Waddling steam engines?

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rue_d_etropal
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Waddling steam engines?

Postby rue_d_etropal » Wed Feb 05, 2014 12:52 am

Rev Awdrey famously called his GWR 060 pannier tank engine 'Duck' because the model he had waddled, possibly due to a poorly built chassis, but real steam loco do waddle when they start running.
Anyway I noticed one of my new locos waddling when i tested it last week, and that was running on a Bachmann USRA 060 chassis with main rods from cylinders connected to rear wheel. As I have been looking for chassis for the locos I am building I notice that many 060s have the middle wheel connected to the cylinders and wondered why.
Obviously connecting rods need to be as short as possible, but which wheel is preferable, is the rear wheel better because there is maybe more weight on that axle, or the middle one because maybe it reduces waddle.
Checked out the Heywood book and the 060 locos all have the middle axle connected to the cylinders. Maybe this helps flexibility on sharp curves.
Many of these locos are also outside framed, but I am not even going to atrtempt that, but do want the feel of the loco to be right, so am at least trying to get connecting rods looking close to original, but probably still wrong for the purist.
With freelance locos we are the designer but still need to make things look plausible.
So which is better middle axle or rear axle?
Simon Dawson
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Postby csundstr » Wed Feb 05, 2014 6:04 am

From what I can determine, the choice of drive axle seems largely dependent on the size of the cylinders (specifically the length of the stroke) relative to the drive wheel diameter. The crosshead of the cylinder on an outside cylinder locomotive generally "overlaps" the first drive wheel in 0-6-0 or other six-coupled designs. If the cylinder stroke is short enough that the crosshead does not extend too far past the axle of the first drive wheel, or the drive wheel spacing is wider between the first two axles than the second two axles, or wider than the minimum in general, the drive can be to the middle axle without the connecting rod from the crosshead to the drive wheel forming an extreme angle (which would make the engine less efficient due to added forces and friction).

The Bachmann USRA prototype 0-6-0ST is a relatively typical early 20th century US heavy switcher or industrial locomotive that needed a lot of power in a small size, so the cylinders are relatively large compared to the driver diameters (28" stroke, thus an approximate crosshead length of about 54", with 51" drivers). The wheels were mounted as close together as possible to minimize the wheelbase for handling curves, thus the cylinder could not drive the middle axle without the connecting rod being at too severe an angle.

Note that they could have driven the centre axle, but only with a drive pin location that has almost no radius from the axle. This was sometimes done in passenger locomotives where the load was light and the wheel speeds were high so that the connecting rod weight could be minimized, but a greater radius on the drive pin location means higher torque and thus more tractive effort potential.

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Postby rue_d_etropal » Wed Feb 05, 2014 9:28 am

There is a lot of science involved, cylinder size is relevant but position of cylinders is also important, just changing that would change relative position to wheels.
Just looked at the WW1 Baldwins and the 460 uses middle axle, whereas the 2-6-2 uses the rear axle, same sized wheels, but cylindes in slightly different positions. I think both designs shared a lot of parts.
Anyway I wonder what effect this had on the ride, working off the central axle is possibly more balanced and being central should cause less initial wobble than connecting to the off centre rear axle.
Interestingly there were issues with the 460 Baldwin which is why it was replaced by the 262. That was more to do with the fixed rear axle not being flexible enough when in reverse. It was also a long way back, supporting the cab, which is probably why the centre wheel was the drive wheel.
Its also something to think about when designing freelance locos as we quite often do. I am sure there was much discussion in engineering circles at he time, not dis-similar to discussions on whether front or rear wheel drive is better in motor vehicles.
Simon Dawson
(Simon D.),
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges.
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Postby Brack » Wed Feb 05, 2014 11:38 pm

In heywood's locos (except the 4 coupled ones) the end axles were the flexible ones, so you'd get extra problems/wear trying to drive on those.

I think that the longer con rod was used on slower goods locos so presumably there is an advantage - perhaps reduced hammer blow if the angle of con rod/cylinders to the rails is shallower? In high speed locos they preferred to reduce the weight of all reciprocating masses (as they'd be moving much quicker), so shorter con rods help with that.

Driving onto the leading axle (or the rear axle in a loco frequently reversed) causes increased wear, this was why PC Dewhurst rebuilt the Colombian Kitson Meyers to have the cylinders at the outer ends of the bogies, rather than the inner end (previously to this the leading axle of the front engine unit was both leading axle fo the loco and the driven axle). Presumably these factors together account for most modern (1930s+) SG locos either driving on the middle (if 6 coupled) or 3rd axle (if 8 coupled), whereas this wasn't always the case as you go look backwards in time.

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Postby rue_d_etropal » Thu Feb 06, 2014 12:13 am

still not sure why the Baldwin 460 locos used middle axle, unless it was an attempt to combat some of the problems when these locos operated cab forward.
Also which arrangement waddles the most?
Simon Dawson
(Simon D.),
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges.
http://www.rue-d-etropal.com

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Postby jefran » Sat Feb 08, 2014 4:35 pm

My immediate reaction on seeing this was to cite cases like the GNR Atlantics and Churchward's Counties notoriously rough riding on account of their short fixed wheelbases and hefty piston thrusts from their outside cylinders, the County had 18 x 30 cylinders as the Saints did. I then reflected that Duck has inside cylinders and a relatively long wheelbase and poked about a bit further; it appears that Duck's creator (Awdry not Collett) had a 00 model pannier in the 1950s with an eccentric wheel which gave rise to a very pronounced waddle and that this model is the source of the name rather than the GWR original.
Outside cylinders are much further away from the longitudinal centre-line than those inside and thus exert a greater yawing moment, (tending to oscillate the loco about a vertical centre line), this is resisted by the friction of the wheels, or their flanges coming up against the rail, so the shorter the fixed wheelbase and the the greater the clearance between the rail and flange, the greater will be the yaw. The Heywood 6 coupled engines have no absolute fixed wheelbase and in consequence yaw quite significantly on straight track.
As to the 2-6-2 v 4-6-0 Baldwins, the Penrhyn railway found their 2-6-2s unable to cope with the mainline curves, and I suspect that their troubles with tube leakage may have been due to the frames being distorted on the sharp bends, whereas the 4-6-0s elsewhere worked quite satisfactorily, though riding badly in reverse.
Andrew Lamin


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