You will see from these results that steel stood this fatigue test,
Vickers' 73 per cent. and Steel Co.'s 68 per cent., better than iron
of the best quality for crank shafts; and I am of opinion that so long
as we use such material as these for crank shafts, along with the
present rules, and give ample _bearing surface_, there will be few
broken shafts to record.
I omitted to mention that built shafts, both of steel and iron, of
large diameter, are now in general use, and with the excellent
machines, and under special mechanics, are built up of five separate
pieces in such a rigid manner that they possess all the solidity
necessary for a crank shaft. The forgings of iron and steel being much
smaller are capable of more careful treatment in the process of
manufacture. These shafts, for large mail steamers, when coupled up,
are 35 feet long, and weigh 45 tons. They require to be carefully
coupled, some makers finishing the bearings in the lathe, others
depend on the excellence of their work in each piece, and finish each
complete. To insure the correct centering of these large shafts, I
have had 6 in. dia. recesses ¾ inch deep turned out of each coupling
to one gauge and made to fit one disk. Duplicate disks are then fitted
in each coupling, and the centering is preserved, and should a spare
piece be ever required, there is no trouble to couple correctly on
board the steamer.
The propeller shaft is generally made of iron, and if made _not less_
than the Board of Trade rules as regards diameter, of the best iron,
and the gun metal liners carefully fitted, they have given little
trouble; the principal trouble has arisen from defective fitting of
the propeller boss. This shaft working in sea water, though running in
lignum vitæ bearings, has a considerable wear down at the outer
bearings in four or five years, and the shaft gets out of line. This
wear has been lessened considerably by fitting the wood so that the
grain is endway to the shaft, and with sufficient bearing surface
these bearings have not required lining up for nine years. It is,
however, a shaft that cannot be inspected except when in dry dock, and
has to be disconnected from the propeller, and drawn inside for
examination at periods suggested by experience. Serious accidents have
occurred through want of attention to the examination of this shaft;
when working in salt water, with liners of gun metal, galvanic action
ensues, and extensive corrosion takes place in the iron at the ends of
the brass liners, more especially if they are faced up at right angles
to the shaft. Some engineers have the uncovered part of the shaft
between the liners, inside the tube, protected against the sea water
by winding over it tarred line. As this may give out and cause some
trouble, by stopping the water space, I have not adopted it, and shall
be pleased to have the experience of any seagoing engineer on this
important matter. A groove round the shaft is formed, due to this
action, and in some cases the shaft has broken inside the stern tube,
breaking not only it, but tearing open the hull, resulting in the
foundering of the vessel. Steel has been used for screw shafts, but
has not been found so suitable, as it corrodes more rapidly in the
presence of salt water and gun metal than iron, and unless protected
by a solid liner for the most part of its length, a mechanical feat
which has not yet been achieved in ordinary construction, as this
liner would require to be 20 ft. long. I find it exceedingly difficult
to get a liner of only 7 ft. long in one piece, and the majority of 6
ft. liners are fitted _in two pieces_. The joint of the two liners is
rarely _watertight_, and many shafts have been destroyed by this
method of fitting these liners.
I trust that engine builders will make a step further in the fitting
of these liners on these shafts, as it is against the interest of the
_shipowner_ to keep ships in dry dock from such causes as defective
liners, and I think it will be only a matter of time when the screw
shaft will be completely protected from sea water, at least inside the
stern tube; and when this is done, I would have no hesitation in using
steel for screw shafts. Though an easier forging than a crank shaft,
these shafts are often liable to flaws of a very serious character,
owing to the contraction of the _mass_ of metal forming the coupling;
the outside cooling first tears the center open, and when there is not
much metal to turn off the face of the coupling, it is sometimes
undiscovered. Having observed several of these cavities, some only
when the _last cut_ was being taken off, I have considered it
advisable to have holes bored in the end and center of each coupling,
as far through as the thickness of the flange; when the shafts are of
large size, this is sure to find these flaws out. Another flaw, which
has in many cases proved serious when allowed to extend, is situated
immediately abaft the gun metal liner, in front of the propeller.
This may be induced by corrosion, caused by the presence of sea water,
gun metal, and iron, assisted by the rotation of the shaft. It may
also be caused under heavy strain, owing to the over-finishing of the
shaft at this part under the steam hammer.
The forgemen, in these days of competition and low prices, are
instructed to so finish that there won't be much weight to turn off
when completing the shaft in the lathe. This is effected by the use of
half-round blocks under the hammer, at a lower temperature than the
rest of the forging is done, along with the use of a little water
flung on from time to time; and it is remarkable how near a forging is
in truth when centered in the lathe, and how little there is to come
off. The effect of this manipulation is to form a hard ring of close
grain about one inch thick from the circumference of the shaft inward.
The metal in this ring is much harder than that in the rest of the
shaft, and takes all the strain the inner section gives; consequently,
when strain is brought on, either in heavy weather or should the
propeller strike any object at sea or in the Suez canal, a fracture is
caused at the circumference. This, assisted by slight corrosion, has
in my experience led in the course of four months to a screw shaft
being seriously crippled.
I show you a section of a screw shaft found to be flawed, and which I
had broken under the falling weight of a steam hammer, when the
decided difference of the granules near the circumference from that in
the central part conveyed to me that it was weakened by treatment I
have referred to. I think more material should be left on the forging,
and the high finish with a little cold water should be discontinued.
Doing away with the outer bearing in rudder post is an improvement,
provided the bearing in the outer end of screw shaft in the stern tube
is sufficiently large. It allows the rudder post to have its own work
to do without bringing any strain on the screw shaft, and in the event
of the vessel's grounding and striking under the rudder post, it does
not throw any strain on the screw shaft. It also tends to reduce
weight at this part, where all the weight is overhung from the stern
of the vessel.
* * * * *