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N. (Nehemiah) Hawkins.

Hawkins' indicator catechism. A practical treatise for the use of erecting and operating engineers, superintendents, students of steam engineering, etc online

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HAWKINS*



INDICATOR



CATECHISM.



7



ENGINEERING LIBRARY




THREE WAY COCK



THIS WORK IS DEDICATED TO

HENRY RAABE, M. E.,

A.S A TOKEN OF APPRECIATION OF VALUABLE ASSISTANCE IN
ITS MAKING UP, AND A REMINDER OF A LONG SUMMER

SPENT AGREEABLY TOGETHER IN COMPILING AND

EDITING THE AUTHOR'S NEW CATECHISM

OF THE STEAM ENGINE.



838786



HAWKINS' -

INDICATOR CATECHISM

A PRACTICAL TREATISE

FOR'THE USE OF ERECTING AND OPERATING ENGINEERS

SUPERINTENDENTS, STUDENTS OF STEAM

ENGINEERING, ETC.



N. HAWKINS, M, E.



TO NGINEERS



IONS FOR ENGINEERS." THE ?tfiw CATECH*SNI OF THE
STEAM ENGINE" *. EVt I ,'



CONTAINING THE DESCRIPTION OF THE INSTRUMENT AND DI-
RECTIONS FOR ITS APPLICATION FOR OBTAINING THE
BEST RESULTS IN THE USE OF STEAM.



THEO. AUDEL & CO.

63 FIFTH AVE. NEW YORK CITY

1903




;: ;.**...' COPYRIGHTED

1898, 1901, 1903,

THEa AUDEL & CO.
NEW YORK.



\
\



CONTENTS.



INTRODUCTION, - ^

PREPARING INDICATOR FOR USE, - - - 23

REDUCING MOTIONS, ? 27

PIPING UP INDICATOR, ** 37

TAKING INDICATOR CARDS, * " 43

THE DIAGRAM, -...<.. ^ 2

FIGURING STEAM CONSUMPTION BY THE DIAGRAM, - 74

REVOLUTION COUNTERS, .... 76

EXAMPLES OF DIAGRAMS, 78

INDICATING AIR COMPRESSORS, 87

DESCRIPTION OF INDICATORS, 89

MEASURING DIAGRAMS BY ORDINATES, - . I0 6

PLANIMETER, - - - - 122

TABLES. - - 143



' ' Much care is necessary in indicator practice, both of
the instrument and its method of attachment, since so very much
depends on a very small area; and the engineer who obeys this
rule, and puts a little thought into his work, will find there is
nothing difficult nor mysterious in the use of the Steam Engine
Indicator. "

THOMAS HAWLEY.



Hawkins Indicator Catechism. ix



PREFACE.

Hawkins' Indicator Catechism, of which this is a
newly copyrighted edition, has been before the public
for a .space of five years, during which time it has
attained a considerable sale and has given satisfaction
to its purchasers; it has proved its value in building
up the student in a sound knowledge of principles and
in the details of indicator practice already thoroughly
established.

And the satisfaction expressed by those who have
used the book, has amply sustained the author' s judg-
ment in the arrangement of the subject matter.

Perhaps the most gratifying expression of commen-
dation is one received from Prof. Alva T. Hill, M. E., of
Detroit, Mich. : " I have read with interest your book
on the Indicator and have obtained more information
on this subject, than from any other source; after a
careful study of your book, I thought it would be of
interest for you to know the progress of work which has
been done in the way of improving the manner of
obtaining indicator cards." This is high praise, con-
sidering its source, and is representative of many
words of approval.

It has been said that he who plants a tree and
causes it to grow in a spot where there was no tree, is a
benefactor to his race. How much more is he a bene-
factor who by the use of the indicator, skillfully and



x Hawkins Indicator Catechism.

PREFACE.

intelligently applied, effects the saving of scores and
hundreds of tons of coal ; the world's supply of coal is
diminishing day ~by day, and the world's Civilization
and comfort depend upon its use, hence the double
merit of economy and philanthropy in its preservation.

A few words relating to the history of the instru-
ment ?nay be properly used in this brief introduction.

The original indicator, as first invented by Watt
(born 1736, died 1819), consisted simply of a piston
working in a cylinder against the resistance of a spring.
The movement of the spring, due to the pressure of the
steam on the piston, was shown by a pointer attached
to the piston rod of the instrument, and pointing to
a scale.

Later, Watt added a board, on which was attached
a piece of paper moving simultaneously with the piston
of the engine ; a pencil was substituted for the pointer,
and a card traced on the paper on the board. Watt
did not use the paper drum, which was added by a
contemporary and generally adopted, and substantially
in this form the instrument remained until the intro-
duction of the Richards Indicator.



THE STEAM ENGINE



The measuring instrument best designed to show the
power of the steam engine is that invented by Watt the
steam engine indicator.



Figs. 5 and 6. THE INDICATOR.




Fig. 7. THE INDICATOR SPRING.

NOTE. The purpose of the instrument is the same as a recording
steam gauge, with this difference, that it records the pressure at each
instant of the engine stroke, and this pressure is usually a variable one
in different parts of the stroke. This is its object, simply to put upon
paper the steam pressure pushing the piston at each point in the stroke,
and at the same time to put on paper what pressure is opposing the
piston at each portion of the piston stroke. That is all it does. The
instrument may be considered as a steam gauge with a spiral spring to
measure the pressure, and a pencil to record it.



., Hawkins' Indicator Catechism.

,' 8 v" r,**v,

, ,, , INTRODUCTION.

r * vfv; '

The indicator is a miniature of the larger cylinder whose

performance it is designed to reveal. The object of the
instrument is simply to record the pressures that are in the
cylinder of the engine at certain points of the stroke, and
from this can be told, first, the action of the valve gear, and
second, the distribution of pressure. This is all that the
indicator shows ; it indicates how much pressure is in the
cylinder, and the exact location of the piston when that
pressure existed in the cylinder.

Figs. 5 and 6 show an exterior and interior view of the
American Thompson Indicator and Fig. 7 exhibits the
spring which is the real measuring factor of the indicator ;
this spring is enclosed in a cylinder which is constructed
so that its area is generally one-half of a square inch.



! i



Fig. 8. INDICATOR SPRING.

Fig. 9 is the piston rod of the Robertson-
Thompson Indicator, shown in Figs. 10 and u.

These two instruments are shown out of a
variety of excellent and reliable devices on the
market, to which the following general descrip-
tion will apply.

The indicator consists of a cylinder of ^ a
square inch area and about two inches long to
Fig. 9. the upper end of which is attached an arm or
bracket which carries a drum around which the
indicator " card " is wound ; this drum has a diameter of
about two inches and is capable of a semi-rotary motion
which is given to it by a cord operated from the engine
crosshead.




Hawkins Indicator CatecJiism.



INTRODUCTION,




Fig. 10. INDICATOR.




Fig. ii. INDICATOR.



The " diagram " is
marked upon the card
by a pencil which is
on the end of a lever
and which is attached
by means of links to
the piston rod of the
indicator. Upon the
piston is mounted a
spring which counter-
acts the steam press-
ure ; these springs
are of different ten-
sions, or strength, to
be used according to
the steam pressure.

To impart a straight
vertical motion upon
the paper card wound
around the drum, a
parallel motion de-
vice, Fig. 12, is
attached to the lever
which carries the
pencil. The one
shown in the illustra-
tion is the " Thomp-
son parallel motion "
and is that most fre-
quently in use.

This brief descrip-
tion of this important
instrument will be
readily understood
by study of the sev-
eral illustrations.



Hawkins Indicator Catechism.



INTRODUCTION.

The operation of the indica.
tor is as follows : steam enters
the cylinder of the instrument
forcing the piston upwards
against the pressure of the
spring, this gives a vertical mo-
tion to the pencil on the paper
mounted upon the drum. The
drum receives its revolving mo-
tion by a cord which is wound
around the lower part of the
drum, its other end being con-
nected by means of a reducing
motion to the cross-head of the
engine, which upon commencing
its stroke exerts a pull upon the
cord, thus giving the drum a
revolving motion. This causes
the pencil to make a horizontal
line upon the paper.

Upon the return of the engine
cross-head the drum is revolved
by a spring which is constantly
held under tension. A section
of the paper drum of the Rob-
ertson-Thompson Indicator is
shown in Fig. 13.

In preparing to indicate an
engine, then, we have two prob-
lems to consider only ; one is to
place the indicator in perfect
communication with the cylin-
Fig. 13. PAPER DRUM. der at each end, and the other

is to rig a reducing motion that

will give to the indicator in its reduced scale an exact repro*
duction of the movement of the piston.




Fig. 12. THOMPSON
PARAI^EI, MOTION.




Hawkins Indicator Catechism. 75



QUESTIONS AND ANSWERS.

QUES. What is the first thing mportant to be known
relating to the Steam Engine Indicator?
ANS. The name of its essential parts.

QUES. What next is of importance?
ANS. To know the special terms, definitions and " shop
names " in ordinary use.

QUES. Cannot the instrument be used without this
knowledge ?

ANS. Yes ; it can be done, but it is next to impossible.

What facts must be noted in connection with the
indicator diagrams ?

ANS. The data needed for a complete interpretation of
the indicator diagrams are :

(i.) Size, name and end of cylinder to which each card
belongs.

(2.) The scale of the spring that has been used.

(3.) The number of revolutions per minute.

NOTE. It is an unwritten law that each art, science and mechanical
contrivance has related to it certain words, phrases and definitions,
which are known mainly and almost exclusively to those who practice
the arts or use the tools. In order to gain the necessary instruction in
the shortest time and in the most thorough manner it is well for the
learner to have already memorized and at his tongue's end those
elementary shop naines and definitions.



16 Hawkins Indicator Catechism.

QUESTIONS AND ANSWERS.

(4.) The pressure from the steam and receiver gauges
and the vacuum.

(5.) Degree ot opening of throttle.

(6.) Time, date, cut-off of cylinder and other details, as
desired.

How do engineers determine that the engine per-
forms its work with good economy ?

ANS. The economy of the engine's performance is
determined from the indicator diagrams and from the
amount of feed water that must be supplied to the boiler,
for furnishing the quantity of steam needed for the con-
tinuous, regular operation of the main engine and its
auxiliaries.

Why is it desirable to know the amount of the feed
water, as well as the indicator diagram ?

ANS. The amount of feed water tells the total quantity
of the steam generated by the boiler, while the indicator
diagram shows only that part of the steam which actually
exerts pressure upon the piston. The amount lost by
initial condensation and leakage through valve, piston,
safety valves, stuffing-boxes and joints, is not learned from
the cards.

However, by comparing the amount of feed water with
the amount of steam shown by the diagram, a percentage
of efficiency of the engine as a steam-user may be found.

The engineer can, therefore, discover whether the engine
is in order to prevent loss of steam or access of air to
those parts in connection with the condenser.



Hawkins' Indicator Catechism.



QUESTIONS AND ANSWERS.

QUES. Is the indicator more properly a machine or an
instrument ?

ANS. It is an instrument because it is a device by which
work is performed or anything is effected ; a machine or
engine implies a much larger piece of mechanism.

QUES. What are the special important uses of the indi-
cator?

ANS. It is used primarily to measure the power of the
steam engine ; second, to show the quantity of steam used
per horse power for a given time ; also, to indicate how to
adjust the valve gear of the engine.

QUES. What other uses can be named for which the
instrument is serviceable ?

ANS. It shows the vacuum obtained by the use of the
condenser and the relative pressure existing between the
steam in the boiler and the pressure of the steam in the
cylinder.

QUES. What are the principal (four) parts of the indi-
cator ?

ANS. The cylinder, the spring, the piston with its con-
nections, and the drum.

QUES. Does the steam required to operate the indicator
go to waste ?

ANS. Yes ; in effect it is so much added to the per-
centage of clearance of the cylinder.



NOTE. The indicator does no work, any more than a guide post ;
it simply records on the diagram the work being performed by the
engine.



i8 Hawkins* Indicator Catechism.

QUESTIONS AND ANSWERS.

QUES. What parts of the instrument will have to be
closely observed when diagrams are being taken ?

ANS. i, The pencil levers, that they do not get bent ;
2, the lead, which must not be too large, and always should
have a fine round point ; 3, the drum spring, which must
not be too slack nor too tight ; 4, the piston rod, which
must not be too short nor too long, .otherwise the pencil
will travel either too high or too low on the card.

QUES. What other parts should be looked after while
indicating or preparing for indicating?

ANS. Care should be taken that no lost motion is in
the reducing mechanism and its connections, for this would
produce serious error on the diagram. In adjusting the
cord, it should not be made too long or too short, and,
when putting the paper upon the drum, it should be
placed smooth, so it will not offer any obstructions to the
pencil.

QUES. When piping up the indicator, what is there
important to be guarded against ?

ANS. No pipe chips, red lead or other foreign matter
must be allowed to remain in the pipes, and when tapping
apertures into the cylinder for indicator connections they
must be so located that the piston will not obstruct the
holes.

QUES. How should the instrument be treated after
use?



NOTE. Also, care should be taken not to allow the pe/icil to press
too tightly upon the paper, which may produce unnecessary friction,
and produce error in the final results.



Hawkins Indicator Catechism. 19

QUESTIONS AND ANSWERS.

ANS. It should be cleaned, wiped dry, and oiled, so as
to be ready for the next test.

QUES. What about the springs ?

ANS. The springs are the most vital part of the instru-
ment, and need particular attention. They must not be
allowed to get rusty, otherwise they will be useless for
accurate work.

QUES. Is there any other simple way of causing serious
error in applying the indicator?

ANS. Error is often produced by not shutting the indi-
cator cock properly. It is not supposed to be pressed
tightly into its seat, as this would soon cut both plug and
seat, and cause the cock to leak. Besides the steam pas-
sage through the plug of the indicator cock, there is a small
hole drilled at right angles to the steam passage, which
communicates with a similar hole in the body of the cock ;
now, when shutting the cock, the plug should be turned so
that this hole will point toward the indicator cylinder, and
thus open communication between the atmosphere and
the cylinder of the instrument, and thus allow the atmos-
pheric line to be drawn in its proper place.

When opening the cock, this hole should be turned so

that the steam will not escape through it.

*
QUES. In indicator practice, what is meant by absolute

pressure ?

ANS. Absolute pressure is the pressure of the steam
above a perfect vacuum.

QUES. What is gauge pressure ?



20 Hawkins 1 Indicator Catechism.

QUESTIONS AND ANSWERS.

ANS. Gauge pressure, or pressure by gauge, is the
boiler pressure, as indicated by the steam gauge, or the
pressure per square inch of boiler surface above the atmos-
pheric pressure.

QUES. What is initial pressure?

ANS. Initial pressure is the pressure in the engine cyl-
inder as shown by the indicator, at the beginning of the
stroke. The initial pressure is ordinarily below the gauge
pressure, on account of the wire drawing in the connection
between the cylinder and boiler.

QUES. What is wire drawing?

ANS. Wire drawing is a term used in describing the
reduction of pressure caused by passing the steam air or
gas through a narrow opening.

QUES. What is the definition of mean effective pres-
sure?

ANS. Mean effective pressure is the average of the
pressures recorded by the indicator at different points of
the stroke above the exhaust pressure. It is the pressure
which would have to act upon the piston throughout the
entire stroke to cause the engine to develope the same
power as under the indicated conditions.

QUES. What is terminal pressure ?

ANS. Terminal pressure would be the pressure at the
end of the stroke, if the exhaust valve did not open before
the piston comes a dead standstill.



NOTE. As in all engines the exhaust valve opens before the end of
the stroke, the terminal pressure is never reached.



Hawkins* Indicator Catechism. 21

QUESTIONS AND ANSWERS.

QUES. What is back pressure ?

ANS. Back pressure, or counter pressure, is the amount
of pressure above the atmosphere during the exhaust
stroke. The back pressure counteracts the forward move-
ment of the piston, and therefore should be avoided. On
an indicator diagram it is found by allowing the instrument
to trace the atmospheric line after taking the diagram.

QUES. What is meant by " ratio of expansion " ?
ANS. Ratio of expansion is the entire cylinder volume,
divided by the volume before cut-off.

QUES. What does clearance mean ?

ANS. Clearance is the space included between the pis-
ton at the end of the stroke and the cylinder head ; to this
is added the volume of the steam ports. The clearance is
generally expressed in percentage of the cylinder volume.

QUES. What is saturated steam ?

ANS. Saturated or dry steam is steam in a perfect gas-
eous state. Any loss of heat will change its condition by
partial condensation.

QUES. What is superheated steam ?
ANS. Superheated steam is such as is heated above the
temperature of saturation.

QUES. What is the difference between an indicator
diagram and an indicator card ?

ANS. The diagram is the outline traced by the pencil
upon a card or paper, and a piece of paper becomes an
" indicator card " when it is used to record one or more
diagrams. A card often contains a diagram from both the
head and crank ends of the steam cylinder.



22



Hawkins Indicator Catechism.



AN INDICATOR OUTFIT.

An indicator outfit consists of the following articles :

The indicator; or if a very accurate test is required,
when both ends of the cylinder have to be indicated
simultaneously, then two instruments are needed.

A good reducing motion, as preferably a pantograph or
reducing wheel.

The cord for connecting reducing motion to indicator.

The indicator cards.

Pencils.

One three-way cock, or two straightway cocks.

Three or more springs, of different tensions.

The corresponding scales for the springs, as shown in
figure below.

A bottle of pure machinery oil.

The necessary wrenches, spanners, screw-drivers, etc.,
belonging to the indicator.

Planimeter.




NOTE. The numbers by which indicator springs are designated
denote the pounds pressure of steam required to raise the pencil one
inch ; the corresponding scales are divided so that each division repre-
sents one pound of steam ; in the figure above the spring is No. 6or= i
inch, which is divided (in the scale) into 60 parts, each of which repre-
sents one pound pressure.



HAWKINS' INDICATOR CATECHISM.



PREPARING THE INDICATOR FOR USE.

In selecting a spring, aim to get as large a card as pos-
sible without undue distortion. If a card be taken with a
20 spring, an error of measurement of jfa of an inch would
influence the results .only one-fifth of a pound. With a
50, spring the same error in measurement would represent
a departure of one-half a pound.

It is, therefore, advisable to have the area as large as
possible.

On the other hand, the allowable movement of both the
pencil and the drum is limited by the effects of momentum.
At high speeds a light spring and long movement of the
drum would result in a diagram so distorted by the effects
of momentum and inertia as to introduce errors much more
serious than those that are likely to occur from inaccurate
measurement of a smaller and more perfect diagram. The
speed as well as the pressure will therefore have a bearing
on the spring selected, and will also influence the selec-
tion as between the standard size of paper drum which is
used for moderate speeds, and the smaller drums which
some of the makers supply for high-speed work. Some
manufacturers furnish two sizes of drums, which may be
used interchangeably upon the same instrument, adapting
it to the highest and slowest speeds.

In changing the spring, unscrew the head of the indi-
cator, hold the carrying ring as shown in Fig. 14, and the
piston and spring may easily be disconnected from the
moving parts and head.



24 HAWKINS' INDICATOR CATECHISM.

PREPARING THE INDICATOR FOR USE.

In some instruments the position of the atmospheric line
is fixed ; in others it is adjustable, so that in indicating a
non-condensing engine the base-line may be lowered, and
the whole of the allowable movement of the pencil utilized
for the height of the diagram.




Fig. 14.

The adjustment for height is effected by lengthening or
shortening the distance between pencil lever and piston.

It is frequently desirable, in condensing engines, to obtain
the lower or condensing portion of the diagram on a
larger scale than that of the spring available with the initial
pressures used. With an initial pressure that demands a
60 spring, a realized vacuum of twelve pounds would be
represented by a line only one-fifth of an inch below the
atmospheric line, Fig. 15, giving a very small area to the
condenser portion of the card. In order to obtain this area



HAWKINS* INDICATOR CATECHISM. fc5

PREPARING THB INDICATOR FOR USE.

upon a larger scale, giving increased accuracy of measure-
ment, showing more clearly the points of release and com-
pression, etc., springs of low tension are sometimes fitted
with bosses or studs, which prevent their closing beyond
a certain point, while they are free to extend to any
amount.




Fig- 15-



In Figs. 15 and 16 are shown two diagrams, the first
drawn to a 60 scale ; and beneath it the shaded portion of
the diagram is shown expanded to a 10 scale. Notice how
much more prominently the points of release and compres-
sion are shown, on account of the more rapid vertical move-
ment with the same horizontal movement ; and how much
less an error of a few hundredths of a square inch in meas-
uring the area of the condensing portion of the card would
affect the result.

Select a hard lead of good smooth quality and of small
diameter, and use only a small piece at a time. At the end
of the pencil lever, where the motion is greatest, the weight
should be reduced to the smallest possible value. If
pointed with a fine file and rubbed down with an emery



26 HAWKINS' INDICATOR CATECHISM.

PREPARING THE INDICATOR FOR USE.

stick, such as is used for sharpening draftsmen's pencils, or
a fine stone, it will wear longer and be smoother and more
satisfactory than if whittled into shape.

For lubricating the bearings of the instrument a light
machinery oil should be used one that will not gum or
corrode. A small vial of such oil usually accompanies the




Fig. 16.

instrument, some makers furnishing porpoise oil, such as is
used for clocks and watches. The piston, however, is better
lubricated with cylinder oil, which must be absolutely free
of grit.

Filter the oil carefully and make sure that the can is
perfectly clean ; a small particle of grit upon the piston of
an indicator will not only throw the diagram into mislead-
ing forms, but may scratch and injure both cylinder and
piston to a serious degree.

Use hard, tough smoothly calendered paper of a width
sufficient to include the highest allowable pencil travel, and
about an inch longer than the circumference of the barrel.



HAWKINS' INDICATOR CATECHISM. 87



REDUCING MOTIONS.

In order to use the indicator, a means must be provided
for moving the paper drum exactly in time with the engine
piston. This movement is usually derived from the cross-
head, and the appliance used to reduce the movement to
that adapted to the paper barrel is spoken of as the " reduc-
ing motion."

The most simple expedient for this purpose is a lever
suspended from the ceiling or other suitable support, and
connected at its lower end with the cross-head in such a


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Online LibraryN. (Nehemiah) HawkinsHawkins' indicator catechism. A practical treatise for the use of erecting and operating engineers, superintendents, students of steam engineering, etc → online text (page 1 of 8)