Stephensons valve gear

There are examples of die up for forward running with Walschaerts but are there any examples of link up for forward with Stephensons? Duncan might know.
Also, I don’t understand why the correct drive line is different for loco and launch links. They are not that different. Don’s book shows the preferred arrangements but doesn’t really explain why, stating that the preferred arrangements limit die slip.
I am designing outside Stephensons for Simplex, just for something different, and even raising the cylinders slightly and angling them will still have the launch link only 3/4” off the rail head in full gear.

On my Ruston Proctor engine, the quadrant link is up when going in the forward direction. If is was assembled in the down position it would just be a matter of swapping the connection of the links to the eccentrics to go forward.

A difference with the Ruston, compared to some other engines is the control lever points up where others point down.

Is there any advantage in "link up for forward running" with Stephenson's? It would of course give you more clearance between link and rail level when running forwards, but you need the clearance in reverse as well. 3/4" clearance seems ample. It is usually much less than that in 3 1/2" gauge.

I'll have to put my thinking cap on, Stephenson is not as intuitive as radial gears. As others have said, 3/4 inch is loads, if you look at a Polly the link seems very low in full gear, and as its right over the 3.5" rail, any oil drips make life difficult for the lesser mortals. I'm a bit swamped out with family today, so don't hold your breath.

Most traction engines (Three Shaft Engines) run link down to save wear and tear on the lifting link pins, on a Four Shaft Engine which the crank shaft runs backwards, in order to get the link down in forward gear the eccentric rods have to be crossed.

Michael

Thanks for replies and pictures. TEs usually have links end suspended so already are sub optimal from a locomotive point of view.
I have put my numbers in Don’s spreadsheet and next will be the Wallace simulator. I played around with the program example for Stephensons 2 making link up forward but it deranged the events. I did not persist. I agree that 3/4” clearance is probably enough (the cylinder drains are much lower) so easiest to go with convention. Reassuring to learn that Polly links are quite low.

As MichaelR says, to get link up with direct drive you'd have to cross the rods. This is not a good feature, it gives lead reducing as cutoff is reduced. Some NBR locos had this and they were indifferent performers. Quite why they didn't rebuild the escapes me, they'd only need new or modified eccentrics and eccentric rods. Perhaps a case of refusing to acknowledge the error.

I must say, outside Stephenson sounds like a pig to make/set up. Two return cranks, one hanging off the other, so you'll have to be bang on with crank throw and return crank length. At least with eccentrics the throw is fixed and you can adjust the angle of advance.

Edited By duncan webster on 10/08/2023 11:54:11

The plan is to have a vertical rocker to reverse the drive to be correct for launch links.
I would make sure I can satisfactorily make the eccentrics arrangement first up.
The background is I was given a full set of Simplex castings, frames and plans, plus boiler barrel. But I have a Simplex which I bought to drive while I made my Springbok. With a saddle tank and unusual valve gear, it would be different enough to be interesting.

Have you considered the layout used on LMS 4767 which has a launch link with direct drive?

Also the version used on some "foreign" locos with two outside eccentrics carried on a return crank? Perhaps a bit bulky, but easier to adjust than a double return crank.

Lms 4767 had piston valves. Do Ashton maintains that for direct drive slide valves you want loco links. Is there room to fit 4 eccentrics inside on the driving axle, then a rocking lever to shift the line of action outside the frames. Like Polly, so fails the unusual requirement

Edited By duncan webster on 11/08/2023 00:28:54

Yes piston valves and quite an angled drive up to the line of the valve rod. It was reckoned to be a good loco.
I don’t understand valve gear enough to do anything that can’t be put into Don’s Excel and then the Wallace simulator. And yes, unusual is the whole point. While the eccentric pins might be a bit flimsy, at my age/health, things won’t have to last very long! And it is only a thought bubble at this point.

No disrespect intended to Don Ashton, but he was a perfectionist, and in my humble opinion he was unnecessarily critical of some designs which work quite well in practice.

Just found this on Middleton Railway site. It has outside Gooch gear, (I think) but very similar to outside Stephensons. Might give you some inspiration

No it's outside Allen straight link, now that would be interesting

The Lewin locomotive "Erith" had outside Stephensons operating through a rocker, but I'm not sure inspiration is quite the word for it. **LINK**

This photo of the RHDR locomotive "The Bug" might be more useful as it shows the valve gear layout very clearly **LINK**

Brian G

The Bug was used during the construction of the RHDR, a\nd was eventually rescued from under a pile of rubbish in a Belfast scrapyard.

Returned to Romney, she was restored ans is used occasionally for special trains.

Apparentlyn the LMS Black 5, with Stephenson va\lve gear was a successful loco, but never repeated.

GWR used Stepehenson gear on all its locos, inside the frames, opereating the valves for the outside cylinders, of four cylinder locos via rocking levers.

I think that in one or two instances the gear was set up with crossed rods, and called scissors gear.

Howard

Scissors gear is totally different. It is a version of Walschearts where instead of a return crank the out phase motion is derived from the crosshead of the other cylinder (on a 2 cylinder loco). It was invented at about the same time by both the GWR and Deeley of the Midland Railway.

Incorrect, the GWR Stars, Castles and Kings (All 4 cylinder) had Walschaerts, after trials with Scissors on the prototype Star.

LBSC described an outside Stephenson gear for Tich in ME, vol. 120, 3012, as an alternative to Walschearts and used it on his own engine. I have only ever seen one other example. It would probably suit a Simplex if scaled up, and the double return cranks would be easier to deal with in the larger size.

Went to Middleton on Sunday to ride behind their Sentinel. Quite an odd experience, Poor performance setting off turned out to be due to not releasing the hand brake! Talk about spark throwing, managed to set fire to the lineside even after all the heavy rain. Anyway, took some pictures of the outside Allen Straight Link gear which might be of interest