Author Topic: Observations & Questions on Module & Gear dimensions  (Read 6169 times)

Offline dcunningham2

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Observations & Questions on Module & Gear dimensions
« on: February 12, 2007, 11:01:04 AM »
Just found the pdf document that JR had posted describing how to calculate Module and the various diameters and geometries of wheels and pinions using that quantity.  This really takes the guesswork out of determining clearances and tooth dimensions of gear trains for me.

In digging into the formulas, I noticed two peculiarities:

1.  There appears to be an error or inconsistency between two of the formulas -- specifically, there is a diagram of a pinion gear in the lower left-hand corner that contains a dimension that conflicts with an earlier diameter formula.  The radial distance from the pitch radius to the outer tip of the gear is defined as 1.5xM.  I believe it should be 0.75xM to agree with the Pinion Outer Diameter formula defined in the same document as Mx(N+1.5).  I believe someone forgot to divide by 2 to account for radius versus diameter.  Also, were this dimension to be 1.5M, there would hardly be any clearance between the tip of the pinion and the root of the wheel (1.55xM - 1.5xM = 0.05xM) when the two gears are engaged.

2.  If I assume my modification described above is correct and calculate the clearances, I discover that the wheel now has twice the root clearance as the pinion.  When fully engaged, the distance from the tip of the wheel to the root of the pinion is 0.4xM, and the distance from the tip of the pinion to the root of the wheel is 0.8xM.  It seems to me that this additional clearance has no value in compensating for misalignment because the wheel tooth will foul in the pinion root before the additional wheel root clearance can be consumed.  It would seem that these clearances should be the same.

If I'm correct, then the wheel teeth can be cut less deeply, which I would think would increase strength and dimensional stability.  I calculate that the formula for the Wheel Root Diameter could be modified from Mx(N-3.1) to Mx(N-2.3).  This would reduce the radial distance between the root and the pitch circle from 1.55xM to 1.15xM,  and the clearance between the pinion tip and wheel root would be reduced from 0.8xM to 0.4xM.

Would anyone who is interested give your thoughts on the feasibility of cutting wheel teeth less deeply.  Also, please let me know if my analysis is flawed.

Thanks !!
Dave Cunningham