Mystery Gearbox

[Archivist]

A recent generous donation to the Archive has been drawings of a couple of gearboxes that have caused a bit of head scratching. They are, without doubt, genuine Austin drawings dated 1929, though these versions were issued by the Austin Service Department to a 750 MC member, John Watt, in 1947. So far, enquiries amongst a few people 'in the know' have drawn a blank. 

You will see from the example I am posting, it is described as a Silent 3rd, 4 speed Austin 7 gearbox. The other drawing I have is for a Silent 2nd 3 speed gearbox. The silent 3rd gearbox is described by Bob WYatt in his History of Austin book, though he doesn't suggest any possible use. They seem to be 'robust' in the extreme, with more bearings than normal for starters. 

I offer them for further discussion and extravagant speculation amongst the crowd here...

Hugh

   

[Chris KC]

The patent might be traceable Hugh?

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I'm more than likely reading it wrong, but based on a few mins of staring at it i would guess that 1st & 2nd seem more or less conventional; top is engaged by sliding the 3rd motion shaft rearward from the position drawn; and that 3rd is perhaps achieved by sliding it forwards, so that the internal teeth on the rear gear cluster engage with the external teeth on the 'floating' cog in the middle, which is supported by the extra bearings (which presumably fit into something which isn't drawn). The key point being that the 'floating cog' is not concentric with the shaft. Heaven knows what ratio that would give. I might be very wide of the mark! One thing I note is a bush in place of that horrid intershaft roller.

[Hugh Barnes]

I did have a poke around in that space, Chris, but I have no experience in that area and drew a blank. If anyone knows more about that sort of searching, I would be very happy if they might find a few minutes to dig on my behalf?

[Mike Costigan]

In 1929 the Austin Motor Company was actively courting the War Department for contracts; could it be a design for an 'off-road' version?

[Dave Mann]

I've looked at this drawing a couple of times and to say the least I'm confused by the apparent eccentric assembly like Chris.

[Dave Wortley]

The eccentric assembly was designed by the same draughtsman who incorporated the notorious “blind spline” on the gearbox input shaft. Must have been a disgruntled Austin employee.

[Chris KC]

I *think* that drive in 3rd is achieved by running small gears with external teeth in mesh with larger gears with internal teeth, which would have to be offset otherwise they would lock solid or not mesh at all. 
It looks rather expensive to produce so my guess is it never saw the light of day.
It may be just a concept drawing, perhaps in support of the patent application; but if so, curious that it has surfaced via the Service Dept.

[Bob Culver]

I woud have expected the design to come from R and M or SKF. It is near my bedtime and my brain is starting to smoke. Presumably nuetral, top, third selection is by moving the mainshaft axially? What gives it the silent feature? Are internal gears inherently quieter? It has acquired the very considerable complexity of a second layshaft surrounding the output mainshaft. Maybe it is what draughtsmen did for amusement in their lunch breaks. The designer was probably familair with hand lawnmowers.
Silent 3rds were fashionable around 1931 acheived wth constant mesh helical or double helical gears as the late RN, early RP? Dunno how helical gears are depicted. Teeth have rounded corners more typical of sliding mesh teeth.
There is a vast difference in machining complexity between the simple 1920s box and the 1933 synchro box

[Chris KC]

Bob, don't forget Austin was working quite closely with Frank Hayes on his auto transmission from the late 20's and I understand numerous patents were filed by Austin, this may have been a direct spin-off from that work.
As for the 'silent' third no, I don't get it either. Perhaps it was a relative term!

[Hugh Barnes]

In other gearboxes I have come across described as Silent 3rd (a euphamism for 'noisy second') the silence comes from dogs engaging which transmits the drives, rather than gears running in mesh.