seat.

When the valve is in its middle position it generally more than covers the steam ports. The amount which the valve projects over the steam port on the outside, the valve being in its middle position, is called the _outside lap_ of the valve, and the amount which it projects on the inside is called the _inside lap_. When the term lap is used without any qualification, outside lap is to be understood. In fig. 58 it will be seen that the valve has no inside lap, and that the outside lap is three-eighths of an inch. The inside lap is generally small compared with the outside lap.

When the piston is at the beginning of its stroke the steam port is generally open by a small amount called the _lead_ of the valve.

The reciprocating motion of the slide valve is nearly always derived from an eccentric fixed on the crank-shaft of the engine. Slide valves are generally made of brass, bronze, or cast iron.

EXERCISE 59: _Simple Slide Valve._--At (_d_), fig. 58, is shown a
sectional elevation of a simple slide valve for a steam-engine, the
section being taken through the centre line of the valve spindle,
while at (_e_) is shown a cross section and elevation, and at (_f_)
a plan of the same. Draw all these views full size, and also a
sectional elevation at A B. The valve is made of brass, and the
valve spindle and nuts of wrought iron.

EXERCISE 60: _Slide Valve Casing, &c., for Steam-engine._--Draw,
half size, the views shown at (_a_), (_b_), and (_c_), fig. 58;
also a sectional plan at L M. (_b_) is an elevation of the valve
casing with the cover and the valve removed. (_a_) is a sectional
elevation, the section being taken through the axes of the steam
cylinder and valve spindle. (_c_) is a sectional plan, the section
being a horizontal one through the centre of the exhaust port. The
inlet and outlet for the steam are clearly shown in the sectional
plan: in the sectional elevation their positions are shown by
dotted circles.

The stroke of the piston is in this case 12 inches, so that from
the dimensions given at (_a_) it must come within a quarter of an
inch of each end of the cylinder; this is called the _cylinder
clearance_.

The piston has three Ramsbottom rings, a quarter of an inch wide
and a quarter of an inch apart.

The steam cylinder and valve casing are made of cast iron.

_Cocks._--A cock consists of a slightly conical plug which fits into a corresponding casing cast on a pipe. Through the plug is a hole which may be made by turning the plug to form a continuation of the hole in the pipe, and thus allow the fluid to pass, or it may be turned round so that the solid part of the plug lies across the hole in the pipe, and thus prevent the fluid from passing. As the student will be quite familiar with the common water cock or tap such as is used in dwelling-houses we need not illustrate it here.