Crab Comet conversion

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Fri Mar 01, 2019 8:40 pm

Julian.
Definitely the Brassmasters Crab detailing kit for my drain cocks. They look far better than the lumps sticking down from the Comet cylinder wrapper castings. Just need to add the wire to represent the pipes.
Dave.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sat Mar 02, 2019 9:16 am

Thank you so much Gavin and Dave. To think, Dave, I didn't realise Brassmasters do a detailing kit! That's saved me so much time. Brilliant example of the uses of the Forum. Gavin I've got some photos etc of interest which I'll post in a couple of days.

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grovenor-2685
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Re: Crab Comet conversion

Postby grovenor-2685 » Sat Mar 02, 2019 10:00 am

Julian Roberts wrote:While I'm thinking about it I'm also thinking about, but rather more hazily, the only bit of wholly cosmetic modelling on the project, the cylinder drain cocks. I'm not terribly acquainted with them visually, and photos don't exactly home in on those details. An impression is all that's really needed, yet I always find it's hopeless trying to create an impression (as one would if making a painting). I wonder if you or anyone else has any close up photos...

Is this any help? (click picture twice to enlarge)
Rgds
Crab_cylinders.png

dal-t
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Re: Crab Comet conversion

Postby dal-t » Sat Mar 02, 2019 12:11 pm

OK, idiot question, but it is an idiot asking it - where's the third pipe come from (I do know Crabs are two-cylinder locos!)? Is there a drain cock on the valve chest as well as the two on the piston?
David L-T

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Sat Mar 02, 2019 1:44 pm

David,
It's not an idiot question and you're quite right that it is the valve chest drain. Generally, these are an automatic drain, which is closed by steam pressure in the valve chest and falls open when there is no pressure, unlike the cylinder drain cocks, which are opened and closed by the driver.
Dave.

dal-t
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Re: Crab Comet conversion

Postby dal-t » Sat Mar 02, 2019 7:34 pm

Dave,

Many thanks - (small) store of technical knowledge now expanded!
David L-T

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Mar 03, 2019 9:29 am

Many thanks Keith. I intended to but forgot to say earlier that you had posted the best picture I had - so thank you for putting it here a second time! Remarkable clarity as you zoom in. Also I was thinking the same question that Dave has answered, why are there three pipes!

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grovenor-2685
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Re: Crab Comet conversion

Postby grovenor-2685 » Sun Mar 03, 2019 10:43 am

That picture was published by the NRM and offered publicly for use as a screensaver.
Regards

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Mar 03, 2019 1:54 pm

My current problem is how to lock down the return crank in such a way it is unlikely to unscrew. It does of course have to lock down at exactly the right position, slightly forward when coupling rods are at the 6 o'clock position (I hope I'm right in saying!). This means the crankpin bush has to be exactly the right length, and so far I'm finding it isn't long enough allowing for the fact that there is an inconvenient rivet on the back of the crank, so there has to be adequate clearance to the connecting rod.

I am of course using the long bushes. (All the crankpin gubbins is Gibson.)

Previously on the thread I showed the valve gear in operation fully satisfactorily. The return crank at that time was glued adequately, but not really positively against something. Maybe 243 would hold firm indefinitely. I'm reluctant to rely on that as should the crank come unscrewed in operation unnoticed the result could be the mangling up of quite a few hours work!

A photo below illustrates the rivet and clearance issue.
Attachments
20190302_112401-1.jpg

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Sun Mar 03, 2019 3:36 pm

Julian.
First, you can reduce the thickness of the rivet head behind the return crank by at least half to reduce that problem. For fixing the return crank at the correct angle, do you have any Ultrascale recessed crank pin retainers - they're a headed crank pin bush with an internal 14BA thread? I use them by soldering the bush flange to the back of the return crank and then shortening progressively to give clearance to the coupling and connecting rods and to tighten at the correct orientation. To be honest, after a few assemblies and re-assemblies, they do tend to tighten a bit further, so it's best to start off a little short of the desired final position. If you don't, then you could solder a Gibson short bush and file to length, assuming the crank pin screws are long enough.
Actually, I always use a short bush through the coupling rods and into the inside half of the connecting rods, then my reversed bush on the return crank also supports the outer edge of the connecting rod hole.
My method only works if you can rock the driving wheels back and forth, to screw the return cranks on or off.
Dave.

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Sun Mar 03, 2019 4:11 pm

Problem solved. Filing rivet was more practical and effective than I expected.
Attachments
20190303_160506-1.jpg
Return crank now satisfactorily screwed down against bush in correct orientation with no fouling.

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Sun Mar 03, 2019 7:07 pm

I am dreadfully sorry to suggest this, and I know that this is a slightly heathen method. I just use a brass 14 BA screw crankpin screw and solder the return crank, Guy Williams style. I have tried the shims methods, screwing the crank down tight, but the solder method is, for me, the most secure.

Philip

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Mon Mar 04, 2019 7:03 am

Dave your reply somehow was not visible when I posted that I'd solved the problem. Thank you for your comprehensive reply which, I am sure, I will find helpful on future builds and may need on this one if problems recur. One of the issues is that the crankpin seems scarcely long enough and I wonder if the connecting rod thickness is over scale. I took a picture at the NRM of a Midland 2-6-4 tank loco return crank details - the same area of the Crab is concealed in their display arrangement.

I have some useful years old sheets of brass shim material of three different thicknesses, and a little piece placed on the wheel over the crankpin before the bush is placed over it can by trial and error result in the return crank screwing down to the right place. And similarly to what you write, on repeated operations it screws down further so a thicker piece of shim has to be employed. Yes the gears have a grubscrew. One of the new generation High Level boxes which all have one.

I take it that you find this is reliable. As I have of course considered Philip's method. Thank you Philip for your reply. I did that on my Welshpool "Earl" but on this loco I'm trying to make everything disassembleable. Not least because I've got to paint it still! Soldering on would not bear much reversal I imagine before the crankpin in the plastic wheel became out of adjustment...or would it? On the Earl I think I tinned it before assembly and was thus able to use 70 deg solder. (Actually the crankpin didn't go into a plastic wheel but some issues were similar).
Attachments
20171119_110311.jpg

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Mon Mar 04, 2019 10:59 am

Julian.
Your shimming idea sounds a good solution to the tightening angle issue. Using the Ultrascale bush gives a longer engagement on the crank pin screw, which helps overcome the screw being too short (if they are too long, I find they won't fit into my GW wheel press). There is still a tendency for one return crank to unscrew, depending on direction of travel, so I always use some type of low strength thread locking substance on final assembly. Usually a tiny drop of Loctite, but paint, nail varnish or non-hardening glue (I've heard of Copydex being used) would be just as effective.
Dave.

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Mon Mar 04, 2019 11:31 am

Julian,

In practice so little solder is used that removal is possible, it’s merely there as a retainer. I have used 270 nut locker (thick and very strong) in a similar fashion. On balance I prefer a smidgeon of solder.

I know it’s a good idea to plan for dismantling but I tend to arrange my chassis, home built as well as RTR conversions, in such a way as that they should not need any attention after final assembly. The wheels and rods are the last things to go on, and everything is painted in a base colour before assembly. I dislike getting an engine running nicely, only to have to break it all down again to get a paintbrush in.

Philip

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Tue Mar 05, 2019 2:27 pm

Philip.
I've always been nervous of soldering near to plastic centred wheels, especially Alan Gibson products, which appear to have a very low melting point. I know this from bitter experience, once with a return crank and once trying to add a packer on top of an axle box to correct ride height. I thought I was being quick with the iron but clearly not quick enough as distorted wheel centres resulted in both cases. I've seen several soldered bits on Ultrascale wheels, so perhaps they use a higher melting point grade of plastic?
Dave.

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Tue Mar 05, 2019 8:21 pm

Dave,

You could well be right about Ultrascale plastic, it’s harder for a start whereas Alan Gibson’s is a bit bendier. I will ask David Rogers sometime. I admit I have had to be quick with the iron, and I am always nervous! I do blacken every part of the crankpin where I don’t want solder to go, so the only place it sticks is around the return crank. I also use 145 solder. In practice when I want to unscrew it I usually give a sharp twist with pliers and the soldered joint breaks.

If the thread on the return crank is a good fit on the crankpin screw then I have used Loctite 270, a gloopy liquid which locks the crank very well. It’s a pig to remove though if it has gone off properly.

I do agree that perhaps I shouldn’t encourage people to run the risk of melting their wheels...

A Schools I rebuilt once (Jamieson) with Sharman drivers dropped a crank when it was charging around Retford in full public view. (At Expo EM in Watford that time years ago with the wonderful (?) catering). The crank had been soldered to the steel Sharman screw, and hadn’t taken properly. When I got home I used Araldite (the proper 24 hr stuff) to secure a longer brass crankpin bush to the screw and soldered the return crank to that. As far as I know it’s still there.

Philip

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Tue Mar 05, 2019 11:38 pm

Apologies for the non sequitur. No glue on this return crank! Also tender operational on bendy bumpy test track. Loctite 243 now applied.


Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Wed Mar 06, 2019 9:32 am

Very nice, Julian. Will look good at the head of a long train.

Philip

dal-t
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Re: Crab Comet conversion

Postby dal-t » Wed Mar 06, 2019 10:02 am

Is it just the video, or do I detect (marginally, in the initial slow running) the presence of the 'iron rule' that locos must always go better backwards than forwards? There seems to be a very, very slight hesitation when the crankpins reach tdc in forward gear, that isn't present in reverse ...?
David L-T

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Thu Mar 07, 2019 7:22 am

I owe some replies! Philip all my locos were made as you suggest, fixed together for good, until my 6th one in P4, a Barclay Tank from High Level. Chris' kit design had me making separable brake gear. Though not his intention, it was easily modified to hold the wheels in their hornblocks up in place in the frames rather than relying on pieces of wire soldered or threaded through tiny holes.

But this showed me in an easy way how to follow one of the many things I have learned from Allan Goodwillie's thread on locobuilding where wheelsets are all able to be dropped out of the frames. I had found one of the really hard things was setting up wheelsets in fixed holes in the chassis. (I had always had one fixed non compensated axle). To make a stand alone wheelset quartered and with correct BB with its hornblocks (and motor/gearset) is much more satisfactory even if it requires careful planning. Allan may well have the hornblocks of one of these wheelsets fixed in place without compensation as if in a fixed hole in the chassis, but they can always be removed.

Why might I want to remove it? Well I have realised for one thing that my BB gauge was inaccurate so I have some wheels slightly undergauge and another pair are minute tad over gauge. The latter, the front pair of my Class 782, caused it to derail in the hot weather at the Perth show last year. To remedy them will require unsoldering brake gear and removing whatever else keeps the wheels in situ with all the associated spoiling of the paintwork thus loads of repainting. Plus it is a voyage into the deep hidden (possibly irretrievable) recesses of my memory of how I put the darned thing together!

With this build I have been making it up as I go having departed from the instructions fairly early on in some aspects. I am needing to test it at every stage and several times I have had to disassemble everything. If I had painted the chassis as soon as it was made, by now it would be a sorry sight I think! Only when I'm sure it works properly on the layouts will I paint it; the dis/reassembly is quite straightforward.

Thank you and Dave for your contributions about the return crank. The whole discussion has really helped me as you can see from the video. (And just to elaborate, the return crank was screwed down just the right amount of tightness to hold its position plus be in the correct place, for about 30 minutes of testing. It was then glued)

Also thank you Dave for asking the question I had in mind, whether soldering the crank will melt the plastic. What occurs to me is, if the end of crankpin was tinned with 145 before being fixed into the wheel, maybe using the appropriate die to remove excess solder from the thread, the return crank could be solder locked down with 70deg. A little piece of wet sponge placed behind the wheel at the other end of the crankpin might act as a heatsink sufficiently to protect it perhaps?

Dal-t the loco needs thorough running in - and also lubrication and clean up of the pickups. But even so I expect there will be a slightly tight spot, I always find there is. It shows to me the need for the highest possible gearing for low speed control, while the top speed shown at the end is faster than I need for where I'm going to run this loco where a 30mph top speed will be all that's required. I've ordered from Chris the 120:1 worm and 1st stage gear to modify this from the current approx 90:1. I hadn't really noticed the difference between backwards and forwards. I wonder if that is to do with the motor being able to move slightly in the boiler, a torque reaction. I think I'll wait and see what cleaning up and modifying the gear ratio do before thinking any more about that issue if it was to bother me.

Dave Holt
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Re: Crab Comet conversion

Postby Dave Holt » Thu Mar 07, 2019 11:09 am

Julian,
For a brand new chassis, I thought your Crab ran very well in the video and I'm sure that any slight unevenness will quickly bed in with some running in and some lubrication. I doubt that any of my locos run any better at that stage (some probably don't ever run any better!).
I'm very happy that you've found some of my suggestions and comments helpful. I'm sure there are lots of equally effective ways of doing most things on a model but we tend to find methods that suit us well and, in my case, I tend to stick with them. But that's not to say my ways are in any way superior to others approaches.
I do think that one of the great attributes of forums such as this, is the facility to freely exchange ideas with the aim to making it easier to achieve satisfying results from our hobby.
Dave.

Philip Hall
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Re: Crab Comet conversion

Postby Philip Hall » Thu Mar 07, 2019 11:28 am

Julian,

Although my preference is not to dismantle an engine after building, you have reminded me that with all the RTR conversions I have done over the years that is precisely the way they are put together! RTR chassis invariably have slots in the chassis block, and these days bearings as well, and as you say it is much easier to assemble a wheelset away from the chassis and drop it in. For a hand built job I still usually don’t arrange for removeable axles, unless that axle is stuck behind cylinders which cannot be removed. I might just rethink that side of things for assembly reasons...

Where I will not change is that everything is painted in a weathered base colour before assembly. It’s easy to do with acrylics and a brush; no point in spraying as you’ll never see the difference in finish in amongst all the gubbins. When the wheels go into the chassis, by whatever means, and the pickups go on, that usually (hopefully!) is the first and only time. Once I’ve got the thing running nicely, as I have said, I don’t want to materially take it apart again. Maybe twenty years down the line, but hopefully not.

Regarding the overgauge wheelset, there might be a way to avoid the dismantling, by having a round block of brass with a 4mm hole in it. The block is clamped in the vice with the boss of the wheel resting on it. Using a light hammer and a pin punch, persuade the axle through the wheel a touch. If you hold the wheel carefully you will be able to feel if it moves. A straight punch should not change the quartering. If it doesn't work, you haven’t lost a lot of time, but it might. It does occur to me that this might take up any sideplay you have built in, so it might not be suitable for this case.

Good luck with the running in.

Philip

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Allan Goodwillie
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Re: Crab Comet conversion

Postby Allan Goodwillie » Sun Mar 10, 2019 8:12 pm

Hi Julian, :D

still following this thread and you have been given a lot of good and interesting advice, which I have enjoyed reading. Darn it! I missed seeing the loco running at the weekend in Linlithgow as so much of my time was taken up on the demonstration stand and I only managed to catch up on a few things right at the end of the day, so missed your crab running. Looking forward to seeing it however running on Calderside in due course, if it runs like any of your other locos it will be a winner indeed. :) The thought and care with which you are building your locos is exemplary.

I know you enjoy good slow speed running and tend to go for the slowest gear set you can manage, but I doubt if I would change your gear set. Too fine a thread can lead to other problems of wear especially if you are wanting to be able to pull heavy/long trains. I have recently rebuilt the WD 2-8-0 which I had fitted experimentally with a 2:1 ratio gear in the tender driving a 40:1 in the loco, both Exactoscale, this was after I had decided to avoid using an 80:1 in the loco itself as Chris had suggested that the gears would be liable to wear out quicker when my WD was pulling long trains on my 1in70 gradients. My intention was to put together a similar loco using a 1:60 gear of Chris's and see what might be the difference long term, however the engine after a few years and before I had enough time to build another was not performing up to the standard I normally like, even from the beginning the motor and gears in the tender were noisy, despite several attempts at quietening it.

I now have a quiet running loco with more of a top speed than I need, but it gives me the slow running I was after and plenty of power to pull long trains when necessary. :thumb

Allan

Julian Roberts
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Re: Crab Comet conversion

Postby Julian Roberts » Mon Mar 11, 2019 10:22 am

Philip Hall wrote:
Where I will not change is that everything is painted in a weathered base colour before assembly. It’s easy to do with acrylics and a brush; no point in spraying as you’ll never see the difference in finish in amongst all the gubbins.....


Thanks for your advice Philip. I've just quoted the bit here to ask - is it not essential to spray a primer onto the metal? - that's what I have always thought. I use Halfords car stuff. Also thank you for your advice re the overgauge wheel. When I get rountuit and adjust the wheel I'll post an exciting thread...but mostly the slight overgauge is less pressing than my desire to get on with the next project!

Allan thanks also for your most kind words...here is a photo from Saturday.

Screenshot_2019-03-11-09-28-57.png


Regarding the gearing, this loco won't ever be particularly taxed on Calderside or Kettlewell. Even with a 40' layout our coal trains are limited by the station loop length. Most of my other locos run on the High Level highest possible ratios without any problems but yes if running on Grayrigg with 70 wagons I see there is an issue. The ratios of Chris' boxes is determined between the three options simply by the worm and 1st stage gear, so it will be just a matter of swopping that gear and worm which I will describe here when I do it.

I was going to post this referring to previously mentioning you. Please accept my apology you now slip into the third person!

I mentioned Allan Goodwillie's loco building thread. His method is even more like the RTR approach, he makes a keeper plate to keep the wheels in the chassis. See viewtopic.php?f=20&t=993&hilit=Loco+building+for+Starters&start=100 - this takes you straight to the relevant page.
The brakes are part of the keeper plate and are sprung over the top hanger crossrods. Allan also describes his method of pickups (using gold!) which I will definitely try on a future build. A very thorough way of doing it, using a pair of screws to hold the keeper plate in place. I didn't feel up to designing all that into the Comet kit. His illustration is a scratchbuild so he has designed the keeper plate retaining nuts into it from the start. His designed-in ease of disassembly is the result of practical experience running locos for long periods on public display where he found ease of access for maintenence became essential.

In total contrast it was my enormous pleasure in Linlithgow on Saturday to test for the first time on an actual layout, as opposed to my 009 (N gauge) test track, a loco completed in 2007 and started in 2001 on Martyn Harrison's wonderfully evocative layout Castle Caerinion. I'm please to say it ran faultlessly the only problem being an incompatible coupling height. I will post a video sometime.

Later.....here it is
Attachments
20190309_120137.jpg
20190309_114835.jpg
20190309_114548.jpg
Last edited by Julian Roberts on Mon Mar 11, 2019 10:08 pm, edited 1 time in total.


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