Stephensons valve gear

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

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

@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.