Draft gear load calculation

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Glenn Brooks
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Draft gear load calculation

Post by Glenn Brooks »

Hello All,

I’ve been exploring making up a two stage polymer spring/friction wedge draft gear design utilizing polymer springs, and some kind of rubber friction wedges in draft gear. The wedges would serve the dual purpose of buffering shock and self centering the coupler.

Similar to these real world examples:
70A19245-8049-408B-BA37-D6BF65CCB5BF.png
CBDB7BF7-9B90-43C0-A8FE-A81D40EA0ECF.png
However, it occurs to me I don’t know how to properly measure max load capacity of miniature train draft gear. E.g. the actual draft and buffering forces acting on the coupler and attached draft mechanism.


any ideas?

I assume a rule of thumb measurement would either be actual load of the attached rolling stock, or some measurement related to tractive effort. Anybody know how draft gear loading actually should be measured?

Also, is anybody using small polymer springs or friction wedges? If so where do you source them?

Thanks much,
Glenn
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Bill Shields
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Re: Draft gear load calculation

Post by Bill Shields »

Sort of depends on load and maximum grade.

Do you have those numbers?
Too many things going on to bother listing them.
Glenn Brooks
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Re: Draft gear load calculation

Post by Glenn Brooks »

Bill, I guess two use cases would be: 7.x “ gauge goods cars maybe 1500# dead weight, and 12-15” ga cars, max weight 5000-8000# max weight. 2.5” ruling grade.

However, looking more for what is the underlying criteria or definition to compute load capacity for draft gear, rather than a specific calc.

Thanks
Glenn
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Bill Shields
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Re: Draft gear load calculation

Post by Bill Shields »

now that you have numbers:

plug them into formula

https://www.arema.org/files/pubs/pgre/PGChapter2.pdf

grade resistance = 20# / ton / % grade

which is generally what we as small scale modelers are most concerned with...getting going!!!!

in reality, the idea of polymer springs in our model scale may not work well...but then i don't know hoot about polymers and always look my springs up on a Lee chart
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STRR
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Re: Draft gear load calculation

Post by STRR »

Glenn,
The rule of thumb for 1:1 railroads is: It takes 2-5 pounds of force to move 1 ton, free-rolling, on straight, level tracks with wheels, bearings, gauge, and alignment all in excellent specifications. 2 pounds/ton at slow speeds and upward to 5 pounds/ton at high speeds. The same should be true on small scales as well, with the weights being scaled down.
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Bill Shields
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Re: Draft gear load calculation

Post by Bill Shields »

Silly ? that I have never explored....

Is it the same for aluminum rail?

Not that the 1:1 guys care
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STRR
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Re: Draft gear load calculation

Post by STRR »

Bill,

That is a great question. I would expect it would be pretty close but a little higher since the aluminum would provide some greater rolling resistance. I do not think it would be a great amount, though.

It would be fascinating to have someone actually do some draft gear pull testing on both steel and aluminum rails. Accurately scaled, they should reflect real-life numbers.
Glenn Brooks
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Re: Draft gear load calculation

Post by Glenn Brooks »

Just FYI, further digging turned up a couple of recent studies of draft gear. It appears the key metric for determining performance in full scale railway draft gear, is tractive effort. Train weight does apply when factoring in grade, but the only case where full train weight need be considered is where you would pull up a 90* (vertical) grade. “Draw gear” including coupler, yoke and draft components, generally is described as a longitudinal suspension system, that deals mostly with towing and braking forces. In other words, shock (buffering), slack, and towing forces.

Here is a link to a report published in Railway Engineering Science (2021):

https://link.springer.com/article/10.10 ... 21-00237-y

So I guess the rule of thumb metric for designing draft gear (for pulling rolling stock) is max train resistance.

Still haven’t found much useful detail on buffering shock loads, other than friction blocks and polymer springs offer greater buffering capability and smoother compression curves than steel spring systems, in prototype equipment. Also haven’t yet located any small scale polymer products that could be useful in our size draft gear. The small plastic die springs sold on Mcmasters Carr and other on line vendors appear to be forms of plastic, nylon, or composite epoxy, not polymer. 1:1 railroad polymer draft systems have better compression characteristics than these materials, in that polymers vary non linearly, according to the force applied to the gear.

The report mentioned above describes the several types of draft gear currently in use, and their operating characteristics.

Glenn
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rkcarguy
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Re: Draft gear load calculation

Post by rkcarguy »

Glenn,

Just a thought, you could incorporate a small automotive or motorcycle shock/strut/spring in the center of the frame and run tie rods to it from each coupler? Fluid damping coupled with progressive springs should work really nice. You may find it's only necessary on your locomotive.
As we go from scale trains to full scale, weights increase by the square. I think you are going to find a prototypical arrangement to be too stiff at scale.
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Re: Draft gear load calculation

Post by Kimball McGinley »

As far as I know, plastics, and nylon (a type of plastic) are polymers. Are we using the term "polymer" here to refer to a solid block of material used as a spring, as opposed to a conventional metal spring?
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Re: Draft gear load calculation

Post by Erskine Tramway »

Kimball McGinley wrote: Mon Sep 27, 2021 2:51 pm As far as I know, plastics, and nylon (a type of plastic) are polymers. Are we using the term "polymer" here to refer to a solid block of material used as a spring, as opposed to a conventional metal spring?
The springs I use on my Heywood cars are Urethane Die Springs, sometimes referred to as 'elastomer' material: https://www.centuryspring.com/products/ ... e-springs/

That might be what he's referring to.
1-29-14 rubber spring.jpg
Mike
Former Locomotive Engineer and Designer, Sandley Light Railway Equipment Works, Inc. and Riverside & Great Northern Railway 1962-77
BN RR Locomotive Engineer 1977-2014, Retired
Glenn Brooks
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Re: Draft gear load calculation

Post by Glenn Brooks »

Mike, very cool to see you are using these solid blocks of polymer on your trucks. indeed that is the type of material I first started looking into.

Do you recall what durometer rating you ordered? And are you happy with the material, and how well does this provide a comfortable ride for passengers? E.g. is a smooth ride or jarring for a passenger?

My original thought was to build a draft gear housing around this type of solid block material. But found the manufacturers generally say the compression is only around 1/8”. So wondering if this short compression length would make for a more jarring ride, compared to say 1” travel of comparable steel coupler springs? Which leads back around to maybe using a wound type polymer die spring - which compresses a greater distance than solid block.

BTW, another plus- all the polymer springs I’ve found so far, do have a published weight bearing load capacity - as opposed to the DIY hardware stuff, which often is undocumented.

Glenn
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
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