Have you exhausted the Dockstader simulations, available via this

https://www.billp.org/Dockstader/ValveGear.html

Firth valve gear was a joint collaboration between Fowlers and T Firth patented in 1905.  It is a development of Hackworth radial valve gear of 1859.  Maybe our resident patents expert can turn something up?  It was only used for the large Fowler compound ploughing engines, never on a roller and doesn’t seem to have been adopted by any other manufacturers or for any other uses.  Worth noting perhaps as designed and used by Fowlers it was employed on piston valves, the HP being inside admission (keeping HP steam away from the rod glands) and outside admission on the LP.  Reference is Traction Engine Design and Construction, G F A Gilbert pages 175 and a layout drawing on 179.  The main difference to the Hackworth gear seems to be Firth adopted curved rather than straight links.  I have a 2” Superba that was owned by my father but not built by him with the Haining version of the gear and it runs OK but I wouldn’t think it’s terribly efficient, it’s quite hungry for coal and water but it’s quite a low boiler pressure for any compound benefit at 90psi.  It’s not been out for a few years now though.  Somewhere in the family archives there is a set of original Haining drawings but I wouldn’t know where to start looking for them!

Summary I guess is it’s quite a rare valve gear with little published detail so getting info is going to be difficult!  Perhaps try a post on Traction Talk?  Ross Bishop (suction hose) is well tuned in to valve gear.

Good luck,

Paul

Andy, I’ve sent you a pm

Andy – I’m interested in this string on Firth Valve Gears.

I have an almost finished John Haining designed Superba, built as a piston valve using the MERL Z7S cylinder piston valve liners.  And, yes – Gilberts books are excellent.

Like you I tried an early version of Onshape and Fusion 360 to reproduce a dynamic model – not then available. And, I built a full-size ply model of JHs design and decided it was no good for piston valves.

I also have the undimensioned GA Fowler drawing and I ignored Rule number One and scaled it off and built it.  However, although the engine has not steamed yet – it runs passably on air.  Ahead running is much neater than reverse.

I did a valve measurement from the model and got similar results to your plots.

I think all the mentioned valve gears in the string are Hackworth radials in basic form, often with their own names and minor variations. And probably you are right the singe eccentric allowed room for a middle bearing on the crank shaft.

What makes this gear unique is the offset for the pivot.

A friend here has a full-size American Case (Hackworth straight guide) and with an offset pivot. This makes the Firth gear and the Case Gears different from the standard Hackworth and variants.

I have tried to get this engine (single Cylinder) to stop on dead centre — it will not!  This appears to be a feature of the gear on the Case.

Both the Superba /Z7S and the Case were designed as single direction machines.

There is a Z7S being restored in the UK with some excellent comments in the Steam Plough Club quarterly publications.

Fascinating thread.

Several reasons why folk invented different valve gear:  patent dodging, reduced maintenance, improved reliability, and being able to fit into a tight space etc.

As I understand it, Stephenson’s gear produces near perfect events, but is expensive to make and a pig to maintain when the gear is under the boiler and between the wheels.   During the 19th century many attempts at alternatives, of which the best is  Walshearts.  Not quite so good as Stephenson events, but much easier to maintain because it’s mounted on the outside of the wheels.

As Walshearts is somewhat complicated, interest in alternatives continued, especially for road vehicles.  They aimed at some combination of being lighter, smaller, with fewer joints, cheaper, and/or simpler to set up and mend.  All compromises, usually with inferior events that waste fuel, and not notably more reliable either.

Fuel economy doesn’t matter if the application favours simplicity, size or reliability, but it does tend to discourage wastefully over-simple valve gear.  I think Firth was a brave attempt at a compromise suitable for traction engines but it didn’t outshine other gear in practice, and never became popular.   It is what it is!   And then internal combustion put an end to traction engines…

Plenty of interest in studying Firth though, and Andy’s clever efforts with FreeCAD, Python and Onshape might well bear fruit.  He could find an optimisation missed by Firth/Fowler because they didn’t have time to wade through all the possibilities.

More please!

Dave

AsDave says, this is a fascinating thread.

Previously I had not even heard of Firth Gear though I know there were quite a variety invented, of link-motions, radial-motions and assorted cam-drives.

I am not sure where the Mann steam-wagon gear fits, but I think radial. It uses a slotted eccentric moved across a square on the crankshaft, so was relatively simple, very compact and might give near-“perfect” valve-events although with harmonic rather than the rapid-action valve-travel of Stephenson’s gear.

Stephenson’s Link Motion was probably costly to make simply because it contains so many parts: 11 or 12 between crankshaft and valve-spindle, omitting the reach-rod and control, and valve-spindle guide; and neglecting assorted pins, keys and fastenings.

Walschearts’ Gear (radial) has about eight per cylinder even if discounting some parts, such as the expansion-link and its lifting arm, being divisible into further bits.

One might imagine the most difficult and expensive component was the curved expansion-link, common to both gears; and that on locomotive’s Walschaerts Gear was sometimes assembled from several pieces. I doubt it was really very difficult, industrially.

Its critical area, the arcuate slot, could be shaped relatively easily using a slotting-machine with power-driven rotary-table, albeit starting with a large, drilled hole then cut in two episodes. I think an ME contributor (Doug Hewson?) pointed out fairly recently the links on some locomotives were made in four parts: two curved arms bolted together with intervening, large spacer-blocks. The really awkward bit may have been machining the outline!

The American company, Case, used a variation of the deceptively-simple Hackworth Gear (radial) on its traction-engines. This, if designed and set correctly, can give fairly good results from very few, easily-made components but such niceties were less of a consideration than sales and servicing cost when applied to very basic, narrow-gauge, industrial-site locomotives such as the Kerr, Stuart “Wren” class.

<p style=”text-align: left;”>As others have said, this is a Hackworth type gear, but as the vibrating lever is quite long it wont suffer as badly from angularity, at least of this component.  I have found with Hackworth that having a curved slide with radius substantially less than the length of the valve rod gives better results. Might be worth experimenting. The angle between the arms of the bell crank will also have an effect.</p>
Amazing how many people lay claim to this gear. As others have said it’s very similar to Southern, and a chap called Brian Remnant had a Sweet Pea fitted with a similar set up but he called it Bremme.

I’m not into animated CAD models, but if anyone wants to have a go at a mathematical model give me a shout and I’ll send my Hackworth written in Python code as a starter for 10.