Valve Gears - please explain to a Numpty

Following on from the Thompson thread, I know what a valve gear does but I have some questions about how & why.

The simplest is clearly a fixed eccentric. Fit two & you get Stevensons Link Motion with the ability to reverse/change cut off.

Now to my simple mind it may be possible to do the same thing with only one eccentric and the expansion link, which the other big player, Walschaerts does in part - the return crank & expansion link is in effect 'Stevensons Lite' but what exactly is the role of all the components driven off the crosshead, how and why does all that lot work?

 

The combination lever in Walschaerts gear governs the length of travel of the valve spindle in relation to the travel of the piston rod.
Its travel is not pivoted at the top, but is a parallel motion, with less movement at the top than at the bottom, as the valve spindle will only move a few inches of travel in any one cycle of operation, whereas the piston may travel up to 4ft.
The top of the combination lever is in forward and reverse motion as governed by the radius rod, driven by the expansion link. When notching up, the die block is moved closer to the centre of the path traced by the expansion link, so its travel, and thus that of the combination lever, becomes less at the top.
But the combination lever still has to move at the bottom, otherwise it would just pivot about the valve spindle crosshead, and impart no movement to the valve.

That's how I see it anyway!

Richard.

 

If you want to find out more about valve gear take a look at the Kent and East Sussex Railway's online MIC. Public access.

http://www.kesr-operating.org.uk/9.html

 

As another simple soul: can someone explain why with Stephenson gear it is inadvisable to have short cut offs?

 

In Walschaerts gear the combination lever provides the Lap plus Lead component.

In theory there is no reason why a well designed Stephensons valve gear cannot be used at short cut offs. The GWR arrangement of Stephensons gear as used on Saints, Halls etc gave excellent valve events

However Lead is not constant on Stephensons, unlike Walschaerts, and there is also a limit to how big the eccentrics can be made without creating undue friction to provide large throws for long valve travel. The above GWR version got round this by using launch links and rocker arms.

Cheers,
Julian

 

According to the BR "Handbook for Railway Steam Locomotive Enginemen", published 1957, and subsequently reprinted by Ian Allan from 1977 onwards, the effect of "notching up" with Stephenson's valve gear is to give an increase in 'lead' which is advantageous at high speed. But as you approach mid-gear, the movement of the valves becomes very small. Unfortunately the 'Handbook' doesn't give any figures as to the optimum cut-offs.
Anyone interested in locomotive valve gear should try and get a copy of this book; it is very infomative

 

Thank you.

 

Thank you.

 

With a conventional Stephensons link gear the link is suspended and lifted up and down. Unlike Walschaerts, it is not pivoted. If you only had one eccentric, the link would just thrash around and not impart any movement to the valve rod. To get it to work as you suggest, you would have to fix the location of the link and when you do that, you are effectively creating the original Walschaerts valve gear, which did not have the combination lever. That came later.

The advantage of variable lead was not originally appreciated and constant lead was once thought to be the best arrangement, hence a preference for the likes of Walshaerts and Baker gears. Lead is a hindrance at starting so it is advantageous to have no lead in full gear. In early days, before the advantages of lead were better understood, Stephenson link gear was often arranged with crossed eccentric rods, which gives maximum lead in full gear and minimum lead in mid gear. An example of this can be found on Manning Wardle 1210 Sir Berkeley.

 

With 'modern' long travel valves, Walschaerts is much lighter and creates much less friction than a comparable Stephensons gear with loco links. With launch links with Stephensons, the gears are pretty much equal in performance to Walschaerts, but you have to be very careful to use the correct arrangement of the gear to avoid excessive suspension offset. The Maunsell 'Q' class suffered from this, and oddly Bulleid copied same on his Q1s.

Cheers,
Julian

 

Is this the point to introduce the unique Stephenson Link fitted Black 5, (4)4767, into the mix? As I understand it, she was always regarded as a free running and strong locomotive, yet remained a one-off. Why? Anybody know?

 

4767 was not a prototype as such; it was Ivatt's little joke over the GWR which used Stephenson's gear for all its two-cylinder locos, but inside the frames where access for preparation, etc. was limited, an issue during and post-WWII. It could not be fitted outside as this, it was said, would impinge on the loading gauge. Ivatt set out - and succeeded - to prove this wrong.

The performance of this loco has a certain controversial element. LMS and BR drivers always reckoned it was a very strong engine, about one coach better than a normal Black 'un. But it was tested at Rugby, where the results were lower than those of the standard engines. Strangely, her record as a strong engine has followed her into preservation, where she is still considered a cut above the rest.

 

Thanks for that - fascinating info there, which I hadn't heard before. Some joke, eh? Aesthetics is another matter, of course

 

It is interesting how often locos and types got reputations (good or bad) which could never be substantiated by real empirical evidence. I suppose first impressions are hard to shift as well - so locos got bad reputations before optimised and never managed to shift them. Others got reputations in comparison to other classes which weren't really comparable (i.e. preceding important developments such as saturated/superheated, different power class). It all suggests to me that a true dispassionate assessment of designs (and hence designers) is all but impossible.

 

I'm sure if the Mail ran an article about Walschaerts curing cancer, being banned by the EU or invented by immigrants, you'd never hear the end of it…



Simon

 

And the "non sequitur of the day award" goes to…………………………………………………

 

And the humourless grouch of the day award goes to…

 

Somewhere in the back of my mind is a tale about Ivatt coming back from a meeting at Swindon and telling his team that the GWR swore by Stephenson gear for slower/hill work - and 4767 was the product of that. Can anyone confirm or is my memory playing tricks?

 

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.