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Fellows Stub Tooth, 9 teeth on a 10 tooth blank? Help..

Fellows Stub Tooth, 9 teeth on a 10 tooth blank? Help..

Fellows Stub Tooth, 9 teeth on a 10 tooth blank? Help..

(OP)
I have a pinion spur gear that I need to mate a new gear with. It is an 8/10 Fellows Stub tooth with 9 teeth on a 10 tooth blank. Pitch diameter the # teeth divided by the diametral pitch. I know for Fellows Stub Tooth gears the numerator of the 8/10 diametral pitch is the one to use to establish pitch diameter (the 8), but what of the # of teeth? The pinion prints says the pitch diameter is 1.25. That means they were using the # of teeth for the blank size (10), not the actual # of teeth (9). Is this correct? I do not understand what 9 teeth on a 10 tooth blank means, and whether to use 9 or 10 to establish pitch diameter.

RE: Fellows Stub Tooth, 9 teeth on a 10 tooth blank? Help..

If I understand you the gear is has 9 teeth but the actual pitch diameter is as the theoretical pitch diameter of 10 teeth. It seems that the designer used a profile shift (tooth correction) to reduce undercut. The amount of correction is such that the actual pitch diamter is 1.25.

For gear transmission rate calculations you need to use the number 9. For center distance calculations you need to use the pitch diameter of 1.25.

RE: Fellows Stub Tooth, 9 teeth on a 10 tooth blank? Help..

One more reason for tooth correction is to make the teeth stronger and maybe to reduce specific sliding.

RE: Fellows Stub Tooth, 9 teeth on a 10 tooth blank? Help..

When you cut 9 teeth in a 10 tooth blank, it usually means that you are using an enlarged od pinion to run with a reduced od gear on standard centers.  It reduces the amount of undercut in the pinion and makes for a stronger tooth.  This is usually done when you have a small number of teeth in the pinion and a large number of teeth in the gear.  You calculate the od for 10 teeth but you only cut 9.  Since you have a print, you cut the teeth to the size on the drawing.  If you don't have a print, it becomes a lot more challenging

RE: Fellows Stub Tooth, 9 teeth on a 10 tooth blank? Help..

There are two questions in your proposed case.

1) The meaning  of Fellows stub – tooth system

     For example for DP 8/10:

a- use DP 8 for pitch diameter calculation
 
d = N / DP = 9 / 8 = 1.125 "

b- use DP 10 for addendum and dedendum calculation

addendum =  a = 1 / DP = 1 / 10 = 0.1”
                        
dedendum = 1,25 / DP = 1.25 / 10 = 0.125”

The above values are the same  as DP 8  stub – tooth  gear with 0.8 / DP addendum (addendum = 0.8 / 8 = 0.1” )
and 1 / DP dedendum ( dedendum = 1 / 8 = 0.125”).

2) The meaning of the use of 10 teeth blank for 9 teeth gear, for example.

This is an old and bad way of specification for one Profile Shift  gear.

The pitch diameter for DP 8 gear with 10 teeth is
  
d = N / DP = 10 / 8 = 1.250”

The pitch diameter for DP 8 gear with   9 teeth is
  
d = N / DP =   9 / 8 = 1.125”

Profile shift = (1.250 – 1.125) / 2 = 0.125” / 2 =  0.0625 = x / DP

x = 0.0625 (DP) = 0.0625 (8) = 0.5

3) Conclusion:

    The  stub - gear mentioned  will be better and clearer specified as:

DP = 8; N = 9; x = 0.5; addendum = 0.8 / DP and dedendum = 1 / DP


 

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