Engineering conditions in 1860. I meet Mr. Allen. Mr. Allen’s
inventions. Analysis of the Allen link.
Before resuming my narrative, it seems desirable to present a brief
sketch of steam engineering conditions forty years ago.
The science of thermodynamics had been established on the foundation
laid in the experiments of Joule, determining with precision the rate
at which, through the medium of water, heat is converted into dynamical
force. This science was, however, as yet without practical results.
The condensation of steam in the cylinder from the conversion of its
heat into mechanical energy was unregarded. The same was true also
respecting the far greater loss from the changing temperatures of the
surfaces with which the steam comes in contact in alternately entering
and leaving the cylinder. The action of these surfaces in transmitting
heat from the entering to the exhaust steam without its doing any work
was imagined by very few.
In the United States economy of steam was sought only by mechanical
means--by cutting off the admission of the steam at an early point of
the stroke in a single cylinder and permitting the confined steam to
complete the stroke by its expansion. By this means a large saving of
steam over that consumed in earlier practice was effected, and with
this gain the universal disposition was to rest content.
America was eminently the land of the cut-off system, an early
application of which was on steamboats. The earliest device for this
purpose was the elegant Stevens cut-off, which still keeps its position
on the class of boats to which it was first applied, though commonly
modified by the Sickles improvement. In this system the exhaust and
the admission valves are operated by separate eccentrics on opposite
sides of the engine, and all the valves have the amount and rapidity of
their opening and closing movements increased by the intervention of
wiper cams, those for the admission valves being very long and giving a
correspondingly greater enlargement of opening. The valves were double
poppet valves, moving nearly in equilibrium in directions vertical to
their seats. This cut-off was found to be capable of improvement in
one important respect. The closing motion of the valve grew slower as
the valve approached its seat, and while the piston was moving most
rapidly much steam passed through the ports at a lower pressure, and
so a great part of its expansive value was lost. This was technically
termed “wire-drawing.” To remedy this defect Mr. Sickels invented his
celebrated trip cut-off. The valve, lifted by the Stevens wiper, was
liberated by tripping the mechanism, and fell quickly to its seat,
which it was prevented from striking forcibly, being caught by water in
a dash-pot. The steam was thus cut off sharply and the economy was much
improved. The pressure used in this system was only about 25 pounds,
the vacuum being relied upon for the larger portion of the power.
On the Great Lakes a pressure of 60 pounds was commonly employed, and
the valves were the four cylindrical rotating slide valves afterwards
adopted by Mr. Corliss. What was called the cut-off was made by a
separate valve located in the steam-pipe somewhere between the engine
and the boiler.
On the Mississippi and its tributaries, much higher pressures were
carried, condensers were not used, and the admission and release of the
steam were generally effected by four single poppet valves, lifted by
cams against the pressure of the steam.
On land engines Mr. Sickels’ invention of the trip cut-off stimulated
inventors to a multitude of devices for working steam expansively. Of
these the one of enduring excellence proved to be that of Mr. Corliss.
He applied the trip cut-off to the rotating slide valve, and arrested
the motion of the liberated valve by an air-cushion. This proved a
satisfactory method, as the valve, moving in directions parallel to its
seat, did not need to be stopped at a determinate point. Mr. Corliss
applied the governor to vary the point of liberation of the valve, and
so produced a variable cut-off, which effected a large saving of steam
and regulated the motion of the engine more closely than could be done
by a throttle valve outside the steam-chest. This was by far the most
prominent of the numerous forms of automatic variable cut-offs, to all
of which it was supposed that the liberating feature was essential.
In England, when the steam was worked expansively, it was cut off by a
separately driven valve on the back of the main slide valve, the point
of cut-off being fixed; and the regulation was effected by means of the
throttle. This system was also largely employed in this country.
The compound engine was unknown in the United States. I once saw at
some place in New York City, now forgotten, a Wolff engine--a small
beam-engine, which had been imported from England. It was visited as a
curiosity by several engineers, and I remember Mr. Horatio Allen, then
president of the Novelty Iron Works, remarking, “It is only a cut-off.”
In the south of England the Wolff system was used to a limited extent.
I was much interested in the McNaught system, devised, I think, by
the same Scotchman who first applied a rotating paper drum to the
Watt indicator. The cotton and woolen mills, as their business grew,
felt the need of additional power, but dared not employ higher steam
pressures in their cylinders, because the beam centers of their engines
would not stand the additional stress. McNaught provided an additional
cylinder to carry a higher pressure, and applied this pressure directly
to the connecting-rod end of the beam. The exhaust from this cylinder
was taken into the old cylinder at the old pressure. This latter
cylinder then exerted the same power it always had done. The stresses
on the beam centers were not increased, but the power of the engine was
doubled, and only a little more steam was used than before. This method
of compounding was known as McNaughting, and became common in the
manufacturing districts of England and Scotland.
There was one feature which was common to all engines in America and
Europe, both ashore and afloat, and of whatever make or name, except
locomotives. That was the piston speed, which varied only from 200 to
300 feet per minute. This last was the maximum speed, to which every
new engine, however novel in other respects, was made to conform.
I come now to the turning-point in my career, and the reflection forces
itself upon me, how often in the course of my life incidents trivial in
themselves have proved afterwards to have been big with consequences;
and how events, sometimes chains of events, beyond my control, of
which indeed I had no knowledge, have determined my course. The same
must be the case in the lives of many persons, and the thoughtful mind
cannot look back on them without being impressed by the mysterious
interrelations of our being.
One morning in the winter of 1860-61, Mr. Henry A. Hurlbut, of the firm
of Swift, Hurlbut & Co., wholesale dealers in hats at No. 65 Broadway,
and who was interested in my governor manufacture, called upon me
to tell me that a friend of his, Mr. Henry A. Burr, manufacturer
of felt hat bodies at the corner of Frankfort and Cliff streets in
New York, had been having trouble with his engine. He thought my
governor was just what he needed, and asked me to accompany him to Mr.
Burr’s office, where he would give me the advantage of his personal
introduction. In the interview with Mr. Burr which followed, I did not
have an opportunity to say a word. After Mr. Hurlbut had explained
the object of our visit, Mr. Burr replied that he had had a great
deal of trouble with the regulation of his engine, and had thought
seriously of getting a Corliss engine in the place of it; but two or
three weeks before the builders of the engine had sent him a very
skillful engineer, and since he came there had been no further trouble,
so he should not need my governor. He invited us to see his engine,
in which--since it had been taught to behave itself--he evidently
took much pride. We found a pair of beam-engines of 5 feet stroke,
running at 25 revolutions per minute, made by Thurston & Gardiner of
Providence. They had the usual poppet valves and the Sickels cut-off.
This was made adjustable, and was regulated by the governor. At the
time of our entrance, Mr. Allen, the new engineer, was engaged on the
scaffold. Mr. Burr called him and he came down, and at Mr. Burr’s
request explained to us the variable liberating mechanism and what he
had done to make it work satisfactorily. The regulation did not appear
to me to be very close, and I made a determined effort to induce Mr.
Burr to substitute one of my governors. I showed him a cut of the
governor, and pointed out its combination of power and sensitiveness,
but all in vain. He was satisfied with things as they were, and I went
away crestfallen, having lost not only the sale of a governor, but also
an opportunity for a triumph in a very important place. But I did not
know to whom I had in fact been talking.
As we were leaving, Mr. Allen asked me if I would call some time and
see him--he had something he thought I would be interested in. I
called soon after. He told me he had a plan for a variable cut-off
with positive movements, which he thought would avoid defects in the
liberating gear. He had had it in his mind a good while, but did not
think it could be used, because the governor could not handle the block
in his link so as to maintain uniform motion, and he had been inclined
to abandon the idea; but when he heard me describing my governor to
Mr. Burr, it occurred to him that that governor would do it, and he
would like to explain his plan to me. He had no drawing, not a line;
the design existed only in his mind. He put down his ideas, as he fitly
expressed it, with chalk on the engine-room floor, and that rude sketch
represented the perfect system.
When his plan came to be analyzed, it was found that everything had
been thought out and provided for, with a single exception afterwards
provided by Mr. Allen, as will be described. But the wonder did not
stop there. Mr. Allen had remedied the defect in the link motion of
making a narrow opening for admission when cutting off early, by
employing a four-opening admission valve of unique design at each end
of the cylinder, and also by greatly enlarging the opening movements.
The four-opening valve required four seats in one plane, and it was
important that these should be as narrow as possible. For this purpose
Mr. Allen employed the Corliss wrist-plate movement to reduce the lap
of the valve, and, by an elegant improvement on this movement, he made
it available also to enlarge the openings. This improvement consisted
in the employment of two rockers having a common axis and separate
driving-arms, as well as driven arms, for each valve. The driving-arms
were made to vibrate a long way towards their dead points, and the
increased opening movement in arc thus obtained was imparted directly
to the valve. This combination of an enlarged opening with a reduced
lap was perhaps the most surprising feature of Mr. Allen’s system.
The four-opening equilibrium valve, afterwards invented by Mr. Allen
and since 1876 always employed, requires but two seats in one plane.
These could therefore be made wider. The division of the driving-arm
was then dispensed with, and the enlarged openings were obtained by
increasing the length of the driven arms.
That this remarkable system of ports and movements should have been
elaborated in the mind of a man who had no knowledge of mechanics
except what he had absorbed in engine-rooms must stand among the
marvels of inventive power.
The accompanying diagram represents the lines put down by Mr. Allen on
his engine-room floor and since retained, except that it is now adapted
to the more simple movement, with a single driving-arm on the rocker,
as previously described.
The eccentric is formed on the shaft coincident with the crank of the
engine, so that the two arrive at their dead points simultaneously.
The angular vibration of the line connecting the center of the
eccentric with the trunnions of the link is the same as that of the
connecting-rod.
The connecting-rod of the length always used by me, namely, six cranks,
makes the piston velocity at the head end of the cylinder 40 per
cent. greater than at the crank end. By this construction the valve
velocities were made to vary in the same ratio.
A connecting-rod five cranks in length would increase this difference
in piston velocities to 50 per cent., and one four cranks in length
would increase it to 66 per cent.
After Mr. Allen had explained his plan to me, I expressed my confidence
that my governor would meet its requirements, and observed that
it would enable a variable cut-off engine to be run as fast as a
locomotive. Somewhat to my surprise, he replied that he wanted his
cut-off compared with the liberating cut-off turn for turn; that it had
an advantage which he thought would cause it to be generally preferred
at the same speed.
[Illustration:
PORTER-ALLEN ENGINE.
DIAGRAM OF ADMISSION-VALVE MOVEMENTS.
RELEASE AND COMPRESSION
¹⁵⁄₁₆ OF THE STROKE
¹⁄₂ CUT OFF ¹⁄₂ CUT OFF
³⁄₈ CUT OFF ³⁄₈ CUT OFF
¹⁄₄ CUT OFF ¹⁄₄ CUT OFF
¹⁄₈ CUT OFF ¹⁄₈ CUT OFF
TO VALVE AT
CRANK END
MEAN POSITION OF ROD
RADIUS OF LINK
TO VALVE AT
HEAD END
A. POINTS OF ADMISSION AND CUT-OFF.
FOR DISTINCTNESS OF REPRESENTATION, THE THROW
OF THE ECCENTRIC IS SHOWN ¹⁄₄ THAT OF THE CRANK.
IN PRACTICE IT IS ONLY ¹⁄₁₂ THAT OF THE CRANK.
The Diagram Drawn by Mr. Allen on his Engine-room Floor.]
[Illustration: JOHN F. ALLEN]
I was then ignorant of his state of mind on that subject, or of what
had produced it. I learned these afterwards, and will state them here.
In one of our interviews, in reply to my question as to what had led
him to make this invention, he told me it was his experience when he
was engineer of the propeller “Curlew,” a freight-boat running on Long
Island Sound, between New York and Providence, which had a Corliss
engine. He became impressed with what he thought to be a serious defect
in the liberating system. The governor did not control the point of
cut-off, but the point of release; this point being at the beginning
of the closing movement of the valve, while the cut-off took place
near the end of that movement. When the engine was worked up to nearly
its capacity, as was the case in a ship, the port was opened wide,
and quite an appreciable time elapsed between the release and the
cut-off. During this interval the piston advanced considerably, and if
the engine ran fast enough it might get to the very end of the stroke
before the cut-off took place. He said that in smooth water they had
no trouble, but in the open ocean, going around Point Judith, it was
always rough, and sometimes in stormy weather the screw would be thrown
quite out of the water, and the engine, having no fly-wheel, would race
most furiously. The faster it ran the further the steam would follow,
and was pumped out of the boiler very rapidly. Springs were employed
to accelerate the closing movement of the valves, but in these cases
they seemed to be of little use, and were continually breaking. He saw
that this difficulty could be avoided only by a positive motion gear
which would enable the governor to control the point of cut-off itself;
and, accordingly, he set himself to work to devise such a system.
We know now that this judgment, formed from observations made under
very exceptional conditions, was not well founded. The difficulty in
question does not practically exist in engines having fly-wheels and
the present improved liberating gear, and running at moderate speeds;
but the experience naturally made a deep impression upon Mr. Allen’s
mind, and led to the invention of the positive motion system.
This he did not tell me at the time, so that I was at a loss to
understand his reluctance to admit what was really the great value
of his invention. However, I told him I would be willing to attempt
its introduction, provided he would allow me to apply it at once to a
high-speed engine; that being a field into which the liberating system
could not enter. We had quite an argument on this point. I told him
his invention interested me only because it would enable two or three
times the power to be obtained from a given engine without additional
stress on any part, the fly-wheel to be reduced in size, and the means
for getting up the speed of machinery to be largely dispensed with.
I represented to him also that a high-speed engine ought to be more
economical and to give a more nearly uniform motion.
He finally agreed to my condition, and I took him directly to the
office of Mr. Richards and engaged him to make an analysis and drawing
of Mr. Allen’s system under his direction, and soon afterwards gave him
an order for the plans for an experimental engine, 6×15 inches, to make
160 revolutions per minute.
As the diagram of the link motion was at first drawn, the center of the
trunnions vibrated in an arc which terminated at points _on_ the line
connecting the center of the engine shaft with the ends of the rocker
arms, and which in the diagram on page 48 is named “radius of link.”
I determined to work out this link motion myself on a large scale. For
this purpose I drew a diagram in which the throw of the eccentric was
4 inches, and the distance from the center of the shaft to that of the
trunnions of the link in their mid-position was 12 inches. I made a
three-point beam compass. Two of these points were secured permanently
on the beam, 12 inches apart. As one of these points traversed the path
of the center of the eccentric, the other could be made to traverse the
arc of vibration of the trunnions of the link.
I divided the former into 40 equal divisions measured from its
dead points, making needle-holes in the circle, in which the taper
compass-points would center themselves accurately. The paper was firm
and the points of division were fixed with extreme care; and they
lasted through all my experiments. I then set out 20 corresponding
divisions in the arc of vibration of the center of the trunnions.
These showed distinctly the modification of the motion at the opposite
ends of this vibration as already described.
The third point was adjustable on a hinged beam which could be secured
in any position. I drew two arcs representing the lead lines of the
link, or the lines on which the link would stand when the eccentric was
on its dead points. The third point was now secured on its beam at any
point on one of the lead lines, when the other points stood, one on the
dead point of the eccentric and the other at the end of the trunnion
vibration.
The apparatus was now ready for use, the corresponding points on the
circle and the arc being numbered alike. By setting the first two
points in any corresponding holes, the third point would show the
corresponding position of that point of the link at which it was set.
I thus set out the movements of six different points of the link, the
highest being 12 inches above the trunnions. These represented the
movements of the valves of the engine when the block was at these
points in the link. The apparatus being firm, it worked with entire
precision. To my surprise, it showed much the larger valve opening at
the crank end of the cylinder, where the movement of the piston was
slowest. That would not do; we wanted just the reverse.
I called Mr. Allen in and showed him the defect. After considering it
a few minutes, he said he thought it would be corrected by lowering
the trunnions, so that their arc of vibration would coincide with the
line of centers at its middle point, instead of terminating on it. This
was done, and the result was most successful. The lead was now earlier
and the opening wider at the back end of the cylinder, as the greater
velocity of the piston at that point required, and the cut-offs on the
opposite strokes more equal. The link has always been set in this way,
as shown in the diagram.
From this description of the link motion, it will be seen that the
correct vertical adjustment of the trunnions of the link was an
important matter. To enable this adjustment to be made with precision,
and to be corrected, if from wear of the shaft-bearings or other cause
this became necessary, I secured the pin on which these trunnions
were pivoted to the side of the engine bed in the manner shown in the
following figure. To hold the wedge securely, the surface of the bed
below was reduced, so that the wedge was seized by the flange. The
correct position of this pin was determined by the motions given to the
valves.
[Illustration: VERTICAL ADJUSTMENT OF SUSTAINING PIN FOR TRUNNIONS OF
THE ALLEN LINK]
I now took a more prominent part myself in steam-engine design. I had
got an idea from Mr. Sparks that took full possession of my mind. This
was the exceedingly unmechanical nature of the single or overhanging
crank. The engines of the “New York,” built by Caird & Co., of
Greenock, were among the first of the direct inverted-cylinder engines
applied to screw propulsion. They were then known as the steam-hammer
engines, their leading feature being taken from Mr. Nasmyth’s
invention. I am not sure but Caird & Co. were the first to make this
application. The forward engine had a single crank. The vital defect of
this construction became especially apparent in these vertical engines
of large power. The stress on the cap bolts during the upward strokes
and the deflection of the shaft alternately in opposite directions
over the pillow-block as a fulcrum were very serious. Mr. Sparks told
me that on his very first voyage he had a great deal of trouble with
this forward bearing, and it caused him continual anxiety. He got into
such a state of worry and apprehension that as soon as he reached New
York he wrote to the firm: “For God’s sake, never make another pair
of engines without giving a double crank to the forward engine.” The
reply he got was, to mind his own business: they employed him to run
their engines; they would attend to the designing of them. He told me
not long after that he had the satisfaction of seeing every ship they
built except his own disabled, either by a broken shaft or broken
pillow-block bolts. He attributed the escape of the “New York” from a
like disaster to his own extreme care. They did, however, adopt his
suggestion on all future vessels, and, moreover, added a forward crank
and pillow-block to the engines already built. This they evidently
found themselves compelled to do. I saw this addition afterwards on the
“Bremen,” sister ship to the “New York.” The added pillow-block was
supported by a heavy casting bolted to the forward end of the bedplate.
I went everywhere visiting engines at work and in process of
construction, to observe this particular feature of the overhanging
crank, which was universal in horizontal engines. In this class of
engines, running slowly, its defective nature was not productive of
serious consequences, because no stress was exerted on the cap bolts
and the shaft was made larger in proportion to the power of the engine,
as it had to carry the fly-wheel. But I was astonished to see the
extent to which the overhang of the single crank was allowed. Builders
seemed to be perfectly regardless of its unmechanical nature. First,
the crank-pin was made with a length of bearing surface equal to about
twice its diameter; then a stout collar was formed on the pin between
its bearing surface and the crank. The latter was made thick and a
long hub was formed on the back of it. I was told that the long hub
was necessary in order to give a proper depth of eye to receive the
shaft. This being turned down smaller than the journal, so that the
crank might be forced on up to a shoulder, the eye needed to be deep or
the crank would not be held securely. Finally, the journal boxes were
made with flanges on the ends, sometimes projecting a couple of inches.
Altogether, the transverse distance from the center line of the engine
to the solid support of the shaft in the pillow-block was about twice
what it needed to be. I also saw in some cases the eccentric placed
between the crank and the pillow-block. Fifteen years later I saw a
large engine sent from Belgium to our 1876 Exhibition which was made in
this manner.
I determined at once that such a construction would not do for
high-speed engines, and proceeded to change every one of these
features. The single crank could not be avoided, but its overhang could
be much reduced.
[Illustration: OLD AND NEW CRANKS AND JOURNAL BOXES.
THE CRANKS ARE SHOWN IN THE VERTICAL POSITION. CRANKS AND TOP AND
BOTTOM BOXES ARE SHOWN IN SECTION.]
The following sketches show the changes which were then made, and all
of which have been retained. The inside collar on the crank-pin was
dispensed with and the diameter of the pin was made greater than its
length, the projected area being generally increased. The shank of the
pin was made larger and shorter, and was riveted at the back. Instead
of turning the shaft down smaller than the journal to receive the
crank, I made it with a large head for this purpose. The keyway could
then be planed out and the key fitted above the surface of the journal,
and the joint was so much further from the axis that but little more
than one half the depth was required in the crank-eye.
Mr. Corliss had already discarded the flanged boxes. He also first
made this bearing in four parts. The wear in the horizontal direction,
the direction of the thrust, could then be taken up. For this purpose
he used two bolts behind the front side box only. I modified his
construction by making the side boxes wider and taking up their wear by
wedges behind both of them, thus preserving the alignment. One wedge
could also be placed close to the crank. The dotted lines show the
width of the side boxes and the location of the wedges. The shaft was
made with a collar to hold the bearings in place, and was enlarged in
its body. The substitution in place of the crank of the entire disk
carrying a counterweight completed these changes. This was the fruit of
my first lesson in high-speed engine designing, which had unconsciously
been given to me by Mr. Sparks. The oil passage in the pin was added
later, as will be described.
I had another piece of good luck. I happened one day to see in the
Novelty Iron Works the hubs being bored for the paddle-wheels of the
new ship for the Collins line--the “Adriatic.” These were perhaps the
largest castings ever made for such a purpose. I observed that they
were bored out only half-way around. The opposite side of the hole had
been cored to about half an inch greater radius, and three key-seats
were cored in it, which needed only to be finished in the key-seating
machine. The idea struck me that this would be an excellent way to bore
fly-wheels and pulleys. As commonly bored, so that they could be put on
the shaft comfortably they were bored too large, their contact with the
shaft could then be only on a line opposite the key, and the periphery
could not run perfectly true.
I adopted the plan of first boring to the exact size of the shaft and
then shifting the piece about an eighth of an inch, and boring out
a slender crescent, the opposite points of which extended a little
more than half-way around. The keyway was cut in the middle of this
enlargement. The wheel could then be readily put on to the shaft, and
when the key was driven up contact was made over nearly one half the
surface and the periphery ran dead true. I remember seeing this feature
much admired in London, and several times heard the remark, “I should
think the key would throw it some.”
To prevent fanning I made the fly-wheel and pulley with arms of oval
cross-section. These have always been used by me. They have done even
better than I expected. They are found to impart no motion to the air,
however rapidly they may be run.
[Illustration: Flanges on the Eccentric.]
[Illustration: Flanges on the Strap.]
As already stated, the Allen valve-gear required the position of the
eccentric to coincide with that of the crank, so that these should
pass their dead points simultaneously. To insure this and to make it
impossible for the engineer to advance his eccentric, which he would be
pretty sure to do if he could, I made the eccentric solid on the shaft.
This also enabled me to make it smaller, the low side being brought
down nearly to the surface of the shaft. The construction, moreover,
was substantial and saved some work.
All eccentrics that I had seen were flanged on each side to keep the
strap in place. I observed the oil to work out freely between the
flanges and the strap. This action would of course be increased in
high-speed engines. So I reversed the design, as shown in the above
sections of these two bearings at the top of the eccentric, putting the
flanges on the strap instead of on the eccentric.
It will be seen that the more rapid the speed the more difficult it
becomes to keep the oil in the first bearing, and the more difficult it
becomes for it to get out of the second one. I ought to have adopted
the same construction for the main shaft journal, but in all the years
I was making engines it never occurred to me. I contented myself with
turning a groove in the hub of the crank, as shown to prevent the oil
from getting on the disk.
The problem of crank-pin lubrication at high speed at once presented
itself and had to be met. I finally solved it in the manner partially
shown on page 54. A wiper was bolted on the back of the crank, and from
it a tube entered the diagonal hole in the pin. This always worked
perfectly. This wiper and the oil cup are shown on page 230. Other
devices have been employed by various makers of high-speed engines, but
I always adhered to this one. It has the advantage of being equally
applicable to double-crank engines. Aside from the above features, the
design for my exhibition engine was made by Mr. Richards.