New 3/4" Scale Track - Part 1

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Re: New 3/4" Scale Track - Part 1

Post by PRR G5s »

Bill Shields wrote: Wed Feb 17, 2021 1:55 pm So the gauge change per degree for a 3/4 inch scale rail setup would be exactly half of 1.5 scale track given the same temperature difference.
Only if 1.5 scale track gauge were 2X 3/4” scale gauge. But now we’re getting into minute numbers, so it probably doesn’t matter. Close enough, maybe? :P
Harold_V wrote: Wed Feb 17, 2021 3:43 pm Coefficient of expansion doesn't change with size, it's linear. Millionths of change per inch per degree.
You are correct Harold
kcameron wrote: Wed Feb 17, 2021 4:07 pm Were these molded ties, with fixed gauge or just plastic stock instead of wood? This is one area I could see the molded ties with fishplates being more of a problem. The plain stock plastic, you could make adjustments for the temperature range for your layout which wouldn't be possible with the molded ones.
These were plain stock plastic.

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Bill Shields
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Re: New 3/4" Scale Track - Part 1

Post by Bill Shields »

It all depends on your definition of within tolerance. 14 / 2 = 6 if your tolerance is 1"

Glad to be confirmed that I have not forgotten everything I learned in school and practice.
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Re: New 3/4" Scale Track - Part 1

Post by NP317 »

The commercial molded ties are not solid. They have voids throughout and open bottoms to lock with the ballast. That might change the thermal stability.
Hopefully someone here has that answer.
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Re: New 3/4" Scale Track - Part 1

Post by Bill Shields »

in reality, solid or not, the growth is the same...HOWEVER..

if there are hollow spots in the 'part'...then these hollows will not grow / shrink with temperature changes...SO THEORETICALLY...a plastic tie that is straight at one temperature will be 'bowed' at another.

how much...well....it depends on your definition of 'straight' / 'bowed'.

in reality...it is probably less of a concern than what wood will do as it ages and / or dries out...

anybody know what the engineering name of the actual material used?

could be that we are chasing undomesticated fowl....
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Re: New 3/4" Scale Track - Part 1

Post by UPsteamfan »

The accutie plastic ties mention are type PP (Polypropylene). For our railroad we are making our own using HDPE (High Density Polyethylene). Ties made out of plastic lumber could be PVC or either of the first two mentioned. All three have different properties regarding strength and expansion from heat.

We choose the HDPE for easy recycling. We have a stock pile of milk jugs that we are turning to ties for our projects.
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Re: New 3/4" Scale Track - Part 1

Post by Harold_V »

UPsteamfan wrote: Thu Feb 18, 2021 6:28 pm We choose the HDPE for easy recycling. We have a stock pile of milk jugs that we are turning to ties for our projects.
Interesting. Are you making the ties, or are you having them made by others?

If you're making them, It would be interesting to hear how you're going about the process, assuming you're willing to talk about it.

H
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Re: New 3/4" Scale Track - Part 1

Post by UPsteamfan »

We are making our own ties, when the time comes that we make the ties i will definitely post how we made them for all to see. Are current plan is to wait until our whole family has the vaccine shot so unfortunately that may not be for a while.
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Re: New 3/4" Scale Track - Part 1

Post by rmac »

Bill Shields wrote: Thu Feb 18, 2021 5:48 pm if there are hollow spots in the 'part'...then these hollows will not grow / shrink with temperature changes...SO THEORETICALLY...a plastic tie that is straight at one temperature will be 'bowed' at another.
Is this really true? Suppose you have a flat plate with a hole in it. As the plate expands and contracts with temperature, I'm pretty sure the hole will do the same, proportionally, so that the overall shape of the thing remains the same.

As an example, think of a bearing pressed on a shaft. If you want to remove the bearing, you heat it up, which makes the whole bearing, including the hole in the middle, expand.

See also the diagram with the green squares here. It's what convinced me.

https://physics.stackexchange.com/quest ... -is-heated

-- Russ
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Re: New 3/4" Scale Track - Part 1

Post by Bill Shields »

yes...this is true if you stop and think about it...it may not be measurable...but theory...

i have personally heated large diameter pump impellers and had the bore shrink.
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Re: New 3/4" Scale Track - Part 1

Post by Harold_V »

I agree---it is true, and it most likely WOULD be measurable. The void areas would not provide a reason to expand/contract, while the area adjacent to the voids would. The item in question, assuming it's slender enough, would definitely bow due to uneven expansion/contraction.

I've always found it interesting that people have the impression that metals are stable. They are far from being so, and plastics, in general, are far worse. If one disturbs one face of a metallic object, they can expect that face to have been relieved of stress. That would cause the piece to bow, generally become convex. Plastic does so even worse.

Heating metallic objects has a tendency to alter their state. That's why it's so difficult to weld things and have them aligned after the fact. I would expect that the bore of pump impeller would change upon heating.

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Re: New 3/4" Scale Track - Part 1

Post by Bill Shields »

the bore did change...after we got everything hot enough, it would eventually grow enough to push the shaft in place and expect it to stay there with 5000+ HP of motor chugging away on it.

after 3-4 heatings we would have to 'rebor' the impeller(s) just enough to get the hole back to desired size so that we could heat it again to get it back on.

All this with duplex stainless...which is not the most stable stuff in the world.

The shafts would come out of the pump crooked...even more so after we had to heat the impellers to get them off -> so we would hang them vertically on an 'eye' and whack away at them with big hammers on blocks of wood.

After a good 'ringing' -> 99% of the time the shafts would be in tolerance to reassemble the pump.

even if we had to put the shaft in a straightening press...we would 'ring' them before checking.

very unstable stuff which we would not have used had it not been absolutely necessary
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