I'm not a mechanical engineer but can advise the amount of contact surface,
rail to wheel, has been is greater with a large wheel than with a small. This
was one of the arguments used by British CMEs to support the ongoing
construction of single-driver locomotives for passenger service to the end of
the 19th Century. This was not their primary argument, though. Primarily, the
amount of resistance for the locomotive to overcome within itself is greatly
increased with coupling rods. It was argued the singles were "freer running",
could accelerate and run continuously at higher speeds.
Particularly in the days of animal/vegetable oil lubrication, the size of
driving wheels was important because wearing surfaces could not stand very
high-speed reciprocation, especially in the valves and cylinders, where animal
and vegetable oils would break down under higher heat from friction. Although
mineral oils were already becoming universal by the time of superheating, the
very high temperatures of superheated steam put an effective end to animal oil
lubrication. It also put an end to the use of slide ("hat") valves, which had
too much bearing surface to be adequately lubricated, and also generated too
much friction from the higher pressure steam that accompanied superheating. Of
course, larger driving wheels could be better counter-balanced, especially with
the limited knowledge of counterbalancing before the Superpower era.
The size of driving wheels in the reciprocating steam locomotive became less of
an issue as pressure lubrication, roller bearings, cross-counterbalancing,
improved crossheads and related modernizations came in. I've ridden behind a
73-inch drivered 4-8-2 at more than 100 mph, something no-one would have
considered possible in, say, 1925. But CN 6060 was built in 1944 and
incorporated all the best of what was known about steam design. [We clocked
her between several mileposts at a top speed of 109 mph, using wristwatches
only. Even if we were 10% off, she was still at or near the century mark. I
was riding the vestibule and darn near hit my head on the ceiling.]
Not that this has anything to do with S Scale model railroading .....
regards ... pqr
----- Original Message -----
From: roy inman
To: [email protected]
Sent: Friday, October 03, 2008 12:11 PM
Subject: Re: {S-Scale List} Steam vs. Diesel
How about size of drive wheels?
Maybe one of the mechanical engineers among us can explain larger wheels VS
smaller wheels on the same track. Is there some sort of mechanical advantage
for the larger steam drive wheels?
Roy, wondering in Lenexa, Inman
From: ed_loizeaux <[EMAIL PROTECTED]>
Reply-To: [email protected]
Date: Fri, 03 Oct 2008 18:26:54 +0000
To: [email protected]
Subject: Re: {S-Scale List} Steam vs. Diesel
> You win! The F couldn't get the train out of the yard before it
started to
> gasp for traction.
> Bill Winans
Bill.....But the basic question remains unanswered. Why is it that two
locos -- steam, diesel, electric, whatever, etc. -- with the same
weight and same number of drivers have a different drawbar pull? I'm
still not sure of what to make of this situation. Did both locos have
powered wheels made of the same material? Was only one loco sprung or
equalized? Did a sudden wind come up? Rubber traction tires? My mind
is going nutz. Cheers...Ed L.
[Non-text portions of this message have been removed]
[Non-text portions of this message have been removed]
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