Internal Spur Gear

[cm777]

I am wondering if the rim thickness factor formula provided in AGMA 2001-D04 Annex B, valid for internal spur gear? Also, does anyone know if the Lewis beam strength equation can be used for checking adequacy of internal spur gear teeth?

 

[dinjin]

There are Y and/or y factors for internal gears as well as for external gears to determine strength.

 

[tbuelna]

cm777,

A general rule of thumb for rim thickness is at least 1 whole depth. The reason for this is to make sure that any fracture originating at the root fillet propagates through the tooth and not through the rim. A single tooth fracture is usually a less catastrophic failure mode than a rim fracture. Here's a good reference from NASA:

http://ntrs.nasa.gov/search.jsp?R=1...=all&Ntx=mode+matchall&Ns=HarvestDate%7c1&N=0

As for checking the stress margin of your internal spur gear teeth and rim, the simplified Lewis approach would not give an accurate result. Besides simple beam bending, the allowable stresses in the gear rim must take into account stress concentrations other than the tooth fillet, whether the gear teeth experience reverse bending, how the gear is mounted, dynamic tooth loads, asymmetric tooth loads along the face due to lead errors or torsional wind-up, etc.

Finally, depending upon the operating conditions and quality of your gear, it may not be limited by tooth bending. It should also be checked for contact stress and scoring.

Good luck.
Terry

 

[mraabe]

I can only tell, that the formulas in ISO 6336-3 are different for internal and external gears. For internal gears a rim thickness larger 3.5 times module is required to avoid loss of strength, for external gears it would be a rim thickness of 1.2 times whole depth.

The formulas are YB = 1.6 * ln(2.242*ht/sr) for external gears and YB = 1.15*ln(8.324*mn/sr) for internal gears, with YB > 1.

So ISO 6335 requires a larger rim thickness for internal gears as for external gears as long the whole depth is smaller 2.9 times module, which is usually the case.

I don't know how they compare to AGMA 2001, I just remember AGMA has only the drawing for an external gear in there.

 

[dinjin]

From mraabe's formulas, the external gear would be 2.70 times the module vs 3.50 for the internal gear.
This assumes that the tooth whole depth is 2.25 times the module. Interesting!