Fits, play and axleboxes

[Sub MandrelParticipant @submandrel]

Tempting to title this a “Play in Three Fits”

[Sub MandrelParticipant @submandrel]

I’m struggling to decide how much play should be in the fit of axles to axleboxes.

 

I made cast iron axleboxes that are a good but very free fit in the horns, almost but not quite rattly. I think that’s probably right.

 

Reamed to 7/16″ I found my dead to size 7/16″ bar was a ‘push fit’ and totally unsuitable for an axle, even in fixed bearings.

 

I turned down some of my 7/16″ bar, but I’m not happy with the finish – I suspect it is some very tough steel. Its ex-whistons then ex-proops bar of unknown composition.

 

The nearest silver steel in my stock is 5/8″, so instead of making a d-bit with a dubious finish I decided to bore the boxes out.

The unmachined 7/16″ rod at 0.437″ now runs in them, but binds the moment one of the axleboxes moves.

 

The 7/16″ turned down to by about 2 is a running fit in the axleboxes, and although it stiffens a little it copes with modest movemnt of the axleboxes.

 

I also have some very fine finished 11mm bar from an old printer. This is 0.433″diameter. Although this is only a teensy bit smaller, it seems to be quite aloose fit in the axleboxes. Although i woudl not worry about it binding, does such a loose fit mean possible issues with poor retention of lubrication etc?

 

LBSC suggested using a hand reamer, but working it at various angles to allow them to move without binding, and I can see this would create a bell-mouthed hole which isd a tighter fit in the middle.

 

This all seems very hit and miss; I suppose my real question is – how much play can I get away with?

 

Neil

[michael howarth 1Participant @michaelhowarth1]

Neil ….I am about to embark on a 2-6-2 loco build in a smaller scale (gauge 1) and I have similar concerns about the required fit of the axleboxes……although I am not at that stage yet. I have had advice ranging from free fitting to close fitting and I think that it will be a case of playing it by ear when I get there. One piece of design advice I have had and which LBSC also secifies on his 3 1/2″ Tich is the single flanged axlebox which allows the axlebox to tilt in the frames. He states that the purpose is to iron out the humps and hollows on uneven track and I suppose this would be much the same as a very loose fitting axlebox. I do wonder though, whether this would be appropriate to a 6 coupled loco…….Tich is an 0-4-0 . I think that I will keep this in mind as a fallback position if things don’t work to plan. Sorry I can’t be of more assistance.

 

Mick

[Sub MandrelParticipant @submandrel]

Interesting – as I’m using Tich horns, but I’ve made double flanged axleboxes…

 

I plan to use loctite for my wheels not a force fit, so if quatering or something else goes wrong I can heat up and try again so i’ll await any advice but I’m not going to ruin my wheels if i take the wrong direction.

 

Neil

[Clive HartlandParticipant @clivehartland94829]

Neil, you have double flanged axleboxes like my Evening Star, I had to relieve the flanges in an arc so that as the axle was lifted one side it stayed free to turn.

Hard to describe but a few minutes with a file relieved at both ends of the flanges solved the binding problem.

The axle lift is very small and not very much metal has to be removed.

 

Clive

[michael howarth 1Participant @michaelhowarth1]

Clive….was it necessary to relieve all of the axleboxes on your Evening Star? I have got the LBSC / Martin Evans book, Evening Star, in 3 1/2″, and I see that it says the axleboxes should slide freely without shake.

 

Mick

Edited By mick H on 11/01/2012 07:20:08

[Clive HartlandParticipant @clivehartland94829]

Hi Mick, yes, I had to relieve all the axle boxes as there are five axles and as they flex up and down all the cranks have an effect anyway.

I read one item about the cranks and connecting rods and this was that play in a full size loco was 60thou.

As the axles tilt the clearance gets less and I would think that the movement is quite small

So I took all this into account and allowed some extra clearance between the axle boxes.

I can press down on the chassi and rock it side to side and move along at the same time.

I have no binding of moving parts at all.

I did find a mistake on the drwgs. about the size of a link , I would have to look back at this to remember.

 

Clive

[michael howarth 1Participant @michaelhowarth1]

Neil……what have you decided to do about your axleboxes ?

 

Mick

[Sub MandrelParticipant @submandrel]

I’m goiong to try the 11mm bar and see what happens. the wheels are now complete and just need shouldered crankpins pressing in and i can quarter and fit them. Hopefully I can try rolling the bare chassis round the living room by tomorrow evening.

 

I will definitely use loctite so that I can disassemble the wheels and try other axles/axleboxes if I have to.

 

Neil

 

[Sub MandrelParticipant @submandrel]

OK, I now have a rolling chassis – but no connecting rods yet. I have used springs that are too weak as a temporary measure, they bottom out if I add the battery and motor. I used a 1/4″ flat bottom hole in the axlebox and a 1/4″ long 1/8″ diameter pin screwed into the top of the horns to locate the springs.

 

The chassis happily bounces up and down while rolling along , quite free but perhaps a little too loose for the expert. At least it did until I fitted hornkeeps which have bottomed the springs, but at least I can pick it up now.

 

Neil

 

[Mark BusParticipant @markbus97330]

There’s a reference to fits and play in the book ` A Beginners Guide to Model Steam Locomotives’ By Tim Coles. In my copy it’s on pages 109 – 110 in the coupling rod subsection. Hope that helps.

Mark

[Sub MandrelParticipant @submandrel]

Thanks Mark,

 

I just need to make the cranks and then some test connecting rods to see what happens. If it seems OK I’ll make some ‘nice’ rods.

 

Neil

[Sub MandrelParticipant @submandrel]

I feel like I’m getting somewhere now – don’t ask where the oilboxes are for the crankpins – I’m going to soft solder some on.

 

I have learned a hell of a lot making those connecting rods (and made a big pile of swarf and scrappers along the way). I tested the unprofiled blanks and they were too short – I made them 1/64″ longer by heating red hot and hitting with a hammer!

 

Neil

 

[DustyParticipant @dusty]

The bearing part of the axlebox should be a good running fit, the same as any other bearing. The problems arise when one wheel or wheelset moves in relationship to the others. If one wheel rises and there is no play in the lateral direction of the axle boxes that axle will bind, thats the reason why Clive has radiused the flanges on his axleboxes. This allows both axleboxes to move around an imaginary point somewhere along the axle. If there was 1.0mm clearance between the axlebox flanges and the horns it would not matter to much. The only thing it might affect is the timing of the valve gear, but this would be of short duration and should have no adverse effect on the running of the loco.

The next thing is the coupling rods, these need clearance in all the crankpin bushes, other than the driving crankpin. Why? because if the leading wheelset on the loco rises(or falls) this has the effect of lengthening the distance between the crankpins and will at best cause erratic running and at worst cause the loco to jamb up. My current loco which has 6 coupled wheels (2-6-2) has .012″ clearance in the leading and trailing rods. It is easier to make it sloppy to start with than have to take the whole thing apart and loosen it up afterwards. Remember Gents its a Loco you are building, not a clock. It runs round a bumpy track. Unless of course it is a glass case model you are building, in which case the above does not apply.

[Anonymous]

Hi Neil,

 

Looking good!

 

Funny thing, but I made a coupling rod yesterday afternoon as a demo part for somebody who was visiting to see my CNC mill before buying his own. I think the thing that stunned him the most was running the (carbide) cutter at 4000rpm in steel.

 

Regards,

 

Andrew

[Sub MandrelParticipant @submandrel]

Grrr!!! My lengthy reply just evaporated!

 

In short, I made the connecting rod bushes 1/64″ oversize (0.016″) and all other fits are ‘generous’.

 

Neil

[Sub MandrelParticipant @submandrel]

Well, I’ve just had a trial run – assembled the gears into the frames, balanced a 7 AH battery sideways so that, even with light springs, it sits level. Brought it into the living room and told my wife “watch carefully – even I haven’t seen this go yet!”

 

Connected up with croc clips, and it shoots across the carpet to collide with the fireplace. A good sign, even jammed up between setee and fireplace the wheels kept turning, so there must be a fair bit of torque – I darn’t try grabbing a wheel with the rods whirling around.

 

I want to remake the stretchers and bearings for the gears and motor mount, as they are rough and ready and could be to tighter tolerances, but even so i was surprised that all the noise was the gears whirring (loudly) not teh rods and axles rattling! Yet I think I’ve got enough suspension travel for a 7 1/4″ Pacific, not a 3″ 0-4-0 according to Keith Wilson’s figures!

 

So great fun. Speed control much needed though. If it’s to high geared I can replace a 28-tooth gear with a 43-tooth one, which should tame it.

 

Neil

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