Coupling rod clearance

[Tony Cleather]

Can someone please advise me what the coupling rod clearance should be for the following situation:

Compensated 0-6-0 chassis, drive on fixed rear axle, halved rods, Gibson wheels with 1.5mm OD bush on crankpins and wheels accurately quartered.

Given the above, what should be the minimum ID of the hole in the rods to achieve totally smooth low speed running?

[Philip Hall]

Tony,

Clearance should be the minimum you can get away with, but this depends on how good your workmanship is, how accurately the rods match the chassis (yes, even with jigs there can be a few thou in there) whether the crankpin throw is exactly the same on all the wheels (not necessarily a given!) and whether the crankpins are exactly at right angles to the wheel centres (see Chris Pendlenton'e recent writing in MRJ).

But I usually go for 1.6mm, which allows for some tiny errors, including the wheels going up and down in the hornguides, and does not show as a lot of slop in the rods. I have a broach with a mark at the right place and I open all the holes out to that mark as a starter. You could start with a touch less, and work up from there. I try to make sure that all the holes in the rods are identical, of course. If you need much more than that, something's probably wrong in the relationship between rods and frames.

Philip

[Chris Mitton]

Hi Tony

I'm no engineer, but.....the rods obviously need enough slop to allow for the movement in the suspension, which isn't difficult to work out using our old friend Pythagoras in three dimensions. So unless I've got something wrong....
Suppose the wheelbase is 8 feet - so in the ideal situation, wheels on dead level dead straight track, the axles and therefore coupling rod holes are 32.00 mm centre to centre.
In a worst case situation, bumpy tight curve, suppose one axle is at the top of its travel, 0.5 mm above datum, while the other is bottomed 0.5 mm below, and one axle is laterally displaced by maximum side play, say 0.7 mm. Then
wheelbase squared = 32.00 squared + (0.5+0.5)squared + 0.7 squared
(sorry the HTML code for superscript doesn't seem to work in this editor)
which my calculator says makes the wheelbase 32.023 mm (or whatever your own numbers come to). Therefore you need at least 0.023 mm of slop so the rods don't bind, plus whatever you need for a running clearance in "normal" operation. That's pretty small, so I'd therefore be tempted to drill the rods the same size as the pins, ie 1.5 mm (on the grounds I've actually got a drill that size!) and very carefully and slowly ream them out with the pointy end of a round Swiss file until they stop binding. Or drill 1.55 mm if you've got such an animal - or is that too much slop? I dare say the engineers on this forum will tell me I'm wrong!

Regards
Chris

[Philip Hall]

A drill will not produce a hole exactly to the size it says it does. I think it usually ends up a trifle oversize but in any event the drills that we usually buy are often slightly undersized. For example, I've just measured a 1.5mm (nominal) bit and it's actually 1.47mm. Which will probably give you that 1.5mm hole!

Too much slop is a bad thing, but 1.55mm is not too much at all. It will usually still work with lots of slop, but it looks awful with the rods floating around all over the place. One thing to bear in mind is that with overlapping rods on the centre crankpin you effectively double the amount of clearance. Properly jointed rods are better if you can arrange them, but I often don't bother when I know I should...

Philip

[Tony Cleather]

Gents, thank you for those comments.
As you might have surmised from my question, I thought I had a problem; from your answers, I now know I have! I jigged the chassis and am confident I got that set-up correct but as you suggested, I think the problem might lie with the crank pins not being vertical to the wheel centres. I suspected a problem in this area when fitting the pins in the first place as countersinking the back of Gibson wheels was not straightforward because the back surface is ridged just where the pin has to be fitted. I didn't think the consequences would be quite so severe!

Looks like I'll have to make new rods and re-prepare the wheels (unless there are any foolproof repairs which can be made to oversize coupling rod holes to bring them back to the clearances you recommend....?)

[Andy W]

Tony, the oversized holes can be filled in with solder, re-drilled with a small sized drill and opened up with a reamer.

[Tim V]

Have you used the Gibson crankpins? Older ones are known to be a loose fit on the screws, making variances in the effective crank throw.

Do not open the rods to take account of this, as every time you reassemble the rods, the bushes will take a different position.

One solution is to wrap the crankpin with PTFE tape. I am sure there are others.

[Paul Willis]

Have you used the Gibson crankpins? Older ones are known to be a loose fit on the screws

Hi Tim,

did you mean to say "...used the Gibson crankpin BUSHES?"?

I'm just trying to mentally construct what you described, and it's not working. Of course, the problem could be entirely at my end!

Cheers
Flymo

[Tim V]

Oops!

Yes I meant their bushes, are a loose fit on the 1mm screws.

I cannot vouch for the current crankpins from Gibson, I make my own these days.

[Tony Cleather]

Tim, yes I have used Gibson crankpins and bushes but I didn't get the impression they were a loose fit. I am more inclined to the view that my problem is caused by one or more pins not being normal to the wheel centre so that is where I shall go next.

Thanks to everyone for their help.

[Philip Hall]

Once or twice lately, I've had crankpin bushes from Alan Gibson which were not drilled concentrically, you can imagine the consequences of this! They do seem to be a tight fit on the pins these days, though. I just check that the bushes are OK by eye, and, as always, if you've got some duds Colin Seymour will exchange.

You say, Tim, that you make your own crankpins these days. Could you give a description of what they look like and how you make them please?

Philip

[Tim V]

I make them on a lathe, not really appropriate for this thread, however, I could put some details on a machine tool thread?

[Philip Hall]

Hi Tim

That would be very useful when you have the time. Sorry to drag you away from building things!

Philip