Calculating Shear Pin dia. with determined load & constant cyclic load

[fuzion]

Hi, I am in a dilemma. I am trying to design a shear pin that would break (shear) with a predetermined load applied. However this shear pin also is seeing a cyclic load. I know that cyclic loading fatigues a shear pin and may cause premature shearing. This shear pin transmits a torque to a centering device, and if the device gets jammed, the user would be able to breakthru the jam and still have function without the centering. I am using a shear pin of .093 inch press diameter, necked down to .080 inch for the shear section which is made of 13-8 H1150 steel. The input is 7.5 inches from center and the shear pin is .875 inches from center. The shear pin needs to break at 90-170lbs at input which is 772-1457lbs at the shear pin. The normal operating loads is 25lbs at input, 214lbs at the pin. and this is a reversing (cyclic) load.
I chose 13-8 because of good fatigue resistance. So how should I go about designing this shear pin to take account the cyclic loading, because in static condition the pin breaks at the predetermined load.
Thanks for your help and time.

 

[desertfox]

Hi fuzion

Have a look at these sites it might help.

http://www.roymech.co.uk/Useful_Tables/Fatigue/Fatigue_index.html

http://www.co-design.co.uk/dpg/sfl/sflhome.htm

regards

desertfox

 

[israelkk]

If the pin shears at 772-1457lbs then it should not fail in fatigue at 214lb which is only 27.8% of the ultimate shearing force.

 

[rb1957]

structural fuses are really fussy to design. i'm not so sure about isaelkk's assessment based on the fatigue loading being a small %age of ultimate.
214lbs shear on a pin 0.08"dia is a stress of 43ksi ... witha Kt of 3 (conservative for your section and loading) ARMMPDS-01 (the replacement for MIL-HDBK 5) which lists the fatigue behaviour of 13-8 H1000, would give an unfactored fatigue life of about 100,000 cycles (assuming cyclic tension),
but your static loading results in a stress of 772/214*43 = 154 ksi ... so you pin will fail at a lower load than you want (fsu for 13-8 H1150 approx 60% of ftu = 80ksi), and this is only about double your fatigue stress.

i think you need H1000 with an fsu of 120ksi, and maybe acceptable fatigue life (do you need infinite life ? what is the Kt for a shear load ? )

 

[fuzion]

Hi first thanks for everyones responses.
5 million cycles under normal operating load which is about 25 lbs at the input.

 

[rb1957]

without doing a thorough analysis, i think your pin is understrength ... i don't think you'll achieve your static load, i don't think you'll achieve your fatigue life, and i'd worry about wear on the housing (we haven't said anything about this yet).

how good (tight) a fit is the pin in the housing ? this'll affect everything

 

[fuzion]

the pin is pressed fit into one part, and there is a .0003 slip into the mating part. The mating part is a steel bushing.

 

[rb1957]

steel on steel is as good as you're going to get to wear (the steels have similar harness ?) ... a bushing is good design (let it wear and replace it, rather than the housing).

i think you still have two problems ...
statically, i think your ultimate shear stress (154ksi by my calcs) exceeds the Fsu of the steel you've choosen, and probably exceeds the Fsu of any steel you're likely to choose. you could go for ultra-high strength steel but you give up fatigue properties as you gain ftu. so i think you have a problem with the 0.08" dia ... why neck the pin ? (shear stress on 0.093" dia is 114ksi)

fatigue, i think you've got a problem with this ... you want 5E6 cycles (presumably safe-life) so you probably want an endurance of about 2E7, with a shear stress of 214/.005 = 43 ksi which would be equivalent to a tension stress of 43 ksi (mohr's circle) with a kt = ? maybe 2 for the stress peaking at the loading interface ...

 

[fuzion]

Hi rb,
I can go up to .200 in diameter of the shear pin.
For the breaking load at the input, of 170lbs static, the pin would be approx .12inches.
But for fatigue, MMPDS only lists the fatigue for H1000. Would you just ratio the H1150 Ftu to H1000 Ftu and use that to get the percentage in reduction in the fatigue?
Thanks Again.

 

[cbrn]

Hi,
rb1957, I suppose the pin has to be necked-down in order to have a preferential location for fracture (if the pin broke in a random location, it could possibly impeach the two connected parts from being released).
Fuzion, in my field (hydraulic turbines), shear pins are one of the most frequent protection device for the guidevanes, but the cycles to which they are subjected is FAR less than 5 milions!!! We usually make them in Anticorodal, because its behaviour is one of the "most brittle" among the materials we can use in our machines.
This point needs care: whichever material you choose, you have to ensure that it doesn't plasticize much before coming to fracture: a material "without yielding" is a must, i.e. you want to dissipate as little energy in plastic deformation as possible.
I agree with the previous posters saying that, with your combination of stresses (average, amplitude, static limit) and service life, you have very little hope in getting your shear pin working... Don't you have the possibility to use another type of security device, such as "Tollok" slip-ring, friction ring or something like that?

Regards

 

[rb1957]

yeah, a .12" dia works much better (shear stress, ult., = 68 ksi) ... H1150 works. you might worry about causing failuer an the minimum failure load (right now you have a +ve MS).

H1000 should have poorer fatigue properties than H1150 (since it has a higher ftu) so i'd use it directly and have a conservative analysis. you might worry about the endurance limit (which should be higher with higher ftu, and so unconservative). a point about the diameter reduction ... is it a "sharp" groove (probably) or a gentle blend ? ... the sharp groove would create a more certain fracture site and a better Kt solution. (you could always develop your own Kt from FEA).

i liked cbrn's point about having brittle fracture, which would suggest a higher strength heat treat.