Talk:Stephenson valve gear

Picture Goof?
According to Vol. 1, of Audel's Engineer's and Mechanics Guide, The Stephenson gear has a curved link, the straight link-variant of the Stephanson gear as is shown in the picture is more correctly called the Gooch gear. This is an important difference, as the curved link tends to give better equalization at varied cutoffs.-WK-139.78.96.115 02:02, 8 March 2007 (UTC)
 * I've just removed a caption by an IP poster from Czech Republic which labelled the motion as "Allan-Trick" valve gear. It was not summarised or sourced, but there is such a system, from that part of the world, and it does look a bit like this.  --Old Moonraker 15:11, 9 March 2007 (UTC)
 * Just found this on Commons.--Old Moonraker 15:55, 9 March 2007 (UTC)
 * OK, I looked when I got home and what is shown is the Allan gear, or, at least, a variant on it. The Allan gear has a straight link as is shown, pivoted at its end and on a pendulumn rod, but from a fixed point. A radius rod is moved up and down on the link to give different cut-off and reverse. The Allan-Trick variant shown here seems to be different in that the link too moves up and down but in the opposite direction as the link and at a different rate, and that it is pivotes at the middle

The gooch link has a curved link. There are endless variants. This site here has downloadable animations of many different steam engine valve gears. [Valve Gear]
 * The Czech IP poster, above, has just drawn our attention to this link. It's in German, unfortunately, and the pictures don't exactly match the photograph accompanying the article. The closest is, as suggested above, the Allan variant. Old Moonraker 15:21, 13 March 2007 (UTC)
 * More from Czech IP poster (who is PetrS. on the Czech wikipedia): referring again to the above image he identifies the difference from the normal Stephenson gear as "the valve stem hangs on the same arm as the link". Now, as the picture here is of the same mech, it does look as though the caption is wrong. Should we label is as a variation of the Stephenson link, included because it is fitted externally and therefore easy to see, or try to find another? --Old Moonraker 09:41, 14 March 2007 (UTC)
 * I think that Allan-Trick (Allan was english, Trick german and they invented this gear in the same time independently) valve gear is an evolution of the Stephensons gear. I am not expert in the branch of steam valve gears but i suppose that the reasons were two - faster operation an better distribution of steam. PetrS.
 * Someone's changed the caption. Old Moonraker 17:08, 15 March 2007 (UTC)

Animation
The animated gif is broken. Instead of the crankshaft rotating around the its center, and the eccentrics doing their thing, the crankshaft is rotating around the crankpin. That results in the drawing losing all meaning. I'd try to fix it, but I don't have the tools to that were used to create the original image.
 * I left a note on the creator's talk page. He promised to fix it, but he's a busy chap... Old Moonraker 11:58, 18 September 2006 (UTC)

I agree the gif had me perplexed for a few moments. I could see something amiss but it took a moment or two for the penny to drop. The article also needs a note about the subtle difference between Launch and Loco links.--7severn7 19:11, 3 October 2006 (UTC)

New edits
Hope these are alright. I have been wondering if there should not be a new article on gab motion; the trouble is that references are rather hard to come by. There were lots of operating systems, often combined with expansion valves worked by fiendishly complicated mechanisms, not really worth the study as they were a blind alley.--John of Paris 12:42, 12 June 2007 (UTC)

Title
This article covers Gooch and Allan valve gears, as well as Stephenson, so I am thinking of moving it to Link valve gears. Please discuss. Biscuittin 21:47, 25 September 2007 (UTC)
 * Sounds like a sensible idea. I've been worried about that too.--John of Paris 15:39, 26 September 2007 (UTC)
 * I think Stephenson gear notable enough to deserve its own article. Other link gears might be grouped together in another. Bill F (talk) 23:01, 30 July 2008 (UTC)

Confusion
I have removed this:

"the launch link was longer than the traditional ‘’locomotive link’’, the latter being used where space was at a premium especially in the small clearance space underneath early low-pitched boilers in Britain."

and part of this:

" ... but in Europe, although occurring as early as 1846, they did not become widespread until around 1900 as boiler centre lines became pitched higher giving greater clearances underneath."

from the 'Applications' section because it makes no sense to me. If someone could explain convincingly why a locomotive link should require less space than a launch link, I would be obliged. What was the source? Bill F (talk) 23:01, 30 July 2008 (UTC)
 * Has this got mixed up through succesive editions? I can imagine that internal valvegear would be contraindicated by the low C of G boilers in Britain until the advent of locos like the Jenny Lind proved low C of G not as important as people thought. Chevin (talk) 06:54, 31 July 2008 (UTC)

GIF has no caption, and other issues
1. What kind of valve gear does the GIF illustrate? Gab? Stephenson?

2. If it is Stephenson, how come it differs totally from the static diagram below it, which is far more complex?

3. If it is gab, then get the GIF out of this article and move it to the/a 'Gab gear' article.

4. It seems to me that articles about valve gear in Wikipedia are still in their infancy. As a man on the street, I understand very little of this article. The second sentence in the 'History' section is unimaginably run-on and unacceptable. Problems with this sentence include:

- 'The gab motion incorporated two sets of eccentrics and rods for each cylinder'. Do not describe gab gear by telling us what a multiple of its components collectively does. In fact, it's worse: you tell us what two orders of multiples of components do: 'sets of' on the one hand, and 'eccentrics and rods' on the other. So where does that leave us? A four-way matrix none of whose individual items is in fact defined. For God's sake, tell us how many eccentrics a cylinder has. And how many rods. Then tell us what you mean by a 'set'. Only then you may name what a multple of a set does.

- I don't know what knowledge the custodians of this article expect the average reader to have, but guess what? I don't what a rod is. I don't know what an eccentric is. Or in fact many of the other terms herein. A schematic -- it can be static -- would be nice. Is an eccentric one of the circular-looking thingies you see in the GIF? Is a rod one of those long vertical elements that connects with that -- I don't know what you call it -- rocker arm?

- Add labels on all individual elements of the GIF. What's the thing on the top that looks like a plunger and moves up and down? Does it sit inside the steam chest? It must connect to the valves somehow, right?

- 'each cylinder; one'. Why the semi-colon? You are starting a new thought. Yes, there are times where semi-colons are called for, namely when a thought is unfinished and you need to 'drop the other shoe'. I use 'em myself -- probably more than I should. But this is not one of those times.

- 'eccentric was set to give'. What do you mean, 'SET to give'? Why don't you just say 'GAVE'?

- How can a single sentence contain all of these phrase?: 'one ... and the other ... and one or the other ... with ... by means of'. This isn't English. It's a mass of images that need to be separated into sentences, and each explained.

- ': -'. I'm sorry, but  is not English punctuation. Again, just start a new sentence here.

- 'vee-shaped ends to the ... catch the rocker ... driving'. On and on. I don't have the least idea of what's being talked about.

- 'ends to the'. I don't understand the preposition 'to'. Are you referring to the ends -of- something? If so, say 'of'. Isn't the phrase 'end to' more valid in speaking about abstract things, e.g. 'will there ever be an end TO this war?' You do not tell a child, "Do not touch the end to this long red-hot poker."

- 'catch the rocker'. What does 'catch' mean? Do you mean to say that the rocker (whatever that is) has gotten loose or detached, and that the vee-shaped ends then grab onto it again?

- 'supposed to catch'. What mean by 'supposed'. Do they catch the rocker or do they not?

Finally, re. the last sentence, 'It was a clumsy ...': That sentence is just fine. That is a clear sentence. Why can't the rest be written like that? (Er ... with THIS exception: 'gave fixed valve events.' Huh? What does it mean to 'give an event'? For me, at any rate, it's not English.)

5. You introduce Stephenson gear only in terms of how it differs from gab. The first thing this article says by way of describing Stephenson's gear is: Two employees 'replaced the gabs'. I mean, huh? I don't want to hear about what Stephenson's is not, at least, not by way of introduction. Can't you simply start in and describe Stephenson gear, period? Or is it too complex for that?

6. And again, why isn't there a GIF that animates the complex drawings seen below? Or is the GIF doing us a big favor by presenting the simplifed, basic core of Stephenson's motion, -without- the copmlexity seen in drawings? If so, then for God's sake say that. Add a caption for the GIF.

Unfortunately, writing of this sort that requires profound revamping is the norm across much of Wikipedia, at least on technical and historical subjects. --Jim Luedke Jimlue (talk) 08:21, 16 March 2011 (UTC)
 * 1. This is a core vertical Stephenson.
 * 2. The drawing shows a horizontal valve gear (to match a steam locomotive
 * right side, the forward direction is typically to the right).
 * The drawing shows a reverser mechanism.
 * The drawing shows an expansion link.
 * The drawing shows just the stem of an Expansion valve, to the far right of the drawing.
 * The drawing shows a mechanism to lift and lower the expansion link.
 * 4. Yes. It might be a "spool" type piston valve, in the steam chest.
 * One eccentric (mechanism) is fitted rotationally advanced (clocked part of +90 degrees) before the drive crank.
 * The other eccentric is retarded (part of -90 degrees) the same amount after the crank.
 * I also don't understand the gab forks, yet!
 * 6. Please see http://www.donashton.co.uk/html/stephenson_s_gear.html
 * for a good animation of this gear and the Walschaerts.
 * I encourage you to edit the non-technical aspects of this article yourself.
 * It seems to me that you have good editing ideas, in general.
 * Wikipedia has several editing tutorials for us.
 * Please use my talk page at user: Douglas Nelson Turner for any questions.
 * I'll add your caption to the GIF (another good idea).
 * I'll try to talk to you at your talk page. Thank you.
 * Douglas Nelson Turner (talk) 03:57, 12 September 2014 (UTC)

Categories
Stephenson valve gear is also widely used in non-locomotive applications, such as marine and stationary. I suggest that Category:Locomotive valve gear be re-named Category:Steam engine valve gear. Biscuittin (talk) 11:10, 30 December 2011 (UTC)
 * Having just purchased a book on marine engineering, in which the sole example given of valve gear is Joy's, I agree with the suggestion. Ning-ning (talk) 17:11, 1 January 2012 (UTC)

Variable lead, an intrinsic advantage (only when the crank pin shown is rotated to the right of top)
The static drawing of the gear in this article shows the gear in neutral (mid-gear). The crank is back dead center (fully left). The expansion link is vertical. If the expansion link were dropped to employ the top eccentric rod exclusively, the valve stem would move very slightly to the right, adding to the lead (not retarding). This is due to the fact that the top eccentric rod big end pivot point is closer, in elevation, to the crank center horizontal plane, than its small end pivot point elevation on the expansion link (14 pixels to the crank plane and 18 pixels to the expansion link pivot horizontal plane). This would be very slightly more lead steam in full gear, than in neutral gear, with the eccentric rods not crossed.

The drawing in this article shows no reduction of lead, with the gear in the full-gear position and the rods uncrossed. This would mean that only one of the two piston strokes in the cycle would benefit from less lead in full gear!. Why not add a statement that only half the stroke cycle in the Stephenson gear benefits from retarded lead in full gear? Too complicated? Douglas Nelson Turner (talk) 01:42, 12 September 2014 (UTC)

WP:BURDEN
Redrose64 is re-adding large unsourced sections and filing edit-warring templates on those who remove them. Does WP:RS not apply to railfan articles? 82.132.234.87 (talk) 15:25, 12 September 2014 (UTC)
 * Your edit summaries simply said "unsourced". That is not sufficient grounds for removal, because none of them were WP:BLP violations. What are your specific problems with this article? -- Red rose64 (talk) 15:30, 12 September 2014 (UTC)

Black 5 no. 4767
This locomotive does have Stephenson valve gear - it was the only one of its class so fitted, see Black 5.

There are two eccentrics driving two eccentric rods, attached one each end of the expansion link; this is suspended from the weighshaft and so lifts to vary cutoff; the expansion link is curved with the inside of the curve to the rear.

In the Walschaerts gear, there is one eccentric and so a single eccentric rod attached to the top of the expansion link. The expansion link is mounted on a fixed pivot, and is curved with the inside of the curve to the front, and cutoff is varied by raising or lowering the radius rod; the radius rod is not attached directly to the valve spindle but to the combination lever, a feature lacking from the loco in the photo. -- Red rose64 (talk) 19:09, 2 October 2016 (UTC)

Sliding link
I'm not an expert in steam engines, but I understood one of the crucial parts of the Stephenson gear to be the sliding link between the two eccentric rods, which moves up or down, therefore altering the valve stroke. However, all of the images in this article (perhaps excepting the photograph on the top, which is just confusing) appear to show a solid bar of metal with pivots on either end connected to a variety of links and pivots. Where is the slot that allows it to slide up and down? To a person who doesn't know anything at all about valve gear, I cant imagine these pictures being anything besides completely confusing. Okay, they let one see how a Stephenson gear actuates a valve, but as soon as we get into talk about variable cutoffs, it becomes meaningless; the crucial piece isn't fully drawn). Or perhaps I'm totally wrong, and there is no slot, and I'm thinking of other valve gears? But no, I remember seeing what I was told was Stephenson valve gear under steam locomotives, and there was always a slightly curved link with a slot that was the centerpiece. If I am right, the average person is not going to come away from here understanding how this device actually works, unless they are very good at visualizing vague explanations (if I am wrong, it certainly hasn't helped me to see how it actually does work, for sure!). While I'm at it, all this talk of 'lead' and 'compression' would be bad enough if any of it was actually explained in the article about Cutoff (steam engine), and I would merely complain that (as usual) a person has to click on another article just to understand what is written in this one. however, they aren't explained in either article, and so are basically meaningless to anyone except someone who is already well-versed in steam engine jargon and operation. Seems sadly typical of Wikipedia articles; people are more worried about showing off their technical knowledge than in explaining it in a way that would be useful to someone who is new to the subject. 64.223.165.28 (talk) 01:55, 31 March 2018 (UTC)

Okay, I looked it up in an old railroad technical book: yes, a Stephenson link as a sliding link, with a groove cut in the center to allow the end of the valve rod to move in it. The simplest way to put it is as it is put in the book: the valve can easily be driven in one direction directly by one eccentric, attached to the valve rod, but only at a fixed cutoff. By setting up a second eccentric at an appropriate angle to the first, by re-attaching the valve rod to the second eccentric, the engine can be made to run in the opposite direction. This is like a gab hook or slip-eccentric engine, fundamentally. But by linking the ends of the two eccentrics together and putting a sliding groove in the middle, by raisin the link up or down on the valve rod one can make it be driven by either one OR the other eccentric, OR any position between which allows you to have variable cutoffs. When the link is fully lowered, the valve rod end is at the top of the link, and is basically being driven directly by the top eccentric rod, and vice versa. In usual service, however, it is generally kept between the two, where the sum of their movement equals the movement of the valve rod. Note that the LINK moves up and down, not the valve rod. I wasn't clear on that. As the book says: "The duties of the link are twofold: first it provides a means of readily reversing the engine, since by means of the rocker it can easily be thrown under the control of one or other of the eccentrics; second, it provides a means whereby the admission of steam to the cylinders can readily be cut off at different parts of the stroke." - A Treatise on the Locomotive and the Air Brake, 1905

64.223.165.28 (talk) 01:55, 31 March 2018 (UTC)
 * If you enlarge [[Media:Black 5 George Stephenson 7 (5441137329).jpg|this image]], the slot is clearly visible. This loco was highly unusual in that the Stephenson link motion was mounted outside both frames and wheels - normally the Stephenson gear was inside, very difficult to examine without the loco being stripped down. -- Red rose64 &#x1f339; (talk) 19:39, 31 March 2018 (UTC)