axlebox clearances

Hi, found some previous discussion on this subject but need to clarify for a current build - what is recommended clearance between axlebox and horn cheeks? i understand that it needs to be a 'nice' sliding fit, too much clearance and it would result in problems with rods binding or jerky motion, but what is a 'nice' sliding fit? I work in metric but can convert (eg 10thuo = 0.254mm). currently have about 0.1mm which is about 4 thou but this seems too loose, and not sure what is considered acceptable.

Geoff,

I know little or nothing about 'axleboxes & horn-cheeks' as such, but I do know that you will be wasting your time trying to do "fitting" by numbers.

A 'nice' sliding fit is just that ... [the word 'nice' conveniently has two meanings, both of which apply here] ...It doesn't bind, and it doesn't slop.

Unless you have the most extraordinary machines at your disposal, this can only be achieved by hand work, and requires patience and sensitivity, as well as a modicum of skill.

... When it feels right, it probably will be right.

MichaelG.

.

Caveat: you may need to consider temperature effects.

You don't say what scale you are building. I am new to this also, am building in 3 1/2" at the moment and just making horn blocks and axle boxes from scratch. Looking at YouTube videos and reading it seems that you make them a running fit then create chamfers so that the axle boxes allow the body of the train to tilt and twist. I am sure there experienced builders on here with a good description of what you need to achieve.

Howard

When was at school we went to the old museum of science and industry at all saints in Manchester ( not the new one). They were building a Newcomen engine as built in around 1750 we were told that the fit of the piston in the bore from the original texts was the 'thickness of a shilling', around 2mm I would guess. In the days before they had pretty much anything we would consider to be a lathe I suppose they thought that 2mm was a nice fit. It would go some way to explain the appallinglyly low efficiencies they got from early steam engines . Still when you have a mine to drain and option 2 is a horse a rope and a bucket I suppose they were well pleased. Just as an asside an old rule of thumb in the petrol engine world was 2 thou per inch clearance for watercooled engines and 4 thou for air cooled.

The difference between a ' tight fit' and a' nice fit' can be no more than the sweep of a file along the length of the axle box.

One more like that, and it will go from 'nice fit ' to 'loose fit', and you will know the difference.

The next sleepless night comes when all the axles have to be bored so that all axles are exactlly square across the frames.

Some people make them a near perfect fit, which is all well and good if you are capable of machining and aligning everyting perfectly first. Any inacuracy anywhere will result in the axle binding badly and the linkages will most likely bind as well. For this reason I opt for what most would describe as a sloppy fit although in reality it is only a few thou. It all depends on what you want from your loco - for display it wont matter but if you are planning running for 200 hours a year for the next twenty years it becomes a little bit more critical.

Hi Geoff,

As you are not building a miniature steam loco you can get away with larger clearances. If I can squeeze a 1.5 thou feeler gauge between the horns and axlebox slides I leave it at that ( so in effect 3/4 thou either side). If much more I shim the horns. However this is for miniature loco work where there is a considerable battering ram effect on the axleboxes as the reciprocal motion is transfered to rotating motion. This wont apply to your loco.

Cheers,

Julian

I'm reading this with great interest as I have it in mind to build a 7 1/2" 0-4-0 starting in about a year. Need to clear up the current mess. And I've never built a loco, nor worked on one yet. I'll get there.....

Anyway, I'm really surprised at the tight clearances stated! How tight are the clearances of full size locos? Even multiplying those clearances by 8 doesn't leave much room.

I have Kozo's 0-4-0 switcher book by the way and am slowly 'absorbing' it.

Please educate this pilgrim!

Thanks,

Pete

Edited By Peter Krogh on 21/08/2016 05:20:20

The sliding fits quoted are to all intents and purposes just up and down movement. This does not take into account the 'Tilt' of an axle, if one side of the wheel train is raised then binding will occur.

My feelings are that the axle bearing should have a degree of tilt by filing an arc on the inside of the axle bearing side guides. It should be easy to visualize this. If you have ever heard a steam train running slowly and the steam is turned off and the loco is coasting then it 'Clanks' and this is caused by the clearances in the cranks and beams.

It is not a watch you are making, so apply some degree of freedom of movement particularly in the bigger gauges.

.

Peter,

I did say as much, in my first response, but I will repeat: I know little or nothing about 'axleboxes & horn-chheks' as such. ... My own response was specific to the general definition of a 'nice' sliding fit; but at least it got the ball rolling.

Like you [albeit for different reasons] I am following the disciussion with interest, and hoping to learn.

MichaelG.

Thanks for all the responses, i should have described the loco which is a 5inch gauge rod-drive diesel similar to a British Rail 08 but different. I was concerned mainly about the 'fore-and-aft' movement of the axleboxes and how that relates to clearances for connecting rods so that i doesn't resemble the sound of a box of nails being shaken when it is running along the track. I appreciate the need for some chamfering of the lateral faces to accommodate tilting of the axles as the loco traverses uneven track, and i have accommodated that already.

I know that clearances for full size steam locos are incredibly small in relation to their size (a few thou each side if some websites are to be believed) but wasn't sure how this relates to our miniature versions.

I think my current clearances may probably be a little large. i have replaceable horn cheeks so i can shim them a bit to take up some of the clearance and see how it goes once the rods have been machined and fitted, which is the next task. I just want to minimise the amount of re-work that may be required; i've done enough of that already just to reach this stage.

This learning curve can be a bit steep at times but it's all good.

The biggest difference in length between axle centres will come when the loco 'leans back' under acceleration or form,wards under braking.

This angle, for a given axle spacing, will be greater for an 0-4-0 than an 0-6-0, for example.

The spacing will reduce in proportion to the cosine of the tilt.

Say the maximum tilt is 5 degrees then cos(5) is 0.996, so for every 1" of connecting rod the distance will reduce by 4 thou.

If the maximum tilt is 2.5 degrees the reduction is closer to 1 thou per inch.

Clearly if you have a 4-6-2 than the impact of any fore-aft tilt is going to be negligible. It's also going to be affected by how stiff your springing is.

Ironically a short wheelbase leads to more tilt and I found with Southam (4" between axles) that if nodded forwards or back it bound a lot and I needed to free the axleboxes up a fair bit, although it wasn't the 16 thou the first calculation would suggest it was more than 4 thou! I also had to allow for the jackshaft being fixed so any up and down movement of the front axlebox affects the distance between it and the front axle as well.

So my instinct is that the proportional changes will get smaller as the loco gets a longer wheelbase or the C of G gets higher but little else will affect it.

I understand Julian's comments to mean that in a no-steam loco with connecting rods this is less critical as motion won't affect valve events whereas excessive movement could be bad news for a steam loco.

Neil

I understand that on a large working Loco that the clearance on the beams was some 1/16".  Perhaps thats why they use very thick oil ?

Clive

Edited By Clive Hartland on 21/08/2016 12:06:00

I understood Geoff's query to be about the front and back tolerance between the horncheeks and axleboxes.

Others have commented on sideplay of same with the wheel assembly, the need to slightly curve the edges of the axleboxes against the edges of the horns to allow the wheelsets to tilt, and what tolerance to provide in coupling rod bearings.

I provide 1/32" sideplay each side, except the driving axle has this halved.

How much play you need in coupling rod bushes depends how well the loco is made and how well the wheels (or if outside frames the outer cranks) are quartered. You ought to be able to start with a 1 to 2 thou clearance on crankpins to coupling rod bushes especially on the driving axle, and a thou on any knuckle joints.

As Neil says, valve setting can be done far more accurately (or shall we say is much easier) if there is no slop especially on the driving axle assembly.

Depending on how good your track is, the tilt requirement on the axles may vary. I allow 1/8" to 3/16" either side (up and down) of the running height on each axle, with stronger springs on the front and driving coupled axles. The rear axle on an 0-6-0 loco I fit softer springs.

The above is for 5"g.

Most designs have far too much sideplay, and if ordinary coil springs are fitted great care to ensure they dont restrict the 'tilt' movement.

Cheers,

Julian

Thank you for the very good information. I was more concerned with the front to back clearance than the side to side, but I need to think about both.

Kozo, on his 7 1/2" 0-4-0 specifies .004" to .010" front to back for the driven axle box. I haven't looked at the coupling rod tolerances yet, nor have I examined the clearance for the front axle.

He is very specific about the tapers in the grooves for the axle boxes to allow for tilting.

This is very interesting, at least for a guy who has not spent any time thinking about the subject!!

Pete

I read in the book for the 3.5" Evening Star that it will go around a 20 foot radius track. working out the side clearance I made this to be 1/16" side movement on each axle.

The cranks all rotate and there is no binding at any point on my Loco. What i did find was that this slack could cause some fouling of the brake hangers though.

Clive