Ffestiniog Bug Boxes and Flying Benches - first run around the garden

[philipy]

For your suggested 2-magnet version, perhaps creating a slight groove between the two magnets would allow for a minature "crow-bar" to slip in and prise them apart?

[3 minutes of fame]

Hmm.
I guess the decoupling handle could even be designed in.

I'll have a think while on my holidays..

[philipy]

If there's enough interest, I'll redraw them to take a pair of 3mm magnets, so that they become bidirectional.

Strength wise, I can pick up the 4 coach train and dangle it from the couplings. That's about 1Kg, so more holding force than I'll ever need on my little railway.

Just to pass the time, I had a play with your stl of the buffer head and modified it to take a pair 3mm x2mm magnets.
Then,
Point1: its a 'mare trying to get the little blighters to go in the holes instead of the second one sticking to the first one and/or the pliers,tweezers, etc!!
Point 2 : it is obviously essential to get the the pairs the same way round each time and the little so-and-so's always want to be awkward....!

However, finally....:

DSC_0035.JPG (2.28 MiB) Viewed 3097 times

The separation force appears to be in the region of 250g. Slightly crude measuring regime of hanging the pair of couplings beneath a digital luggage scale and pulling vertically downwards until they separate. Did it half a dozen times with a result between 240g and 260g each time.
So I guess your train would part between the 1st and 2nd coach!

Separation of these isn't a problem, simply push a small screwdriver into the joint between the two faces and twist, but that might not work for stronger magnets.

Screenshot 2023-06-27 16.35.55.png (55.84 KiB) Viewed 3097 times

I've got some 4 x3mm magnets on order to see if they work better and are small enough not to distort the buffer head too badly.

[3 minutes of fame]

Nice work!
Can I suggest you get the magnets such that they just protrude, and make full contact. That will significantly improve the holding force.

It's probably easier to fit all North magnets using superglue first, and then do the South magnet once the North magnet glue has dried. Closing up the holes by about 0.01mm will help with the interference fit.

[GTB]

The separation force appears to be in the region of 250g. Slightly crude measuring regime of hanging the pair of couplings beneath a digital luggage scale and pulling vertically downwards until they separate. Did it half a dozen times with a result between 240g and 260g each time.
So I guess your train would part between the 1st and 2nd coach!

No, the force on the coupler when hauling a train is the rolling resistance of the train on the track, not it's total mass.........

As an example a coupler with a coupling force of about 250g would just about be able to pull my through goods on straight and level track.

Each bogie wagon has a rolling resistance of 25g and an average mass of about 600g, so for the ten wagon train the rolling resistance is 250g for a total train mass of 6kg.

The rolling resistance of normal wheelsets doubles on a curve and on a gradient a fraction of the wagon mass is added to the rolling resistance, as it has to be lifted up the hill. That's why my rollingstock wheelsets are modified so one wheel is free to turn independently of the other, which means the rolling resistance is the same on straight and curved track.

A worked example. On a 1:50 grade, 1/50th of the wagon weight is hanging on the coupler, so in the above case the rolling resistance increases from 25g to 37g per wagon on the grade and the 250g coupler can only handle six bogie wagons.

With normal wheelsets running on a curve with a 1:50 grade, the rolling resistance becomes 62g per wagon and the 250g coupler will just handle four wagons.

On a track with curves and grades the coupler would need a rating of 650g to move the aforesaid ten bogie wagon through goods train on a curve with a grade of 1:50.........

The other consideration is that the drawbar pull of a loco with metal wheels on metal rail is about 1/5th of the mass on the driving wheels. Which is why I need a loco with 3.5kg on the drivers to move a train with a rolling resistance of 650g. In Roundhouse loco terms a SRRL #24 will do the job, but a Lady Anne or smaller won't cut it.


Experimentation beats theory any day. Given the size of loco you usually build, I'd imagine a track test with a set of 250g couplers will show that they will be more than adequate for any train you would normally run......

Regards,
Graeme

[philipy]

Thanks as always Graeme.

I'm afraid that the maths of mechanics is way beyond me ( like a lot of things ), but I understand what you are saying in principle. Soooo.. having printed half a dozen heads, and mindful of comment about keeping the magnet faces slightly proud, I can confirm that you are correct! Still a bit of a lashup, because I didn't want to go and start modifying perfectly good vehicles without good reason. However a few paperclips allowed me to use the pin holes to fix the magnetic heads on a temporary basis and as you will see, a 4 wagon train weighing in at 2.5Kg, seems to be fine. I haven't messed about with different wheelbases, bogies, etc, and at 4Kg the couplings didn't want to know, but no way will I ever need that kind of grunt!

The 3d prints are the same as the pics I posted the other day.



Apologies for the quality, same old problem of camera in one hand and controller in the other!

BTW, sorry about the thread drift and hijack. Perhaps I'd better split it into anew topic?

[3 minutes of fame]

Thanks for the video and testing under load. Would you mind sharing your design? I would like to add it to the files I've put on Thingiverse.com if you have no objections.

Thread evolution is a natural part of all forums, so no apologies or new thread required

[philipy]

Thanks for the video and testing under load. Would you mind sharing your design? I would like to add it to the files I've put on Thingiverse.com if you have no objections.

No prob.
I've actually attached two stl's, one for the 2x3mm magnet version and one for a 2x4mm magnet version. Both use your standard 5mm square mounting peg.
I must stress that I haven't actually tried the 4mm version yet. I had it drawn and test printed ready for when the magnets arrive but since the 3mm version seems to be adequate I don't think I'll take it any further at this stage, unless I get any problem with weakness.

Actually, I lied when I said that my test train used the same prints as my prototypes. I had forgotten that I put a small reinforcing rib at the back, "just in case"!

I mentioned yesterday that keeping the pairs of magnets the right way round whilst mounting them was a bit of a 'mare but I've now discovered the easy way to do it.
1) Put a pair of magnets into one buffer head and mark it for future reference.
2) Offer two more magnets to the face and they will both ping on to the front of the mounted ones.
3) Press another buffer head on to the front and squeeze the two together to force the 2nd pair of magnets into their holes in the 2nd one.
4) Carefully twist them apart and apply glue. Then repeat for as many as you wish

Two Mag coupling head 3mm.stl

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Two Mag coupling head 4mm.stl

(207.16 KiB) Downloaded 101 times

[GTB]

I'm afraid that the maths of mechanics is way beyond me ( like a lot of things ), but I understand what you are saying in principle. Soooo..

There are large chunks of mathematics that are now beyond my ken.......

I'm glad the theory worked out in practice.

Graeme

[FWLR]

Looks good that Philip.

[philipy]

I have now posted the files for these magnetic buffer heads on our "3D Print Files" section. For the sockets to mount them, you will need to download the original Thingiverse files at https://www.thingiverse.com/thing:6090339