
Re: How are bolt FOS's calculated?
Dougal Hiscock Mar 22, 2011 5:43 AM (in response to Chad St.Louis)Are you interested in FOS for yield or ultimate strength?

Re: How are bolt FOS's calculated?
Chad St.Louis Mar 22, 2011 7:51 AM (in response to Dougal Hiscock)The strength data being used is tensile strength area, bolt strength and Factor of Safety.
For an M16 grade 10.9 bolt for example, I calculated these to be:
.24 in^2
120000psi (proof load stress)
1.15 (arbitrary)
I don't think tensile or ultimate is used in this case. Proof load is used for bolt strength.

Re: How are bolt FOS's calculated?
Dougal Hiscock Mar 22, 2011 4:36 PM (in response to Chad St.Louis)Chad, strength and FOS always comes down to yeild or ultimate tensile strength of the material. Some rough calculations show you are indeed using the yield strength of the bolt material for your FOS.
The 16mm bolt shank will yield in pure tension around 181kN (210mm^2 area, 900 MPa minimum yeild).
The 14mm minor diameter of the threads will yeild (neglecting stress concentration) around 139 kN (154mm^2, 900 MPa minimum yield).
0.24 square inches is the 154mm^2 of the minor diameter.
120,000psi is 827 MPa which is roughly 93% of minimum yield. Because 900 MPa is the minimum yield and bolts can be higher your proof stress based on ~80% yield does make sense.



Re: How are bolt FOS's calculated?
Paul Kellner Mar 30, 2011 9:18 AM (in response to Chad St.Louis)Are you modeling the bolt in Simulation?
The FOS calcs normally used are based on uniaxial stress. An FEA model will include 3D effects.
Bolt FOS are calculated based on ultimate strength. Frequently this means some stretch in the bolt. This implies a nonlinear analysis where stress will increase to yeild and no further.

Re: How are bolt FOS's calculated?
Chad St.Louis Apr 1, 2011 1:24 PM (in response to Paul Kellner)Hi everyone,
Thanks for the comments. The following link explains the procedure used by SW.
The equations are taken from "Criteria for Preloaded Bolts", NASA Document MSTS 08307; Revision A: July 6, 1998. SolidWorks formulation is based off the formulation described in this document. It is not the exact methodology.
It was not clear to me how the axial, bendingmoment, and shear were combined to determine an overall FOS.
Hope this helps.
Cheers,

Re: How are bolt FOS's calculated?
Paul Kellner Apr 1, 2011 5:00 PM (in response to Chad St.Louis)There is only one type of element that will easily yield those values and that is a beam element. So they calculate that ratios for axial, bending and shear and combine them according to the following equation. The R values are the individual factors of safety given just that type of loading.
1 / ((Ra + Rb)^2 + Rs ^3) > SF
For axial:
Ra = F/(At*S)
S is the allowable tensile stress for the material in the axial direction. F/(At) is the axial stress on the gross section.
It would certainly seem that there is something wrong with the equation because it doesn't yield sensible results in the case of a single nonzero load. For example if Ra worked out to be 1/2 and the others were zero then the calculated value should be 2. Instead it would be 4 if you used their pass fail equation. It looks like a typo, the equation should take the square root of the denominator. I would definitely run a test case to see what is really happening.

Re: How are bolt FOS's calculated?
Adrian Tayne Mar 7, 2012 2:33 PM (in response to Paul Kellner)This is old but I found this helpful and wanted to add to what Paul mentioned. There is definitely a mistake in the SW help equations, which are still that way as of today. The error stems from a discrepancy in how the ratio values (Ra et. al.) are defined. SW defines them as not being multiplied by FOS while the aforementioned NASA document (very useful) defines them including the FOS. For example, NASA defines Ra as the ratio of actual load to acceptable load multiplied by the FOS.
Ra = FOS * (P/At*S)
whereas SW defined it as
Ra = (P/At*S)
So in the equation for Rcombined in the SW help, each term on the left needs to be multiplied by FOS. This then needs to be less than or equal to 1.
Also, NASA states that the fastener must meet all 4 criteria listed (tension, shear, bending, and combined loads), not just combined load like SW implies.
What I don't know is whether SW is actually calculating this wrong or if the help just has a typo. A user should take the listed bolt and pin loads and check the (4) criteria by hand to make sure.

Re: How are bolt FOS's calculated?
sandeep pawar Mar 8, 2012 1:30 PM (in response to Adrian Tayne)I agree. Also, I have found discrepancies in the FoS between what's given in the check plot and calculated FoS based on the above equation.
The FoS is highly mesh dependent and usually increases with mesh size. Same thing for edge weld.

Re: How are bolt FOS's calculated?
Paul Kellner Mar 8, 2012 2:01 PM (in response to sandeep pawar)If the FOS depends on mesh size then there needs to be some way to determine if the mesh is converged.

Re: How are bolt FOS's calculated?
Dougal Hiscock Mar 8, 2012 3:38 PM (in response to Paul Kellner)Surely in direct tension the effect of mesh size is quite small? It is in bending where I expect to find the biggest discrepancies.

Re: How are bolt FOS's calculated?
sandeep pawar Mar 8, 2012 4:19 PM (in response to Paul Kellner)Aren't the loads/reaction foces supposed to be relatively insensitive to the mesh size? so that you can use those to calculate the stresses based on code requirements.
When a bolt connector is defined, I usually have trouble applying the local refined mesh around the bolt hole. coz software draws it's own split surface for bolt head/nut. So I end up refining the whole mesh.
one more problem is, when analyzing the bolted joint, the loads carried by bolt is as important as the load carried by the joint material. currently the only way to access that info is to look through the .vs0 file. I think it should be presented along with the bolt loads.




Re: How are bolt FOS's calculated?
Chris Fernald Mar 21, 2012 1:49 PM (in response to Paul Kellner)I’ve also had issues with bolt connector FOS. I made a post a while back about a similar subject: https://forum.solidworks.com/thread/44790
Regarding the help file formula, I come to the same conclusion; there seems to be an extra squared term in the SW equation. So the problem I am having is twofold:
 FOS given by solidworks is dominated by the preload, which is generally very close to the proof load. This means the FOS is always slightly greater than 1, even when the applied load is very small. (see hyperlink)
 Even if I could separate the FOS from being dominated by preload (for example, set the preload to 0), the formula used to calculate that FOS is x^2 higher than I would expect in the simple axial case. Therefore, it is suspect.
When I presented these questions to our SW reseller I was referred to the same NASA document. At this point, I continue stuggle with this problem, as I haven't found a clear answer.

Re: How are bolt FOS's calculated?
Adrian Tayne Mar 21, 2012 5:04 PM (in response to Chris Fernald)Based on my conversation with my VAR, the equations that SW actually used to calculate the bolt stresses are accurate. It's only the equations in Help that are inaccurate. Someone made a typo... So I am surprised that your numbers are coming out wrong

Re: How are bolt FOS's calculated?
Chris Fernald Aug 6, 2012 5:05 PM (in response to Adrian Tayne)I just checked in this thread to see if there was any resolution. My VAR also referred to the NASA document which says the FOS has a squared relationship to load. A few people posted that it would be a good thing to run a comparison model, so below is a basic hand calc compared to the SW results. Since it is still not clear to me why the two methods don't match, I wanted to post this for documentation purposes. Seems like predicting the FOS to be falsely high could be a real problem, so I hope it is either fixed or better explained in the future.



