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Fire Fly valve gear




I joined my grand-children for a visit to ‘Thomas the Tank Engine’ at Didcot Railway Centre on 7th October. During the afternoon, I slipped away for a while, for a look inside the Broad Gauge shed.

Fire Fly replica at Didcot

After my exploration of early valve gears, while working on my 'Waverley' model, I decided to look at some gear 'in the flesh', so to speak, since I find it hard to read engineering drawings sufficiently well to get a real 'feel' for the hardware,.

It's always a bit of a shock to make the transition from tiny slivers of brass and strands of wire, to the reality of 12" to the foot scale. Those tiny levers and shafts turn into rather hefty lumps of iron. As someone once commented "real valve gear ain't so dainty"

It seems a pity that this splendid replica locomotive, which I watched having so much care and attention lavished upon it, now seems to be left at the exposed end of its shed, where corrosion and accumulation of debris are taking their toll. As my photos show, there are various old cans amongst the motion, which may be covering various oil cups, as well as providing homes for several spiders, with their extensive webs. I've not tried to use Photoshop to clean-up the artefacts, in case I falsify some hidden components.

To start with a drawing: 'Fire Fly' has gab motion, which is clearly derived from the layout used by Stephenson on 'North Star'. There are three transverse shafts, the central one being called the 'weigh bar' which carries the levers that operate the valves, and two others, arranged to raise and lower the gabs so as to obtain either forward or reverse gear.

EDIT 11th.Oct. - I have cleaned up this drawing a little, to clarify the linkages to the gabs and forks, while removing some extraneous lines. This has made it clear that the reverse eccentrics are towards the outsides of the engine and the forward eccentrics towards the centre.

Fire Fly valve gear

The drawing shows the arrangement, which ensures that, when the reversing lever is operated, one set of gabs is lowered and the other is raised, to engage the valve operating mechanism. The large forks guide the gabs into position, removing the need for manual alignment of these parts. The three transverse shafts are marked in red.

Turning to my photos of the Didcot replica: a lot of the details are hidden by the frames, since the photos were all taken from the platform alongside the locomotive. These views are from the right-hand side (RHS) of the locomotive, with the wooden boiler cladding visible at the top of each frame. The forward eccentrics (with the gabs engaged) are near the centre-line of the locomotive.

Each side of the two-cylinder engine has the motion for its cylinder carried between bearings mounted on two frames, so that there are four frames across the whole width of the engine. Achieving precise alignment of all the bearings in these four frames must be a critical assembly task. The use of these multiple frames does, however, reduce the load on each frame and this helps understanding why they look somewhat insubstantial, in comparison with later plate frames.


I found it was very useful to see how the transverse shafts are supported in bearings carried on shaped pedestals above the frames. The bearings themselves are then held by keepers, with adjustment bolts.


A horizontal view across the width of the engine shows the shafts running through the bearings on every frame. In between, the tips of the forks which 'grab' the valve rods when the appropriate gear is engaged can just be seen, although the main rods from the forward eccentrics, with their gabs (notches) are unfortunately hidden.


A more oblique view taken from near the front right-hand side of the locomotive shows the layout of the linkage to the valves more clearly. The arched bracings between the inner and outer frames on each side are also seen, as well as the means of attachment of the frames to the rear side of the smokebox.


My last photo is a view looking back along the engine, with the somewhat rusty big end visible on the crank axle.


I hope that others will find these views helpful, in gaining an understanding of the layout of early valve gears.


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Agreed, seems to need a lot of cleaning and TLC. 


Interesting pics though, sometimes I wish I was working in the bigger scales to really build engineering models.

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Very interesting and very helpful. Thank you. Pity it didn't have a good smothering of grease before it was left.

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Pleased this is of interest.and helpful. 



There was a recent Broad Gauge Society journal which looked at the early valve gears.


Indeed - I wrote it!  I took the photos, since the Didcot replica is the closest I could get to seeing early valve gear 'in the flesh' :)

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Sorry, I didn’t make the connection, very informative article, and these photos are good, although it’s sad to see the state it’s in.

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Thanks for sharing those photos Mike, very informative.


Running Didcot must be a constant battle with time and resources. A bit like gardening I suppose. As soon as you've got the weeding done it all starts growing back.

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Thank you for looking in, Mikkel.  I should like to see Fire Fly being given more protection from the elements, now that it is not in running order.

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(disclaimer - I volunteer at Didcot)


I suspect you'll find that those cans are deliberately there to provide some protection for vulnerable things like oiling points etc, such that when it does reach the top of the overhaul queue it's a *bit* of an eaiser job as the oil reservoirs won't be full of water... Not elegant I agree, but probably effective. Of course, like anywhere, there are always the odd visitors who will just leave their rubbish lying about too :-\


I don't know where else it could go to get more protection though -- the broad gauge road is full, so something has to be at the open end of the shed.

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Thank you for that information, Fraser. It's good to know that some care is being given to this locomotive. Perhaps some sheeting could be arranged at the open end to give a little protection from the forthcoming Winter weather? I'll edit my comment on the cans in my original entry.

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I'll mention it Mike... I'm (a lowly) part of the group that looks after the "currently out of service" engines in the main shed, but I'm not sure if the broad gauge stuff has a similar arrangement. Perhaps we should take them under our aegis if not.

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Anyone know where I can get a complete set of drawings for Firefly? Plans to build one (or two) in 2mm scale.

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