Drilling out coupler shanks

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Busdriver
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Joined: Sat Nov 30, 2019 3:03 pm

Drilling out coupler shanks

Post by Busdriver »

I have the 5/16” holes drilled into the frame built to Tom Bee specs. Now for the question. I have older Tom Bee couplers that haven’t been drilled and have no dimples or marks in the castings to tell me where to drill for the proper swing. Can anyone tell me how far out from the frame tube the couplers should extend. This is my first attempt at car building. Thanks to everyone. Should I use a 5 1/16 bit and 1/4” bolt ?
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rkcarguy
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Re: Drilling out truck shanks

Post by rkcarguy »

I used 3/8" bolts through my couplers, and have 1" of the shank exposed beyond the end of the rectangle tube.
If you don't have very sharp turns on your RR, you can probably have less protrusion. I'm modeling in 2" scale on 12" gage with 40' radius curves, so thought I might need a little more room between the cars.
jeffsmith
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Re: Drilling out truck shanks

Post by jeffsmith »

I would use a 5/16" bolt. Drill 21/64" hole.
Glenn Brooks
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Re: Drilling out truck shan

Post by Glenn Brooks »

The hole in the coupler (and the frame mounting hole) needs to have at least one diameter of material between the edge of the hole and edge of material, for rigidity, strength, and long term wear. 2-3x is better, at least fore and aft. So if you choose say a 3/8” diameter bolt to secure coupler to frame/pocket, center the hole in the coupler at least 6/8” (3/4”) from the back edge of the coupler. This way, you can use any reasonable diameter size bolt to secure your coupler. For 7.x gauge I would think 5/16” hardened bolt is a minimum size you would want to use. You could easily go up to 3/8”, even 7/16” for greater strength, (repetitive shock load resistance) and longevity.

In terms of how far to extend the coupler away from the frame; prototype length was sometimes up to 3’ proud of the coupler pocket. So you could go easily 2-3” out from the pivot point at 1/8 scale and be good.

FYI, an alternative option is make up draft gear, by drilling the rear of the coupler shank along the horizontal axis, then tapping a 1/2” threaded, spring loaded shank that comes out the rear of the coupler - 6-8” long. This way, your couplers actually become draft gear that absorbs a great deal of the shock and jarring when you first start the consist in motion. I do this on all my cars (12” ga) and it makes a big difference for passengers in smoothing out the ride. Also reduces the repetitive shock load on the coupler attachment point.

Put one spring inside and one spring outside the coupler pocket wall, and the draft gear acts in both directions, e.g. starting and stopping the train set.

Below is an example of a 3” scale flat car (40’ prototype) I built last spring. The frame is flipped over as I was just welding up the draft gear. The attachment point for the draft gear is the rear of the coupler pocket. The shank extension is a 7/16 threaded round stock, screwed into the rear of the coupler shank. I am very happy with how it works.
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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....
Pontiacguy1
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Re: Drilling out truck shanks

Post by Pontiacguy1 »

Your cast iron coupler will break long before a 3/8" bolt will in that application.
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Steggy
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Re: Drilling out truck shanks

Post by Steggy »

Busdriver wrote: Thu Mar 05, 2020 4:05 pm I have the 5/16” holes drilled into the frame built to Tom Bee specs. Now for the question. I have older Tom Bee couplers that haven’t been drilled and have no dimples or marks in the castings to tell me where to drill for the proper swing. Can anyone tell me how far out from the frame tube the couplers should extend. This is my first attempt at car building. Thanks to everyone. Should I use a 5 1/16 bit and 1/4” bolt ?
I use a 5/16" diameter hitch pin, which is a standard item from McMaster-Carr and others. I do not recommend a bolt, other than a stripper bolt, unless the bolt is of sufficient length to keep the threads out of the coupler shank and the holes in the draft pocket. Threads will gradually wallow out the holes and produce a sloppy fit.

For perspective, a 5/16" diameter, mild steel, solid pin has a double shear strength of approximately 4000 pounds. That simply means shearing such a pin requires at least 4000 pounds of drawbar pull, which is far beyond the capabilities of our locomotives. My F-unit can develop about 225 pounds of drawbar pull on steel rail at start-up, which is above average for most Live Steam locomotives. The pins that secure the drawbar that couples the control car to the locomotive have absolutely no sign of wear, despite many hours of pulling long and heavy trains up-grade.

Holes in draft gear and pockets should be drilled to the size of the pin, not over-size. Over-size holes will become more over-size from wear and from the peening effect that occurs from loosely-fitted parts shifting under load. Couplers should be given sufficient vertical support to prevent them from flopping up and down as the train negotiates uneven track.

Last, but not least, shouldn't this topic's name be "Drilling out coupler shanks"?
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Busdriver
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Joined: Sat Nov 30, 2019 3:03 pm

Re: Drilling out truck shanks

Post by Busdriver »

Yes. You are right. I had trucks in the post originally but found my mistake and corrected it. Unfortunately I didn’t correct the title. Thanks for pointing that out and thanks to you and everyone else for the wonderful responses.
rkcarguy
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Re: Drilling out truck shanks

Post by rkcarguy »

BigDumbDinosaur wrote: Fri Mar 06, 2020 11:58 am
Busdriver wrote: Thu Mar 05, 2020 4:05 pm I have the 5/16” holes drilled into the frame built to Tom Bee specs. Now for the question. I have older Tom Bee couplers that haven’t been drilled and have no dimples or marks in the castings to tell me where to drill for the proper swing. Can anyone tell me how far out from the frame tube the couplers should extend. This is my first attempt at car building. Thanks to everyone. Should I use a 5 1/16 bit and 1/4” bolt ?
I use a 5/16" diameter hitch pin, which is a standard item from McMaster-Carr and others. I do not recommend a bolt, other than a stripper bolt, unless the bolt is of sufficient length to keep the threads out of the coupler shank and the holes in the draft pocket. Threads will gradually wallow out the holes and produce a sloppy fit.

For perspective, a 5/16" diameter, mild steel, solid pin has a double shear strength of approximately 4000 pounds. That simply means shearing such a pin requires at least 4000 pounds of drawbar pull, which is far beyond the capabilities of our locomotives. My F-unit can develop about 225 pounds of drawbar pull on steel rail at start-up, which is above average for most Live Steam locomotives. The pins that secure the drawbar that couples the control car to the locomotive have absolutely no sign of wear, despite many hours of pulling long and heavy trains up-grade.

Holes in draft gear and pockets should be drilled to the size of the pin, not over-size. Over-size holes will become more over-size from wear and from the peening effect that occurs from loosely-fitted parts shifting under load. Couplers should be given sufficient vertical support to prevent them from flopping up and down as the train negotiates uneven track.

Last, but not least, shouldn't this topic's name be "Drilling out coupler shanks"?
Agreed. I did use a 3/8" bolt, but I used longer bolts so there was no threads in the "shear plane". I drilled the holes 11/32" and then ream drilled them 3/8", and greased the couplers when I installed them. They are just free enough to pivot nicely without any droop or slop. I chose 3/8", because of the cross section of the coupler shank was still far more than the cross section of the bolt so why not?
Another point to discuss is shock loading. While our locomotives will likely never have enough power to shear a 5/16" bolt, repeated shock from stop-n-go sure can. I'm planning on having a sprung drawbar (similar to what Glenn shows there, but with springs on both sides and no couplers) instead of couplers between my locomotive and riding car to help negate this.
Glenn Brooks
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Re: Drilling out coupler shanks

Post by Glenn Brooks »

Sided loading is an even more immediate concern. I’ve had a new, 6” long 3/8” bolt break within 20 minutes of operation, as it were stressed by repeated bending motion from contact with the connection to a coupler pocket. One reason why draw bars are usually wide and flat, or if rounded, have a flat reinforcing ridge along their length - resist torsional failure.
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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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