Copyright
American Technical Society.

Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) online

. (page 14 of 30)
Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 14 of 30)
Font size
QR-code for this ebook


When the pressure of steam forces the piston upward it com-
presses the spring above it ; the amount of compression varies with
the strength of the spring. The rise of the piston causes the
pencil to rise because of the system of levers. The cylinder to
which the paper is attached rotates by means of a, cord which is
fastened to some part of the engine, the crosshead for example.



194



INDICATORS.



11




While the drum revolves and the steam pressure forces the piston

to rise, the pencil, lightly touching the paper, describes the diagram.
The parallel movement of this indicator is obtained by a link

attached directly to the lever. It is

so constructed that there is but little

lost motion, hardly any friction and

no appreciable error within the limited

movement of the pencil.

The paper cylinder is so con-
structed that the tension of the coiled

spring within the drum may be altered

for different speeds of the engine. By

this means the cord can be kept taut

with little trouble. The cord is led

through a hold and kept in contact

with the scored wheel by another

small one. By this means the cord

can be run to any angle. It is con- Fi =- 6 -

venient to have the cards of about the same size. If we used a

spring of such a tension that the
pencil would move one inch for
every 60 pounds pressure, and there
was but 30 pounds pressure in the
engine cylinder, the diagram would
be but one-half inch high. This
diagram would be too small for
accurate work. For this reason
indicators are provided with sets of
springs of varying stiffness which
may be used according to the steam
pressure. If a spring is of such
stiffness that the pencil moves 1
inch for every 50 pounds pressure
it is called a 40 pound spring.
Others are called 10 pound, 20
pound, 30 pound, etc., springs.
In selecting the spring for a given




To Change the Springs.



pressure, care should be taken that it will easily stand that press-



195



INDICATORS.



ure. A safe rule to follow : multiply the scale of the spring by
12 and subtract 15 for the vacuum.

For example: The maximum pressure for a 50 pound spring
is 110 pounds, because (50 X 2|) 15 125 15 = 110.
Springs are made in the following scale: 8, 10, 12, 16, 20, 24, 30,
32, 40, 48, 50, 56, 60, 64, 80, 100. For pressures from 70 to 90
pounds a 40* pound spring should be used, as 80 pounds pressure
on a 40 pound spring will raise the pencil 2 inches, and this is a
good height tor the diagram.

If very high pressures are to be indicated, an extra piston,
having an area of ^ square inch, is used. This
doubles the allowable pressure on the spring. For
instance, if a spring can be used for 110 pounds pres-
ure when tbe piston is -^ inch in area, it can be used
for 220 pounds pressure if the ^ inch piston is used.

When the spring has been selected it is placed
in the indicator. First unscrew the milled nut at
the top of the steam cylinder and take out the piston
with arm and connections. The pencil lever and
piston are disconnected by unscrewing the small-
headed screw which connects them. If a spring is
connected to the piston it should be removed, the
selected one substituted and the indicator put
together. The spring should always be firmly
screwed to the shoulder or the pencil will not
properly indicate the pressure.

Care of the Indicator. Before attaching the indicator to the
engine cylinder it should be taken apart, cleaned and oiled. If
each part works freely and smoothly the spring may be put in and
the indicator put together. After connecting to the cylinder,
admit steam to it, but do not take cards until it is thoroughly
warmed and blows dry steam through the relief. It is not neces-
sary to use lead in connecting as it is likely to get into the indi-
cator. After using, take the indicator apart, clean and oil. Only
porpoise or fine watch oil should be used.

THE CR05BY INDICATOR.

The internal arrangement of the parts of the Crosby Indicator
*NOTB In practice it is better to use a somewhat stiff er spring.




Fig. 7.



196



INDICATORS.



is shown in Fig. 8. The piston, 8, is formed with shallow channels
on its outer surface to retain oil which prevents leakage and
lubricates the piston.

The socket in the center of the piston is supported by a cen-
tral web and projects both upward and downward. The upper




Fig. 8,

portion is threaded inside to receive the lower end of the piston-
rod. It has a vertical slot which allows the ball bearing on the
end of the spring to drop into a concave bearing on the upper end
of the piston-screw 9 which is screwed into the lower part of the
socket.

The piston-rod, 10, is made hollow, with the lower end
threaded. When the piston-rod is connected to the socket, the
former should be screwed into the socket as far as it will go.

The height of the atmospheric line on the diagram depends



197



14 INDICATORS.




upon the amount the swivel head, 11, is screwed into the top of
the piston-rod.

A small projection on the lower side of this cap is threaded
to screw into the top of the spring and hold it firmly in place.
The moving parts are kept in line by this cap. The pencil
mechanism is supported by the sleeve 3, which surrounds the
upper part of the cylinder ; it turns freely and is held in position
by the cap.

The pencil mechanism is made as light as is consistent with
strength and stiffness. The pencil moves exactly
parallel to the piston because the fulcrum of the
mechanism and the point of attachment to the
piston-rod are always in a straight line. The
pencil point moves six times as far as the piston
because of the multiplying levers.

The drum 24, is one and one-half inches in
diameter, and is rewound when the string is pulled,
by a short spiral spring 31.

The piston spring is made of a single piece
of steel wire wound from the middle into a
double coil. The ends are screwed into a brass head Fi g

having four radial wings. At the bottom of the
spring a small steel bead is firmly attached to the wire. This
forms a ball and socket joint with the lower end of the piston-rod.
This joint is light and allows the spring to yield to pressure from
any direction. These springs are made in the following scale
8, 12, 16, 20, 24, 30, 40, 50, 60, 80, 100, 120, 150, and 180.

To Insert the Spring. First unscrew the cap- 2, then lift the
connected parts free from the cylinder by means of the sleeve.
The hollow wrench should be held in an inverted position and
the piston-rod inserted until the hexagonal part engages the
wrench. Then, having the spring shown in Fig. 9 inverted,
insert the combined wrench and piston-rod until the steel bead
and the end of the spring rests in the concave seat. Now invert
the piston and pass the transverse wire at the bottom of the spring
through the slot until the threads at the bottom of the piston-rod
engage those inside the socket of the piston. With the wrench
screw it in as far as it will go*



198



INDICATORS. 15



The piston screw should be loosemd slightly before the
piston-rod is screwed in, and afterward set up against the bead
lightly to prevent lost motion. Then with the sleeve and cap
upright, engage the threads of the swivel bead with those inside
the piston-rod and screw it up until the lower projection of the
cap engages the threads inside the spring top; continue the process
until the spring is screwed up firmly against the cap. Holding
only by the sleeve 3, turn the piston and the connections until
the top of the piston-rod is flush with the shoulder on the swivel
head.

Now that the piston and all the connections are in their
places, the whole may be inserted in the cylinder and the cap
screwed down, which will fix all parts in their -proper places.
If there is a spring in the cylinder first detach by reversing the
above process.

THE TABOR INDICATOR.

The Tabor Indicator is shown in Fig. 10. It is used exten-
sively in the navy. The principle of action and details of con-
struction are similar to the indicators already described ; the chief
peculiarity being the means employed to obtain a straight line
movement for the pencil. Inside the steam cylinder is a lining in
which the piston moves. This lining can expand when heated.
In the side of the cylinder small holes allow any steam which may
leak by the piston to escape. The piston-rod is connected at one
end to the piston by means of a ball and socket joint; the other
end is connected to the pencil mechanism.

The pencil mechanism consists of three pieces, the pencil
lever, the back link, and the piston-rod link. The two links are
parallel for every position of the pencil. Thus the lower pivots
of these links and the pencil point are always in the same straight
line. The straight line movement of the pencil is obtained by
means of a curved slot in a stationary plate. The pencil bar is
provided with a roller which is fitted in such a manner that it can
roll from one eiid of the slot to the other. The curve of the slot
guides the pencil bar and is of such a radius that the pencil is
caused to move in a straight line. The curve compensates for the
tendency of the pencil point to move in an are of a circle. The



109



16 INDICATORS.



pressure of the pencil on the paper is regulated by a screw which
strikes a stop plate attached to the frame. The end of the pencil
bar is formed for either a pencil lead or a metallic marking point.

The drum for the paper is made similar to those of other
indicators. The backward movement is obtained by a flat spiral
spring placed under the
drum. The tension of
this spring is altered by
loosening a thumbscrew,
lifting the carriage, and
winding or unwinding.
A simple pulley guides
the driving cord in any
direction.

The indicator is at-
tached by a coupling
having a single thread.

The springs of the
Tabor "indicator are of
the duplex type, that is,
they are made of two
spiral coils of wire. A
50 pound spring is
shown in Fig. 11. The
wire terminates in fit-
tings at each end. The spring is attached to the upper side of the
piston by means of threads cut on the inside of the fitting and on a
projection on the piston. The top of the spring is attached to the
under side of the cover in a similar manner. The springs are
made in the following scales, 8, 10, 12, 16, 20, 24, 30, 32, 40, 48,
50, 60, 64, 80, and 100 pounds. The maximum safe steam pres-
sures (absolute) to which these springs may be subjected are
respectively, 10, 15, 20, 24, 40, 48, 70, 75, 95, 112, 120, 140, 152,
180, and 200 pounds.

Change of Location of Atmospheric Line. Unscrew the cap
and lift the sleeve and connections from the cylinder. Then turn
the piston to the left or right according as the pencil is to be
raised or lowered. One revolution causes the pencil to rise
inch.



200




INDICATORS. 17




Care of the Indicator. Before attaching the indicator to the
engine, steam should be blown through the pipes and cocks so
that all particles of dust may be removed. After using, the indi-
cator should be carefully wiped and oiled. The cylinder cap
should be unscrewed and all the parts connected to the piston
removed. The piston spring and piston-rod should be detached,
carefully wiped dry, and then oiled. The inside of the cylinder
also should be oiled. Then the piston and piston-rod should be
placed in the cylinder and the spring placed in the box. If the
indicator has not been used for some time the oil may have become
gummed. It may be easily cleaned by wiping with
a cloth saturated with naphtha or benzine. It must
be oiled again before using. A good test that the
indicator is in proper working order is to detach the
spring and after replacing the piston and piston-rod,
raise the pencil to the highest point. When allowed
to fall it should descend to the lowest point freely.
The pencil should always have a smooth fine point. ,

To Attach the Indicator to the Engine. Usually
all first-class engines are prepared for the indicator
before leaving the factory. Holes are drilled and
tapped in the cylinder and have plugs screwed in them. These
plugs are easily removed and the indicator connections screwed in.
When this is not the case, any engineer can perform the
work. Before drilling the holes, in the cylinder, the heads should
be removed so that the exact positions of the pistons and the size
of the ports and passages may be known. Also with the heads off
all chips and particles of dirt from drilling may be easily removed.
If it is impossible to remove the heads, a little steam admitted to
the cylinder just before the drilling is completed will blow the
chips out.

Each end should be drilled and tapped for a one-half inch
pipe thread. The holes must be drilled into the clearance space,
so that the piston at the ends of the stroke will not cover them.
They should also be placed so that currents of steam will not reach
them. Before deciding just the points at which to drill the holes,
it is well to consider every plan of indicating the engine. The
type of engine, the position of the steam chest, the kind of cross-



201



18 INDICATORS.



head, and the position of the eccentric and its connections should
all be considered, as well as the most convenient place in the
engine room. The holes should not be drilled until the plan
shows the proper connections with the reducing motion, conven-
ient access and free passage of steam to the indicator.

AVhen the plan has been adopted, the engine should be placed
on dead center, to determine the clearance. The holes should be
drilled into the middle of the clearance space.

In common practice for horizontal engines the holes are
drilled in the side of the cylinder at each end. Short half inch
pipes with quarter upward bends into which the indicator coils
may be screwed ars inserted in these holes.

It may be more convenient to drill and tap into the top of
the cylinder and attach the indicators directly.

For vertical engines, the upper head or cover and the side of
the cylinder are often drilled and tapped for the upper and lower
indicators respectively. It is preferable to connect the indicators




Fig. 12.

to the sides because less pipe and fittings are required and better
results obtained.

If only one indicator is to be used for both ends of the cyl-
inder, it may be connected by side pipes and a three way cock.
By this method both diagrams are taken on the same card and
with the loss of but one revolution. Fig. 12 shows the section of
a three way cock.

Reducing notion. As we have already seen the length of
the card represents the travel of the piston. As the length of



INDICATORS.



19



card is obtained "by the rotation of the drum the motion of the
drum must be taken from some part of the engine which has a
motion coinciding with that of the piston. The crosshead is the
most common, reliable and convenient part. The length of the
card is much less than the travel of the piston since the stroke is
longer than the circumference
of the drum so that the move-
ment of the crosshead must
be reduced to the length of
the diagram.

There are several devices
employed to obtain this re-
duced motion.

A simple form of reduc-
ing motion, called the panto-
graph, is shown in Fig. 13.
Four links, , 5, c, rf, are
joined in the form of a par-
allelogram. One link, , is
prolonged and pivoted at the
crosshead C. The point where
l> and c join is pivoted at the
fixed point E. The cord is
fastened at 1) on the link d.

The point I) must be in the straight line connecting E and C.
Then letting A B represent the. stroke and h the length of the
indicator diagram, we have

A B : h = E C : E D, from which




E D = JL-'



X EC



A B

Another form of pantograph is shown in Fig. 14. It is placed
horizontally with the pivot, B, resting on a support opposite the
crosshead when in mid-position. The pivot A, is attached to the
crosshead ; usually by having the stud A inserted in a hole drilled
in the crosshead. If the pivot B is adjusted to the proper height
and at the right distance from the crosshead, the cord from the
indicator may be attached to the pin E without any pulleys. The



20



INDICATORS.



length of the diagram is varied by adjusting the movable bar C D;
the pin E must be in the straight line from A to B.

The pantograph is likely to become shaky and loose on
account of its many joints. If well made it gives perfect motion.

The reducing motion shown in Fig. 15, called the Brumbo
Pulley, is easily and quickly made and can be used on almost any
engine. The wooden rod A is usually about twice as long as the
stroke. It is pivoted by a bolt or screw at B, a fixed point. At the
lower end it is connected by the wooden link, C, to the crosshead.




Fig. 14

This link C is usually about one-half the length of the stroke. The
sector S may be made either of wood or metal. It should have a
groove in the circular edge for the cord, and is made fast to the upper
end of the lever A. Its center should coincide with that of the
pivot B. The length of the radius of the sector may be found as
follows. Divide the length of the lever by the length of the
stroke, multiply the result by the length of the desired diagram
and the product will be the radius of the sector. For instance, if
the lever is 60 inches long and the stroke 30 inches and we wish
the diagram to be 3 inches long, the sector should be 6 inches in
radius for,

60



"30"



X 3 = 6.



To avoid the use of guide pulleys the lever should be hung
so that it will swing in a vertical plane parallel with the guides and
in line with the indicator. When the crosshead is at mid-strokg,



INDICATORS.



'21



the lever must be vertical and the point D must be below the axis
of the cylinder because it conies above the line at the ends of the
stroke.

The reducing lever used for large quick running engines is
shown in Fig. 16. The rod A is made of pine wood, tapering
toward the lower point and about one inch in thickness. The
length is about one and one-half times the length of the stroke.
It is suspended by a bolt or screw from some fixed point above the
engine, and should swing edgewise and parallel to the guides of
the crosshead. The steel stud at the bottom of the rod has a T
shaped slot in an iron plate which is attached firmly to the cross-
head. The slot should be long enough to retain the stud when
the crosshead is at the end of the stroke. To find the point at
which the indicator cord should be attached, divide the length of





Fig. 15.



Fig. 1C.



the lever by the length of the stroke of the piston and multiply
the quotient by the length of the desired diagram. The product
is the distance of the point from the pivot at the top of the lever.
Example. A lever is 45 inches long, the piston stroke 30
inches and the diagram to be 3J inches long. At what distance
from the pivot should the indicator cord be attached ?

ijlx 8J = 4J inches.
Having placed the indicator in position and obtained the



205



22 INDICATORS.



reducing motion, the length of the cord must be so adjusted that
the drum will not strike the stops at either end of the stroke.

For convenience an approximate length of cord is first found
and the cord cut in two parts, one attached to the reducing
motion ; the other to the indicator. A hook should be fastened to
the free end of the piece attached to the indicator. A loop is
made in the free end of the piece from the reducing motion.
The hook is then attached to the ^op and the extra length
of cord taken up by tying knots. Anocher method for adjust-
ing the length of the cord is the arrangement shown in Fig.
17. The hook A is attached to the indicator cord. The cord B
from the reducing motion passes through the holes in the plate P
as shown. To adjust the length of the cord it is slacked at the
point B and the plate slipped along the cord.




Fig. 17.

To Take Indicator Diagrams. The indicator should be in
good working order before attaching to the cylinder ; it should be
clean, well oiled, and the levers and springs should work smoothly.
The pencil point should be sharp and the pressure adjusted to
make a distinct fine line.

The spring should be selected that will give a diagram 1*
to 2 inches in height. If the spring chosen is too light, the lines
are likely to be wavy from the vibration of the pencil levers.

When the indicator is in position a satisfactory reducing
motion obtained and the cord adjusted, the paper should be
wrapped smoothly around the drum. The edges projecting over
the clips should be folded back so that they will not touch the
pencil lever. Before taking the card, allow the steam to enter
the indicator and move the piston up and down until the parts
have become thoroughly warmed. Then pass the pencil against
the paper long enough to take the diagram. Some engineers
allow the pencil to remain in contact with the paper during but
one revolution of the engine; others trace the diagram two or



206



INDICATORS.



more times. After the diagram is taken the cock is shut and
without unhooking the cord the pencil is again pressed to the
paper to take the atmospheric line. The cord is then disconnected
and the card removed from the drum. The scale of spring, the
dimensions and speed of the engine, the date, and all useful partic-
ulars are written on the cards.

If one indicator is used for both ends the three way cock
shown in Fig. 12 is opened to admit steam from one end, the
diagram taken ; then opened for the other end and that diagram
taken. Then the steam is shut off from both ends and the atmos-
pheric line taken.

As has been said before, the indicator is of great importance
to the engineer. It is used to find the indicated horse-power of
the engine, and by comparison of the indicated horse-power with
the brake horse-power, the mechanical efficiency is obtained. The
indicator card shows several other things ; the time and manner
of the four events of the stroke, namely, the admission, cut-off,
release, and compression. These four events make up what is
called the steam distribution. It shows faults in the setting and
working of the valves.

We have seen how the indicator diagram represents the net
work done on the piston in one stroke.

Work is equal to pressure multiplied by the distance through
which it acts. The distance is the length of stroke multiplied by
the number of strokes per minute. The jiressure is the average
net pressure acting on the piston during the stroke. This average
net pressure is called the mean effective pressure. If we know
from the indicator card the mean effective pressure per square
inch, we can find the total pressure by multiplying it by the area
of the piston in square inches.

The distance per minute is equal to the length of stroke
multiplied by the number of strokes.

Let P ==. mean effective pressure in pounds per square inch.
A = area of piston in square inches.
L = length of stroke in feet.
N = number of strokes per minute.

Then the work done per minute,

W = P L A N.



207



24 INDICATORS.



Since one horse-power is the rate of doing work when 33,000
foot-pounds of work are done per minute the indicated horse-power
of an engine is obtained by means of the formula,

H.P.= PLAN



33,000

The length of the stroke, the area of the piston and the num-
ber of strokes are easily found. Then all that remains to be
determined before the horse-power is calculated is the value of P.
or the mean effective pressure.

Suppose one side of the piston is in communication with the
boiler during the entire stroke, the mean pressure is then the boiler
pressure. But if the supply of steam is cut off before the stroke
is completed, the mean pressure will not equal the boiler pressure.
In Fig. 1 we saw that the area of the shaded portion was equal to
the length multiplied by the height. The area of a figure of any
shape can be reduced to that of a rectangle having a length equal
to the extreme length of the figure. Then whenever we know the
area and length, we can find the height or mean height by dividing
the area by the length.

Suppose a diagram like that shown in Fig. 2 has an area of
5| square inches and its extreme length is 3i inches ; then the
height is 5.25 divided by 3.5 = 1.5 inches. Then with any indi-
cator card the area of the diagram is equal to the area of a
rectangle, the length of which is known and the height can easily
be computed.

Suppose that we have taken a card and know that the mean
height is 1|- inches. In order to find the horse-power we must
reduce the 1|- inches to pounds pressure. If we multiply the



Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 14 of 30)