Minimum Radius for locomotive

[allanferguson]

I have a loco, an 0-4-4T (Caledonian Railway), and I'm trying to ensure it will go safely round my curves. The frames and wheels re already (about 20 years ago.....) erected, so I can't easily modify the sideplay. I've looked at the various notes about curves and 6 wheelers, but my brain is about bursting trying to apply the theory to my particular case. (I have read Derek Genzel's article in Scalefour Digest 40.2.1.) My problem is that the theory applies to the displacement of the centre axle in a symmetrical wheelbase, but I have a non symmetrical wheelbase, and one end is fixed, with the other two having sideplay

The wheelbase of the loco is 30mm between the drivers, and 48mm rear driver to bogie centre. Sideplay available is zero on the leading drivers, +/- 0.5mm on the rear drivers, and +/- 0.7mm on the bogie.

I can't cope with the maths, and I'd be grateful if anyone could tell me what the theoretical minimum radius for this loco should be. I am using gauge widening on the relevant curves (about 19.1mm).

I know that I should try it and see; I have done this and it seems to cope with 1000mm radius. But I do like to tie in the theory to the practice.

On an associated subject, does anyone have thoughts about side control springs on bogies? I have tried this on a 4-4-0 and it gave a much smoother motion through curves, but the strength of the spring is critical.

Allan F

[Russ Elliott]

It's a complicated bit of trigonometry involving the angle taken up by the bogie on a radius and an offset versine. The resulting equation would be stupidly complex, and I'm not sure it would give you a sensible answer anyway, because the limiting factors are dictated by the slop of wheelsets on track, so the back-to-back setting will have a significant effect on the calculation. Crudely, one can add half the wheelslop, say (19.1 - 17.7 - (2 x 0.4))/2 = 0.3mm to the mid-chassispoint versine (about 0.6mm, say, in your case; your asymmetry works in your favour), and if treating the overall setup as a '3-axle' one with a 78mm wheelbase, that gives a limiting radius in the region of 900mm. That limiting radius represents an interference fit of the wheels on the radius, so would not in itself provide a comfortable traverse. In practice, I think your bogie pin slop (which is critical) will tend to work in your favour - in effect, you have something akin to a bogie vehicle, so some factors are working in your favour and other factors work against it.

That's a long-winded way of saying if you've found empirically a 1000mm radius is ok, I don't think the sums are going to give you a better answer.

[Russ Elliott]

Btw, good question on bogie side-control springs, Allan: whilst some principles seem fairly clear (and there are other approaches using coils), it's probably true to say that we can't put any kind of figures on their 'desirable' strength. Coil springs occupy less real-estate, but the straight or bent beam approach is arguably more easily adjustable.

[MarkS]

This what I used for side control on the bogie of my O2 0-4-4t. (There is a slot in the bogie under the washer)
In reality it doesn't really do too much other than preventing the bogie from clunking the steps... It will go around a 3' curve easily enough...

P1030965.jpg

[Philip Hall]

I have put on side control springs in the past, but recent experience has been that if the bogie (of a 0-4-4 or 4-4-0) takes some of the loco weight, the action of the bogie rubbing on the bolster imparts a degree of control akin to a side control spring. This stops the loco wagging from side to side, and seems to work on the proprietary conversions I've done, although I don't think it will lead the loco into curves quite as well.

I'm currently in the middle of rebuilding a Finney M7 chassis to make it work in P4. It had been built rigid (!) with 00 width bogie, and when it came to anything remotely resembling a curve... and I will see how that behaves with or without the springs. I always found that the spring pressure was critical, because if it's too fierce it stops the loco from going round the bend.

Philip

[HowardGWR]

This what I used for side control on the bogie of my O2 0-4-4t. (There is a slot in the bogie under the washer)
In reality it doesn't really do too much other than preventing the bogie from clunking the steps... It will go around a 3' curve easily enough...

P1030965.jpg

Mark, I have stared at this for minutes on end and I can't see any washer anywhere.

I can see you have an interesting axle keep system on the bogie and I can see that the axle is bushed in the slots (horns).

Could you point out what I am failing to understand please? Regards, Howard

[allanferguson]

My thanks to Russ for his response. I'm reassured that If he thinks it's difficult, then I haven't a hope!

I've been trying to attach a picture of my attempt at bogie side control springing; but the system won't upload my picture.... I don't know why. Is there any problem at the moment?

Allan F

[grovenor-2685]

Mark, I have stared at this for minutes on end and I can't see any washer anywhere.

I think its under the big screwhead in the middle of the bogie, not a very big washer, the two spring wires look to be soldered to it.
Regards
Keith

[MarkS]

Howard, as Kieth has observed correctly, there is a small washer, (to which I soldered the springs) barely larger than the large screw head.
The axle bushes are glued into the compensating beams, which are in turn sprung using transverse springs, which in fact is the original Wills system, from nearly 50 years ago...
...but you cannot see it in the picture!