REFURBISHING THE SHADOW SPEEDOMETER RIGHT ANGLE DRIVE GEARBOX
Paul Ennis
MPH 669
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Mike Chipperfield's failed on his grand American tour: Peter Merrick advertised for a replacement for his broken one in
June's MPH. Mine gave up on the way to Anglesea last year and I suspect most of them are past their `best before' date. I'm
referring, of course, to the Shadow clock right-angle drive gearbox (also used on series `D' 3" clocks).
After the Anglesea ride I mixed and matched my spare gearboxes to get one to work - that lasted a mere 40 miles before
locking up, breaking the speedo cable in the process, so a proper repair was now a necessity. Having been in the VOC for 36
years I can't remember ever reading anything about repairing these gearboxes, so what do you do? Surely there can't be that
many second hand spares out there, can there? I know that Dave Hanson had a pair of gears made for his Shadow some
years ago but where does the average VOC member find a gear cutting company prepared to set up a machine to cut two
small helical gears?
Model engineers tend to collect all sorts of things that `might come in useful one day' and amongst my collection I have a 15
year old catalogue of standard gears: but with over 600 pages, could I find anything suitable? By measuring my worn out
gears and comparing specifications from the catalogue I concluded that the gears were "0.5 module 20° pressure angle
crossed axis 45° helix angle, left hand helical gears". Quite a mouthful, but amazingly they are available. I ordered 6 gears
in EN32 steel (their standard) and although they were not in stock at the time of ordering, they arrived within 14 days.
Before dismantling your gearbox I'd advise you to measure the position of the brass drive gear on the output shaft with
respect to the casing. It is a push fit on it's splines and, being only 40 thou thick, it is important that it is refitted to mesh
correctly with its mate in the speedo head. If in doubt take the speedo mechanism out of the case to check the mating on
reassembly. If you've ever seen a speedo head full of brass dust, this gear is usually the source.
The original Smiths gears are about 12mm long, but the mating length of a pair is about 5mm so, unsurprisingly, the new
gears are 5mm long and come with a 3mm centre hole as standard. My photograph shows the gears as supplied on the right,
next to a worn out set of Smiths gears.
Step 1: My method of fitting the new gears to the old shafts requires the gear centre hole to be opened from 3mm to 5mm
diameter. It is important to keep the hole true, so I made a simple split collet to hold the gears in the 3 jaw chuck on my
lathe. I used a piece of 1/2" AF brass hexagon bar, but any round or hexagon offcut will do. Just drill through about 7 or
8mm diameter and then bore a recess for the gear to slide into (9.5mm diameter by 5mm deep). Now mark the position of
the bar in the chuck so that it can be replaced in exactly the same place. Remove the bar and make a cut along its length into
the centre hole with a hacksaw or slitting saw. The finished collet can be seen on the right of my photograph.
Now with the gear in its recess and the collet in its original position in the 3 jaw chuck it will clamp the gear accurately
enough to drill out the centre hole to 5mm diameter. Chamfer the edge of the hole that will fit against a shoulder on the
modified shaft. At this stage I case hardened my gears using Kasenit compound. If you can't do this I'm sure that well
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lubricated EN32 gears will last many years - and if they do wear out it should be easy enough to fit new gears once the
shafts are machined to suit.
Step 2: As the Smiths gears are case hardened the next task is to grind off the worn gears. This sounds daunting but it's
quite straightforward. Using a bench grinder, freehand grind to just below the root of the teeth, that is until you can't see any
of the teeth. This will still leave the shaft above the 6mm diameter of its bearing part. Take care not to damage the end of
the shaft where the seat for the thrust ball bearing is located.
Step 3: Put the shaft in a 3 or 4 jaw chuck so that it runs true (shim under one or two jaws of a worn 3 jaw chuck to ensure
that it's running true - model engineers dodge!). Now turn the rough ground part of the shaft to 1/4" diameter. Yes, I know
I'm mixing Imperial and metric but I'm not going to suggest turning to 6.36mm! The logic behind not turning to 6mm, the
main shaft diameter, is that by using 1/4" we leave a step as a reference for the thickness of the phosphor bronze space
required later.
Step 4: Now turn the end of the shaft to 5mm diameter by 6.6mm long (say ± 0.1mm). Aim for the new gear to be a `light
press' or `sliding fit' on the shaft and use Loctite to secure the gear in position against the shoulder.
Step 5: Finally we need to make 2 phosphor bronze thrust bearings, 9mm diameter, 1/4" diameter centre hole and about
1/16" thick (this is the length of the 1/4" shaft below the gear). Cut a generous chamfer on the edge that fits against the gear
to ensure it doesn't foul the mating gear when assembled. I found it easier to part off a handful of spacers of different
thicknesses and select those which gave zero end float without being tight when the end cap, thrust ball bearing and circlip
were fitted.
All the above should be well within the scope of anyone who is competent with a lathe and, to be honest, provided that the
gear centre hole is not bored hopelessly out of true, none of the turning is all that critical. The gear meshing is already set by
the bores in the casting body.
So, where do we get the gears and how much do they cost? The company is: HPC Gears Ltd., Unit 14 Foxwood Industrial
Park, Chesterfield, Derbyshire, England S419RN, tel: 01246 26808, e-mail [email protected]. The part number is
PHO.5-12/LH - steel, description: helical gear crossed axis plain.
Expect to pay around £24 for two gears including postage and VAT, but with generous discounts starting at just 6 off and
proportionally lower postage costs it could be advantageous to get together with other VOC members to make a larger
order. I paid a total of £52.91 for 6 gears.
Feel free to call me 0121 443 5218 if you need any of my explanations clarifying.
Although my gearbox is the most common type there are bound to be some oddballs out there. I know Bob Plant's shadow
clock has a clockwise input drive whereas most speedos have an anticlockwise drive, so compare your old gears to my
photograph and if the direction of the teeth look the same you should be alright.
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