Crab Comet conversion

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Mon Mar 11, 2019 11:41 am

Julian,

Yes I do use a primer but usually spray some into the can lid and brush it on with an oldish brush, diluting with cellulose thinners if it gets a bit thick. I don’t want to have paint near hornblocks, bearings and the like, so brushing enables you to control this more easily. The brush will not give such a good finish as spraying but on a chassis it will not show at all. I will chemically blacken moving bits with an etch market pen where bare metal can be seen. It is surprising when you are looking an a model from normal viewing angles how little of the works underneath and inside can be seen, so no need at all to paint it.

I should say that apart from cans, which I use outside in the fresh air, I do not use an airbrush at all.

Philip

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Mon Mar 11, 2019 9:59 pm

Thank you for the clarification Philip. It's just car spray, preferably outside, and then the brush for me too.

It had to happen in suppose...the screw thread of the LHS return crank is done for. Whether it's that or the outer end of crankpin screw thread that's shot doesn't really matter. It's obvious there isn't enough depth or area of contact to withstand my repeated taking of the crank on and off.

Thanks again to Dave for your post the other day, I can see what to do. I have to unsolder the Gibson article from the crank and replace it with the Ultrascale recessed crankpin. With an appropriate rethink of the bush in use - probably replace long bush with short one. So a set will be ordered ASAP. Bad news is that an auto message says delivery 8 months! Hopefully such a basic item is not subject to this delay.

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Tue Mar 12, 2019 12:34 am

Julian,

Such 'basic items' are still hand made, so if there are not stocks, they will have to be made. If you have no luck or cannot wait, I could let you have a couple of the nuts, but not the washers they fit into, as I often don't use those so don't order them. PM me if you wish, but do ask Ultrascale first.

Philip

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Horsetan
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Re: Crab Comet conversion

Postby Horsetan » Tue Mar 12, 2019 11:33 am

Julian Roberts wrote:....Bad news is that an auto message says delivery 8 months! ....


It was 10 months when they re-opened their order books, so it looks like they've caught up a bit.
That would be an ecumenical matter.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sat Mar 16, 2019 6:41 pm

I have thanked Philip for his kind offer but was lucky to be offered a pair from a local source. The waiting time at Ultrascale is only two months for this item I'm glad to say.

I have jumped ahead of sequence on this thread with all the talk about valve gear and drain cocks. Nearly three weeks ago now, the loco was tested on two layouts. First, on Calderside Exchange Sidings, the part of the WS4G layout Calderside that is currently erected in our new clubroom. (There isn't space for all 40 feet of it!). The loco derailed at just one place, a turnout set for the straight route. The tender derailed at a couple of spots running tender first.
Perth2018 - DAS - IMG_2817-1-1-1-1.jpg
Exchange Sidings overview

As my 0-6-0 Barclay Tank runs through all the layout without derailing, as do other locos and tenders, I wasn't satisfied with this, even if the derailments could be said to be caused by track faults. (We found the turnout straight route was undergauge through the switch - a very typical problem.) I really want to prove to myself it is possible to get a 2-6-0 to hold the rails as reliably as an 0-6-0.
20190312_201846.jpg
Suspect turnout on straight route

20190312_201826.jpg
Suspect housing of blade to stockrail

The second layout visited was John Stocks' layout Kettlewell. Firstly this was for structure gauging tests. Despite the large cylinders there were no issues.

John constantly looks out for track faults. Where any derailment takes place he makes checks and replaces the track if there is any fault. The Crab ran almost perfectly here. The only problem was that the front wheel sanding gear was so close to the rails it caused a short where inevitably the track levels are not absolutely perfectly matched on the entrance to the turntable/fiddleyard.

That's an illustration to me of the sort of track imperfection a model has to be able to cope with and where faithful realism has to be compromised. No other stock of mine or his has the same problem. So the sanding gear was modified to run above the brake cross rod rather than below it as per the prototype. It's completely unnoticeable to all intents and purposes.

From the Calderside tests I concluded the tender suspension would be more reliable with a fairly simple modification, to make it exactly conform to the 3 legged stool principle. So the springs on the front wheels were removed and a central resting place made for the axle (which runs in a tube). Getting this the correct height was the only difficulty. The middle axle spring remains.
20190302_102628.jpg
Tender chassis underside

The loco derailment was caused by the pony wheel derailing. Only by running dead slow would it stay on the rails at this (undergauge) spot. People have varying opinions about such derailments and many accept them as just a normal fact of P4 life. Roger Sanders recent articles would suggest the problem is the Back to Back setting. My hypothesis remained that there was, despite the modifications I had already made, still insufficient weight on the front pony wheel, and that this is a typical problem with a front pony or bogie.

This loco, as I've made it, has a central roller above the pony bearing a proportion of the loco weight, and as per Comet design the front driving wheels are sprung with individual coil springs. These are modified to exert less force than the Comet design. (See page 4 31st December '17) With still not quite perfect trackholding it was just a matter of "tuning" the front driving wheel spring suspension to take still less loco weight, which results in more weight resting on the pony.

So the front wheel suspension was again modified by yet further raising the top of the spring fixing place in the frames by 1mm, thus reducing the spring thrust and (therefore) increasing the weight born by the pony.
20190228_144014.jpg
Spring holder modification stage 1 showing amount done on first channel before matching amount done on second one.

This resulted (on my bendy bumpy test track) in the front driving wheel not staying fully down on the rails at all times. So this was easily "retuned" by placing a 14BA washer above the spring, roughly halving the reduction of spring force I had just made.

With the front wheels out of the frames, it was opportune to reconsider the matter of the hornblocks I visited earlier - does the shape of the slot influence how well they work? With a reduction of spring force it is that much more important they work properly and freely. What I observed previously was that the front wheel downward movement was better one side than the other, and I'd found that it wasn't caused by any difference of spring force.

(This refers to my post on May 4 2018 on Page 6)

To briefly recap, the original hornblock design (as per Mike Sharman) had slots that were slightly V shaped. This enables the wheelset to not only move up and down as a pair but also to tilt where a single wheel needs to move up or down more than its opposite partner. MJT hornblocks I bought years ago are made to this design. But recent acquisitions including those with this kit are not. (Mike Sharman's Flexichas book starts with a whole spiel about the importance of the axle being able to tilt, so I think it must be fundamental to his concept.)

I could now see that, as I had guessed, I had modified one hornblock but not the other. So that fettling was done. Very carefully so as not to deepen the grooves. I don't have an ideal file which would probably be a very small ended curved triangular one with a safe edge. I put these fettled wheelsets in the frame and with Brasso tried to replicate the type of movement I was seeking to further wear the slots to shape.
20190228_204022.jpg
Hornblocks grooves modified

The result was what I'd hoped, the front wheels now actively descend into the track dip of my test track with similar vigour, in contrast to the video I posted earlier.




The video posted earlier this week (which was chiefly to show the working valve gear, set to about half way between full and midgear), shows that the loco and tender can run at full speed with no problems. The test track is quite challenging as I showed earlier in the thread but testing on the layouts demonstrates that - for me - there are always further hurdles to be overcome and modifications made before the model can be said to work properly. I prefer not to paint it before reaching that point because of all the handling needed in making modifications. But to go back to Philip's point of painting it as you go, if I was making another Crab from the same kit I would probably copy exactly what I've eventually found does work, so would be able to paint it at each stage.

Using 24hr Araldite footsteps have at last been added, using those supplied with the Bachmann model and located as per the locating holes and pegs. The pony wheel is more or less up against them on my test track 4ft curve.
20190303_162444-1.jpg
Pony on 4ft curve

If I were to add guide springs there might be 1mm clearance enabling a sharper minimum radius - if the guide springs worked! - but I'm content with an already probably sharper than prototypical 4ft minimum. (I wonder how to find out what the actual minimum radius figure was?) So for Gavin's interest here is a picture of the tightest curve on Calderside. I don't know the radius but it's pretty tight. The Crab easily went round it but that was before footsteps were added.
20190312_213540.jpg
Sharpest curve Calderside Exchange Sidings

I might add that I could probably couple the tender a bit closer to the loco for a 4ft minimum. But the 243 Loctite seems to have locked up the EMGS adjustment screw coupling very firmly indeed and I might do damage to its fixing in the loco chassis if I try to use force here. Unless anyone knows how to unlock Loctite 243...?! Not sure it's worth the bother for the very minor adjustment that could be made.

The loco was tested a few days ago on both layouts again. The shorting problem on Kettlewell was solved, and the loco and tender no longer derailed at any place on the Calderside exchange sidings even at full speed which I estimate a scale 45 - 50mph, in either direction. Sadly I could not take a video to prove it here but will do sometime. Interestingly the sharpest curve was still no problem even with the footsteps now added so it may be less sharp than I surmised.

So basically I'm fairly happy to say it works and it's now ready for strip down clean up and painting with just the return crank issue to complete and cylinder stuff to add first.

My conclusion, with due apologies to Bill (page 3) and Will (page 6), is that the pony wheel does need to have substantial weight on it, much more than its own weight, to reliably stay on the track, however that weight is applied. (Note the hefty looking springs just visible on the picture of David Franks' Crab (Page 3 October 14th).) And probably much more as a proportion of the front driving wheel's weight, or total loco weight, is needed on the pony than the prototype, as the issues are completely different. And (with due apologies to Roger) that Back to Back exact accuracy of 17.75 is irrelevant. However obviously the downside is a loss of adhesive weight. To test how much loss, the next step must be to apply for a haulage capacity trial on Grayrigg to see how it compares...Allan!? :?:

davebradwell
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Re: Crab Comet conversion

Postby davebradwell » Sat Mar 16, 2019 8:51 pm

Julian,

I have just read your struggles with the pony truck and I agree completely with your conclusion that some of the loco weight must be taken by carrying wheels if they are to stay on the track reliably. I believe you are nearly there and that if you replaced the strange rocking bar above the pony with a flat slide and spaced your pony springs out as far as possible you would find it much improved. These springs might be easier to adjust if they were cantilevered out from the legs of the A frame which pivots the truck but it doesn't matter much. Hardest bit with an existing model is getting the height of the slide right so that the wheels ride in the centre of the pony frame slots.

For some reason, probably dynamic, a single bearer on ponies and bogies just doesn't quite do the job. If in doubt, why not copy the prototype - they worked quite well.

DaveB

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Mar 17, 2019 10:22 am

Dave thank you for your advice. I think what I need to do is buy your K1 kit to understand how a 2-6-0 is properly done in a pukka way in a locomotive kit where everything is thought out. It would not be out of place on our West Scotland layout. However while I don't want to be smug, I've convinced myself this Crab works adequately well. Unless you've read a lot of these 9 pages you may not appreciate that this is now not a sprung loco, but compensated. The pony is the 3 legged stool single leg, the pair of legs are a pair of beams between the two rear wheelsets. The front drivers are the only sprung wheelset. Thus the strange rocking bar above the pony is critical to the way this works! The pony bears much more weight than the prototype would, and the front drivers much less, for the reasons illustrated above. I know compensation may be anathema to most people but there it is! The springs in the pony truck do practically nothing by the way.

Regarding the Ultrascale recessed crankpin retainer for the return crank, I found it would go on the crankpin only about half way. Looking up the Gibson catalogue I found the bolts/crankpins are 1M thread. Having for years used 14BA nuts on these crankpins until the final completion when I've used the proper Gibson retainers I hadn't realised this. The Ultrascale system uses 14BA thread. Asking Google "14BA compatibility with 1M" took me straight to a thread on this Forum, which I now cannot find, where I gleaned that over a short distance (like a nut) the threads are adequately compatible but with a longer item like these crankpin retainers there will come a full stop.

Mentioning this here before I try forcing the recessed crankpin retainer through the sticky spot full stop, which I fear may destroy the thread, but which may just force a new thread that will work adequately. I imagine, Dave Holt, that you use the Ultrascale crankpin system. The thread that I can't find had a reply from Philip saying that he uses these Ultrascale 14BA retainers on 1M Gibson crankpins for their intended purpose, the front driver behind the slidebars where space is limited; but in this situation the distance is short and the retainers are shortened so the issue doesn't arise.

davebradwell
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Re: Crab Comet conversion

Postby davebradwell » Sun Mar 17, 2019 11:02 am

Julian,

Yes, I admit to skipping much of the advice offered in the middle of your thread as I found it completely depressing but you are where you are and I still believe the pony will be improved with a flat slide. Of course, if it works on the layouts to which you have access then there's little incentive to change it. You'd end up with a sort of Crampton, with a rigid bit at the back and the front propped up on 4 springs. Strange but not unsound but there's a limit to how much you can keep changing a model before you either get fed up or it falls apart. You have a model that runs which is a fine achievement. Have you mastered the tender fall plate yet? Another invention of the devil!

DaveB

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Sun Mar 17, 2019 1:36 pm

Julian,

When I use the Ultrascale recessed crankpins on a Gibson wheel there are differences to when they are used on an Ultrascale wheel. One is the thread, of course, but the other is that I remove the bottom turned down end of the crankpin. This is designed to go through the turned washer that Ultrascale supply as a bearing for the rod, into the recess in the Ultrascale wheel which centres the crankpin securely. On a Gibson wheel, I use a very small thin washer, about 14BA or maybe a bit less, and the crankpin nut tightens against this or sometimes goes through, but a much smaller amount. Thus there is less thread to conflict with the M1 screw. Another dodge might be to put a mousetail file into the turned down end to take off a thread or two.

The easiest solution is to use a 14BA screw as the crankpin in that place, but often I forget!

Philip

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Sun Mar 17, 2019 2:11 pm

Julian.
Yes, you're quite right; I use 14BA crank pin screws - either from the Ultrascale packs or from a bag of 100 i bought from Squires some years ago. I have to admit that I was under the impression that 14BA and M1 were sufficiently similar to be compatible, but apparently not. On the subject of Ultrascale crank pin bushes, they now only appear to supple complete assemblies, i.e. screw, front and back washers and the appropriate length bush or, in the case of the recessed type, the screw, back washer and threaded bush, whereas they used to supply all the components separately, a much better arrangement for the way I use them.
Since I fit them mainly yo Gibson wheels, like Philip, I also modify them by soldering the rear washer to the back of the bearing tube and then filing flush with the back of the washer, to make a sort of top hat shape. In the case of the recessed type, I file the bush very nearly flush with the back of the coupling rod and fit an Ultrascale front washer between the wheel boss and the bush. I find 14BA washers are too sloppy a fit on the crank pin to be used at this location.
Regarding the issues you've described with the tender and front truck, I also have had problems where I have made a single point rocking support at the tear of tenders. I think this arrangement, allowing unrestrained rock of the axle can unload tone of the wheels on twists and dips too easily. My solution has been to add supplementary wire springs each end of the axle or avoid that arrangement all together. On pony trucks, I mainly spring these, reacting against prototypical bearing pads, even on my compensated locos.
I'm glad you've managed to overcome the derailment problems on the Crab, but the lightly loaded front coupled axle would, in theory at least, restrict haulage capacity as the loco slips when the least loaded axle loses traction.
Dave.

davebradwell
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Re: Crab Comet conversion

Postby davebradwell » Sun Mar 17, 2019 3:23 pm

While I'm here.......when I use the Ultrascale recessed crankpin on a Gibson wheel, I just drill a shallow 1.3 hole in the wheel face. The drill centres itself in the crankpin hole and the pins can then be fitted exactly as intended. It's better if a drill jig is used but I've done plenty without. The original screw hole can be enlarged to a clearance as the new plain hole does the location, and much better than on the screw thread. I use Ultrascale type crankpins universally with the wheels adjusted as above although I now make a tapped version for the driven wheel so that the return crank can be screwed more securely on the end. I've found the original design comes loose, probably as the screw head settles into the plastic wheel centre. Dave Holt described something similar so I kept out of it. I knew I could rely on him with the pony truck pad - anything that can rock freely is a potential disaster.

Sorry to butt in,

DaveB

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Sun Mar 17, 2019 5:23 pm

I agree with Dave about the sloppy 14BA washers - I have some which are a tighter fit, no idea what size they are or where they came from. If I run out I suppose I could turn up some more but such small things are a bit of a pain to make!

Philip

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Will L
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Re: Crab Comet conversion

Postby Will L » Sun Mar 17, 2019 5:47 pm

Julian Roberts wrote:...
My conclusion, with due apologies to Bill (page 3) and Will (page 6), is that the pony wheel does need to have substantial weight on it, much more than its own weight, to reliably stay on the track, however that weight is applied...

Don't think we are disagreeing here, the only question was where is the sufficient weight was coming from. My preference would be to not attempt to transfer body weight to the pony truck with all the complexity that involves or to try any sort of side to side spring control for that matter. Just fit a suitably large lump of lead in the pony truck, which you then find won't need to be all that great as you don't have any other forces being transmitted between pony truck and the rest of the chassis to complicate matters. I think this all comes under the KISS principle.

Whatever, I do think you are displaying the basic persistence which I reckon is the hallmark of the successful modeller

bobwallison
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Re: Crab Comet conversion

Postby bobwallison » Mon Mar 18, 2019 8:40 am

DaveB,
I like the idea of drilling Gibson wheels to take Ultrascale crankpins, but how then do you get the quartering right? The slot for the crankpins on the GW wheelpress only takes a 14BA screw.
Regards,
Bob

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Mon Mar 18, 2019 9:53 am

The Gibson wheels would not be drilled to take the Ultrascale crankpins; the hole moulded in them is the right size to enable a 14BA screw to tap its own thread, as with the Gibson M1 screw. Both these are effectively the same outside diameter so will fit in the GW jig, so the quartering is not affected. The GW jig was originally designed for Sharman wheels which used a 14BA screw.

An Ultrascale wheel has a clearance hole, not a tapping hole, to take the 14BA screw. That hole has a shallow recess at the front which takes a turned down end of the crankpin bush. This precisely centres the crankpin rather than relying on the screw cutting a thread accurately to centre. This works because the Ultrascale wheel is moulded from a much harder grade of plastic. I think Dave B was suggesting that drilling this shallow recess was helpful if using a Ultrascale crankpin on a Gibson wheel.

In practice, the differences don’t matter much. The only critical thing is ensuring that the crankpin screw in a Gibson wheel is precisely at right angles to the wheel face. Sometimes it isn’t. I just bend ‘em straight, but others prefer a better engineering solution like drilling out the crankpin hole and plugging with filler before re-drilling using a jig.

Philip

davebradwell
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Re: Crab Comet conversion

Postby davebradwell » Mon Mar 18, 2019 10:51 am

Bob,
Yes, I see the issue with the jig. You could add the holes after quartering I suppose as it's quite a delicate operation to avoid going right through. It all depends on how much you want to use Ultrascale crankpins. I haven't looked recently but the diagram on their website showed a hole deeper than necessary and close to breaking into the rear csk.

Philip,
You've lost me here, I'm afraid. We were discussing the Ultrascale recessed crankpin some form of which is essential for leading wheelsets and other places too if there's more than 3 axles and which is tapped through. Another reason for using the Ultrascale 'pins is that a long one is available. I believe there will be some useful bits in the Exactoscale crankpin system when it is re-introduced.

And Will, I note you have chosen to completely ignore Dave Holt's experience and warning which, especially with your acronym, is clearly meant to be provocative. I had suggested to Julian that with a simple modification his model could be improved if he so chose, an upgrade if you like. His considered reply explains the design but also happens to completely rule out your floppy pony truck. If he needs a bit more weight for the leading axles he can stick the lump of lead in the smokebox. Isn't it worth trying to produce something a bit nicer than 00 r-t-r for all the work we put in?

DaveB

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Mon Mar 18, 2019 11:16 am

Just for clarity, when i said I generally spring pony trucks, I should have said that I spring the axle in pony trucks. I don't allow the truck frame (or bogie frames from that matter) to bob up and down relative to the main frames. I don't like to see daylight between trucks/bogies and the loco chassis. This is both visually and functionally un-prototypical for me.
Obviously, floppy trucks (and bogies) can be made to work in terms of track holding but make every loco into an 0-N-0 in function as they provide no support or lateral guidance. However, I can see that this approach might suit CSB suspension as it removes the carrying axles from the beam pivot calculations?
Dave.

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Mon Mar 18, 2019 12:06 pm

Sorry if I haven’t been clear. I was just saying that drilling a recess for the end of the flush Ultrascale crankpin does not affect the use of the GW jig. The jig uses the 14BA or M1 crankpin screw running in a slot to set the quartering, so merely putting a little recess in the wheel to accommodate the turned down end of the recessed crankpin (or a standard Ultrascale crankpin) will not compromise its use. I actually often use Dave Holt’s method as they both work.

Philip

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Mon Mar 18, 2019 2:34 pm

I must thank everyone for their most erudite replies. Much to try to take in. I'm going to Scalefour North so hopefully I can continue the conversations "live" so to speak for the details I'm not quite understanding if any of you are there - I see DaveB you will be.

This isn't a properly considered reply but I'm just doing a hasty one. Bill Bedford was saying much the same thing as you DaveB, that I should understand the prototype and copy that. As I don't I didn't! But I would love to! - which is why I will come and chat to you, maybe buy the K1 kit too. But I am not convinced that the rocking beam over the pony is a disaster waiting to happen. This thing belts through A6 turnouts and over colliery type trackwork with no sign of instability - at a totally unrealistic speed. But the weight on the pony I estimate at least twice that on the front driver, probably a lot more - the amount could only be determined by a clever weighing device. The point is that I had to have that much weight for fully reliable trackholding of that pony wheel, which is why my conclusion is that the prototype considerations are not wholly relevant. Noel said earlier carrying wheels take in general around half as much load as driving wheels: - this is the other way round. Basically my thought is now - the problem the prototype has is all the weight that has got to be carried, while the problem with the model is that there are all these wheels with not enough weight to carry to keep them reliably on the rails. But I'm just a musician and I don't expect to be taken seriously by engineer types! I just do this for a strange kind of fun.

Will thanks also, just a point that I said earlier having the same discussion with you (Nov 30 Page7) that the pony wheel assembly weight on the rails, with as much lead weight as I can conceal in the truck and A frame, is only 15g. We reckon 25g per wheelset for our unstable SWB wagons is the minimum I think...

Returning to the return crank :D I have found the problem isn't quite as bad as I thought and I'm getting there I think. Very very exciting details will follow if I'm successful.... :cry:

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Mon Mar 18, 2019 3:00 pm

I rarely use Ultrascale wheels, but they were supplied for a Panier chassis I did recently in EM for Pendon. I don't recall any problems using my GW wheel press, so the crank pin screws must have been secure in the wheels. On this basis, i would question whether their wheels have a 14BA clearance hole, unless the Pendon wheels are a special product.
On one occasion, I did modify some Gibson wheels to take the extension of the Ultrascale bushes but found it hard to control the depth of the front recess to still leave enough plastic to grip the screw. This prompted me to adopt my now standard procedure.
We do seem to have hijacked Julians thread about his Crab conversion, but I think these questions relate to his project in a good way.
Dave.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Mon Mar 18, 2019 3:20 pm

Apologies for seeming flippancy in previous post! I very much appreciate all your detailed replies and it's all thoroughly helpful. I am not expecting my slow progress details to be so useful...!

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Mon Mar 18, 2019 4:42 pm

Sackcloth and ashes time, I’m afraid. Dave Holt’s comment about Ultrascale wheels not having a clearance hole for the crankpin but a tapping hole suddenly made me think, have I got this wrong? So I went to the workshop and checked. And of course you’re right, Dave, it is a tapping hole, and the 14BA screw cuts its own thread just as a M1 does in a Gibson wheel. Which is why your screw when you checked was very secure.

Why I wrote that makes no sense given the number of times I have used Ultrascale wheels, some very recently, and also that it would have to be this way to be able to use a GW press for them.

I could blame it on advancing age but that would give my wife and friends even more excuse to comment that way and which they do very effectively already!

Philip

bobwallison
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Re: Crab Comet conversion

Postby bobwallison » Mon Mar 18, 2019 5:28 pm

Thanks Dave and Philip.

I feared that Ultrascale crankpins would require either a different wheel press tool or opening out to 1.3mm after the GW wheelpress had done its bit. With the latter option, I would be concerned that the threaded crankpin hole would throw the drill off centre.

So its back to Alan Gibson crankpins for now. I take heart from the fact that many people (including me) have produced sweet-running locos with them, in spite of the slightly dubious engineering of bushes on self-tapping screws in soft plastic.
Regards,
Bob

davebradwell
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Re: Crab Comet conversion

Postby davebradwell » Mon Mar 18, 2019 9:09 pm

Your money's safe at Wakefield this time, Julian, as I don't have a K1 this year. It'll be good to have a chat, however and musicians are very welcome.

DaveB

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Will L
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Re: Crab Comet conversion

Postby Will L » Tue Mar 19, 2019 12:20 am

Dave Holt wrote:... I don't like to see daylight between trucks/bogies and the loco chassis. This is both visually and functionally un-prototypical for me.
You have to agree with that and through keeping the pony truck/bogie in prototypical contact with the main chassis is certainly a fair way to achieve that, I don't believe it actually necessary to have the acers of empty space between pony truck/bogie you do some times see when weight is not being transferred.
Obviously, floppy trucks (and bogies) can be made to work in terms of track holding but make every loco into an 0-N-0 in function as they provide no support or lateral guidance. However, I can see that this approach might suit CSB suspension as it removes the carrying axles from the beam pivot calculations?

It isn't including the carring wheels into the CSB calculation that is the problem, its implementing the result which can have practical difficulties, but the effect is the same and yes it is a lot easier to produce 0-N-0. But its horses for courses and It has to be done for 4-4-0s and their ilk where the support and guidance is certainly an issue, but with the possible exception of 2-4-2 tanks I doubt that the complexity of load bearing pony truck will repay the complexity they introduce with any visible improvement in the running properties of the loco, CSB fitted or otherwise.


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