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Working Depth and Whole Depth of Metric Helical Gears
2

Working Depth and Whole Depth of Metric Helical Gears

Working Depth and Whole Depth of Metric Helical Gears

(OP)
What is the rule of thumb for the root diameter for metric helical gears?  Isn't it (#teeth-2)*module for spur gears?

RE: Working Depth and Whole Depth of Metric Helical Gears

It depends on the manufacturing method / "standard rack" used.
Usually it is (#teeth-2.25)*module, what creates 0.125*module clearance between the meshing gears' root and tip.

RE: Working Depth and Whole Depth of Metric Helical Gears

gearguru,

I agree with your formula for root diameter, but assuming the usual 2*module working depth, you would get a tip to root clearance of 0.25* module, not 0.125*module.

RE: Working Depth and Whole Depth of Metric Helical Gears

The formula above means, that the root DIAMETER is 2.25*m smaller than the pitch diameter.
Root RADIUS is half of that value, it  means it is 1.125*m smaller than pitch radius. The meshing tooth' addendum = module, subtracting the addendum we'll get the clearance (0.125*m)
Of course, we are still talking about the standard ("N") teeth on both meshing gears; the pitch diameters (z*m) are tangent (they are operating diameters as well).
(the symbols in formulas above mean: z = # of teeth, m = module)

RE: Working Depth and Whole Depth of Metric Helical Gears

The dedendum is 1.25 and the addendum is
1.00 so .25 is the clearance.  You do not
divide the 2.25 ratio by 2.
Philrock is correct about this.

There are also two ways of cutting helicals
in Europe so caution is needed.
If a rack cutter as Gearguru suggests is
used then the above applies.   

RE: Working Depth and Whole Depth of Metric Helical Gears

Diamondjim,
If dedendum is 1.25*m (depends on the rack) then
root diameter = pitch diameter - 2.50*module.
In such case I agree that the clearance is 0.25*module.
But in my example above the root dia = pitch dia - 2.25*m,
I calculated with the rack with 1.125*m addendum. For my example the clearance is 0.125*m.
The original question was "What is the rule of thumb for the root diameter", I answered it and calculated the clearance using that answer.
You did not mention that in your case the root dia is pitch diameter - 2.50*module

RE: Working Depth and Whole Depth of Metric Helical Gears

Gearguru,
Well I did read that too quickly.
Most module gearing was 1.166 times
the module until the last 8 years
where 1.25 times the module was adopted
for full depth gears.  I have not seen
1.125 times the module on a rack so I
made the wrong assumption.  Sorry!

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