Coupler height

chrishappe
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Re: Coupler height

Post by chrishappe »

Regarding the Standard S-2 for coupler height, the dimension of .281” for TT Scale and .219” for TT narrow gauge are from the rail head to the center of the knuckle. This dimension is scaled from the prototype, and is best measured with a caliper or a gauge. The tolerance of .013” is plus or minus and indicates the coupler knuckle should not be higher or lower than the ideal dimension by more than .013” If it is then unexpected uncoupling may occur on dips and humps in trackwork or on the top or bottom of grades when cars go from the slope to the level. This may occur on long cars if the transition is abrupt rather than gradual as it should be.
The NMRA technical group is still studying this situation of grade transition as it affects coupler performance. Of course in the scales smaller than HO the couplers are generally smaller and so uncoupling is more likely to occur. However it is difficult to hold tolerances smaller than those listed as there is often more slop than that in the coupler shank. With that in mind a suggestion was made at one time that the small scales should use a tolerance of 25% of the knuckle height and that is the dimension that is listed in the last column of the TT scale standard, as the ¼ Rule .0225”. That was never accepted or approved and shouldn’t be there. It got left in that notes column in error, just to confuse everyone. I will try to get it out of there.
So no matter whether you use the 1025 or the 714 couplers the center of the knuckle should be .281” above the rail. This means that some adjustment may be necessary in the coupler pocket mounting on the underside of the car to achieve that dimension. And of course if you change trucks or wheels you may need to adjust the car height to keep the coupler knuckle where it should be.
I made my own gauge for the 1025’s that I use. It sits on the rails and one end of the gauge (.3285” high) should just slide under the car end. The other end of the gauge has a coupler mounted on it for checking that everything lines up after the couplers are installed on the equipment. This is just like the 1055 gauge that Micro trains sells and in fact you could probably just use that by gluing a .0475” (3/64”) thick spacer to the bottom of it. That would be slightly more for the 714’s. I don’t know the thickness of their draft gear, but I would guess it is .125” or .030” thicker than the 1025 so the gauge would need to be .015” thicker for the 714’s
For more information than you wanted to know on couplers, their sizes and scales see http://home.comcast.net/~brmetcalf/rr/coupleri.htm In there the Author shows that for TT Scale the most accurate coupler is the Micro Trains Z scale coupler. Now that could create a whole new set of coupler problems.
Cheers, Chris Happé

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Rob M
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Re: Coupler height

Post by Rob M »

Thanks for the clarification Chris. Sounds like we're on the right track with what we came up with.
chrishappe wrote:the ¼ Rule .0225”. That was never accepted or approved and shouldn’t be there. It got left in that notes column in error, just to confuse everyone
And that it did. Spent a good hour Googling yesterday trying to figure out what that meant. :lol:

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ConducTTor
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Re: Coupler height

Post by ConducTTor »

Thanks Chris!
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areibel
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Re: Coupler height

Post by areibel »

I'll add a big Thanks too!
Funny, I was just reading through a back issue of the TT EMpire, and it mentions that Chris had volunteered for the job of TT Standards coordinator for the NMRA- back in 1983! It's nice to have a true expert helping us out!

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ConducTTor
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Coupler height

Post by ConducTTor »

I think he's still mad at me for arguing with him about the module standards.....


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ConducTTor
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Re: Coupler height

Post by ConducTTor »

All my trucks are updated and available again on Shapeways (except for the Dalmans....still have work to do on them). The cars will follow with updates in the coming days and will be available again.

Cheers!
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ConducTTor
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Re: Coupler height

Post by ConducTTor »

Back on topic. Rob, when you measured my trucks as being 0.2mm too high - was that with 36 or 33 inch wheels?

And by the way - I'm leaving the coupler mount height on my cars as is. Just too many variables there to settle on an exact height....wheel size, coupler type.....
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Rob M
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Re: Coupler height

Post by Rob M »

I measured them with 33" Gold Coast wheels.

I feel your pain with trying to settle on an exact height :lol: :wall:

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ConducTTor
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Re: Coupler height

Post by ConducTTor »

Oh by the way, the price of the trucks went up a tiny bit - I changed materials to get a bit more detail into them.
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WillYart
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Re: Coupler height

Post by WillYart »

We just did something really outrageous. We're going with a coupler that's 0.15mm too high, hopefully this will not result in horrendous rail disasters. There are tankers in this train!

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