Stephensons valve gear

[Perko7Participant @perko7]

I've been having trouble finding a resource that explains the difference between launch-type links and locomotive-type links as used in Stephensons valve gear. Can anyone on this forum enlighten me please, or point me to a suitable reference source? Thanks.

[Perko7Participant @perko7]

[Simon CollierParticipant @simoncollier74340]

Look up Don Ashton’s stuff online. It would be best to buy his book on valve gear also. With loco links, the eccentric rod pin holes are on the line of the expansion link slot center, and with launch links, the pins are behind the slot. Loco links need more space vertically, so much less appealing, but there are rules for which is preferred. Loco for direct drive to outside admission valves (slide valves) and launch for direct drive to piston valves with inside admission. This is reversed if the drive is indirect, with eg., a 180 degree rocker through the frames to outside valves. Look for pictures or animations- much better than words.

Edited By Simon Collier on 28/11/2021 20:22:05

[DMBParticipant @dmb]

See Charlie Dockstader s live animations on internet and Don Ashtons book, as above. There is also a book on Walschaerts gear, for those interested.

[RedsetterParticipant @redsetter]

A locomotive type link has the eccentric rod pivots on (or about) the curved centre line of the link and beyond the ends of the slot in which the die block fits. A launch type link (which is not the same as a true launch link) has the eccentric rod pivots to the rear of the link, behind the slot. Both type have pros and cons but the main advantage of a launch type link when used in a locomotive is is that the eccentrics will be smaller for a given valve travel.

There is, unfortunately, a lot more to it than that.

[Nigel Graham 2Participant @nigelgraham2]

The main difference is in where the eccentric-rod pins lie in relation to the rest of the expansion-link.

Locomotive type: The pins are on the slot's centre-line, beyond the ends of slot. This necessitates a total eccentric travel greater than the full-gear travel; and makes the mechanism rather "broad in the beam".

The expansion-link is suspended at its mid-point, on the lifting-links.

(Ideally a pair of lifting-links, embracing the expansion-link. Some of LBSC's designs needlessly skimped on that by using a single-sided suspension, greatly helping wear and magnifying its effects.)

Launch-link (True) : The eccentric-rod pins lie behind the slot but within its ends. In full gear this often puts the valve-spindle link in direct line with the salient eccentric.

The suspension point is on the slot centre-line but above its end.

Common on traction-engines, which have limited room below the motion but don't habitually run backwards for long distances at high speed! Typically these have the lifting-eye at the top, with the lifting-links meeting the weight-shaft's lifting-arms down below, just above the boiler. A loco's weighshaft is normally above the motion.

Launch-type link: As above but the suspension point is as the Loco-type, in mid-link. It allows long valve travels with large laps, suited to express locomotives, with relatively compact valve-gear and symmetrical motion for both directions.

Going into the subleties the suspension-point on Loco and Launch-type is often slightly aft of the slot's centre-line, for geometrical reasons, but that is a matter for the engine's designer rather than builder.

'

My source?

Martin Evans' Manual of Model Steam Locomotive Construction (MAP, latest reprint 1970). This is as good as any reference but I don't know its availability now, so you'd probably be looking for a second-hand copy.

Also browse through TEE Publishing's catalogue. I do not have any of their books on locomotives and valve-gears but can vouch for those they sell on workshop processes. Perhaps others here can suggest particular choices.

[Perko7Participant @perko7]

Many thanks for the prompt replies, all clear to me now. Will follow up the resources mentioned as well.

[Perko7Participant @perko7]

Ok, I've been reading up on this subject for a little while and have viewed the Dockstader simulations and read most of what Don Ashton has written, but there is still one thing that puzzles me. Most (if not all) the simulations and design texts assume that the valve rod is suspended in a fixed position and the expansion link is lowered or raised to provide forward and reverse running. The original drawings for the loco I am constructing have the expansion link suspended in a fixed position and the valve rod raised or lowered to provide forward and reverse. None of the simulations seem to accommodate this arrangement. The expansion link slot in the simulations appears to be curved to approximately match the radius of the eccentric rods, so when the piston is at dead centre, there would seem to be minimal movement of the valve rod between full forward and full reverse as the expansion link is raised or lowered. In my example, with the valve rod being raised or lowered instead, would the curved slot in the expansion link need to face the other way (as in say a Walschearts link) so that there is minimal valve movement as the valve rod is raised or lowered when the piston is at dead centre. If the curve is the normal way there would be significant movement of the valve between the mid-point and the two extremities of the expansion link.

Hope some of you more experienced loco builders can point me in the right direction.

[Perko7Participant @perko7]

On further examination, I think the arrangement I am looking at is more like the Gooch valve gear. I will look at the simulations for that and see if it works.

[RedsetterParticipant @redsetter]

OP, it sounds as though you have Gooch valve gear.  Out of interest, what is the locomotive?

Edited By Redsetter on 27/12/2022 09:01:44

[Nigel Graham 2Participant @nigelgraham2]

To clarify and summarise the differences:

Stephensons' Link Motion:

Expansion Link concave towards the eccentrics.

Expansion-link is moved by the lifting-links. Its suspension-point on a locomotive is usually in the centre of the link*.

Valve-rod stays on its axis, as a rigid component terminating in the die-block clevis; and indeed is better constrained to reciprocate in a rigid guide rather than using an intermediate radius-rod suspended from a swinging-link, as LBSC's designs tended to use.

'

Gooch Gear:

Expansion Link concave towards the valve-chest.

Expansion-link suspended from its mid-point by a swing-arm hanging from a fixed bearing.

Valve-rod is moved up and down the expansion-link by the lifting-links, necessitating a separate radius-rod to link die-block clevis and valve-spindle. A guide, like a small cross-head, on the valve-spindle end would be advantageous.

'

For completeness, there is another variation:

Allen "Straight Link" Gear

Expansion Link straight, not curved.

Expansion-link and valve-rod radius-rod are both moved, in opposite directions.

This entails a valve radius-arm and rather complicated lifting-link set, with more parts and pin-joints. Rarely used – it might appear on a miniature locomotive faithful to a prototype so fitted, but I do not know of any such examples.

.

* The expansion-link's suspension point on many traction-engine and marine engines is at one end of the link. It may give less effective valve-events in reverse but these applications run nearly always forwards. A railway locomotive needs be as efficient in both directions.

[Andy AshParticipant @andyash24902]

I have come to the conclusion that the generic valve gear names are useful in casual conversation, but not for much more. The complexities of building a model locomotive valve gear are many. Generally one has to think of them as highly imperfect compromises.

Probably, some designs are better than others. If you're actually going to build it, you have to check the design you have before you use it. The devil is in the detail, and every detail matters. One detail that makes a big difference for example is suspension travel, and no-one ever seems to think about it.

Don Ashton's simulator is good and interesting, but it is less useful when it comes to simulating an actual valve gear implementation, because the details are frequently different. It can be hard to get the Ashton simulator to match one's own gear exactly. Many CAD tools support kinematics, but they can be expensive. If you want a free kinematics tool which is very good, you could try;

Download Linkage Free

I have no affiliation with this tool, but I have used it and found it to be good. For me, I found that the sliding length of my expansion link wouldn't be long enough to clear the die block for all reverser settings. The crash was only 10 thousandths, but enough to make the expansion link bearings wear excessively.

Obviously the linkage tool won't do steam diagrams, but for a model I think that is all a bit academic anyhow. If you can make all the parts fit properly and perform their generally intended functions, you won't go too far wrong.

With the kinematic simulator you will be able to see why the gear behaves differently in forward and reverse. You can also experiment with the static settings like eccentric and valve positions.

[Nigel Graham 2Participant @nigelgraham2]

I fear this thread is veering into too deep thinking. I don't know if it's still in print but Martin Evans' Model Steam Locomotive Construction probably tells us all we need to know, especially if building to a published design. It was my source for my previous answer.

Perko simply asked the difference between valve-gear types, and specifically expansion-link types.

Really, you need delve far into the geometry only if designing a specific engine. Otherwise problems including conflicts between moving parts, are more likely to stem from drawing errors than geometry.

Yes, you do need understand how the mechanism works, and simulations are an interesting route into that; but for the most part if you are building to an established design you should be fine – although some of LBSC's smaller locomotives do contain mechanical weaknesses likely to be a source of trouble from premature wear .

[JAParticipant @ja]

Martin Evan's "The Model Steam Locomotive" has four chapters on valve gear (one on Stephenson type gear). It is pretty thorough and goes into the different Stephenson's gears. I bought a copy about 18 months ago.

(I started this reply an hour ago and then the phone rang. I have just seen Nigel's sensible comments but I will continue since this may be of use to someone)

I have found the online simulations difficult since I like a lot of numbers. The maths involved is not complex (Stephenson's is easier that Walschaerts&#39 but you need a computer to do the sums at a lot of crank angles and reversing lever settings (i.e. large spread sheets).

Finally if anyone wants to know about the varieties of locomotive gears, supperficially, there is an article in the 1943 "Journal of the Inst. of Loco. Engineers" The Evolution of Locomotive Valvegear. It covers 101 valve gears over 80 pages. You probably won't find a copy for sale on the internet but if you contact the IMechE library they will probably download you a copy for not too much money.

JA

[DMBParticipant @dmb]

Copies of valve gear drawings may be available from York museum or if not, will be able to advise where from. A friend of mine was building GWR No. 40 with the Scissors valve gear, some years ago but cannot remember him completing it. He showed me a copy of the valve gear which I think he obtained from one of the Institutes mentioned above.

I have a copy @1.5ins scale of GWR 2251 Class from Paddington, back in the 60's?, which shows Stephens inside vg with lifting links attached to the top of the expansion links and weighshaft underneath. As far as I can recall. Stored away in loft for decades.

[JAParticipant @ja]

Posted by DMB on 27/12/2022 20:50:50:

He showed me a copy of the valve gear which I think he obtained from one of the Institutes mentioned above.

Mothers have institutes. Engineers have institutions.

Drummed into me during my HND days.

JA

 

Edited By JA on 27/12/2022 21:32:50

[Andy AshParticipant @andyash24902]

I'm pretty sure many of the full size engines had really odd arrangements. Some probably never even had names. I think the scissors gear is also known as "Gab Gear". Joy gear, is one of the most fiddly to get right, but also one of the most useful if you can. Baker is pretty practical. The Bullied chain driven gear probably wasn't.

Some early Midland and North Western engines had valve gear with a bad reputation. From what I remember the Midland Compounds were pretty terrible.

Probably my favourite arrangement is "Bagnall Price". That has an "expansion link" that appears to operate counter to all the links around it. Actually there is a crank inside the frame which can't be seen at first glance.

For dogmatic reasons modern approaches were resisted in England for a long time. With the advantage of modern thinking it is now obvious that cam operated valve systems like Caprotti had a significant technical edge. I think some of it was the difficulty of making the necessary bevel gears and cams compared with usual simplistic railway locomotive components. There are plenty of examples of complex components in other industries of the time, so dogma must have been a significant issue.

By the time of the BR standards, engines had become so complex anyway, that such technicalities as Caprotti gear happened with relative ease. At the time, I'm not sure that Caprotti really outshone Walschaerts from a performance perspective. I think if this generation of engines had reached their proper service life the cam operated gears would have shown significant performance benefits with less maintenance.

Hopefully that's a less technical response.

I still think it misses the point because it is a bit disingenuous. The exact placement of the pivots and method of support for the expansion link in Stephenson's, Gooch and Allan gear makes a huge difference to the performance of the engine. There aren't enough names (I think) to cover all the potential possibilities.

Edited By Andy Ash on 27/12/2022 22:03:46

[LukerParticipant @luker]

Posted by Perko7 on 27/12/2022 08:35:58:

Ok, I've been reading up on this subject for a little while and have viewed the Dockstader simulations and read most of what Don Ashton has written, but there is still one thing that puzzles me. Most (if not all) the simulations and design texts assume that the valve rod is suspended in a fixed position and the expansion link is lowered or raised to provide forward and reverse running. The original drawings for the loco I am constructing have the expansion link suspended in a fixed position and the valve rod raised or lowered to provide forward and reverse.

This does sound like the Gooch valve gear, or a variation of it. Regardless the first principles will be the same. The expansion link radius matches the length to the CL of the eccentric rods for constant lead. If you want to increase lead when back notching this radius needs to change, but then you could just as well modify the setup to be the Stephenson’s valve gear. Because you need to split the expansion link the material thickness needs to be thicker than true scale to prevent pin wear and slack on the valve gear. The mid-gear position isn’t on the CL relative to the driving axel, so I suggest cutting the reversing lever stand slots right at the end. The CL needs to be designed slightly lower to improve balanced valve opening.

Unfortunately, you’ll probably only find passing mentions of these types of valve gears due to die slip issues that was difficult to design out on paper. But with more modern techniques you can actually design a very satisfactory valve gear. Some of my simulations…

[LukerParticipant @luker]

[PatJParticipant @patj87806]

I have studied Don Ashton's papers on valve gear, specifically the Stephenson's link.

I thought I would just build a simple Stephenson's link for my bottle engine design, and I found out there is a lot to just a simple Stephenson's design.

Open rods, crossed rods, launch link, loco link, and that all important pin location.

While I still have a long way to go before I finish my Stephenson link design, the one thing I did get from Don Ashton's paper was that if you set it up correctly, with the correct pin location, you can get almost equal valve travel in both forward and reverse, and also equal valve travel in various link positions.

I have considered simulating a Stephenson's in Solidworks motion study, but I think perhaps a quicker method is to make a mock-up of the valve gear using wood sticks and a cardboard link, with rods perhaps 36" long.

.

[Nigel Graham 2Participant @nigelgraham2]

On the matter of Lead:

Every text I have seen (including ones written for professional use) states that with notching up, the lead with –

– Stephenson's Link Motion ( and presumably its similar ones like Gooch and Allen) increases;

– Radial Gears (Hackworth and its equivalents, Walschaert's / Baker, etc). remains constant.

In the simpler radial gears the die block locus is an ellipse whose horizontal (valve-spindle aligned) axis in mid-gear is the [2(lap + lead)] part. The ellipse is tilted to increase the component to add the port opening. It is somewhat distorted by having to limit the expansion-link length. Walschaearts and the similar Baker Gears use the combination-lever to give the lap and lead travel.

The link motions give the lap, lead and port opening as a direct resultant of the eccentrics' throws.

'

Some simple narrow-gauge locos use Hackworh Gear, but this is heavily perturbed by the chassis suspension, minimised only by very stiff driving-axle springs. However, these machines ambled around within industrial sites where efficiency could be compromised a bit for simplicity and economy of manufacture, use and servicing. The more sophisticated link and radial gears used on Standard Gauge locomotives designed for best efficiency and performance practicable, are better at handling the modest axle suspension travels.

'''

I don't know its name but the gear on Luker's video image appears a combination of Stephenson's and Gooch; the reverser moving the valve radius-link (G); but the expansion link curve faces the eccentrics (S) from a fixed suspension (also G). The animation shows the gear in one position so I could not determine if this is so.

''''

JA points to the huge number of valve-gears invented to control variably, reciprocating valve travels and phases. All doing the same thing! I imagine some were as much patent avoiding as trying to make a better valve-gear./ Why for example Baker gear? It is Walschaerts with relatively minor difference, as far as I can see. Marshall, Bremme and Case are all variations of Hackworth's.

Perhaps the most original was the Heywood gear, designed to squeeze under the low running-plates on Heywood's very narrow-gauge estate-railway locomotives. It uses a set of pivoted links in place of the expansion-link.

'

Yes, if you want to write a computer-simulation of a gear like Walschaerts' you need use a spread-sheet to handle far more cut-off settings than any real reverser-stand will give; but how did M. Egide Walschaert himself manage?

Slide-rule and log.trig. tables drawn and primed, I think he would have calculated or drawn on paper, the full, mid-gear and a couple of intermediate cut-offs at dead-centre and mid-stroke. If these are all consistent, the intervening points should work properly. They are quite long-winded calculations, with a lot of trigonometry. Though at least the equations were direct, not wrapped in 'Excel'-ese!

(I used 'Excel' frequently, at work…. Chasing formulae brackets I'd miscounted, while cursing its writers for not knowing 0º = 360º or giving proper label editing tools, on polar graphs they called "radar charts" ! )

The simplest and theoretically most efficient eccentric-driven, reversing valve-gear may be the Mann Gear, in which the eccentric is slotted to move across a square on the crankshaft, by a cam arrangement. As far as I know It was only ever used on the Mann steam-wagons; never on locomotives.

[DMBParticipant @dmb]

PatJ,

Full size loco. designers used "wood sticks" intended for apprentices tuition but I wonder if also used in the design layout for a new class.

Andy,

"Scissors" was scissors, GWR or Midland versions but not known as "Gab Gear" as the latter were a pair of open ended curved V shaped forks, used, I think, on one or more, early GWR locos.

One Sweet Pea builder used the American "Southern" gear.

John

[LukerParticipant @luker]

Posted by Nigel Graham 2 on 28/12/2022 10:17:55:

On the matter of Lead:

Every text I have seen (including ones written for professional use) states that with notching up, the lead with –

– Stephenson's Link Motion ( and presumably its similar ones like Gooch and Allen) increases;

– Radial Gears (Hackworth and its equivalents, Walschaert's / Baker, etc). remains constant.

I don't know its name but the gear on Luker's video image appears a combination of Stephenson's and Gooch; the reverser moving the valve radius-link (G); but the expansion link curve faces the eccentrics (S) from a fixed suspension (also G). The animation shows the gear in one position so I could not determine if this is so….

Hi Nigel, the standard Gooch valve gear is a constant lead valve gear. The animation and video is also for a standard Gooch valve gear I designed and built. The orientation of the expansion link is, as you rightly pointed out, reversed but this doesn't change the valve gear motion. It’s just the eccentric offset relative to the driving crank that changes.

I’ve designed and built a few different Stephenson’s valve gears and the most notable difference to the Gooch valve gear is the expansion link that is lifted instead of the valve rod. Some nice examples are the inside cylinder and frame classical Stephenson valve gear…

The outside cylinder with rocker (changes the set point to get the lead correct when back notching)…

And the most difficult Stephenson’s valve gear I’ve had to design so far (more fun than I’ve had designing anything else!)…

Edited By Luker on 28/12/2022 13:33:10

[Andy AshParticipant @andyash24902]

@DMB, Well I just had twenty minutes to try and find out a bit more about the scissor gear, but I'm still none the wiser. I had heard about it before but I don't think I have ever seen a picture or diagrammatic representation.

As far as I can tell it was fitted to North Star. As a four cylinder engine, I imagine it would be what I would generally describe as a "conjugated gear", where the valve motions of some cylinders are generated from valve gear directly set for another cylinder or cylinders. The mechanics of the conjugation may look like a pair of scissors.

Indeed I think I had thought that scissors was gab, because someone had said that to me a long time ago. Probably like forty years ago if I'm honest. Gab certainly looks like two pairs of scissors, back to back. That was probably enough for me at the time.

Today, there does seem to be some suggestion that Churchward and Deeley had a bit of a thing about Midland railway patents in relation to valve gear. Perhaps those dodgy early Midland railway engines were the origin of the mysterious scissors.

I suppose one must be particularly intent to go searching for the geometry of a potentially difficult valve gear. Personally, I have long since moved on from the time where I try to improve my own bogus efforts. For what it's worth, I learned it was easy to come up with something bad, and even harder to make it better! Nowadays I just try to make whatever it was work O.K. That's enough for me, if I am lucky enough to succeed.

If anyone has a picture or a diagram showing what scissors actually was I would be quite interested to look, if only to relegate my mental image of gab gear. I understand there is a picture in the Martin Evans book. Maybe one day I'll buy it, but I always was more of a Curly fan than a Martin Evans one. No slight to Mr Evans of course, a nice person and an exceptional engineer for sure.

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