Crab Comet conversion

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Mon Apr 01, 2019 8:58 am

Hi Allan! The Crab chassis is being painted but I hope to be able to arrange a test on Grayrigg before long with you. What I mean about modelling weight is the way a train stops and starts. It's got to be able to go slowly to before there is even a possibility it can start and stop realistically. DCC control can help of course but I haven't gone down that route. As can various types of controller even with straight DC .

I did make a video with my Q1 on Dubbieside of exactly what I mean but it is somewhere inaccessible on my pc. Here is what I think is a realistic minimum speed but it doesn't show a start and stop. This is the third loco I made in P4. There is no flywheel - there's no room!



Here is my fourth loco (just a new chassis under a 2nd hand body). It can run just as slowly but it is up to the operator to create the illusion by the quality of the start and stop....or not as in this case! :( This one does have a flywheel.



Both these locos have 1:108 gearing.

If the second video doesn't work it's at https://youtu.be/ApsQA44XqaE

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Thu Apr 04, 2019 9:00 am

Jol Wilkinson wrote:.... does anyone else find that the loose fit of the crankpin bush on a AGW (or even Sharman Wheels if you still have any) can cause a problem.

Loco "chassis" that will can be propelled or run smoothly when the crankpin nuts are loose sometimes develop a tight spot when the nuts are tightened. Loosened off, wheels rotated a quarter turn or so, or rocked back and forth and re-tightened and the problem may be cured. I should add that I use a GW quartering jig and LRM alignment jigs for setting up, having assembled the rods and broached them for a close fit on the crankpin bushes. However, there is more play between the crankpin and the bush than the crankpin and the rod. Opening up the coupling rod crankpin holes is usually the solution, but not an ideal solution. I wonder also if the bush holes are sufficiently concentric, especially on the brass Sharman ones.

I can always get over the problem but it usually takes a little time even with four coupled locos. So getting a consistent and close "fit" of the rods, pins and bushes seems a bit of pot luck. A threaded, concentric, bush which fits on the pin accurately would seem to be the answer.
.


I wrote to Colin at AGW to ask if he'd ever considered producing the bushes in a threaded version, though I described my reasons for wanting one (a long threaded bush would be useful if soldered to the return crank) rather than for the reasons Jol has described above. He replied (perfectly understandably) that this is a bit specialist, and that he could make a thousand and have them hanging about for years. (Perhaps if I'd quoted Jol's post above he might have been more interested...)

But if anyone else thinks this is a good idea, and is going there, maybe Scalefour North this weekend is a good time to have a chat about it with Colin, whether for my reason or Jol's - or any other for that matter.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Thu Apr 04, 2019 12:31 pm

Having disassembled the chassis I ascertained the middle wheel crankpins were not going to be removed without ruining them. So I got on with making my own equivalent of the Ultrascale recessed crankpin as previously described. Otherwise the chassis is ready to paint. The gears are now 1:120 and I have used another motor, abandoning efforts to remove the 1:90 worm from the previous one.

As well as reading it in the books, I have noticed for myself on today's trains vividly that a black underframe is anything but black and is covered with a film of yellowy/reddish dust that highlights all of the shapes. Yet I subconsciously still think it's "black" - I think it's the transparency of this film of dirt that mixes up my perception.

20190404_125755.jpg


Anyway I sprayed the chassis with primer and then satin black. After a couple of days I mixed up about four different enamels till I had roughly what I wanted and sloshed it everywhere, thinned down with white spirit, and a thinner still slosh over the loco body, with a brush, blowing to recreate the effect of the train moving through the air and the dust swirling around. I was aiming for something reminiscent of the dust from the red earth of Ayrshire, and rust.

Really my aim isn't so much realism as an excuse to make all the various bits I've patiently attached stand out - or at least be visible. All those sandboxes and sandpipes that took such hours - no point if you don't see them. Of course it's difficult to know whether I've overdone it or not done it enough. The great thing is that it can always be modified, especially if you're lucky enough to be in Allan Goodwillie's orbit!

20190402_205157.jpg


With trepidation I soldered the "Roberts Retainer" to the return crank and managed not to unsolder the rivet holding the eccentric crank. I then modified the second return crank to have the same arrangement. I was lucky that the return crank was just big enough to accommodate the retainer I had made.

20190403_110834.jpg


So now I am back to where I was several days ago, determining how many shims are needed to get the return crank to screw down to the correct orientation. Now however the return crank serves as a bush for all the rods too and winds on around 20 times.

20190403_112613.jpg


20190404_120637.jpg

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Fri Apr 05, 2019 5:57 pm

20190405_171825-1.jpg

I'm pleased to say the screw-on return cranks that incorporate a bush for all the rods have proved their worth (in terms of the time and effort to make them) and, with nail varnish applied as a glue for the last two or three turns, are staying screwed down and in position through three or four hours running on my auto-reversing test plank. Thus I can say that, subject to them staying put, the loco is now a goer and ready testing in service... :!: :D The running is smooth enough that I feel reasonably sure if they come loose it is not because of a fault, and that some 243 Loctite Retainer will do the job more durably and they shouldn't loosen off again till I need them to.

20190405_174338-1.jpg


The gearing gives a more than reasonable top speed on my home layout controls, though strangely the same voltage gives less speed on club layouts. Yes there is motor whine at top speed, inaudible at an exhibition, but the loco seldom goes at that speed on the layouts for which it is intended. The slow speed is much better now with the higher ratio, improved yet further probably by the total clean up involved in stripping it all down for painting.

The videos are pretty duff I know but are there to prove something at least. Radio 4 was on at an ordinary listening level underneath the board to give some idea of noise level.


Regarding the weathering, I don't know if I'm in a hole but I'm definitely not digging any more till I next meet up with Allan who has promised a weathering session for our WS4Group mutual improvement classes. Sometime I'll get Rountuit and renumber the loco and give it a local shedplate; and finish final details like coal and coal rails, and some some further blackening.

Very many thanks to you Forum people especially Philip Hall, Davids Holt and Bradwell, and Allan Goodwillie, for your encouragement and many useful tips. Especially the tip re the Ultrascale recessed crankpin. Although I had to make my own (to be compatible with the AGW 1M thread) I don't know if I'd have thought up how to solve the problem of the return crank in that way otherwise. Next time I know to use the (Ultrascale) 14BA screw for any wheel that has a return crank, and their recessed crankpin.

Attached below a few more shots below taken at the same time. In ordinary light and at a distance the weathering doesn't seem extreme to me and does what I intended. Still to do is to somehow highlight the moving valve motion in a similar manner.
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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Fri Apr 26, 2019 7:59 pm

Hi Julian, :)

First time I have been on the forum for a while - just been trying to catch up on my own project. My goodness the crab is fairly coming on! I will be at Bonnybridge show tomorrow (Saturday) just to enjoy the show. :D

I notice Jol mentioned a few postings back about how sometimes when you tighten up the nuts the coupling rods sometimes bind, and most people open out the holes in the coupling rods a little more assuming that will cure it , and sometimes it does, but this is something I brought up in my loco building thread for beginners and the thing to check is the thickness of the connecting rod boss compared to the depth of the bush around the crankpin. What you want to make sure of is that the nut when tightened comes into contact with the end of the bush before there is any chance of it contacting the connecting rod boss. There has to be a working clearance/tolerance between the two otherwise they will bind. Opening out the holes increases the amount of slop and will lead to earlier wear and tear than is necessary. I know this is a very simple piece of advice, considering the complexity of what you have been building, which has been very absorbing, but worth mentioning as there will be others who have only built maybe an engine or two and have not even thought of this as being something to check as in most descriptions of how to get free running coupling rods nearly always concentrates on opening out the holes.

Grayrigg will be out of commission until next year I am afraid after the problems with the garage roof and the damage done. It is still leaking and some of the baseboards will have to be scrapped and started again, however all the scenery lifts off and I will take off the wiring looms before lifting the boards and seeing what can be salvaged - I think it may be next spring, if you can wait that long, sorry. :cry:

Allan :)

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Apr 28, 2019 8:20 pm

Hi Allan thank you for getting back to me. I'm really sorry to hear of the damage to Grayrigg. Especially when you're wanting to concentrate on your new layout. Anyway the Crab performed its first goods test services very satisfactorily on Kettlewell and was passed for running there. I was pleased that propelling forwards (as well as backwards) while shunting there were no buffer locking issues. But the trackwork is kind to stock on that layout. Calderside will present a stiffer test. I won't post a video owing to the banter in the background - not for family viewing...

I fully see what you mean regarding tightening down the retaining bushes. I find as a normal experience the short AG bush is not long enough. I use a long one and file it down so it's just a little proud of the rod. So I have to buy extra supplies of the long ones and have lots of surplus short ones! I think the Crab is finished apart from the coal, weathering and changing the number to a Scottish one. Apparently this one was shedded at Fleetwood during the early BR emblem time. Now I've started completing the LMS Compound (some way to go...)

Couldn't come to Bonnybridge as I was cycling this weekend including along part of the Fort Augustus line.
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Last edited by Julian Roberts on Sun Apr 28, 2019 9:16 pm, edited 1 time in total.

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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Sun Apr 28, 2019 9:07 pm

Hi Julian, :)

The Fort Augustus line - that must have been lovely this weekend - Dave and I were planting out the back garden today and I did go along to the exhibition at Bonnybridge. I think the lads put on a good show and it was well attended on Saturday and with the AMRSS meeting being on on the Sunday I am sure they will have done alright financially. Not to worry about Grayrigg It will be repaired in time, but just not this year I still have water coming in from my neighbour's garage, but Davy has promised to get his roof done as well, so I will not be doing anything until after then anyway.

A discussion I had with the exhibition manager at Bonnybridge was that he was finding that he was being offered more and more small layouts and less bigger ones as the years are moving on. I am trying to make Scott's Road be compact, yet have lots of movement as it did in real life. This time it is the exchange yard that is central to the operation, but the length will be around 24/30 feet which makes it medium size and as big as I think I can manage in the car. :)

Taking Jean for a meal and attending the annual musical follies at Fingask Castle on Saturday, should be fun. ;) NO trains however, but beautiful views out over the river Tay from high up on the north shore. Been there twice before-the castle and gardens are stunning, would make a nice model.

Allan :)

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Re: Crab Comet conversion

Postby Julian Roberts » Sat May 18, 2019 3:43 pm

This is to (somewhat belatedly) acknowledge the mention in the Snooze from Will. My main reason for doing this thread was because I became known for the thread on gauge widening, where it was justifiably said "Why don't you do some modelling". This was to correct the impression that I never did any. I did a warts and all blog kind of thing, to encourage others to persist too. Hope I didn't have the reverse effect. Sorry that I'm no engineer and it all looks so crude compared with most modellers. And again many thanks for so many helpful hints.

The natural successor to this project is a 2-6-2 wheel arrangement. This will be a V1/3 from a well known kit designer where all the wheels will be individually sprung.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sat Jul 27, 2019 3:33 am

To more or less wrap up this thread here is a video of the Crab stretching its legs at a certain iconic location that some may recognize. 7 coaches were handled with no slipping on the 1:100 gradient. I wasn't running at max speed just in case of any mishap (which did not occur) while I was distant from the controller - this was about 3/4 regulator.

Our group were guests of Chris Pendlenton for the day. The layout runs on DCC so the Crab couldn't run except on its own with the layout switched to DC. As well as preventing the realistically high speed running desirable on a big layout such as this, I can well see why my high gearing is unnecessary for low speeds with the sophisticated DCC control on this layout, and the motor whine counterproductive to the soundscape DCC enables, as in the next video, a 9 coach boat train leaving Collingwood Yard for the dock, top and tailed by big pugs. (Unfortunately I didn't record the completely believable start and stop which is the whole reason I want good low speed control).

We had many hours of faultless running. Big trains of wagons were propelled over the complex pointwork.
20190718_174920-1.jpg

I could forget this was P4 and I was as though 'really there'. Such a standard comes through attention to detail. For example, every one of the 200+ wagons has spring suspension and sprung buffers. Chris has written plenty over the years in MRJ about how he gets such running. But he doesn't write on this Forum and his methods are to some extent below my radar, relying as I do on material easily accessed on my phone while travelling about. His locos have individually sprung wheels, not the AG type of very light buffer coil spring where just the end stop is adjustable, but more near in design principle to those in Dave Bradwell's kits. Fiddly to build perhaps, but each wheel is therefore adjustable, just as the real thing was, to get correct weight distribution, as well as ride height. If there's anything I'm sure about after this build it's that the weight borne by ideally each wheel needs to be adjustable, particularly in the case of the front pony wheel which needs to have plenty of weight on it, to attain fully reliable track holding.




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Horsetan
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Re: Crab Comet conversion

Postby Horsetan » Sat Jul 27, 2019 1:30 pm

Julian Roberts wrote:.... a V1/3 from a well known kit designer where all the wheels will be individually sprung.


Not Worsley Works? (That's the only one I can think of....)

Julian Roberts wrote:.....His locos have individually sprung wheels, not the AG type of very light buffer coil spring where just the end stop is adjustable, but more near in design principle to those in Dave Bradwell's kits. ....


That's because Dave more or less productionised the Pendlenton ideas. The tail of the leaf spring is supposed to fit into the hollow Exactoscale grub screw, but sometimes it falls out of the bottom when you're picking the model up.....
That would be an ecumenical matter.

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PeteT
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Re: Crab Comet conversion

Postby PeteT » Fri Aug 02, 2019 9:58 am

Horsetan wrote:Not Worsley Works? (That's the only one I can think of....)


I would think it will be the Dave Bradwell kit - as aluded to with the springing - as seen at Scalefour North earlier in the year.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Tue Aug 27, 2019 10:09 am

Yesterday I started the process of properly fixing the RHS return crank for small scale exhibition next week. Main thing was getting the correct amount of shims to bring the crank to the correct height/orientation when fully screwed down. And, to get rid of all the oil that was going to prevent a threadlock from working.
I don’t know whether I ever put the threadlock on that I meant to do in April. I suspect not as there is a layer of nail varnish on the top surface of the crank. So I used nail varnish remover to thoroughly get rid of this and clean up.
Fully screwed down the crank was 180 degrees away from where it needed to be without any shims. All the shims that had been on it came to 5 thou (measured by the micrometer) – but this was not quite enough as it was screwing on too far.
My final result of the shims required was 8 thou, though this did mean tightening the crank quite hard.
Stupid thing was that this amount of tightening probably needed no threadlock. The feeble amount of tightening I’d been able to do for the Pendlenton day was enough for its running on that occasion – a good 10 minutes or more.
I blackened the connecting rod but this didn’t go anywhere near the thread of the pin before I put water on it to stop the blackening process.
I wound the whole lot together to check the shims, then wound off about half way, and filled the hole with Expo Threadlock. I took about a minute making sure I’d put enough on and started to wind it back on, and found I could hardly move it in either direction. As it slightly loosened I thought I’d fix it on properly and continued to wind down. Only half a turn or so and suddenly it was loose. I’d worried I might break the return crank but in fact I’d broken the crankpin, right at the point it comes out of the wheel.
Subsequent experiments with this threadlock on a brass 14BA screw and nut, and a Gibson steel crankpin and crankpin bush retainer did not have them locking up in seconds. I wonder if it was the trace of nail varnish remover and the cleaning up yesterday with (already used) white spirit when combined with the Threadlock that caused the dramatic locking up. Also of course the locking surface on the crankpin was about twice as long as the screw in one of these nuts.
No way out of this except to start again with the wheel - strip it off the axle and get a new plastic centre for the steel tyre from Colin. Luckily the centre wheels are the ones that will drop out of the chassis with least general disassembly. And start again with the return crank if I can't get the remains of the crankpin out.
Well, at least I'm on the way to not having a return crank unscrewing in exhibition service....but not by next week!
Last edited by Julian Roberts on Wed Aug 28, 2019 1:52 pm, edited 1 time in total.

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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Tue Aug 27, 2019 5:01 pm

Sorry to hear about this Julian, :(

We were all looking forward to seeing it at the weekend show doing its stuff. I have had similar happen to me. Soldering can have just as many problems. What happens is that too much heat and the crank pin comes loose within the wheel and therefore the crank malfunctions - happened to my A4 when I was building it. I am sure all will be well in the end, your engine performed well at Chris's.

I have been looking forward to going down to see just how his layout has developed and wasn't it splendid! Well worth the wait.

Allan :)

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Tue Aug 27, 2019 7:05 pm

Julian.
Yes, sorry to read of your problems. It's so frustrating when a finished model gets damaged or malfunctions like that. It sound as if you have a solution, even if it involves a fair bit of re-work.
Best wishes for a successful repair.
Dave.

PS. Sooo envious of your visit to Chris;s layout.

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Re: Crab Comet conversion

Postby davebradwell » Wed Aug 28, 2019 11:13 am

Normally you would be able to unwind the remaining piece of screw from the back of the wheel, Julian, so do I conclude that this has also been nutlocked in place? All through my career I've been suspicious of these adhesives and especially so when outside their 12 month shelf life. There's a lot of variables, especially with small parts. The (model) web is full of tales of folk claiming their 30year old Loctite is as good as new but I'm not convinced.

A traditional thread locking compound is shellac, often sold as knotting. It can be released with a touch of meths. Good shelf life, too.

I'll continue on the 9F page:

DaveB

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Wed Aug 28, 2019 1:51 pm

20190827_093735.jpg
Crankpin clean break. Visible screwed into the return crank

Allan and both Daves, thanks for your thoughts! I was going to say, and Dave B you have beaten me to it by asking, yes the Gibson crankpins were Araldited (24hr stuff) in place. They're absolutely immovable, that stuff really works.

Not quite sure whether to continue the conversation here or again usurp Dave H's 9F thread viewtopic.php?f=20&t=6527

Shellac is commonly used in the musical instrument repair business so that is an option that's attractive as I should have some and it's easy to use, but I'd never associated it with modelling. One question is, I sometimes slosh white spirit about to clean the chassis, or even put it in a shallow bath of the stuff, to get back to square one with cleanliness before re-lubricating. Would that be likely to act like meths on anything retained with shellac?

I'd said on the 9F thread that I'd been concerned whether the brass thread (which I tapped) in the return crank could strip, and that was my concern when getting it to absolutely stay put at the right orientation. (By the way I will correct my previous post to clarify this). I now see that I did not need to worry about that and that the crankpin was more likely to break than any threads strip. I can see just a tiny bit of locking is needed if any at all given a firm screwing down of the crank.

From the 9F thread:

My reasoning is that if I want the screw to be in the right position at the front then the back of the wheel is the wrong place to start winding when it must tap its own thread.


Dave B, Where the hole in the Gibson wheel is the normal size, not just a pilot hole as on some of the old stock, I think I've screwed the crankpin into the wheel from the front for it to make its thread, then removed and reinserted from the rear. (I don't have a lathe). Marks out of 10?

Seems to me if using a pillar drill which I do have it is better to have a wheel with just a pilot hole. I did the wheels for one loco in this way. It does go well though now has problems from trying to adjust the BB and needs similar surgery to the Crab.

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Re: Crab Comet conversion

Postby davebradwell » Wed Aug 28, 2019 4:18 pm

Don't know why you glued the screw in the wheel, Julian, is this standard out there? If the screw was loose you could adjust the ret crank angle this way. Of course it probably improves the security of the crank fixing so there you are. Doesn't sound as if white spirit should shift shellac but try it first - I'm no chemist. I just know that the opposite of what you expect is the usual outcome.

Any thread can strip, especially in thin brass or n/s. 2 layers helps, of course but this is why ChrisP fixes bits of bolt in his return cranks and crankpin nuts and screws them into the crankpin - there's even more thread engagement. Care is required when deburring the tapped hole in a ret crank as flat mating surfaces are required to discourage the thing from coming undone but a small 0.2 chamfer each side will remove a good % of the threaded length. It's probably worth producing a tighter tapped hole by not taking the second tap right through and possibly using a slightly smaller tapping drill - commercial nuts and bolts produce a very loose fit. The trick is producing a stack of bits that won't come loose even after a few years and some systems will need locking, in fact it's normally a company design rule to lock every screwed fixing in some way. Difficult with these tiny parts.

Julian, you score very highly for your technique for fitting the screw and even more if you countersunk the back of the hole to give you a chance of picking up the thread - this also prevents the head from catching the axlebox corners. I feel, though, that the less depth of thread you are making then the better the screw will follow the hole. I'm chasing down the odd thou' here so it may sound a bit extreme. Colin moulds his crankpin holes pretty true so I don't worry about this if drilling. The face of the moulded wheel boss must be very flat as the sit of the crankpin bush flange determines the angle of the crankpin. I've said elsewhere that with an outside cylinder loco the face of this boss should be flush with the tyre to give maximum clearance for the whirling bits and particularly the crosshead. There will be a few exceptions, of course - check with the GA but remember our wheels are slightly wider than scale (except Exactoscale). I must be careful!

DaveB

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Will L
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Re: Crab Comet conversion

Postby Will L » Wed Aug 28, 2019 10:42 pm

Julian Roberts wrote:Shellac is commonly used in the musical instrument repair business so that is an option that's attractive as I should have some and it's easy to use, but I'd never associated it with modelling.

When I first got interested in model trains, reading the Modeller and the Constructor it seamed that there was very little modelling that didn't call for shellac one way or another. Things have moved on since.

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Re: Crab Comet conversion

Postby Julian Roberts » Sun Sep 08, 2019 5:22 pm

Having eventually a suitably calm mentality and spare two hours I've successfully removed and disassembled the Crab middle axle. I was surprised how easily the wheels came off the axle which I thought I'd attached securely with 24 hr Araldite and a cross-pin. (Page 4, 15th Dec '17) So in advance of getting new wheel centres from Colin I'm thinking about an old question, how to really securely attach the wheels to the axles in such a way they will not possibly move once quartered and gauged, in normal use at least.

Chris Pendlenton in MRJ 28/9 mentions how surprisingly securely he found the wheels attached when he had filed (or done in the lathe) a groove around the axle where the wheel fixes on, thus giving Loctite something to bite into - suspecting that without a groove the interference fit otherwise simply pushes any Loctite aside so it cannot function.

In the days until recently that High Level gears sometimes had to be Loctite fitted to the axle, I have filed a flat on the axle at the appropriate place and used 24 hr Araldite to fix the gear wheel, and this has been entirely successful. The flat gives a more chunky area for the adhesive and also a key. So I wonder about a similar idea for wheels, using Araldite.

Meanwhile I'm also tossing up the various approaches to attaching the return crank generously suggested here and the 9F thread.

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Le Corbusier
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Re: Crab Comet conversion

Postby Le Corbusier » Sun Sep 08, 2019 5:47 pm

Strangely to date I have just push fitted the axles into the gibson moulded centres and never had a slippage. On the odd occasion I have had to remove a wheel from an axle I have had to apply considerable force to break the adhesion .... the centres appear to shrink fit on to the axle over time.

I have yet to have a problem with wheels slipping and coming un-quartered. Interested to know if your approach is belt and braces or have I just been lucky? ... my test sample is pretty limited in both time and numbers :?
Tim Lee

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Sep 08, 2019 7:00 pm

Belt and braces I think Tim! Thanks, yes I have thought the same as you but I think time does things. These wheels seemed less difficult to remove from the axle than I expected and I have wondered if on some older locos (say eight years or so) things move. Talking about Gibson wheels by the way. But don't let me put you off - I'm just getting more neurotic probably with age!

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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Sun Sep 08, 2019 7:11 pm

Hi Julian,
you are much more likely to have a wheel go out with combinations of the following -

1 A heavy locomotive pulling a heavy load (something may have to give especially where gradients are involved)
2 Engines with larger wheels from about 5' 0" upwards all to do with leverage and torque.
3 Mishandling a locomotive on full power especially if it has real power and good adhesion.
4 Taking the wheels on and off too often during construction.

Allan :)

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Re: Crab Comet conversion

Postby Julian Roberts » Sat Sep 14, 2019 6:19 am

I've thanked Allan for his post above. I might start a new thread on that subject.

Friday 13th gave me some time to bite the bullet with the possibly wrecked return crank. I was fearing I would have to make another one.

To my surprise, application of the soldering iron to the remains of the crankpin loosened it enough that I could unscrew it from the crank with pliers.

The heat disturbed the perpendicular orentation of the crank on the tube/bush (which is tapped 1M for its entire length) and I had to separate them and clean them up to get the crank fixed on perpendicular in two planes.
20190913_173840.jpg

20190913_175740.jpg
Crank hopefully parallel to wheel in two planes, end on and side on

So now a resume of how I fixed it to the wheel last time round and propose to do again unless 2 Daves persuade me otherwise. These photos are using the undamaged wheel though I've opted to buy a new pair.
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davebradwell wrote:Don't know why you glued the screw in the wheel, Julian, is this standard out there? If the screw was loose you could adjust the ret crank angle this way. Of course it probably improves the security of the crank fixing so there you are. Doesn't sound as if white spirit should shift shellac but try it first - I'm no chemist. I just know that the opposite of what you expect is the usual outcome.

Any thread can strip, especially in thin brass or n/s. 2 layers helps, of course but this is why ChrisP fixes bits of bolt in his return cranks and crankpin nuts and screws them into the crankpin - there's even more thread engagement. Care is required when deburring the tapped hole in a ret crank as flat mating surfaces are required to discourage the thing from coming undone but a small 0.2 chamfer each side will remove a good % of the threaded length. It's probably worth producing a tighter tapped hole by not taking the second tap right through and possibly using a slightly smaller tapping drill - commercial nuts and bolts produce a very loose fit. The trick is producing a stack of bits that won't come loose even after a few years and some systems will need locking, in fact it's normally a company design rule to lock every screwed fixing in some way. Difficult with these tiny parts.

Julian, you score very highly for your technique for fitting the screw and even more if you countersunk the back of the hole to give you a chance of picking up the thread - this also prevents the head from catching the axlebox corners. I feel, though, that the less depth of thread you are making then the better the screw will follow the hole. I'm chasing down the odd thou' here so it may sound a bit extreme. Colin moulds his crankpin holes pretty true so I don't worry about this if drilling. The face of the moulded wheel boss must be very flat as the sit of the crankpin bush flange determines the angle of the crankpin. I've said elsewhere that with an outside cylinder loco the face of this boss should be flush with the tyre to give maximum clearance for the whirling bits and particularly the crosshead. There will be a few exceptions, of course - check with the GA but remember our wheels are slightly wider than scale (except Exactoscale). I must be careful!

DaveB


Yes DaveB the crankpin is countersunk in the wheel (after first screwing in from the front to create the thread) and fixed for good with 24hr Araldite. The return crank screws right down to the wheel and if the orientation is incorrect thin brass shims sit between crank and wheel. (It happens that the orientation is correct in the previous photo but that may not be the situation on the new wheel). The return crank assembly acts as a bush for all the rods.
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Sad news from Iain Rice this morning. I hope Mike would have approved of this Crab which is based on his compensation principle.

davebradwell
Posts: 84
Joined: Fri Jan 18, 2019 3:48 pm

Re: Crab Comet conversion

Postby davebradwell » Sun Sep 15, 2019 5:05 pm

I could well award that an ok, Julian. Your shim washer saves it by spreading the force from the small diameter end of the tightened flangeless crankpin over a larger area so that it doesn't slowly sink into the plastic wheel boss with time and let the return crank loosen. A drop of shellac and it might not matter, although I don't do it myself. It's not a bad plan to reduce the throw of the return crank a bit as this will reduce the stress on the whole valve gear but don't overdo it or I'll call you a wimp. One thought is that the shim will become dished - can the rough corners catch on any bits of the rod boss?

You've lost the flange from the back of the crankpin bush (which would normally spread the load) - is rod alignment with other wheels still ok?

DaveB

Julian Roberts
Posts: 746
Joined: Sat Jan 09, 2010 11:33 pm

Re: Crab Comet conversion

Postby Julian Roberts » Tue Sep 17, 2019 11:00 am

davebradwell wrote:I could well award that an ok....

DaveB



Well I'll take that as high praise indeed Dave, coming from you! Yes and thank you for pointing out the issues. Of course the loco has already run successfully. The shims I thought acted as the missing bush flange between wheel boss and rod and I thought any slight misalignment of rods to the other wheels wouldn't matter. But maybe it does? But I see the rods must be clear of the wheel boss.

What you call the throw of the return crank I have been calling its orientation. I understand the less the throw the less the movement of the valve gear. I have long wondered how to know what is the correct amount of throw other than trying to copy and photo or drawing. What position should the driving wheel be such that the eccentric rod is parallel to the connecting rod?

I have re-read Chris P's "wheel derangements" article in MRJ 219. All of his refinements are beyond me with no lathe and not a lot of inclination to go to those lengths, and a quite high degree of trust in the quality Colin now produces. I do agree with his point that the fix of the wheel on axle will suffer many stresses and strains. Even if it is fitted only once as I do now using the GW jig. The only practical improvement I could do is along the lines of the groove in the axle he made which gave Loctite a better means of gripping the wheel even more securely than the interference fit. With just kitchen table tools I can quite easily file a flat and I wonder whether two of those per wheel would be an "ok" plan.
Attachments
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Flat something like this, to be filed centrally in the 2mm wheel width.


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