Valve Gears - please explain to a Numpty

Beckford · 21 Mai 2016

LMS2968 a spus: ↑

From 'Under 1o CMEs Vol 2', page 39, E.A. Langridge (2011) Oakwood Press, Usk ISBN 978 0 85361 716 7:

One day when he was looking round the development office with Coleman, Ivatt said, 'Those Swindon boys are always telling me that our class '5' would be a good engine if it had Stephenson gear.' By the next time he came we had drawn out a gear with two return cranks. It required surprisingly little modification to castings used for the Walschaerts gear. Ivatt was amused and said nothing, but a week or so later asked Coleman for the drawing. He must have got other opinions and then decided to fit one of the current batch, No. 4767 of Lot 199. It will be noted that the eccentric rods and the centre line of the expansion link, in mid-gear, also the suspension link bottom centre, all lie on the centre line of the outside cylinders, and that the valve rod works at an incline to the piston valve, which, as with the Walschaert's engines, is parallel to the cylinder centre line. Of course we gave it the 1/8 in. negative lead to which some of the DO strongly objected - not having had anything to do with long travel Stephenson gear. Thus the arrangement was a deliberate copy of the Swindon standard Stephenson gear layout, but outside the wheels and not inside. In this way it was a gentle reply to Swindon's remarks to Ivatt, showing them how they should have built their 4-6-0s! Ivatt had a puckish sense of humour and delighted to chuckle over something that he had done like this; he felt he had scored a point over the rest. From what I heard the engine got a good name, some said it was better than the standard and even official opinion was not derogatory, and it is pleasant when something you feel you started has been preserved in working order.
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Brilliant! Thank you.

johnofwessex · 21 Mai 2016

Thank you everyone

Now to think of some more questions!

Jimc · 22 Mai 2016

There's a Don Ashton analysis of the (4)4767 Valve gear here. http://www.donashton.co.uk/html/44767.html Mr Ashton seems of the opinion that in the end it wasn't a direct copy of the Swindon layout.

RLinkinS · 22 Mai 2016

If you follow links on Don Ashton's website there are computer simulations of Walschaerts' and Stevenson's valve gear available. It is quite informative to watch the animations. Although Don is mainly concerned with the design of valve gear for models he, and others, have put a lot of effort into studying valve gears. I used the simulation programme created by Dr Allan Wallace and entered the dimensions for the Southern moguls. This gave an analysis of the valve events which matched that on the valve gear drawing so I am convinced it is an accurate model. I am also extremely impressed by the quality of the original design, especially as they probably worked out the valve events using a wooden model.

LMS2968 · 23 Mai 2016

Mr Ashton's suggestion that the basis for 4767's construction is unknown is not consistent with Eric Langridge's story as related above, and since Mr Langridge was a senior draughtsman at Derby DO at the time, there is no reason to doubt his words.

I certainly would not though challenge Mr Ashton's analysis of the gear as applied to the loco.

Jimc · 23 Mai 2016

LMS2968 a spus: ↑

Mr Ashton's suggestion that the basis for 4767's construction is unknown .
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I think he actually said that little appears to be known. It would seem the Langridge comment was unknown to him when he wrote that.

A lesson, perhaps, of the Thompson thread is that there is valuable information scattered about in all sorts of obscure and unlikely sources, and perhaps only if you gather a large body of reasonably well read people do you have a chance to capture them all. You then have the problem of disentangling entrenched opinions and misconceptions, original or modern, from the truth of the available information, but that's a better problem than not having the information at all.

K14 · 23 Mai 2016

A couple of Swindon Engineering Society lectures on the subject, given by the chap responsible for implementing the gear used on the 'Stars':—

No. 70 - Valve Motion Developments, W. H. Pearce - 03/04/1906

No. 73 - Stephenson versus Walschaert Valve Gears, W. H. Pearce - 18/12/1906

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Valve Gears - please explain to a Numpty

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Valve Gears - please explain to a Numpty

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