FEA for Gear with high stresses at rim 1

[Skyhawk989]

Hi, i am analyzing a gear using FEA methods to assess the strength capability of various regions except for tooth bending which is analyzed using AGMA standards. My question: If the the FEA shows high stresses below the gear tooth does this area need to be assessed considering the rim thickness factor is taken into account within the AGMA standard for determining tooth bending stresses?

It seems to me that I dont need to assess this location below the tooth assuming AGMA shows tooth bending to be acceptable. A crack would never initiate below the tooth(i.e. at inner diameter of rim), it would initiate at the tooth base radius....let me know if I am wrong.

 

[desertfox]

Hi

This might help, look at page 22 it's shows the root of the tooth and yes there is a possibility that a crack could form there. I am not sure whether AGMA covers this or not.

http://www.dtic.mil/get-tr-doc/pdf?AD=ADA380656

 

[gearcutter]

In answer to your question - yes, this area should be assessed as geometry of the rim can play a large influence on the final result and is an area that the standards don't cover.
The AGMA standards directly refer to this as the 'backup ratio' but, as mentioned, does not cover the specific geometry of the part.

Some handy info here from Cornell Uni.
The work done is similar to that shown in the NASA document that desertfox provided.

http://www.cfg.cornell.edu/projects/spiral_bevel.html

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

The geometry of the rim/web/hub can have a huge effect on the overall stress levels, and should be considered in combination with tooth root stress. With high PLV gears, the rim mass itself can add significant dynamic stress. Wide face gears that are supported by a narrow central web/hub will have lower radial stiffness at the outboard rim edges, which will affect the mesh load distribution across the gear face, and thus the tooth root bending stresses. The rim mass properties can also affect the structural vibration characteristics of the gear. If the gear rim has an unfavorable structural mode that couples with a meshing frequency, these dynamic forces can produce substantial additional stress in the gear structures.

Lastly, as desertfox and gearcutter noted, for safety critical gear applications it is important to ensure that your gears never experience a fracture between the tooth root fillet and the rim ID. This type of failure is usually catastrophic and results in complete loss of gearbox function. Instead, the rim should always have sufficient thickness to ensure that any fractures originating in the root fillets propagate through the tooth rather than through the rim. A gearbox can often continue to function with the loss of a single tooth from a gear. As a rule of thumb, gears designed for critical applications usually use a minimum rim thickness below the root equal to 1.0 times the tooth whole depth.

http://www.dtic.mil/dtic/tr/fulltext/u2/a398842.pdf

http://www.dtic.mil/dtic/tr/fulltext/u2/a312521.pdf

Hope that helps.
Terry

 

[israelkk]

Effects of Rim Thickness on Spur Gear Bending Stress at

http://www.dtic.mil/dtic/tr/fulltext/u2/a239500.pdf

 

[Skyhawk989]

thanks for all the replies. I will analyze the rim within the FEA since it looks like its not covered by the standard. Best to be safe than sorry.