P4 steam loco driving wheels centre outside boss

Julian Roberts
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P4 steam loco driving wheels centre outside boss

Postby Julian Roberts » Sun Oct 06, 2019 1:32 pm

davebradwell wrote:The wheel seat is 2.5 mm long at most and less if you file off the protruding front boss, as I do, on outside cylinder engines to allow the prototype valve gear alignment.


I've just assembled a wheelset from Alan Gibson (aka Colin Seymour). Each of the outside bosses were 0.6mm (or just over) thick, and with the correct Back to Back setting the axle didn't go right through. Having gone to the trouble of attaching the crankpins as well I'm not going to go back to square one, and will make do with this.

But I'm curious as to how we should know whether the wheel we get from whatever source needs the fettling Dave mentions above. Should the boss be filed (or sanded) flush with the wheel rim before crankpins are inserted? If so, why aren't they made like this in the first place? Dave infers this is the case with outside cylinder locos only. If the answer is to refer to the General Arrangement, is there any suggestion as to how to proceed if I don't have a GA? Should all the driving wheels of a particular loco be so treated?

Filing the boss flush will give quite a useful extra depth behind the crosshead on future builds if that is the correct procedure.
Last edited by Julian Roberts on Mon Oct 07, 2019 7:03 am, edited 1 time in total.

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steamraiser
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Re: P4 wheels centre outside boss

Postby steamraiser » Sun Oct 06, 2019 2:26 pm

IIRC Colin only produces a single length axle for inside framed locos.

I have had a few axles that appear slightly short when fitted to wheels with thick bosses.
It is not a problem, just fill the indentation with filler before painting.

Gordon A

Julian Roberts
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Re: P4 wheels centre outside boss

Postby Julian Roberts » Sun Oct 06, 2019 6:40 pm

Hi Gordon - yes, sure, and I've got no problem with filling up the slight indentation. My question is whether the boss should be this extra thickness. Perhaps it varies from prototype to prototype.

PhilipT
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Re: P4 wheels centre outside boss

Postby PhilipT » Sun Oct 06, 2019 6:45 pm

Julian Roberts wrote:Perhaps it varies from prototype to prototype.

It does - compare, say, an LMS Duchess with an LNER A1, the former is much more pronounced than the latter.

Philip Hall
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Re: P4 wheels centre outside boss

Postby Philip Hall » Sun Oct 06, 2019 7:52 pm

As Gordon says, sometimes AG axles are a little short, depending upon prototype. In those cases, I cut new ones from 1/8" or 3mm stock I keep handy. I prefer to have the axle flush with the boss (unless it's one of those engines where the axle actually protrudes slightly). I think this gives a tiny bit of extra grip although this is probably being a bit bonkers on my part. I have found on occasion that if an axle, being too short, is not inserted all the way into a wheel the wheel is more likely to wobble, but this has been in extreme cases.

Philip

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Horsetan
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Re: P4 wheels centre outside boss

Postby Horsetan » Sun Oct 06, 2019 10:25 pm

If it helps, I have occasionally ordered alternative axles from Ultrascale, who can turn them out relatively quickly, where an indented or bored face is required. All they ask is for the measurement over the wheel boss face, a matter of setting a wheel on either side of a btb gauge.
That would be an ecumenical matter.

Julian Roberts
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Re: P4 steam loco driving wheels centre outside boss

Postby Julian Roberts » Mon Oct 07, 2019 7:12 am

Thanks for your replies, folks. I'm getting the message that you take the thickness of the boss as correct, and you get or make a suitable length axle. Not the inverse, taking the axle as the given, and filing or sanding down the boss to the length of that axle. Nor is anyone so far going with Dave and filing the boss down to be flush with the wheel rim.

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John Bateson
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Re: P4 steam loco driving wheels centre outside boss

Postby John Bateson » Mon Oct 07, 2019 8:19 am

Philip,
If my memory serves me correctly, a conversation I had with Alan Gibson well before Colin took over said quite clearly that the axle length would vary with the wheel - those with a boss being longer.
John
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Dave Franks
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Re: P4 steam loco driving wheels centre outside boss

Postby Dave Franks » Mon Oct 07, 2019 8:23 am

Yes Gibson axles can sometimes be short and in the past I asked for P4 axles for EM wheelsets and turned them down slightly, it's compounded now by the fact my EM wheels are set to 16.65 B2B so axles are always short. Ultrascale do lengths of ground steel axle material and I just chop to suit whatever wheels I'm doing EM or P4.
Just finished a lovely wee P4 loco for someone yesterday so piccies soon.

Dave Franks.

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Re: P4 steam loco driving wheels centre outside boss

Postby grovenor-2685 » Mon Oct 07, 2019 8:59 am

Julian Roberts wrote:Thanks for your replies, folks. I'm getting the message that you take the thickness of the boss as correct, and you get or make a suitable length axle. Not the inverse, taking the axle as the given, and filing or sanding down the boss to the length of that axle. Nor is anyone so far going with Dave and filing the boss down to be flush with the wheel rim.

It depends on the prototype, some have flat face wheels, some have the boss protruding. Whe the wheel you need is not available as a flat face you may need to remove the boss. For example the LMS Garratt where the closest wheel is the 4F wheel but does need the boss removing to give a flat face.
Rgds

davebradwell
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Re: P4 steam loco driving wheels centre outside boss

Postby davebradwell » Mon Oct 07, 2019 11:53 am

Might I suggest a different approach, Julian and do a sketch, not to scale, with the dimensions on. Do a section, or plan, of one side - put in the centreline and the tyre outer face with a dimension (10.85ish). Work back the other way with cylinder centre distance from CL, width of crosshead on wheel side, thickness of rods, spacing washers, etc and do a simple sum. Chances are you'll be glad of the space removing the wheel boss gives especially when you subtract an allowance for axle side play and clearance. No point in building anything without this sort of check first. If you start adding expansion link and motion you'll wish your drg was to scale - I've some A4 pads with 5mm squares for doing such schemes freehand or use CAD.

DaveB

Philip Hall
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Re: P4 steam loco driving wheels centre outside boss

Postby Philip Hall » Mon Oct 07, 2019 4:31 pm

One of the problems with outside cylinder engines in P4 is there really isn’t much clearance at the front end. Even if the prototype had a pronounced boss, I would not be averse to filing it flush with the tyre face to gain a few thou extra. It doesn’t help that driving wheels are commonly 2mm wide, the top end of the P4 limit, rather than 1.85mm which is (I think) more correct. But usually there are ways around these things.

Even an inside cylinder 4-4-0 is not immune to these oddities. For the Hornby SR T9, for example, I reduce the boss on the Ultrascale wheel by about 0.5mm so as to have some chance of getting the crank pins inside the coupling rod splashers without having to rebuild them out of shim!

Philip

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Re: P4 steam loco driving wheels centre outside boss

Postby Julian Roberts » Mon Oct 07, 2019 6:32 pm

More thanks to all. I'm getting my head round this now. Up till this escapade, I'd taken manufactured wheels on trust, but now I see that they have to be treated with the same scepticism as everything else and may need to be adjusted to suit the purpose. For correct appearance, as Keith says, as the nearest wheel may be for a different prototype. For practicality, as Dave says, for behind the slidebars, with perhaps some cheating so the front wheel may be flat at the boss even if not quite true to life, to give that extra half mm or so. (I didn't do any fettling with the Crab front wheel and it worked with just enough room, but if the wheels had had the same thickness boss as these replacements for the centre ones, I don't think it would have done. Probably in future I'll always slim the boss to the minimum on the front wheel, might not bother to draw it out!). And as many of you have said, to regard the provided axle as likely to need to be adjusted or replaced, having measured over the outside of the bosses (after having fettled them if required) with B2B gauge between the wheels. Your point about the 4-4-0 Philip is helpful too as I've got replacement wheels for my Compound (being dissatisfied with the quartering of the present set up), where the same issue may apply.

Thanks too for the idea where to get the plain rod to cut to length. For what it's worth John, the axles were correct for the original wheels for this Crab, but with the replacement centre wheels the axle was short, and bosses appreciably thicker.

Cheers everyone, thanks again.

Dave Holt
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Re: P4 steam loco driving wheels centre outside boss

Postby Dave Holt » Mon Oct 07, 2019 7:53 pm

An interesting topic which has prompted quite a range of approaches. I generally use Gibson wheels on my locos and, until very recently, have never reduced the outside boss, despite a few wheel sets where the axle doesn't quite come right to the outside of the wheel boss, as already mentioned. So far, this has resulted only in a cosmetic error rather than a functional problem.
That said, I always measure the width over bosses by holding a pair of wheels against my back-to-back gauge, as suggested by Horsetan, and then produce sketches and calculations of rods, spacing washers, etc., per Dave Bradwell, to check clearances to cylinder centres. This enables adjustments to be made before committing to metal.
I find it a bit odd when people shy away from setting things down on paper and then find that parts of the rods or valve gear go clunk. Surely, it's so much easier to make any necessary alterations before assembly than having to try to disassemble delicate parts in order to modify them? Each to his (her) own.
Experience with a friend's Brassmasters Royal Scot - clearances inside splashers and connecting rods inside expansion links - caused me to get the wheels for mine reduced to 1.8 mm rims and the centre bosses reduced to just protruding beyond the rim face, in order to give a bit more clearance.
BR Standards, some LNER and Bulleid pacifics had flat fronted wheels, but even they had bosses slightly proud of the rim, as far as I know. Might be wrong about this.
Dave.

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Re: P4 steam loco driving wheels centre outside boss

Postby Philip Hall » Tue Oct 08, 2019 9:22 am

Julian,

The Ultrascale wheel for the T9 is a true representation, and the boss on that engine is very pronounced indeed. The combination of a true scale wheel centre and a plastic moulding for the footplate produced the clearance issue, hence my reducing the boss. The LMS compound may not have this issue.

Dave H,

Am very interested you managed to have tyres reduced to 1.8mm. Gibson wheels I have had over the years have generally had a very thin front face to the tyre, seemingly not enough to have anything turned or filed off. Maybe the ones you had had a shallower recess in them?

Philip

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Re: P4 steam loco driving wheels centre outside boss

Postby Dave Holt » Tue Oct 08, 2019 4:17 pm

Philip.
I don't know about any changes to Gibson wheel rims. The wheels in question (two different types - original Scot and Stanier types) are probably some years old. They appeared to have sufficient thickness in the lip outside the plastic centre to take a 0.2 mm reduction and, indeed, the machining has not resulted in any of the lip breaking away.
Dave.

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Re: P4 steam loco driving wheels centre outside boss

Postby Philip Hall » Tue Oct 08, 2019 7:22 pm

Thanks Dave, useful to know when something tight next comes along (like a King Arthur or S15).

Philip

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Re: P4 steam loco driving wheels centre outside boss

Postby davebradwell » Wed Oct 09, 2019 8:29 am

I dug out sample LNER Pacific wheel drgs, Dave, and was surprised to find the face perfectly flush with the tyre - I expected a tiny boss like you suggest. Perhaps on lesser classes? I noticed that the crankpins have a 1/2" thick base which compares to our 0.3 flange or washer here so with thicker tyre and crankpin we've already lost at least 0.3. Add 0.5 if you've left the boss on the wheel and you're 0.8 down before you even get to the rods. Then there's the mystery where 2 laminations of 0.45 give 1mm or greater thickness and you're certainly doomed, especially with one of my 4 layer big ends.

Actually, any loco with a leading front bogie is less of a problem as the front crankpin doesn't pass behind the crosshead. There may still be the valve gear, though.

The down side to paring down the clearances is the coupling rod can shave the paint off the tyre faces on tight curves, although filing the rear of the rods can help.

DaveB

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Re: P4 steam loco driving wheels centre outside boss

Postby Dave Holt » Wed Oct 09, 2019 3:01 pm

Dave,
The Bulleid BFB wheels are nominally flat on the front face but the boss protrudes 3/16" beyond the face of the tyre - effectively flush at our scale. The boss local to the crank pin is a further 3/16 beyond that, described as a "wear face".
Dave.

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Re: P4 steam loco driving wheels centre outside boss

Postby decauville1126 » Wed Oct 09, 2019 7:35 pm

Unread postby Dave Franks » Mon Oct 07, 2019 8:23 am

Ultrascale do lengths of ground steel axle material and I just chop to suit whatever wheels I'm doing EM or P4.

Dave Franks.



En1a leaded ground stock. Machines like a dream! If anyone has a source without buying-in huge quantities then please share here! But well worth tacking onto an Ultrascale order if your lead (pun not intended) time permits.

Julian Roberts
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Re: P4 wheels centre outside boss

Postby Julian Roberts » Fri Oct 11, 2019 1:39 pm

Philip Hall wrote: I have found on occasion that if an axle, being too short, is not inserted all the way into a wheel the wheel is more likely to wobble, but this has been in extreme cases.

Philip


Touche Philip (don't know how to do an " E acute" accent!). The pair of wheels has a wobble with max BB of about 17.85 and minimum about 17.65. When I was pushing the pair together on the GW jig they slid together surprisingly easily. Maybe because I was using Araldite having filed a small groove all the way round near the ends. The BB gauge was difficult to remove and I found the B2B was around 17.25. So I tapped the axle end (wheel supported on the vice), aiming for max 17.75, and I thought I'd got that. Not for the first time mysteriously the reading seems different now that they are fully cured. Anyway I'm going to go with them and if I get problems I'll know the cause.

Dave Holt wrote:I find it a bit odd when people shy away from setting things down on paper and then find that parts of the rods or valve gear go clunk. Surely, it's so much easier to make any necessary alterations before assembly than having to try to disassemble delicate parts in order to modify them? Each to his (her) own.
Dave.


I'm sure you're right Dave but I find the idea too daunting. I do wonder whether I'd have foreseen all the fouling issues I have had with the Crab if I'd drawn it. Today reassembling the chassis with the new centre pair of wheels I found I had some new fouling with just connecting and coupling rods attached, arising from the thicker bosses. The front coupling rod was fouling the bottom of the motion bracket, though only when the crank was at the forward position, on a curve such that the centre wheel was deflected outwards. The issue is that the motion bracket has a little too much "meat", though it may be prototypically correct. A few strokes of the file sorted it. I just don't think I'd be able to work out what these dimensions would be from looking at it on the flat in the fret. Most of the issues I had were like this, quite minor. I think I take the kit maker on at least some trust. There have not been many issues with the valve gear. It would be interesting to know what particular problems there are to look out for in future, maybe worth a different topic? - you have mentioned a couple like expansion link outside the connecting rod.

20191011_115614.jpg


Dave Franks wrote:Just finished a lovely wee P4 loco for someone yesterday so piccies soon.


Look forward to that Dave!

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Re: P4 steam loco driving wheels centre outside boss

Postby Dave Holt » Fri Oct 11, 2019 2:42 pm

Julian.
Good luck with the Crab rectification. I can't now remember which loco required the inside of the expansion link tail to be thinned to miss the con rod, but it definitely wasn't my Crab.
Going back to the question of wheel boss protrusion, I had assumed that there would be some consistency in locos from the same designer, e.g., Stanier LMS locos. However, not so. Black 5's and 8F's show the boss only 1/2" outside the rim, but Coronation pacifics appear to have about 2" to the face local to the crank pins and a further 1/2 round the axle. Quite a whopping difference which I suppose is related to the cylinder diameter/centre spacing.
The Gibson wheels on my current build (Black 5) certainly have bosses which stick out more than a scale 1/2", so I hope there are no clearance issues. I built one previously and had no problems in this respect, so fingers crossed.
Dave.

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Noel
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Re: P4 steam loco driving wheels centre outside boss

Postby Noel » Fri Oct 11, 2019 8:02 pm

Dave Holt wrote:I had assumed that there would be some consistency in locos from the same designer, e.g., Stanier LMS locos. However, not so. Black 5's and 8F's show the boss only 1/2" outside the rim, but Coronation pacifics appear to have about 2" to the face local to the crank pins and a further 1/2 round the axle. Quite a whopping difference which I suppose is related to the cylinder diameter/centre spacing.


In the case of the 8Fs it may have been a case of not reinventing the wheel, as the Fowler 7Fs had wheels of the same diameter and, I think, spoke pattern. Dunno about the other two, though.
Regards
Noel

Dave Holt
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Re: P4 steam loco driving wheels centre outside boss

Postby Dave Holt » Sat Oct 12, 2019 11:55 pm

Noel,
From the loco drawings, it looks like the only things in common between the Austin 7 wheels and the Stanier 8F is the tread diameter. Without actually measuring it, the 7F wheel has a boss protrusion of at least 2" vs the 1/2" on the Stanier (bevel rim and different crank throw, also).
Dave.

davebradwell
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Re: P4 steam loco driving wheels centre outside boss

Postby davebradwell » Sun Oct 13, 2019 9:04 am

Yes, certainly on classes of my acquaintance, mainly ex-NE, wheels for inside and outside cyl engines were quite different. The crank webs inside the frames displaced the axleboxes outwards as locos grew larger and the backs of the wheels were usually recessed to accommodate them. This in turn pushed the wheel boss further outwards to give the shape we see. The GER seems to have been the star at this and a B12 GA shows the boss protruding by almost the tyre width. Stroke was less, too, presumably in an attempt to reduce out of balance forces. By contrast, with cylinders outside, there's usually no space for an extended boss on front so this is at the rear of the wheel.

DaveB


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