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

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.

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.

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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.

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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.

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* 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.

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 .

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.

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

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.

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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.

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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.

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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.

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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.

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