Tale of two chassis by Tony Wright


I was hankering to build a battery diesel outline and as there is little in the hobby based on Hudswell, Clarke prototypes, I turned to that section in my library. The first page in the first book I looked at settled my mind, on the frontpeice was a classic Hudswell, small front gearcase, tall narrow bonnet fronted with a heavy radiator and capped with a steam loco chimney.  Right, I had the prototype, but no drawings, so a general arrangement was derived from the photograph based on a few assumed dimensions. So a generic Hudswell would fit the bill rather than a slavish scale model. As often happens fate intervened and at a visit to a show I came across Worsley Works’ stand. On the stand I found a ‘scratch-aid’ etch for a Hudswell Rail Tractor based on twelve tractors for New Zealand.  I had discounted these as a mundane and fairly unattractive prototype to build, but the etching was irresistible and was bought. The ‘scratch-aid’ etch is just that, an etching, no castings, chassis, etc and no instructions!  I now had two Hudswell project on the go both needing chassis’s which is the first bit of a project I tackle as if the chassis is duff then you haven’t built a body for it. I decided that I would use as much proprietary equipment as possible limiting machined components’ to a minimum. 

Here is the story of the building of the two chassis:  

To Start. The two loco’s are completely different, Ethyl is a large 0-6-0 outside framed bruiser with side-rod drive via a jackshaft, Greymouth is a small chain drive four wheeler with the wheels hidden by an external frame. Where Ethyl’s frame is on full view, Greymouth’s would be totally hidden. I wanted all wheels driven and to have the cabs free of control equipment to allow detailing, Ethyl would be radio controlled whilst Greymouth would be manual. Both presented problems with fitting in the equipment in the bodies so the drives would have to be encompassed in the chassis’s footprint. The Frames. Ethyl’s frame was made first, the only plate large enough I had I stock was a piece of 2.5mm thick Corten cold reduced plate. This was marked out and machined to size.

I used a 12’ Eliot shaper to do this but it could have been done by hand cutting and filing. Greymouth’s frame was milled out of a piece of 4mm bright drawn steel flat I pulled out of a clients scrap bin!

Both were drilled for axle bushes on an old Wolfcraft drill press fitted with a Black and Decker hand drill. On Ethyl, having six wheels, the line-up of the holes parallel to the rail is vital. Here the Vice was clamped to the drill base and the pair of frames moved in the vice to the next hole position. The axle holes were drilled and dreamed to size for the bushes.  

Tip – reamers are expensive and if you don’t use them often a costly waste. You can ream a sized hole by drilling to the nearest size below your finished sized and then with a brand new drill and plenty of cutting fluid drill the hole to size. BUT the job must be done in a machine – not by hand, the feed and speed kept to a minimum. The finish and size will be almost as good as a reamed hole and good enough for our needs. 

The axle bushes for Ethyl are Essel standard axle bushes, where as Greymouth’s were turned up from brass. The bushes were pressed into the frames with a dab of Loctite to ensure security. Ethyl’s frames being visible are detailed and GRS white metal tender axle boxes were Araldited onto the frame. These were gently drilled through one size undersize, the frames were re-assembled and the axle boxes hand reamed to finished 3/16th size.

Greymouth’s axle bushes were done the same way but without the detailing. 

Tip – cutting tools for brass should have a zero cutting or rake angle on the face of the tool, this prevents the tool biting into the brass and snatching. This is difficult with drills as the helix angle of the flutes automatically creates the rake angle at the cutting point. Credit card sized diamond honing files are now cheaply available, use one to hone off the rake angle on the drills cutting point. Hold the card parallel to the drill and half a dozen strokes across the cutting edge will produce a small flat. Cheap drill sets which claim to be HSS drills but prove to be very brittle in steel drilling are idea for brass and once altered will give you a brass drilling set. 

The Drives. My preferred drive is to use MFA Como motor gearbox units and on Ethyl there is plenty of room in the frames to fit one, I also detest worn drives as a cut in motor power locks up the wheels.  With Ethyl I wanted to try out cross helical gears as a drive, these have many advantages over worm drives. Firstly many more ratios are possible from greater than 1:1, i.e. step up drive through 1:1 drive (which is what I used) to less than 1:1.  

In Photo 4 the drive shaft is shown and it’s all purchased items. The Motor gear box, the 4mm coupling, the 4mm shaft, and the two ball bearings in the aluminium housings are all from MFA Como. The two cross helical gears are from HPC gears and came with 4mm bores so no machining. The gears were positioned to axle centres and held with Loctite before pinning with 1/16” taper pins. Cross helical gears impart an axial load on the shaft when driving so the end bearing is locked to the shaft by a KS tube held with Loctite trapping the bearing between the gear and the tube. The wheels sets for Ethyl are Essel wheels on 3/16th silver steel axles, the crossed helical gears needed the bores opened up from 4mm to 3/16th and this was done on the lathe with undersize drill and finishing size reamer. The gears were held in place with Loctite prior to taper pinning. The wheels are held by their grub screws in dimples in the axles.

Greymouth presented different problems; its small size was an issue ruling out MFA Como units. An internet trawl turned up Precision Micro Drives and two diminutive motor gearbox units were sourced, (Photo 6), these were coupled to Slater’s gearbox units.

I had reservations over the Slater’s units as they are small and seemed light for rigours in the garden, but I reason that two of them would be okay for this small loco. These units also use cross helical gears but with a 1:3 ratio, ideal with the ratio of the motor gearbox units. The gear boxes were assembled with Loctite onto a 1/8th axle, KS tube spacers were cut to give the Essel wheels the correct gauge. Trial assembly of Ethyl and testing on a 38in radius curve proved to be too tight on the flanges so the middle wheel set was de-flanged. To save splitting down the wheel set I turned the wheelset by gripping a scrap piece of brass in the tailstock Jacob’s chuck and with drill and then reamer in the lathe chuck bored out the brass to provide a centred journal to support the axle end. Nip the other wheel tread in the chuck, slide the brass over the axle end, oil, and skim off the flange, reverse and repeat for the other end. 

Frame Assembly. Ethyl’s frames are held apart by two substantial frame spacers, these were again made from available off-cut material. Two additional frame spacers were made, these are to support the two drive shaft bearing blocks, the positioning of these is crucial to get the correct engagement of the crossed helical gearing. However I felt that if these two spacers were flush with the top of the frame them this would facilitate the location of the body. Measuring the centre height this would leave the mesh of the gears too shallow by 032”. This was over come by adding two 015” shims under each bearing to reduce the over height to a more than acceptable 002”. A bracket for the motor was made up from brass angle and sheet and fixed at the correct height to align with the drive shaft.

Greymouth’s frames are a much simpler affair, the two frames were clamped together with the axle holes aligned with dowels. A tight clearance hole for 6BA was drilled at each end and two spaced turned up from hexagonal bar, the length of these was 005” larger than the wheel sets overall width.  

Tip – Clamping small components for through drilling can be tricky, for example the buffer beam brackets were very fiddly to clamp. The outer bracket was drilled first using Veroboard to space the holes. Then it and the rear bracket were super-glued with Zap-a-gap CA+ into place on the frame. These can then be drilled through, the brackets will then snap off (one needed a waft of heat from a micro butane touch to break the seal) then a  quick clean up with emery cloth. 

Trail assembly and a modification. After trial assembly of Ethyl the unit was run under power. I found that the drive had a distinct whirr, whirr, whirr note to it. Clearly there was a misalignment to the drive. I adjusted the setting of the MFA gearbox several times and could not totally rid the drive of the whirring. The problem seemed to be the mounting bracket supplied with the unit was less than accurate and quite flexible. A secondary bracket was mounted at the back of the motor and an 8BA screwed into the convenient hole in the motor end-cap. This stiffened the unit but I decided that in best engineering fashion to add a drive coupling between the gearbox and the drive shaft.

In Photo 8 shows the inverted frame with the modified drive and Photo 9 shows the two halves of the coupling.

The two halves are 24mm outside diameter and use four 8BA bolts with nylon sleeves cut from a water bottle filler pipe to provide the drive spigots. Running after assembly was sweet with an almost silent drive. The axle ends have Essel cranks fitted as these are almost identical to the original cranks. The jack shaft cranks are more substantial and have balance weights, these yet to be made. The side coupling and connecting rods are also pending as I have ordered these from a laser cutter, the original had split brass cottered bearings and  are a complicated shape. Greymouth’s assembly was easier as there are only two stretchers, but the Slater’s gearbox units presented problems. The units have no means of counteracting the torque reaction of the drive. I therefore decided to clamp two plates either side of the gearbox with longer screws, the torque of each unit is now cancelled out by the other unit, (Photo 10).

Between the gearbox a carden shaft using Slater’s universal couplings is fitted to keep the units in synchronisation. The two motors are mounted outboard of the gearboxes and their torque is taken by two brackets with square pockets that slip over the square side-plates of the gearboxes. The axle extension will carry a Delrin chain drive, this is cosmetic but will represent the originals drive system. 


Having worked on 12” to the foot scale Hudswell’s I have always admired their no-nonsense solid engineering. I some way building these two chassis I have captured this spirit, particularly with Ethyl. The next stage will be the building of the bodies, Ethyl will follow my usual 28 thou nickel silver soldered construction. Greymouth will be new to me as I have not built an etching before. Hopefully by the 2013 summer running season both will be finished and making appearances in the garden.































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