23 degree pressure angle on stub teeth ???

[MRSSPOCK]

I have a gear of unknown definition in my hand.
I measure base pitch and find it is 6.12mm.
I am confident in this measurement.
I know this 6.12 value is a fundamental dimension of the rack cutter, for example, and is fixed regardless of profile shift etc.
When I do some calculations, it appears to me that the pressure angle has to be 23°, otherwise the module becomes some obscure value.
Is it common to use 23° pressure angle on stub teeth? I can't see any mention of a 23° pressure angle anywhere on line.
This is part of a vintage German motorcycle gearbox.

6.12 ÷ ((cos 23) x 3.142) = 2.11667 which is DP12 (This is the only logical pressure angle and module I can come up with, although I was surprised that Germans use DP)

If I use 20 degree pressure angle it suggests a module of 2.07

If I use 25 degree pressure angle it suggests a module of 2.15

Any enlightenment appreciated. Thanks

 

[spigor]

I have seen a 23PA once or twice, but a 22.5PA would be far more common. A deviation of .5° could easily be explained, e.g., by profile crowning or wear (assuming it is a used gear). I'd have the base pitch confirmed by a second measurement (e.g. over wires and span) and all checked on a Gear Measuring Machine (P-Maschine) to see the microgeometry.

 

[MRSSPOCK]

Thanks for your reply. A few further points with regards to your answer.

a. I can't understand how an over pins measurement can be used to determine base pitch, since the amount of profile shift, (if present), will effect the over pin dimension, and that profile shift is an unknown for now.

b. I have assumed that if base pitch is equal to; a tooth space + a tooth width, then would that not suggest wear has no effect, i.e. since a wider space by default produces a narrower tooth, then the two added is still the same?

c. I can't see how crowning effects the base pitch either.

My over pins measurement is equal to 56.92mm, and that is with Ø3.98mm pins

It is a 23 tooth gear.

The gear has very visible correction.

I calculate the correction as 0.692, (with zero backlash, 0.703 with 0.05 backlash).

With this correction and the quoted over pins measurement, the base pitch equates to 6.14mm, which at a DP of 12 results in a pressure angle of 22.5°

This all makes much more sense, now knowing that a pressure angle of 22.5° isn't unusual. Thanks again

 

[mfgenggear]

Mrsspock

now take this gear and have it verified at a shop that has an involute checker, if the data is correct it will produce a straight involute. that is most positive way to verify it is correct.

 

[spigor]

Hi MRSSPOCK

a: A measurement over wires can be used to confirm other measurements.
b,c: The wear can be (and usually is) not uniform along the tooth's height. If your calipers touch the teeth at their tips, your result might be e.g. smaller than when they touch the teeth mid-way. The profile crowning works similar.

The gear data as you proposed (DP12 22.5PA 23T x=0.692, spur gear) match a MOW=56.92 over 3.98 wires. A final test with an involute checker, as suggested by mfgenggear, would certainly be something to consider.