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NYO&W steam railcar
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 Posted: Tue Jan 12th, 2016 07:13 pm
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oztrainz
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Bernd wrote:
Any particular reason for 5 spokes instead of 4 or 6 I wonder?

Bernd


Hi all,
a belated reply for a topic that has just raised its head again.

There is a very valid metallurgical reason why there is an odd number of spokes in the driving wheels. These wheels were cast in sand moulds. The molten metal was fed to the spokes from the hub and the rim with risers of molten metal feeding the hub and rims and then into the spokes. If this molten pool was closed off by solidification before the spoke was fully solid, then as the spoke cooled and contracted, then internal stresses would build up in the spoke. If the stresses became too great, the spoke would break. This is called a "cold short" failure. If the spokes are offset the amount of stress caused by this is reduced rather than being at a maximum when you have spokes that are directly across from each other.

In the early days of iron casting when these and similar wheels were being cast, the metallurgical processes inside the spokes was not understood, but they had less failures with an odd number of spokes. Wheel castings with an even number of spokes came later once the process of how to ensure that you got enough metal into the spokes was better understood. This gets into mould design as well as the temperature of the pour and a whole lot of other factors as well.



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 Posted: Wed Jan 13th, 2016 11:15 am
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Herb Kephart
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Contraction problems are why some old pulleys were cast with curved spokes, also.

Herb



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 Posted: Thu Jan 14th, 2016 12:44 am
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Tramcar Trev
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Herb Kephart wrote:
Contraction problems are why some old pulleys were cast with curved spokes, also.

Herb

Ahhhhh well the theory behind the curved spokes on some flywheeels is that if the wheel is driven "into" the curve of the spoked the spokes will be in compression so theoretically the wheel will not explode due to centrugal forces trying to stretch the cast iron which is not particularly stong under tension.....



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 Posted: Thu Jan 14th, 2016 06:53 pm
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Herb Kephart
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I have heard of that, Trev.

Also read (where????) an engineering dissertation written back in the 1890's that gave a lot of calculations to prove this false. Also said that if pulleys ran at a speed any way near their failure rpm, the leather belting would fail long before this from heat, generated by it's near instant bending around the pulley at those sort of speeds.

Have helped my son with pulley and belting instillation in water mill restoration, and it is amazing the amount of warmth that the pulley faces acquire. Might be from the belt bending, might be from the slight slippage caused by running on the crowned surface.  Talking 4'' to 20'' wide belts here--but they get comfy warm after running--even when not transmitting any useful work.

Ya gives yer money and ya takes yer choice

Herb



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 Posted: Thu Jan 14th, 2016 07:09 pm
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oztrainz
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Herb Kephart wrote:
Contraction problems are why some old pulleys were cast with curved spokes, also.

Herb

Hi all again,

The shrinkage problem was one of the reasons for the development of curly spoke rail wagon wheels, especially in the smaller sizes used on narrow-gauge mining skips etc. For these smaller sized wheels, the spokes would solidify well before the rim or hub.

Because the curved spoke is longer than the straight distance between the hub and the wheelrim, any shrinkage stresses are taken up by the rim rotating slightly with respect to the hub during the cooling process inside the mould. The amount of rotation here is probably quite small (probably of the order of < 1/16"), but it is sufficient to relieve the stresses and prevent a "cold short" from developing in the spokes.

Now back to bull-catching steam-powered rail cars..



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 Posted: Fri Jan 15th, 2016 08:59 pm
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Tramcar Trev
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That contrivance looks like something out of a Mad Max movie.....



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