Best Of Notes on body-mount couplers (work-in-progress)

[GaryHinshaw]

Hi Bryan,

Thanks for the thoughtful (and thought-provoking) comments!  Feedback like this is exactly why we posted the drafts here.  A few quick comments:

* First, this is in no way intended to be a commercial product, just a curiosity-driven experiment.  I have ~50 pair of FT couplers that I'd like to put to use, but my experiments with body-mounting the stock FT draft gear have been so-so.  I'm hoping this is a better, but still cost-effective, solution.

* Design - I am quite concerned about the support post for the coupler.  The idea at present is to use a 00-90 screw to hold the cover and serve as the post (it is just the right size for the FT shank).  For non-extended draft gear, this screw can extend into - and attach to - the car under-frame in most cases. In this instance, both the top and bottom holes would be 00-90 clearance holes.  Alternatively, the top hole could be tapped with (a few) 00-90 threads, but this is marginal, and probably renders this approach a poor solution for extended draft gear.  (One could use a 00-90 nut over the top cover of extended gear, but it would look ugly.)  In all case, I imagine that a second mounting screw further back would form the main box attachment point.  Note also that the tab & slot design for the cover plate is intended to provide longitudinal support to resist torquing the screw/ coupler post.  It remains to be seen if that is sufficient.  I'm not sure I see the issue with the housing: do you think this tab & slot design will lead to unsightly gaps?  I think we just have to see how that goes.   I'm all for trying stainless steel, and/or thicker stock.

* FT couplers - As noted above, I have a big stock of FT couplers I want to put to use and this is intended to address that.  If it seems promising, I would like to lobby FT to sell couplers without draft gear (just like McHenrys).  This may be a pipe dream, but at least I'd like to make better use of my existing stock.  If someone thinks this is worth resizing to fit MTs or McHenrys, great!

* Artwork - I'm sure Ed N. is taking notes.    The reason this thread languished for so long is because I haven't had time to do a proper CAD drawing.  When Ed posted a draft, I decided to spit some ideas out in a simple drawing program to get some concrete ideas exchanged.  I think your CAD suggestions are great.

Thanks again,
Gary

[ednadolski]

Thanks Bryan for sharing your thoughts!   

How are you going to anchor the pivot post without it being visible, especially on the extended draft gear of a cushioned underframe?

We could add back in the top strip that was in one of the earlier sketches.  The length of the screw would have to be just right, in order to avoid protruding thru the top.  A threaded post might be worth considering, tho it would be a bit more complex.

Another option could be to simply make the parts in 2 lengths, regular and extended.

How are you going to firmly secure the pivot post to brass stock so thin so that the load of pulling a long freight consist (if the cars are up front near the motive power) will not compromise the integrity of the post and or draft gear?

Good Point. If the 0.010" isn't enough, we could consider a heavier gauge brass (perhaps just for the lid).  The K&S #262 tubing is a 0.014" wall, and that looks pretty beefy for N scale.

Gary, is there a way we can whip up a quick confidence test?  Something maybe like a strip of the 0.010" brass, bent over itself into a U shape, and with an FT coupler mounted on a screw going thru two holes drilled into the ends of the brass strip.  This assembly gets anchored on one end of a test track and coupled to a consist of locos. Then run the locos to put a tension load onto the coupler/brass assembly, and keep adding locos to see how the assembly holds up.

I'm not sure what the average drawbar pull is for a typical N scale loco, but we probably want to show this can handle something like 6 locos at a minimum, if we want it to work in long trains.  This would also demonstrate the viability of the FT coupler for N scale applications.

How do you fold up the three sides and attach the lid so that it looks like a complete rectangular housing, with no gaps between the folded sides and the lid?

The lid actually extends past the sides, to simulate the look of the prototype (as per the MM pic).  To eliminate visible gaps, there is the tab/slot arrangement, plus the half-etch in the lid creates a recess to accept the edges of the pocket sides.

use red/black or magenta/black to represent back/front.

I was basing the sketches on the PPD examples.  For actual artwork, I'll do something layered up in a CAD drawing, as per PPD.


Thanks again for the great input!   


Ed

[ednadolski]

Found this on http://www.atlasrr.com/Reviews/gp40.review.htm, for the drawbar pull of a single N scale loco:

   

"The drawbar pull of .8 ounces equates to about 20 typical N scale cars on straight, level track."

So 6 locos would be 4.8 ounces.  If the FT coupler can lift a weight this size, it should be fine for most N scale uses.

Ed

Edit:

My order of FTs just arrived so I was able to do a quick test: the FT holds up fine with hanging a 10-ounce weight from it.  (Ditto for an MTL and a McHenry).  I probably could have done more, if I wanted to push it to the failure point. The 10 oz. is the drawbar pull of 12 locos, so the strength of the coupler itself doesn't seem like a problem.

Now I gotta find me a strip of the 0.010 brass.... 

Ed

[GaryHinshaw]

Good timing on the FT delivery Ed.  I definitely agree that the couplers themselves are plenty robust, so the screw/post support is really the main issue for loading.  I think the concept as sketched is still viable, but here's another idea to kick around:

We could invert the design and make the top be the cover piece and have it made of thicker stock.  This would allow more threads in the top hole, for cases where one didn't want the screw to protrude through the top.  This would preclude any bottom flange detail, and would be a bit more unwieldy to assemble.

One benefit of thinner stock is that it's more compatible with lower ride height.  For example, .010 stock is just the right thickness for mounting to an IM Trinity 5161 hopper with BLMA trucks to match the MT coupler height gauge.  Anything thicker would require increasing the ride height, shaving the under-frames, or compromising on the coupler height.

Just a reminder: back on p. 1 I did some experiments with set screws in rectangular tube stock and those seemed very robust, even though they were only screwed though one surface.  This gives me hope...

-Gary

Note added:  By the way, here is my simplistic picture of the physics of the current design:



The red vectors show the forces acting on the screw/post: drawbar pull to the right, counter-forces applied by the top & bottom box plates to the left.  If the top & bottom plates are held rigidly together in the horizontal direction by tabs & slots, then the top & bottom counter-forces are equal and there is no net torque about the (top) threaded screw position.  In that case the only function of the threads is to maintain tension on the bottom plate to engage the tabs in the slots.  This is a very modest requirement.

[GaryHinshaw]

One other comment: I'm thinking we should reduce the number of half-etch dimples by ~50%.  I'm a bit worried that the dense spacing we have now will almost form another fold line, and I doubt we need that much resolution on potential mount points.

-gfh

[ednadolski]

Seems to me that as long as the screw goes thru both the top and bottom plate (both of which probably want to be threaded BTW), then the strength/integrity should be fine as you say.  The drawbar load on the screw/post would be distributed onto both plates, and there should be no opportunity for enough torquing that could cause either plate to deform under the load.

So the case of concern is the one where the screw only goes thru the bottom plate and is held only by the tap threads.  I.e., that is the extended draft gear case.  To handle that, we could avoid the single-plate case and do something where the screw still goes thru both plates, and then just add a small sheet of cosmetic brass (or even styrene) on the visible portion of the top plate to cover up the top screw hole and the end of the screw.  It would look something like this proto pic: (http://www.mellowmike.com/Prototypes/Coupler.jpg)  Since this cosmetic cover is only on the visible portion, it doesn't change the ride height. The only slight twist is that the screw would have to be trimmed flush with the top of the pocket before applying the cosmetic cover, but that should be manageable.  If the screw is provided at the right length, then trimming could be avoided.

Thoughts?

Ed

[GaryHinshaw]

Hi Ed,

I definitely agree that the screw needs to go through both top & bottom plate.  In addition to torque issues, I don't see how the bottom cover would stay in place if it weren't screwed to the top somehow.

I like the idea of a cosmetic cover for extended draft gear.  We could add a half-etch dimple at the appropriate position to accommodate a bit of the screw tip.  This could also serve as an alignment pin for gluing the cover on.

-gfh

P.S. I downloaded Google Sketchup/Layout to play with.  I really don't care for the DraftSight interface very much...

[ednadolski]

Hi Gary, a quick update from my end: I'm coming along on the DS curve, I've figured out how to set up the scale/grid/snap, as well as the layers that PPD uses.  Takes a little getting used to but overall not too bad.  For this first draft I'll figure on 0.010" brass, so I will draw the half-etch fold lines at 0.010" wide.

I was going over the dimensions for the FT coupler box and I noticed that the exterior dimensions of the FT coupler pocket are pretty close to the #262 brass tubing.  So I was wondering:  have you tried the FT couplers in the brass tubing?  The interior height dimension is larger on the tubing, but that could be taken up by a washer or shim.

Along those lines:  have you tried the FT in cars that have an integral body-mount coupler pocket?  I'm thinking of newer cars like the BLMA Trinity reefer, the Athearn 2-bay hopper, and the new MT PS2 high-side.  These pockets are all sized for either the MT or the McHenry, with an internal pivot post that is too large for the FT.  So while they will not need an aftermarket pocket, it seems these cars still will need some work to convert to the FT, minimally to remove the post and drill/tap for the 00-90 screw, plus a shim as above.  Thoughts?

Thanks,
Ed

[GaryHinshaw]

With Ed's help, I finally got over the hump and put these drawing into a properly layered CAD drawing.  The picture below is an image exported from the .dwg file showing the layout, with black being full metal (.01 brass), red being a top-side half-etch, and blue a bottom-side half-etch. 



The concept hasn't changed much since the last cartoons, but I thought I would post in case the experts looking on see any gaffes that would render this hopeless before we get it ready to submit.

I'm not really thrilled about the screw/post combo for extended draft gear, but that isn't a primary application I have in mind.  In any case, I'm almost thinking I could live with a pan head screw being exposed on the top, and that the bottom cover could be fastened with a nut, which wouldn't be easily visible.

For the growing number of cars that come with body mounts, I'm thinking an FT conversion could be enabled by using a channel shaped insert, where the channel serves to narrow the interior pocket width, and provides a new bottom cover.  This could still retain the tabs for air line and cut lever, if desired, but it would have to be a new, redrawn part.

-gfh

[Sokramiketes]

Wow, I missed a couple months worth of discussion in this thread.  Thought good threads came to "Best of Wire" when the discussion was complete but it's apparently a living archive...

Gary, the etching artwork isn't quite right if that's meant to be the production drawings.  (And if it's not and it's just to illustrate the fold lines in one shot, just let me know)

There should only be two layers worth of color for the actual part.  One layer for the top "mask" and one layer for the bottom "mask".  If there is a half etch from the top, you just don't include masking on the top layer.  Likewise for the bottom.  Having three colors will confuse PPD.

(Though they do like the tabs to be on a separate 3rd layer, green) 

What CAD was used?  Maybe I can help.

[Chris333]

I'm sure not a CAD pro, but if you want I can e-mail you a file that I have sent to PPD.

[GaryHinshaw]

Thanks for the feedback.  The drawings were done in DraftSight, which is a free 2d CAD package that works on a Mac, but I believe the .dwg it writes is portable... at least that's the intent.

The working drawing is actually spread out over 6 layers for ease of use: 3 outline layers and 3 fill layers.  The image above shows the 3 fill layers, and I was imagining exporting those to a separate file for PPD, but I hadn't quite gotten to that stage yet.  But now I'm confused about your description (and I should reread the PPD guidelines).  I think you're saying that I should think of the process as two half-etches, one from each side, and anywhere the half-etches overlap is a full etch, correct?  So I need to generate a top layer and a bottom layer for the two half etches, each with their own color.

Chris, I'd love to see a sample drawing, just to make sure I have it right.  You can email me using the envelope icon.  I'll forward it to Ed too.

Thanks!
Gary

P.S. I'd be happy to start a fresh thread.  But I always felt this topic lacked closure in my mind and I'm hoping to correct that.

[GaryHinshaw]

They're here....



The first sample of boxes from PPD showed up in today's post!  So far everything seems to have been rendered perfectly as per the drawings.  Now I just have to hope that the drawings were right, but a quick comparison to the FT box looks promising.  Updates as they become available.

-gfh

[GaryHinshaw]

I got a chance to fold and assemble the first box and it's looking good so far.  I'll give the first sample a B+ for now and will proceed with some trial mounts on various cars.  Here is a quick look at the folded parts and one of the assembled units - first the outsides of the box and lids (there were two lid designs run):



and the insides:



The main box structure on the left folds into a channel that has shaped sides and a channel shaped extension for mounting flexibility.  The extension can be readily trimmed to length as necessary, and there are several dimples for starting screw holes where desired.  There is also a fold-up flange and a fold-out loop for an air brake line.  The two lid designs are identical except for the fold down loop that accommodates a cut lever: one design folds longitudinally, the other transversely.  The latter required etching a gap in the lid which might lead to lid weakness, but so far it seems ok.  Here are some shots of an assembled box:









Some evaluation notes:

* The basic box dimensions are perfect for the Full Throttle coupler.  The coupler has pretty much the same functionality in this box as it does in the stock plastic box.  (The problem with the stock box is that it is difficult to body mount in most N scale situations - hence this project.)

* It is very rugged. Even the fold-out loops are quite strong.  So this should stand up to abuse on the layout.

* There were a few fit issues:

- The two screw holes ended up being slightly larger than drawn.  The lid hole was supposed to be a .055 clearance hole for a 00-90 screw and it's a bit larger, but it still holds the screw head, so it's not really an issue.  The top hole in the channel was supposed to be a .040 tap hole for a 00-90, but it ended up almost as a clearance hole, so the threads are barely biting in the top hole.  I'll shrink the holes in the drawing for the next run, and I'll work around it for these samples.

- Same for the air line hole.  I was hoping it would be a press fit that didn't require glue (or just a dab).  It fits quite nicely, but it will definitely require glue to stay put as is.  I'll shrink that a bit too.

- Two issues on the lid: 1) there is a small interference in the front corner under the air line loop.  Easily fixed with a spot of half etch, but for now a bit of minor filing on the loop will do.  2) The fold-up back interferes with the flanges on the extension.  I need to notch the corners of the back.  Can't believe I didn't notice that ahead of time... 

In general, I'm really excited about these and can't wait to put some in service and abuse them a bit.  More soon.

Cheers,
Gary

P.S. James at PPD Etching was great to work with.  Once I got my basic drawing skills set up, the whole process was very simple.  I highly recommend them!

[James Costello]

Sounds good and looks even better Gary!

Seeing as our paths didn't cross at Tehachapi this year, I'll have too see how I can get some of the FT couplers of you or direct from FT.

I just got my desktop upgraded before Christmas, so hope to move on to my own etching projects in the new year. That etched fret is definately motivation.