ACME screw actuator failure analysis

[beatcan]

We have a linear actuator manufactured in China. We recently have been getting many failures at a stress concentration area of the screw.
The basic design is a 0.75" ACME screw. Threads machined off to a diameter of 12mm (.472&quot where a 5mm (.197&quot hole is drilled. A pin through this hole transfers the geared motor load and absorbs the thrust load. Our units are rated for a maximum load of 1000lbs. I have worked the max tortional load (lifting) to be 112 in.lbs. The screw has been breaking in half at the pin location. Material for the screw is 1015 and for the pin is SUM22 (apperantly the same as AISI 1213). RPMs are low (approx. 30rpm) Using the max shear theary I come up with 9763 psi shear load for the screw and 18131psi for the pin. I am not sure what the stress concentration factor should be for the tension load of the screw? The failures seam to be fatigue failure. Failures are occuring in the field at loads of 400lbs.

Any suggestions as to how to reduce failures without any major changes to the parts?
Do my stress analysis values seem right?
The screws are manufactured at the plant (threads are rolled). Therefore it is cold worked then the cold rolled thread is machined off. Should the new machined surface be cold rolled again?
What fraction of the yield strength should my max allowable stress be?
Will going to a higher carbon steel make the material more brital, thus more susceptible to failure?

Please help. Let me know if you need more info, I can provide it.
Thanks,
Beatcan

 

[ecampos]

Sounds like you are neglecting on of the two loads here.
I think that if I read it right, you have torsion from the motor torque and bending on the pin from the thrust load. Is this combined loading considered in your stress calcs?

Regards
Edson Campos

 

[beatcan]

I used 1/2 the tension load for the max shear theory (1/2 of 12210psi), combined with the 338 psi with a stress concentration factor of 10 (so 3387psi) because the diameter to diameter ratio of the hole to screw diameter was off the stress concentration chart. I also used a shafting constant Cm of 1.5 for gradually applied loads. These two values in a Mohr diagram gave me a shear stress value of 9767psi. I think my stress calculations are correct, but I don't know about the stress concentration factors.

 

[plasgears]

It looks like you should give serious consideration to mating the acme screw with a proper acme nut. Our actuators have actuation tubes with an acme nut on one end and clevis pin holes on the other.

Engineering design can take either of two extreme directions: conservative and stout, or design for low cost. The low cost approach can be a mine field for failures and delays in the program. I wouldn't actuate thru a small shear pin on an acme shaft. Good luck.

 

[Metalguy]

Can we assume that your failures are via fatigue cracking?

If so, you can forget about yield strength and concentrate on UTS, because YS doesn't correlate to fatigue strength/limits very well. Try and reduce the stress conc. at the hole. Your materials are pretty soft-you can raise the C (etc) a bunch before you'd have to worry about brittle fracture.