Stress Concentration Factors for Threads 1

[trallee]

I am designing a drive line that must be threaded. I have a size constraint (maximum)on the diameter that must be used. I have tried to use V-Threads but when I add the stress concentration factor of 2.8 to the calculation my stress is well above allowable. I am looking for stress concentration factors for round form threads. I thought about using the concentration factor for a radius grooved shaft. If any one has any ideas I would appreciate the help.

 

[ecampos]

I think that your idea of using a radiused shaft is a good approach.&nbsp;&nbsp;There is a good section on calculating the kt factor in the Machinerys handbook 25th edition page 98.<br>I am wondering where you came up with the 2.8 stress concentration factor?<br>I will keep looking for you.<br>

 

[Stress]

A good book for Stress Concetrations is a book by Peterson. You can always build a FEA model. Depending on the application you may be over designing the part. Stress concentrations are typically looked at during LCF, I am not sure how you margins of safety are being calculated.

 

[DonLeffingwellPE]

You might also want to check into using a different material, i.e., one with more strength, and also look into hardening the material. I realize these are more exspensive approaches, but sometimes when you have dimensional constraints, these are the only alternatives.<br><br>Don Leffingwell PE<br><br><A HREF="mailto:dleffingwell@snet.net">dleffingwell@snet.net</A>

 

[trallee]

To reply to ecampos and stress I used the stress concentration factor from Peterson's Stress Concentration Factors for axial tension in a thread. I tried the FEA model and after the 4 th time it crashed I realized the FEA software I have is pretty lame.<br><br>Don,<br>I tried the material approach on the customer and he about croaked when I quoted him prices on the harder and stronger materials.<br><br>I guess the next question I need to ask is there a resonably priced FEA program out there that can handle threads?

 

[MikeVV]

Drive shafts typically experience cyclical loading so materials with an endurance limit are needed, otherwise no matter what you do in terms of design, the shaft will eventually fail.&nbsp;&nbsp;Stress concentration factors are typically related to the part geometry and loading condition only, no affect of material properties is included.&nbsp;&nbsp;The constraints that you have for geometry will force your design problem into a material or loading issue.&nbsp;&nbsp;Hardened materials are stronger but they also have no ductility so they crack when lower strength materials deform.&nbsp;&nbsp;A hardened material might be worse than a lower strength but ductile material.&nbsp;&nbsp;Can anything be done to reduce the load?&nbsp;&nbsp;Also, <A HREF="

http://www.cadreanalytic.com"

TARGET="_new">

http://www.cadreanalytic.com</A>

offers a low price FE product that is relatively easy to use.&nbsp;&nbsp;It uses beam and plate elements.&nbsp;&nbsp;You might be able to mesh up the drive shaft cross section and look at the state of stress at the thread form.&nbsp;&nbsp;Such analysis gives you a &quot;sharper pencil,&quot; that is, a more accurate evaluation of the true stress state than can be obtained from &quot;closed form,&quot; or handbook calculations.&nbsp;&nbsp;Such accuracy allows you to relax safety factors.&nbsp;&nbsp;Be warned however, this can be dangerous to those who use the equipment and testing must be done to verify that you did not make a mistake during analysis.<br><br>Mike Van Voorhis<br><A HREF="mailto:MJVanVoorhis@CS.com">MJVanVoorhis@CS.com</A><br>