Locomotion (7.25 Gauge)

Overview
'Locomotion' is a 7.25 gauge locomotive designed by David Piddington around 1997 or 8, for A J Reeves.

Dimensions
Locomotive length: 19 inches

Tender length: 17 inches

Drawing errors
On the prototype, now in Darlington, the pump is shown mid way between the 2 wheelsets. In 1928, when Henry Greenly produced a design for 'Locomotion' in 7 1/4" gauge, for reasons I don't understand, he moved the pump forwards so that it was very close to, almost fouling, the front right hand wheel. The Piddington design replicates this pump position, as does the German Atelier design ion 5" G. Later, on sheet 4 (wheels) there is a note to reduce the flange depth of the front right hand wheel to 1/8", from the designed 5/32", to aid clearance of the pump, and on sheet 6, showing details of the pump, there are more notes about filing flats on the pump body to clear the wheel. I don't see the point of this, and am going to move the pump rearwards by 1/2", so that it clears the wheel. The length of the tender water tank top plate is shown as 5 7/8". This should be 6" to fit the tank body. This has several amendments dated 1998 and 1999, which I believe increased the wall thickness of the firebox tube, and gave additional clearance under the firebox, following adverse comments from boiler inspectors. There are a large number of thickening plates shown on the inside of the barrel for numerous bits and pieces to be screwed on, as the boiler is in effect the chassis, and everything fits on it. The idea was, with a 10G barrel and 4mm thickener plates, for blind holes for the fixings to be drilled and tapped into the copper, and then the whole assembly to be caulked with Comsol after assembly. My boiler inspector wasn't happy with this and neither was I, so my approach is going to be to fit 3/8" long, 3/8" OD phosphor bronze blind bushes, screwed 3/8 x 32TPI instead. For high load items such as the axle mountings I will retain the thickener plates, but where its only low stress such as for footboard mountings, I'll leave out the thickeners. As noted above the pump position needs to be moved back to 2 9/16" behind the front axle, instead of 2 1/16 as shwon. and the pump stay moved back correspondingly. There are 3 bushes under the barrel for blowdown and 2 feed clacks. Firstly theres no need for 2 feed clacks, as the pump feeds directly into the boiler without an external delivery pipe, and that only leaves the hand pump. This position does correspond with the prototype, but a worse position for a feed clack I can't think of. Therefore, I'm going to delete the 2 clack bushes and fit one on the backhead, 1/2" below boiler centreline, and 2 1/4" to the left of centreline. Not ideal I know, but better than under the barrel. I'd move the blowdown too, if I could, but there isn't any room at the bottom of the backplate. The safety valve is a single deadweight valve to match the prototype. I'm not happy with this, so am going to in corporate 2 bushes on top of the barrel between the backhead and the rear cylinder, for 2 modern low profile valves. Then the deadweight can just be a dummy.
 * Sheet 1 - general arrangement right hand side.
 * Sheet 2
 * Sheet 3 - boiler
 * Sheet 4 - Loco Wheels and axleboxes

The wheel tread profile given doesn't correspond with the 7 1/4" gauge Society recommendations for standard gauge prototypes. The wheel width needs to be increased from 3/4" to 13/16", and the flange depth from 5/32" to 3/16", with slight alterations to the profile of the flange. There seems to be enough metal in the castings to enable this to be done. Back to back dimension from the 7 1/4" Society is 6 13/16". Nowhere on the drawings is this dimension given, you have to calculate it by adding and subtracting various dimensions, and it doesn't work anyway! Width over the axleboxes is 5 3/4", so allowing 1/32" sideplay each side, the hub projection at the back of the wheel should be 1/2" beyond the back face of the wheel, or 21/32" from the wheel centre which is recessed by 5/32" from the back of the wheel face. Therefore, you would think, the length of the axle between wheel seats should be 5 13/16", but its shown as (after some calculation as 5 5/8", with a wheel seat length of 1 13/32", which should I think, be in fact 1 3/8" allowing for a 1/16" chamfer at the ends of the axles. This lot should have some effect on the length of the overhead crossbeam which the con rods fit to, but I can't make any sense of this at all, and will wait until I've got the wheels and crankpins done, and measure them before attempting the cross beams. Locomotion has an odd valve gear, both valves being driven from one slip eccentric on the front axle. The front cylinder valve is driven directly from the eccentric strap by a long rod to a rocking shaft at cylinder level. The rear cylinder valve is driven from the side of the eccentric strap via a bell crank, and then by another long rod up to the rocking shaft at cylinder level. An original sketch done at Robert Stephensons works in 1825 showed that they appreciated that the 2 valve operating rods should be as near as possible parallel to give similar valve events. The prototype roughly achieves this. Greenly showed the position of the pivot of the bell crank to be well to the rear, behind the rear left hand wheel. This resulted in the 2 valve operating rods being considerably out of parallel, so angularity effects on the rear valve would be considerable compared to the front valve. David Piddington's design and the Attelier one both replicate this position of the bell crank. To correct this means moving the position of the bell crank forwards significantly. How much I haven't yet worked out, but probably about 1". This means redesigning the bellcrank mounting, and probably hanging it off the boiler rather than off the axle mounting saddle. The bell crank itself would benefit from longer arms to reduce errors induced as it rocks. A work in progress!
 * Sheet 5 No comments at present
 * Sheet 6 Valve gear layout and pump details.
 * Sheet 7 Cylinders and chimney

I really haven't looked at the cylinders yet. Julian has expressed some doubts about the draughting in the chimney, as drawn, and certainly a blast nozzle size of 3.7mm for 2 no. 7/8" bore x 3" stroke cylinders seems a bit on the small side. So do the exhaust pipes which are 3/8" OD 16G pipe and over a foot long. I'm considering 7/16" OD 20G, which would still be reasonably to scale but free up the exhaust considerably. Still a work in progress. Julian also thinks that the exhaust manifold in the chimney could be more streamlined, to eliminate sharp 90 degree bends. The same issues as the loco wheels described for sheet 4 above. Width should be 13/16", flange depth should be 3/16". Machined to the drawing dimension of 3 3/4" tread diameter, the wheel tread ends up looking very thin. It might be an idea to increase the tread diameter to 3 7/8", to give a more robust wheel. There's enough metal in the casting, and an increase in tender ride height of 1/16" isn't going to be noticed.
 * Sheet 8 Boiler lagging and ashpan details. The only thing I've noticed on this (work on these items is about 5 years in the future) is that the boiler lagging planks are shown a 30 No 5/8" wide ash strips to cover the top half of the boiler only. For a 6" OD barrel, 30 strips need only be 5/16" wide, and this looks to be about 'scale' to the prototype.
 * Sheet 9 Tender GA and tender wheels

Width over the axleboxes is 6 1/2", so allowing 1/32" end play each side, should need a 1/8" long boss at the back of the wheel, not 1/4" as shown. The axle length between wheel seats should be 6 9/16", with a wheel seat length of 29/32. (the tender wheels are clamped onto the axle by a bolt and washer on the axle end, so the wheel seat is 1/32" less than the wheel width.)

As drawn, the tender has no springing at all, in accordance with the prototype. I'm incorporating a couple of 9/16" long x 1/4" dia coil springs to each wheel, pocketed 1/4" deep into the top of the axlebox and 1/8" deep into the top flange of the axlebox pedestal, to give a movement of 1/8". Not much, but better than nothing. Its worth commenting that if you are using the Reeves axlebox castings, theres no machining allowance worth speaking of across the faces that slot into the pedestals. With care, and reducing the 7/8" dimension to 27/32", its possible.
 * Sheet 10 - tender body

Not much here to honest, except that only 2 inner stretchers are needed, not 4 as shown.

As designed, the front and rear drawbars bolt to wooden cross stretchers, so the whole tractive effort is taken via a couple of small mortice and tenon joints. I 'm going to fit a couple of long threaded bars, 3/16 dia, running the whole length of the tender, to directly connect the front and rear drawbars. A bit safer I think, even though 'Locomotion''s tractive effort at 50 psi is likely to be in the 'won't pull the skin off a rice pudding' category.


 * A lot of the valve gear has a recommendation that temporary flat steel links are made first, only making the final ones when satisfied that they work.  veranderingen