In the release of MecaStack v5614 we altered the way in which the structural flange preload was calculated. Generally the preload forces are greater now, which can sometimes lead to problems. This brief article is intended to explain this change and how you can address any problems this may create.
The structural flange design is following "Mechanical Engineering Design" by Shigley. This book recommends that preload (Fi) be between 60% and 90% of the proof load for the bolt. The proof load is generally the load that will produce about 85% of the yield stress in the bolt.
Prior to v5614 MecaStack was actually treating the "Allowable Load" for the bolt as the proof load, which is generally much lower than the proof load. The user can specify the percentage (Fp) within MecaStack that they wanted to achieve, and then that Fp value was multiplied by the Allowable Load for the bolt to achieve the bolt initial load. Basically the allowable bolt tensile load was being treated as the Proof load, which isn't exactly what Shigley had intended. We don't recall exactly why we had set it up that way, but it had been that way for some time and generally resulted in lower preloads for bolts.
In v5614 it was determined that the Allowable Load was being used instead of the proof load, and so we corrected the software to use proof load. Since the percentage entered (Fp) by the user was unchanged, and the proof load is higher than the allowable load, the bolt preload (Fi) is increased. Here is an example:
If you want to replicate the old values, then adjust the Fp value to give you the same preload that you had in your previous analysis. This can be determined by trial and error through adjusting the Fp value each time, or by performing a calculation similar to that above.