By G. W. MANUEL.
[Footnote 1: A paper read before the Institute of Marine
Engineers, Stratford, 1889.]
Being asked to read a paper before your institute, I have chosen this
subject, as I think no part of the marine engine has given so much
trouble and anxiety to the seagoing engineer; and from the list of
shipping casualties in the daily papers, a large proportion seem due
to the shafting, causing loss to the shipowner, and in some instances
danger to the crew. My endeavor is to put some of the causes of these
casualties before you, also some of the remedies that have tended to
reduce their number. Several papers have been read on this subject,
chiefly of a theoretical description, dealing with the calculations
relating to the twisting and bending moments, effects of the angles of
the cranks, and length of stroke--notably that read by Mr. Milton
before the Institute of Naval Architects in 1881. The only _practical_
part of this paper dealt with the possibility of the shafts getting
out of line; and regarding this contingency Dr. Kirk said that "if
superintendent engineers would only see that the bearings were kept in
line, broken crank and other shafts would not be so much heard of." Of
course this is one of those statements made in discussions of this
kind, for what purpose I fail to see, and as far as my own experience
goes is _misleading_; for having taken charge of steamers new from the
builders' hands, when it is at least expected that these shafts would
_be in line_, the crank shaft bearings heated very considerably, and
_continued_ to do so, rendering the duration of life of the crank
shaft a short one; and though they were never what is termed out of
line, the bearings could _not_ be kept cool without the use of sea
water, and occasionally the engines had to be stopped to cool and
smooth up the bearing surfaces, causing delays, worry, and anxiety,
for which the engineer in charge was in no way responsible. Happily
this state of what I might call _uncertainties_ is being gradually
remedied, thanks being largely due to those engineers who have the
skill to suggest improvements and the patience to carry them out
against much opposition.
These improvements in many instances pertain to the engine builder's
duties, and are questions which I think have been treated lightly;
notably that of insufficient bearing surface, and one of the principal
causes of hot bearings, whereby the oil intended for lubrication was
squeezed out, and the metal surfaces brought too close in contact; and
when bearings had a pressure of 200 lb. per square inch, it has been
found that not more than 120 lb. per square inch should be exerted to
keep them cool (this varies according to the material of which the
bearing is composed), without having to use sea water and prevent them
being ground down, and thus getting out of line. I have known a
bearing in a new steamer, in spite of many gallons of oil wasted on
it, wear down one-eighth of an inch in a voyage of only 6,000 miles,
from insufficiency of bearing surface.
Several good rules are in use governing the strength of shafts, which
treat of the diameter of the bearings only and angles of the cranks;
and the engine builder, along with the ship owner, has been chary of
increasing the surfaces by lengthening the bearings; for to do this
means increase of space taken up fore and aft the vessel, besides
additional weight of engine. Engine builders all aim in competing to
put their engines in less space than their rivals, giving same power
and sometimes more. I think, however, this inducement is now more
carefully considered, as it has been found more economical to give
larger bearing surfaces than to have steamers lying in port, refitting
a crank shaft, along with the consequences of heavy bills for salvage
and repairs, also the risk of losing the steamer altogether.
Proportioning the bearings to the weights and strains they have to
carry has also been an improvement. The different bearings of marine
engines were usually made alike in surface, irrespective of the work
each had to do, with a view to economy in construction.
In modern practice the after bearings have more surface than the
forward, except in cases where heavy slide-valve gear has to be
supported, so that the wear down in the whole length of the shaft is
equal, thus avoiding those alternate bending strains at the top and
bottom of the stroke every revolution. Another improvement that has
been successfully introduced, adding to the duration of life of crank
shafts, is the use of white bearing metal, such as Parson's white
brass, on which the shafts run smoothly with less friction and
tendency to heat, so that, along with well proportioned surfaces, a
number of crank shafts in the Peninsular and Oriental Co.'s service
have not required lining up for eight years, and I hope with care may
last till new boilers are required. Large and powerful steamers can be
driven full speed from London to Australia and back without having any
water on the bearings, using oil of only what is considered a moderate
price, allowing the engineer in charge to attend to the economical
working of both engines and boilers (as well as many other engines of
all kinds now placed on board a large mail and passenger steamer),
instead of getting many a drenching with sea water, and worried by
close attention to one or two hot bearings all the watch. Compare
these results with the following: In the same service in 1864, and
with no blame to the engineer in charge, the crank shaft bearings of a
screw steamer had to be lined up every five days at intermediate
ports, through insufficient bearing surfaces. Sea water had
continually to be used, resulting in frequent renewal of crank shaft.
Steamers can now run 25,000 miles without having to lift a bearing,
except for examination at the end of the voyage. I would note here
that the form of the bearings on which the shafts work has also been
much improved. They are made more of a _solid character_, the metal
being more equally disposed _round_ the shaft, and the use of gun
metal for the main bearings is now fast disappearing. In large engines
the only metals used are cast iron and white brass, an advantage also
in reducing the amount of wear on the recess by corrosion and grinding
where sea water was used often to a considerable extent.
[Illustration: Fig. 1
Fig. 2]
Figs. No. 1 and No. 2 show the design of the old and new main
bearings, and, I think, require but little explanation. Most of you
present will remember your feelings when, after a hot bearing, the
brasses were found to be cracked at top and bottom, and the trouble
you had afterward to keep these brasses in position. When a smoking
hot bearing occurred, say in the heating of a crank pin, it had the
effect of damaging the material of the shaft more or less, according
to its original soundness, generally at the fillets in the angles of
the cranks. For when the outer surface of the iron got hot, cold
water, often of a low temperature, was suddenly poured on, and the hot
iron, previously expanded, was suddenly contracted, setting up strains
which in my opinion made a small tear transversely where the metal was
_solid_; and where what is termed lamination flaws, due to
construction, existed, these were extended in their natural direction,
and by a repetition of this treatment these flaws became of such a
serious character that the shafts had to be condemned, or actually
gave way at sea. The introduction of the triple expansion engine, with
the three cranks, gave better balance to the shaft, and the forces
acting in the path of the crank pin, being better divided, caused more
regular motion on the shaft, and so to the propeller. This is
specially noticeable in screw steamers, and is taken advantage of by
placing the cabins further aft, nearer the propeller, the stern having
but little vibration; the dull and heavy surging sound, due to unequal
motions of the shaft in the two-crank engines, is exchanged for a more
regular sound of less extent, and the power formerly wasted in
vibrating the stern is utilized in propelling the vessel. In spite of
all these improvements I have mentioned, there remains the serious
question of defects in the material, due to variety of quality and the
extreme care that has to be exercised in all the stages during
construction of crank or other shafts built of iron. Many shafts have
given out at sea and been condemned, through no other cause than
_original defects_ in their construction and material.
The process of welding and forging a crank shaft of large diameter now
is to make it up of so many small _pieces_, the _best shafts_ being
made of what is termed scrap, representing thousands of small pieces
of selected iron, such as cuttings of old iron boiler plates,
cuttings off forgings, old bolts, horseshoes, angle iron, etc., all
welded together, forged into billets, reheated, and rolled into bars.
It is then cut into lengths, piled, and formed into slabs of suitable
size for welding up into the shafts. No doubt this method is
preferable to the old method of "fagoting," so called, as the iron
bars were placed side by side, resembling a bundle of fagots of about
18 or 20 inches square.
The result was that while the outside bars would be welded, the inside
would be improperly welded, or, the hammer being weak, the blow would
be insufficient to secure the proper weld, and it was no uncommon
thing for a shaft to break and expose the internal bars, showing them
to be quite separate, or only partially united. This danger has been
much lessened in late years by careful selection of the materials,
improved methods of cleaning the scrap, better furnaces, the use of
the most suitable fuels, and more powerful steam hammers. Still, with
all this care, I think I may say there is not a shaft without flaws or
defects, more or less, and when these flaws are situated in line of
the greatest strains, and though you _may not_ have a hot bearing,
they often extend until the shaft becomes unseaworthy.
[Diagrams shown illustrated the various forms of flaws.] These flaws
were not observable when the shafts were new, although carefully
inspected. They gradually increased under strain, came to the outside,
and were detected. Considerable loss fell upon the owners of these
vessels, who were in no way to blame; nor could they recover any money
from the makers of the shafts, who were alone to blame. I am pleased
to state, and some of the members here present know, that considerable
improvement has been effected in the use of better material than iron
for crank shafts, by the introduction of a special mild steel, by
Messrs. Vickers, Sons & Co., of Sheffield, and that instead of having
to record the old familiar defects found in iron shafts, I can safely
say no flaws have been observed, when new or during eight years
running, and there are now twenty-two shafts of this mild steel in the
company's service.
I may here state that steel was used for crank shafts in this service
in 1863, as then manufactured in Prussia by Messrs. Krupp, and
generally known as _Krupp's steel_, the tensile strength of which was
about 40 tons per square inch, and though free from flaws, it was
unable to stand the fatigue, and broke, giving little warning. It was
of too brittle a nature, more resembling chisel steel. It was broken
again under a falling weight of 10 cwt. with a 10 ft. drop = 12½ tons.
The mild steel now used was first tried in 1880. It possessed tensile
strength of 24 to 25 tons per square inch. It was then considered
advisable not to exceed this, and err rather on the safe side. This
shaft has been in use eight years, and no sign of any flaw has been
observed. Since then the tensile strength of mild steel has gradually
been increased by Messrs. Vickers, the steel still retaining the
elasticity and toughness to endure fatigue. This has only been arrived
at by improvements in the manufacture and more powerful and better
adapted hammers to forge it down from the large ingots to the size
required. The amount of work they are now able to subject the steel to
renders it more fit to sustain the fatigue such as that to be endured
by a crank shaft. These ingots of steel can be cast up to 100 tons
weight, and require powerful machines to deal with them. For shafts
say of 20 inches diameter, the diameter of the ingot would be about 52
inches. This allows sufficient work to be put on the couplings, as
well as the shaft. To make solid crank shafts of this material, say of
19 inches diameter, the ingot would weigh 42 tons, the forging, when
completed, 17 tons, and the finished shaft 11¾ tons; so that you see
there is 25 tons wasted before any machining is done, and 5¼ tons
between the forging and finished shaft. This makes it very expensive
for solid shafts of large size, and it is found better to make what is
termed a _built shaft_; the cranks are a little heavier, and engine
framings necessarily a little wider, a matter comparatively of little
moment. I give you a rough drawing of the hydraulic hammer, or
strictly speaking a _press_, used by Messrs. Vickers in forging down
the ingots in shafts, guns, or other large work. This hammer can give
a squeeze of 3,000 tons. The steel seems to yield under it like tough
putty, and, unlike the steam hammer, there is no _jarring_ on the
material, and it is manipulated with the same ease as a small hammer
by hydraulics.
The tensile strength of steel used for shafts having increased from 24
to 30 tons, and in some cases 31 tons, considering that this was 2
tons above that specified, and that we were approaching what may be
termed _hard steel_, I proposed to the makers to test this material
beyond the usual tests, viz., tensile, extension, and cold bending
test. The latter, I considered, was much too easy for this fine
material, as a piece of fair iron will bend cold to a radius of 1½
times its diameter or thickness, without fracture; and I proposed a
test more resembling the fatigue that a crank shaft has sometimes to
stand, and more worthy of this material; and in the event of its
standing this successfully, I would pass the material of 30 or 31 tons
tensile strength. Specimens of steel used in the shafts were cut off
different parts--crank pins and main bearings--(the shafts being built
shafts) and roughly planed to 1½ inches square, and about 12 inches
long. They were laid on the block as shown, and a cast iron block,
fitted with a hammer head ½ ton weight, let suddenly fall 12 inches,
the block striking the bar with a blow of about 4 tons. The steel bar
was then turned upside down, and the blow repeated, reversing the
piece every time until fracture was observed, and the bar ultimately
broken. The results were that this steel stood 58 blows before showing
signs of fracture, and was only broken after 77 blows. It is
noticeable how many blows it stood after fracture. A bar of good
wrought iron, undressed, of same dimensions, was tried, and broke the
first blow. A bar cut from a piece of iron to form a large chain,
afterward forged down and only filed to same dimensions, broke at 25
blows. I was well satisfied with the results, and considered this
material, though possessing a high tensile strength, was in every way
suitable for the construction and endurance required in crank shafts.
Sheet No. 1 shows you some particulars of these tests:
Tensile Elong. Fractured Broke Fall
Tons. in 5" Bend. Blows. Blows. In.
A = 30.5 28 p. c. Good 61 78 12
In order to test the comparative value of steel of 24¾ up to 35 tons
tensile strength, I had several specimens taken from shafts tested in
the manner described, which may be called a _fatigue_ test. The
results are shown on the same sheet:
B = 24½ Good 64 72 7
B -- -- -- 48 54 12
C = 27 25.9 p. c. Good 76 81 12
D = 29.6 28.4 p. c. Good 71 78 12
E = 30.5 28.9 p. c. Good 58 77 12
F = 35.5 20 p. c. Good 80 91 12
The latter was very tough to break. Specimen marked A shows one of
these pieces of steel. I show you also fresh broken specimens which
will give you a good idea of the beautiful quality of this material.
These specimens were cut out of shafts made of Steel Co. of Scotland's
steel. I also show you specimens of cold bending:
Tensile Elong. Fractured Broke Fall
Tons. in. 5" Bend. Blows. Blows. In.
G = 30.9 27½ p. c. Good 59 66 12
H = 29.3 30 p. c. Good 66 90 12
I = 28.9 28.9 p. c. Good 53 68 12
I think all of the above tests show that this material, when carefully
made and treated with sufficient mechanical work on forging down from
the ingot, is suitable up to 34 tons for crank shafts; how much higher
it would be desirable to go is a question of superior excellence in
material and manufacture resting with the makers. I would, however,
remark that no allowance has been made by the Board of Trade or Lloyds
for the excellence of this material above that of iron. I was
interested to know how the material in the best iron shafts would
stand this fatigue test compared with steel, and had some specimens of
same dimensions cut out of iron shafts. The following are the results:
Best iron, three good qualities, rolled into flat bars, cut and made
into 4½ cwt. blooms.