I was still waiting for my laser welding repairs to be done and having just about completed the clutches, I decided to rebuild KTT 305s gearbox. I had also ordered a bunch of new bearings for the 3 gearboxes that I was rebuilding.
I have replaced every bearing that is questionable and these KSM bearings are good quality and not too expensive so there is no point in being a cheapskate on gearbox rebuilds. I have also replaced the sleeve gear bearing on this gearbox – available from Velocette Spares Ltd.
After having the gearbox shell vapour blasted it turned out to be in pretty good condition apart from one detail, namely the 2 small screws that hold on the BK-52 wire clip for the thrust cap – the bits shown below.
The threads were oversize – 3/16” instead of the 1/8” that they should be and one of these was partially stripped in the casing. Somebody had made a repair in the past but either this was carried out badly or had not withstood the test of time and so it was time to make a proper repair.
I ended up making 2x shouldered studs out of silver steel (not heat treated – it’s just that bit tougher than mild steel) that were ¼” BSW on one end and 4BA (which at 0.142” OD is slightly larger than the original 0.125” and is a good fit to the new wire) on the other and the case was tapped ¼” BSW to accept the new studs.
The studs were screwed in using 2x 4BA locked nuts.
The gears that were in the ‘box were standard ratios. The gearbox would originally have been fitted with either close ratios or TT ratios and by a stroke a luck, I just happened to have a complete new close-ratio gear cluster on the shelf.
….it wasn’t really a stroke of luck - I had taken the opportunity of buying this set of gears many years ago when a Velo owners club member commissioned a few sets.
The gears appear to have been well made; however, a bit of fettling was needed to get them to actually fit.
The first problem I found was that the dogs on the sliding gear on the mainshaft would not engage with those on the sleeve gear because the OD of the protruding bush on the sleeve gear was slightly greater than the ID of the dogs.
The easiest way to resolve this was take a few thou off the inside of the dogs using a Dremel
to get a nice smooth engagement.
The next little problem was that the new low gear BK-9/3X would not fit on the end of the mainshaft. The existing mainshaft, which has a nominal diameter of 5/8” was already slighty undersize through wear (measured at 0.624”) and the ID of the gear was found to be 0.620” …so no chance! Luckily, I have a 5/8” hand reamer and by mounting the gear on-centre in the lathe using the dogs I could open up the hole
to get a perfect fit.
The next issue with this gear was that it was too wide!
and extended beyond the shoulder (which reduces to ½”
diameter to enter the bearing) on the mainshaft. If it were fitted like this
then the gear, which needs to be free to rotate on the shaft, would be clamped to
the shaft as it would be sandwiched between the shoulder at the end of the splined
section on the shaft and the bearing inner when the ½” nut on the outside of
the bearing was tightened. The width of the gear was therefore reduced by a
few thou to ensure that the shaft rather than the gear would be clamped to the
bearing inner.
Last, but by no means least, the housing for the B22/3 bearing in the layshaft, shown below,
had been machined significantly undersize – the interference fit would have been 0.0045” which is far too much. Even big-ends and main-shafts in flywheels are not normally assembled with such a tight fit and this level of fit would lock the bearing solid.
But how to fix this? I could not see any straightforward way of setting up and supporting the layshaft robustly and accurately in either my lathe or milling machine for a quite delicate machining operation to remove this small amount of material. I have therefore cheated and reduced the OD of the bearing itself, which is much more straightforward.
A mandrel was machined in the lathe to accept the bearing and for which the bearing outer could be firmly clamped
and the tool post grinder was then used to reduce the diameter to give a 0.0005” interference fit.
Perfect!
This does of course mean that any replacement bearing will
have to undergo the same procedure but as I am replacing bearings that are
probably original and fitted 95 years ago and with the amount of usage that
these bikes are likely to get in the future I don’t believe this will ever be a problem
in practice.
A brief note of caution on using toolpost ginders; turning on and off the grinder should never be done with the grinding wheel immediately adjacent to the workpiece. The rated speed of the motor on my grinder is 45,000 rev/min and somewhere in the middle of this speed range there is a resonant speed that induces oscillations into the spindle/grinding wheel. The resonance passes quickly but if the wheel is nominally in contact with the workpiece then it can cause damage. The lathe also needs to be covered and cleaned down immediately afterwards to avoid carborundum dust causing damage.
The next task was to check/setup the layshaft end-float. This is important because: if there is too much then the layshaft would not be properly located on the kickstart shaft (that goes through the centre of the layshaft) and if there were no (or “negative”!!) end-float - which is a worse situation, then the layshaft would be clamped between the kickstart shaft bush, B-15, in the gearbox shell and the bearing, B-23/3, in the cover.
Various methods have been proposed by VOC members, notably in Fishtails 333 and 334, for checking the end float with the gearbox in the frame. So, here’s another method, this time with the gearbox on the bench.
The first issue to sort out was that the kickstart shaft was incorrect. There are 2 versions of this, shown below.
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The upper shaft, part number B-11/4, is for a KTT (this one is from KTT 55) that does not have a kickstart. The lower B-11/3 shaft, which is from KTT 305, is for a gearbox that would normally have a kickstart and the arrangement at the end shown in the above picture would have contained a couple of springs and rollers and a B-14 end cap. This gearbox did not have any of those parts – just what is shown above. In the absence of those parts, the kickstart shaft was hard up against the end of the B-25 bush resulting in the layshaft having nearly ¼ " of end float. At the other end, it would have been longer as someone had chopped off the kickstart part.
Rather than just copy the dimensions of the original KTT shaft I have used the incorrect shaft that was fitted to work out the dimensions for a new shaft to give a desired end float (I have opted for 0.010") and to position the shaft correctly. The easiest way to do this was to take a series of measurements. First, the distance between the mating surface and the inside of the bearing (determined from the measurement shown below minus the measured thickness of the bearing).
I have had to use my milling machine table as a surface plate as my actual surface plate is currently occupied with a pile of Velo bits and pieces.
The second measurement is the gasket thickness
which came out at 0.035”, estimating the pressure to which it would be compressed.
And now, by putting shims onto the kickstart shaft (to act as a spacer to stop the end of the shaft abutting against the inner end of the bush),
and measuring the distance from the mating surface to the shoulder on which the bearing will contact
a bit of simple arithmetic gives the required dimension of a spacer that will give a desired end-float and which includes the gasket. This turned out to be 0.21” to give 0.010” end-float and a spacer with dimensions 0.75” OD, 0.5” ID and 0.21” thick was made
that fits on the end of the kickstart shaft. There is a small radius on the inner side of the spacers hole to accommodate the radius at the change of section on the original shaft.
The assembly with the spacer rather than a bunch of shims was
then checked to confirm that the end-float was 0.010” as intended. Why bother to make a spacer? Well, because it's a 10 minute job and as the layshaft assembly is being put in and taken out a number of times to make the various checks and the chances of misplacing a shim or 2 is quite high (which would screw up the whole exercise) it's just easier.
The final step was to ensure that the teeth on the sleeve gear and layshaft gear align. The gearbox was partially assembled with the sleeve gear bearing and sleeve gear
and, using the vernier for a depth measurement, the difference in mesh was measured at the location shown. This turned out to be 0.054” and the sleeve gear was shimmed, shown below,
with 2x 0.019” plus a 0.011” (= 0.049”) shims to align the gears. These shims can be found on eBay at very reasonable prices in metric sizes and are available in various thicknesses between 0.1mm to 1mm.
I mentioned at the beginning of this section that the objective was to figure out the exact dimensions for a new kickstart shaft to give a desired end float - which I now have and the last part of this was to make the shaft longer to avoid the partial entry into the B-4/3 end cover of the truncated original.
The final step was to make the new kickstart shaft.
This is a short video showing the half-machined shaft in the lathe to check the fit to the bearing - a good tight push fit
For comparison, in the above picture the upper shaft is the incorrect original that was fitted and with the paraphernalia used to figure out the end float, the one in the middle is from KTT 55 and the lower one is the new shaft, made from EN24T, for KTT 305.
Just for the record, the length of the larger diameter section on the original KTT 55 kickstart shaft was 0.69".
The required length of this section on the new shaft is 0.716".
If I had simply copied the dimensions of the original B-11/4 shaft I would have ended up with 0.036" end float.
The positioning of the layshaft was checked and the sleeve gear, the other gears and mainshaft assembled, the selector plunger and spring inserted and the end cover put on.
All gears selected perfectly and so the remaining little jobs – making a clutch thrust pin (C30) from silver steel and heat treat the ends
and assembling the thrust cup assembly (C29/26) with the new thrust bearing
just about complete this gearbox
and this is the pile of bits that is left over.
Looking at this pile of left-over bits reminded me of the time that I started with Perkins Engines in Peterborough as an engineering student back in 1969. All students and craft apprentices spent the first 4 months in the Trade School - lathes, milling machines, welding, electrical stuff etc. and one of the parts of the course was stripping and rebuilding a diesel engine. It was not uncommon to remove the sump and find a pile of bits and pieces that had been stuffed in there by the previous students that had stripped and "rebuilt" the engine, either because they got bored with the exercise or didn't know where they fitted.
Anyway, at the time of writing this blog posting, the laser welding repairs have just been completed on 2 of the other gearboxes so I can now get back to working on these.
