Helen Longish

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Helen Longish is going to be my version of Helen Long, the 4-8-4T tank locomotive jointly designed by LBSC and James Joslin. A brief construction series appeared in Model Engineer volumes 56 and 57. The loco was intended for the experienced builder and having read the articles I can see why! The construction notes and drawings are pretty vague and there seems to be a lot of discrepancies between the details. According to LBSC quite a few Helens were built but where are they all? There are a few photos on the 2 1/2" gauge forum of an unfinished Helen which I believe was sold by Station Road Steam some time ago. Gerald Chandler informs me that Ron Warren built a Helen some years ago but never steamed her. Apparently he has now decided to get her out of mothballs, so hopefully we may see her running in the future.

I took a fancy to Helen when I first saw the design but decided to build a version incorporating some ideas of my own such as needle roller bearings on all axles, piston valve cylinders, and Gresley type conjugated valve gear for the middle cylinder. I will be producing a set of drawings as I go along which I will make available to the N25GA if the design is successful. I will also try and produce a set of drawings for Helen Long as described in ME. The original full size ones seem very hard to find.

Work begins!

The first job was to find some suitable wheel castings. I got in touch with Roger Palmer, who is in charge of the N2.5GA castings service and he suggested using 'Black Five' castings for the driving and coupled wheels so I ordered a set of 6 coupled and 2 drivers. They will only finish to about 3-5/16" in diameter ( *Helen's should be 3-3/8"* ) but that's not a problem. The Association is having a batch of bogie wheels made which are designed to finish at 1.5" diameter but they are not available yet. I could probably use a larger size but will wait for the new ones I think.

*(Note - Although the initial description of Helen Long in Model Engineer for January 6th 1927 gives the driving wheel diameter as 3-3/8", the later construction notes actually give the diameter as 3-5/16")

A start was made on machining the coupled wheels and the work went very well considering these were the first wheels I had ever turned from scratch! The castings had been ground fairly true on the outer edge of the flanges so they were set up in the 4 jaw chuck with the jaws gripping this ground edge and the outside of the wheels facing the tailstock. The casting was then adjusted so that the inside edge of the rim ran true. A skim was then taken over the tread so that the casting could then be reversed and held in the 3 jaw chuck for facing the back of the wheel and boring the axle hole. This ensured that the rims of the wheels would be truly concentric with the axles and not run eccentrically which looks dreadful!

The rear of the wheel castings were faced off, taking care that the amount removed would ensure that the spokes would be central in the rim when the front of the rims were finish turned. The axle bore was drilled and finish reamed to 5/16". A skim was also taken off the edge of the flange to make sure it was concentric with the bore. All 8 wheels were finished to this stage in one go.

The castings were then held by the flange with the back of the wheel casting hard up against the steps on the jaws to ensure true running. The centre bosses were then finished to the correct thickness. This was when a major snag showed up. The 2 driving wheels were from a different batch to the 6 coupled and were a lot thicker. Reducing the boss thickness on these 2 resulted in losing the nice shape of the centre boss as I had to take too much off. I could have got away with it if I had taken more off the back of the castings but it was too late now. I decided to scrap these 2 wheels and ordered two more of the coupled wheels.The balance weight will be made bigger to suit. At least all 8 wheels will then be identical as the spokes on the 2 odd castings were slightly different as well.

A jig to finish the front of the rims, treads and flanges was then needed. I had a spare chuck backplate that I had used to true up the wheels on the Flying Scotsman so I decided to use this again. Ideally, the back plate wants to be slightly smaller than the diameter of the wheel flanges so that both edges of the flange can be finished at the same time but I didn't want to have to turn down the backplate. I had some aluminium blanks 3-3/4" diameter by 1/4" thick so I fastened one of these to the backplate with 0BA screws, faced it up and turned a step on the front to 1/4" diameter less than the finished diameter of the wheel flanges. The centre was drilled and tapped to take a stub of 3/8" dia. steel bar. This was then turned down to a good fit in the axle bores in the wheels and the end threaded to take a nut for holding the wheel casting for finishing ( as described by LBSC)

The wheel turning jig

Each wheel was then mounted on the jig in turn, the rims finished to 5/16" thick, and the flanges to 1/16" thick. The treads were turned to 1/4" wide and the flanges to 3/32" deep. The top-slide was set at an angle of 2 degrees for finish turning the treads. The diameters of the rough turned treads were measured and the wheel with the smallest diameter was finished first. The others were then turned to exactly the same diameter using the micrometer dial on the cross-slide. The edges of the flanges were rounded with a fine file, a small chamfer turned on the edge of the rims, and a small groove machined between the rims and the balance weights with a small 'vee' shaped tool.

Finish turning the wheels on the jig

Crankpin holes were drilled in my small vertical milling machine using the normal sort of jig and finish reamed to 7/32".

Drilling for crankpins

All that remains to do on the wheels now is clean up the spokes and enlarge the balance weights for the 2 driving wheels. The wheels were cast in a 2 part mould split on the centre line of the spokes and there is a lot of flash that needs to be removed. I've found that a decent set of diamond coated needle files are best for this job as the skin on the castings is very tough and ordinary files hardly touch it. You could use diamond coated burrs in a small Dremel type drill but I prefer to do it by hand. It's a long and dirty job either way so I'll do it bit by bit in-between other jobs.

Crankpins are turned from 1/4" dia. silver steel. The original Helen's pins are 1/4" dia for the driving crankpins but only 3/16" for the coupled wheels but I decided to make them all 1/4" for a longer life.

Finished wheel and crankpins


The next job to be tackled was the axles and axleboxes. I had already decided to use drawn cup needle roller bearings on all axles so looked around for a suitable supplier. I use RS quite a lot for electronic components etc. and found that they sell suitable bearings but only in metric sizes, not that that really matters. I decided to use 10mm bore bearings for the main axles and 8mm bore for the bogie axles and 8 of each were ordered along with some lengths of 10mm and 8mm silver steel.

The three rear axles are simply lengths of 10mm silver steel turned down at each end to 5/16" dia. to a tight fit in the wheel bores. These were turned by holding the silver steel rod in a split bush bored in the 3 jaw to ensure the wheel seats were concentric to the main axle surface. My 3 jaw chuck does not hold bar stock accurately enough when gripped directly in the jaws. I will probably use Loctite for eventually securing the wheels in place on the axles.

The crank axle is a built up job with 1/4" mild steel webs. I used Loctite 638 to 'stick' it all together and put 2 1/16" taper pins through each web into the axle for added security. I decided to leave the axle in one piece for the time being in case I decide to press fit the wheels instead of using Loctite. The centre of the axle between the crank webs will be removed after the wheels etc. are fitted.

I found an article in Model Engineer by Phil Bevan (Vol. 158, No. 3792) on making simple Cannon type axleboxes for 5" gauge locos and decided to make mine along similar lines. The principal behind Cannon axleboxes is that the axle is fully enclosed inside a solid axlebox which goes the full width of the frames, thus protecting the bearings from ingress of dirt, grit etc. I thought this would be ideal for Helen's 2 rear axles as they pass under the firebox and the bearings would be exposed to ash etc. The 2 front axles cannot have a continuous axlebox across the frames due to the crank for the centre cylinder and the 2 eccentrics for the 2 axle driven pumps being in the way. Instead the axleboxes will extend as far as the crank and eccentrics and have seals both ends to keep out dirt.

I thought about turning the axleboxes out of square steel bar of a suitable size to fit the horns but eventually decided to build them up using round bar for the actual axle 'tubes' and loctiting these into conventional type axleboxes milled from rectangular steel bar. The 'tubes' are drilled through with a hole to clear the diameter of the axles and each end bored out to take the needle roller bearings. Anybody thinking of using these drawn cup needle roller bearings should note that they must be force fitted into a housing several thou undersize so that the bearings are compressed. If this is not done, the axles will be a very sloppy fit!

Drawing of proposed full width axlebox

The two full width tubes were made from 11/16" dia. mild steel and drilled and bored in the 3 jaw chuck to take the bearings. The bearings are recessed into the ends of the tubes so that seals can be fitted on the outside. The tubes for the front axles are similar but shorter so that the inside ends butt up against the crank webs and the pump eccentrics. They are bored on the inside end to take an 'O' ring seal.

Crank axle with bearing tubes, seals, and bearings

The 2 pump eccentrics were simple turning jobs from 1" dia. mild steel bar, the groove being turned with a parting tool set square to the lathe axis and the embryo eccentrics parted off. They were then held in a split bush for skimming the parted off face. The eccentrics and bush were then transferred to the 4 jaw and offset by 3/16" to bore the hole for the axle.

Boring eccentrics in the 4 jaw

Crank axle, eccentrics and bearing assemblies

I haven't any suitable material in stock for making the axleboxes so will order some after Xmas.

The drawings for the loco are being done as I go along so the final details of the frames, such as positions of all fixing holes etc. won't be known until the chassis is complete. I am still working out the design for the cylinders which I'm hoping to fabricate from cored gunmetal bar and brass sheet.

The outline for the frames has been decided on though so I printed out a full size dimensioned drawing to work from. The frame steel came from Reeves and I chose 3mm thick as they didn't stock a long enough size in 2.5mm and I couldn't be bothered to get a piece specially cut! I think the extra thickness will give the frames a bit more strength as they are not very deep (2-1/4") for their length (28") and the horn slots will be quite deep. Also the rear section of the frames over the trailing bogie is quite flimsy. I think a substantial stretcher will be needed here to stiffen it up. Incidently, the frame drawing, which was printed out on a HP Designjet 250C, is absolutely spot on to the correct size.

Frame steel with full size drawing


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