[Footnote 1: By Samuel Porcher, assistant engineer motive power
department, Pennsylvania Railroad. Read at a regular meeting of the
New York Railroad Club, Feb. 19, 1891.]
It goes almost without saying that for any given service we want the
best car wheel, and in general it is evident that this is the one best
adapted to the efficient, safe and prompt movement of trains, to the
necessary limitations improved by details of construction, and also
the one most economical in maintenance and manufacture.
It is our aim this afternoon to look into this question in so far as
the diameter of the wheel affects it, and in doing it we must consider
what liability there is to breakage or derangement of the parts of the
wheel, hot journals, bent axles, the effect of the weight of the wheel
itself, and the effect upon the track and riding of the car, handling
at wrecks and in the shop, the first cost of repairs, the mileage,
methods of manufacture, the service for which the wheel is intended
and the material of which it is made.
Confining ourselves to freight and passenger service, and to cast iron
and steel wheels in the general acceptation of the term as being the
most interesting, we know that cast iron is not as strong as wrought
iron or steel, that the tendency of a rotating wheel to burst is
directly proportional to its diameter, and that the difficulty of
making a suitable and perfect casting increases with the diameter.
Cast iron, therefore, would receive no attention if it were not for
its far greater cheapness as compared to wrought iron or steel. This
fact makes its use either wholly or in part very desirable for freight
service, and even causes some roads in this country, notably the one
with which I am connected, to find it profitable to develop and
perfect the cast iron wheel for use in all but special cases.
Steel, on the other hand, notwithstanding its great cost, is coming
more and more into favor, and has the great recommendations of
strength and safety. It is also of such a nature that wheels tired
with it run much further before being unfit for further service than
those made of cast iron, and consequently renewals are less frequent.
The inference would seem to be that a combination of steel and cast
iron would effect the desirable safeness with the greatest cheapness;
but up to the present this state of affairs has not yet been realized
to the proper extent, because of the labor and cost necessary to
accomplish this combination and the weakness involved in the manner of
joining the two kinds of material together.
Taking up the consideration of the diameter of the wheel now, and
allowing that on the score of economy cast iron must be used for
wheels in freight service, we are led to reflect that here heavy loads
are carried, and there is a growing tendency to increase them by
letting the floor of the car down to a level with the draft timbers.
All this makes it desirable to have the wheels strong and small to
avoid bent axles and broken flanges, to enable us to build a strong
truck, to reduce the dead weight of cars to a minimum, and have wrecks
quickly cleared away. The time has not yet come when we have to
consider seriously hot journals arising from high speed on freight
trains, and a reasonable degree only of easy riding is required. The
effect on the track is, however, a matter of moment. Judging from the
above, I should say that no wheel larger than one 33 in. in diameter
should be used under freight cars. Since experience in passenger
service shows that larger cast iron wheels do not make greater mileage
and cost more per 1,000 miles run, and that cast iron wheels smaller
than 33 in., while sometimes costing less per 1,000 miles run, are
more troublesome in the end, it is apparent that 33 in. is the best
diameter for the wheels we have to use in freight service.
When we take up passenger service we come to a much more difficult and
interesting part of the subject, for here we must consider it in all
its bearings, and meet the complications that varying conditions of
place and service impose. In consequence, I do not believe we can
recommend one diameter for all passenger car wheels although such a
state of simplicity would be most desirable. For instance, in a sandy
country where competition is active, and consequently speed is high
and maintained for a length of time without interruption, I would
scarcely hesitate to recommend the use of cast iron for car wheels,
because steel will wear out so rapidly in such a place that its use
will be unsatisfactory. If then cast iron is used, we will find that
we cannot make with it as large a wheel as we may determine is
desirable when steel is used. And just to follow this line out to its
close I will state here that we find that 36 in. seems to be the
maximum satisfactory diameter for cast iron wheels, because this size
does not give greater mileage than 33 in., costs more per 1,000 miles
run, and seems to be nearer the limit for good foundry results. On the
other hand, a 36 in. wheel rides well and gives immunity from hot
boxes--a most fruitful source of annoyance in sandy districts. It is
also easily applicable where all modern appliances under the car are
found, including good brake rigging. In all passenger service, then, I
would recommend 36 in. as the best diameter for cast iron wheels.
Next taking up steel wheels, a great deal might be said about the
different makes and patterns, but as the diameter of wheels of this
kind is not limited practically to any extent by the methods of
manufacture, except as to the fastening of the wheel and tire
together, we will note this point only. Tires might be so deeply cut
into for the introduction of a retaining ring that a small wheel would
be unduly weakened after a few turnings.
On the other hand, when centers and tires are held together by
springing the former into the latter under pressure, it is possible
that a tire of larger diameter might be overstrained. But allowing
that the method of manufacture does not limit the diameter of a steel
wheel as it does a cast iron one, the claim that the larger diameter
is the best is open to debate at least, and, I believe, is proved to
the contrary on several accounts. It is argued that increasing the
diameter of a wheel increases its total mileage in proportion, or even
more. Whether this be so or not, there are two other very
objectionable features that come with an increase in diameter--the
wheel becomes more costly and weighs more, without giving in all cases
a proportionate return. We have to do more work in starting and
stopping, and in lifting the large wheel over the hills, and when the
diameter exceeds a certain figure we have to pay more per 1,000 miles
run. I am very firmly convinced that the matter of dead weight should
receive more attention than it does, with a view to reducing it. The
weight of six pairs of 42 in. wheels and axles alone is 15,000 to
16,000 lb.
The matter of brakes is coming up for more attention in these days of
high speed, heavy cars and crowded roads, and the total available
braking power, which has hitherto been but partially taken advantage
of, must be fully utilized. I refer to the fact that many of our
wheels in six-wheel trucks have gone unbraked where they should not.
As the height of cars and length of trucks cannot well be increased
for obvious reasons, it is necessary to keep the size of the wheels
within the limits that will enable us to get efficient brakes on all
of them that carry any weight. This is not easy with a 42 in. wheel in
a six-wheel truck, which is usually the kind that requires most
adjustment and repairs after long runs. The Pullman Co. has recognized
this fact, and is now replacing its 42 in. wheel with one 38 in. in
diameter.
A 42 in. wheel with 4 in. journal has a greater leverage wherewith to
overcome the resistance of journal friction than the 38 in. wheel with
the same journal, and even more than the 36 in. and 33 in. wheels with
33/4 in. and 31/2 in. journals respectively, but the fact remains that the
same amount of work has to be done in overcoming the friction in each
case, and what may be gained in ease of starting with the large wheel
is lost in time necessary to do it, and in the extra weight put into
motion.
A large wheel increases the liability to bent axles in curving on
account of greater leverage unless the size and weight of the axle are
increased to correspond, and the wheel itself must be made stronger. A
four or six wheel truck will not retain its squareness and dependent
good riding qualities so well with 42 in. wheels as with 33 in. ones.
Besides the brakes, the pipes for air and steam under the cars
interfere with large wheels, and as a consequence of all this 42 in.
wheels have been replaced by 36 in. ones to some extent in some places
with satisfactory results. On one road in particular so strong is the
inclination away from large wheels that 30 in. is advocated as the
proper size for passenger cars.
On the other hand, there is no doubt a car wheel may be too small, for
the tires of small wheels probably do not get as much working up under
the rolls, and therefore are not as tough or homogeneous. Small wheels
are more destructive to frogs and rail joints. They revolve faster at
a given speed, and when below a certain size increase the liability to
hot journals if carrying the weight they can bear without detriment to
the rest of the wheel. Speed alone I am not willing to admit is the
most prolific source of hot boxes. The weight per square inch upon the
bearing is a very important factor. I have found by careful
examination of a great many cars that the number of hot boxes bears a
close relation to the weight per square inch on the journal and the
character of lubrication, and is not so much affected by the size of
wheel or speed. These observations were made upon 42 in., 36 in. and
33 in. wheels in the same trains. We find, furthermore, that while a
3-3/8 in. journal on a 33 in. wheel is apt to heat under our passenger
coaches, a 33/4 in., even when worn 3-5/8 in., journal on a 36 in. wheel
runs uniformly cool. In 1890 on one division there were about 180 hot
boxes with the small wheel, against 29 with the larger one, with a
preponderance of the latter size in service and cars of the same
weight over them.
I do not know that there is any more tendency for a large wheel to
slide than a small one under the action of the brakes, but large
wheels wear out more brake shoes than small ones, if there is any
difference in this particular.
My conclusions are that 42 in. is too large a diameter for steel
wheels in ordinary passenger service, and that 36 in. is right. But as
steel-tired wheels usually become 3 in. smaller in diameter before
wearing out, the wheel should be about 38 in. in diameter when new.
Such a wheel can be easily put under all passenger cars and will not
have become too small when worn out. A great many roads are using 36
in. wheels, but when their tires have lost 3 in. diameter they have
become 33 in. wheels, which I think too small.
There are many things I have left unsaid, and I am aware that some of
the members of the club have had most satisfactory service with 42 in.
wheels so far as exemption from all trouble is concerned, and others
have never seen any reason for departing from the most used size of 33
in.
One more word about lightness. A wrought iron or cast steel center, 8
or 9 light spokes on a light rim inside a steel tire, makes the
lightest wheel, and one that ought to be in this country, as it is
elsewhere, the cheapest not made of cast iron.
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