Clearances on Locomotives

[tenor]

I am in the early stages of building a 5" gauge narrow gauge Kerr Stuart 0-6-2 Baretto class to my own design.  It should be a fairly chunky beast with roughly 6" diameter wheels, 6" boiler barrel and about 4' long overall.  I have been a road steam nut these last 35 years, so rail mounted stuff is a bit of a mystery again.

Can anybody help with clearances in axleboxes for going round corners - one assumes there should be some axial play of wheelset in axleboxes?  Axle centres are 140mm and I reckon + or - 1mm on two axles only will get me round a 6.5m radius curve (In theory!!!)

Should that clearance be on all axleboxes?  It would seem better to me to have the crank axle (rear most in this case) with minimal axial clearance to avoid having the connecting rods having to take up side to side movement by squinting around the little end.  Then give the remaining coupled axles end float.  Or is that rubbish?

If you have axial float of wheelset within axleboxes, should the coupling rod pins also have end float built in?  If not, there will be some big strains as the coupling rods attempt to take up the movement.

The crank and coupling rod pins will all be nominal 12mm diameter and I propose to ream the split brasses to 12mm H7.  How far down from nominal 12 mm will be needed for the pins to take up the inevitable inaccuracy in quartering, axlebox positioning, crank throw etc.  I still have nightmares over my Rob Roy for which I dutifully folllowed words and music with 1/4" silver steel pins running in 1/4" H7 reamed coupling rod bushes.  Did it go round?  Did it heck!

All thoughts gratefully received.

Martin

[chris vine]

Hi Martin, ignoring inaccuracies you still need a bit of clearance in coupling rod bushes to allow one wheel up and its neighbour down. For 5” gauge I would think about .2mm on diameter would be about right.
Others may say different!!
Chris

[suctionhose]

Hi Martin,

These are a pair of 5"g 0-6-2's I made in the 1990's (a couple of friends driving). Throughout the recent decade or more that I have strayed into road steam, the green one has remained my most loyal servant!



As 2ft gauge prototypes from the sugar industry in Queensland, they may be larger in the superstructure than your engine but mechanically they are similar.

To further prolong any useful answer to your question, I will mention that coal water is carried in the driving trucks and lengths of 65mm square steel billet are carried in the tanks, along with 30 odd kg of lead and steel added amongst the frames to locate the CoG and make it heavy (240kg on the drivers plus 10 kg on the back truck!).

The weight is borne on coil springs above plain bearing axle boxes oiled up the axle centre (with differing spring rates to provide 4 corners of support and vertical movement of the centre driver). My engine regularly pulls 2000kg trains on 1 in 60 with curves and can deliver all available power from the 2" cylinders to the track without slipping unless on rusty or damp rails. I run with 100psi boiler.

Now to your questions. No special endfloat is allowed for the axles. Running clearance - maybe 10 thou - sufficient to allow one side to rise or fall without the axleboxes binding. The engine will be steered on the track by the flanges. My centre wheels are flangeless. Therefore no special endfloat is needed in coupling rods etc just running clearances. Actually track aberrations over 300mm driving wheel base are not great. More so over the whole length of the engine...

My little ends make use of spherical plain bearings - commercially available - to relieve twisting of the crosshead due to rocking of the "spring mounted crankshaft". These are really good and used by several builder / runners here. Initial diametral clearance on the 12mm crankpins I would suggest not more than 0.05mm. No point making everything too tight - you only have to ease it out later - but neat fits are useful for final alignments, tweaking straightness of rods etc

The only thing I'm unhappy with is the tendency for my engine to rock when pulling hard at slow rpm's. Limiting the vertical travel of the driving boxes would stop this and I may well pack some polyurethane under them to see if that helps. All other boxes are free to follow the track on their springs.

The trailing truck has significant overhang and must accommodate +/- vertical travel associated with traversing vertical curves. Worth mentioning also that there is a tendency when pushing in reverse for the drawbar to jack knife throwing the back end to one side. Not pronounced on my engine due to having a proper swing link bogie but the Black engine does not have such constraint and the effect is noticeable.

Options to avoid such a situation are either adopting a version of prototypical side control for the truck or attaching the draw bar closer in towards the driving axle (ideally at the bissell point). The latter is useful too for pulling around tight curves. Otherwise considerable side load is applied to the front flanges.

You have my email. Always happy to hear what you're up to!

[tenor]

Thanks,

Chris, you are brave for naming a figure. In real money that's about 8thou on diameter (I can't get my head around clearances in metric - I know how rattly 8 thou feels, but I don't instinctively know what 0.2mm feels like).

Ross, They are a couple of beefy engines - 2' gauge in 7.25" gauge, roughly 1/3 then. My smaller beast will be 2'6" gauge on 5" gauge, so 1/6 scale. 0.05mm (2 thou) clearance sounds pretty much like toolmaking to me, but knowing your standards I am not completely surprised. Thanks very much for the honest assessment of the going round corners problem - non existent. I see the potential problems on the rear truck - the Baretto just has a radial truck pivoted under the ashpan, suspension on coil springs and no side control. The spherical bearings sound like a good idea if I can work them in.

The project is going to be a trip down memory lane as I restored Superior from a pretty poor state over 50 years ago. I have been fortunate to get a Kerr Stuart piping drawing from the Hunslet archive and I have some very fuzzy B & W photos of various componentry I took 50 years ago. That being so, the intention is to make it close to scale with all the blobs and gadgets. Heres a good photo:
live.staticflickr.com/2169/5696517919_68f23a5272_b.jpg
I helped build the shed as well! Appy Daze.

Martin

[ettingtonliam]

I thought the normal practice was to keep the axles a reasonable running fit in the axle boxes, but to radius the flanges of the axle boxes where they fit to the horn blocks. This allows for the axle boxes twisting as one side goes up and the other goes down.

[suctionhose]

Martin, my 0-6-2s pictured are 5" gauge. 1/5 scale. Wheels 160mm at 180 centres. Cylinders 2" X 3.5ish.
Boiler 7.5" I think.
Ross

[chris vine]

Hi Martin, I was aiming for a suggestion of around 10 thou on the diameter.
Bongo in 7.25” gauge has 15 thou on coupling rod bushes.
In truth I don’t think it matters too much but don’t go all tool room on them!
From memory I think I made the coupling rod bushes on the driving crank to be a running fit, but again I don’t think it matters too much…
Chris

[oldnorton]

Hello Martin

I will repeat some advice I received when I started that has proven all too true in my attempts to make engines work - 'don't make all the fits to engineering tolerances or it simply won't go round!"

The axle boxes should not have any side to side play in the horns (well, it will be a couple of thou in reality), but, as said above, the retaining flanges need a large radius relief so that the axle set can rise on one side only. The wheel and axle sets should have an ability to move side to side in each pair of axle boxes. The backs of the wheels might have a washer on the axle each side to bear on the axle boxes, or the boxes might have machined shoulders. Don Young specifies about 0.030" (IIRC) on his Black Five. This is so that the three coupled axles can adjust their position when on a track radius. You are going to have to measure and get this float correct when you first try the axles and boxes in the horns.

When we build our engines we have to behave like old-time fitters. See what goes together and adjust all the fits to make it as tight as we can, but loose enough to allow adequate movement in use. When you put the axle sets in the axle boxes for the first time you may have to ease the horn faces to allow easy movement. Then when you add the coupling rods the play in coupling bushes can be adjusted until it all rotates freely. Then you check for each axle being displaced from horizontal and returning freely, and the whole lot still being free to rotate. All this being checked before the springs are added. Free moving suspension is essential if the wheels are to grip the track properly, and free moving motion is essential for an ability to pull away smoothly.

Norm

[tenor]

Thanks Gents,
Apologies for delay in reply, but life is getting in the way of modelling at the moment.  Anyway thoughts as follows:

ettingtonliam
It's not the axlebox diametral clearance I am too worried about but the axial play.  I agree about easing the clearance to the horns so that an axle can tilt up one side and down on t'uther.

Ross,
They ARE big for 5" gauge (as the actress said .......!).  My proposed beast will be about 15% smaller on linear dimension so about 60% of weight of your jobs.  I certainly shan't be adding ballast weights as the object of this exercise is something more portable for this ageing body to manhandle.  For those that don't know me, the current steed is a 7" SCALE steam lorry which weighs in at 1100 kg empty.

Chris,
I knew the name when I saw you respond, but hadn't linked it to Bongo.  I think the 10 thou clearance sounds reasonable on a 12mm journal.  I may aim for a shade tighter on the basis it can be eased out.

Norm,
The 30thou axial clearance sounds readily achievable but that would mean the coupling rods could be bending by 0.060" on tight bends.  So do you put similar axial play on the coupling rod bushes / pins?

All,
I went back to the CAD model and realised that wheelsets can ride to one side or the other a fair bit within the limits of 5" gauge wheel standards and a nominal 5" track, and that increases if gauge is widened a little on tight bends (as it should be).  I also realised that the axle tilt case that ettingtonliam cites (and Ross with regard to little end bearings) would dictate a minimum axial clearance, plus a minimum small end clearance (if the con rod similarly tilts) and how much needs to shaved off axleboxes to allow that tilt.  So I need to do a bit more drawing.

I realise some of this might be seen as a bit OCD, but I feel with all the CAD power at our disposal we should be able to do rather better than Evans, Young, LBSC et al. who all left rather too much to the hapless builder.  Anyway, I am a designer by profession so that is what I do.

Many thanks for the input, I hope to report back in due course,
Martin

[ettingtonliam]

After WW1, several armaments manufacturers, including Armstrong Whitworth and Woolwich Arsenal took up locomotive building in order to keep their works occupied. They got some substantial orders, and built their locos to their usual high standards. Its been related several times that these locos performed much better once they had been taken into the railway company works and given a good 'freeing up'