Author Topic: Gearbox pinion pressure angles  (Read 3611 times)

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Offline RolloTurner

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Gearbox pinion pressure angles
« on: 02 Mar 2012 at 14:02 »
Does anyone know what the pressure angle is on a 1936 gearbox and the tooth size? Pressure angle would appear to be 14.5 degrees - is this correct does anyone know and tooth size  might be 12 DP I think. But if anyone can shed some light on this that would be very useful.

Offline Jim

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Re: Gearbox pinion pressure angles
« Reply #1 on: 04 Mar 2012 at 12:05 »
Hello Rollo,
 I am no expert, but I think you will find that the gears are 12/14 stub tooth which means that they have a 12 DP tooth form and a height of a 14DP tooth which makes for a stronger tooth form, the pressure angle would be 20 degrees, I don`t think they came in 14 1/2 angle. Douglas seem to have started with stub teeth with the EW model as did several other makes in the mid twenties. I haven`t seen a 1936 gearbox but I don`t think they would go back to single DP teeth,  regards Jim

Offline eddie

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Re: Gearbox pinion pressure angles
« Reply #2 on: 04 Mar 2012 at 13:43 »
I don't know if this is going to help, but about 30 years ago I was talking to the then LDMCC prewar spares man. He mentioned that the Club was getting a batch of small gears for the layshaft made, and that they would be in stub tooth form for the earlier boxes and involute tooth form for later boxes. Other than that, I cannot help with any information regarding pressure angles , etc.


Offline Doug

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Re: Gearbox pinion pressure angles
« Reply #3 on: 05 Mar 2012 at 00:06 »
They would all be involute form. The alternate tooth form to involute would be cycloid. Both share a constant velocity motion, but the involute will tolerant a wide range of shaft center distance without affecting the velocity.

20 degree pressure angle seemed to be more prevalent prior to WW2, and 14.5 more common after that. I have not checked the mid-thirties gearboxes, but timing gears from the EW through the mid-thirties used 20 degree pressure angle. There was something that did change part way through the 4-speed gearbox's evolution, something to do with the tooth form, that prevents the third generation gears from being mixed with the second generation. But I seem to recall hearing that one can swap out the entire contents.

Stub teeth, are as Jim mentioned, a stronger variation that found favor in transmissions. They are formed by altering the pitch diameter. All the systems I know of, American Standard, Fellows, and obsolete Nuttal system are 20 degree pressure angle. Basically the pitch diameter is shifted outward, the tooth depth made shallower (or both.) This makes the teeth 'stubby' and stronger, at the expense of a few other functions. The alternative, particularly on smaller pinions with minimal number of teeth, would be a tooth profile with a weakening undercut at the root of the tooth.  The ratio is unchanged, since that is a function of the number of teeth. The 12/14 designation is one used by the Fellows system. The numerator is the pitch used to calculate pitch diameter, tooth thickness and number of teeth. The denominator is the pitch used to calculate the depth of the tooth. The other systems use alterations to the gear calculation constants for tooth depth.

And yes I know, this still does not tell you exactly what form your '36 gearbox uses!   


Offline RolloTurner

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Re: Gearbox pinion pressure angles
« Reply #4 on: 05 Mar 2012 at 08:03 »
Thanks to everyone for your answers on this. My knowledge of gear teeth has been expanded immeasurably!