Accurate Representation of Fasteners 1

[jvian]

I am trying to get a feel for how Pro/M is handling the fastener idealizations that I have in place. I am using advanced fasteners with an applied preload and contacts interfaces to achieve a proper assembly clamp force. My issue comes in with the following statement taken from PTC's suggested technique for using fasteners (wildfire 5.0 release), it reads as follows:

"To achieve exact preload value,
1. Apply desired value, run the analysis.
2. Look at the bolt tensile force measure (it’s value will be less than the desired preload due to the “give” in the components)
3. Proportionally increase the Preload value in the fastener to achieve the desired tension value in the fastener
4. Re-run the analysis and double check the tensile force measure"

I have been under the assumption that historically engineers have used the concept that if an applied preload is greater than the resultant tensile load of the fastener, then the fastener will not likely fatigue. What I am trying to understand is if Pro/M is suggesting that in real application my preload should be "increased proportionaly" due to the "give" in the parts or is it merely for analytical purposes only?

Bottom line is I do not fully understand the quantity the fastener_tensile_force is reporting. My origional assumption was that if my preload was greater than this force my fastener would not fatigue but this PTC statement has me confused.

Any help would be greatly appreciated and thanks in advance.

 

[seymours2571]

This is what you are looking for:

http://www.ptc.com/cs/cs_27/howto/mst12163/mst12163.pdf

Helped me out a ton.

Good luck,

Steve


Stephen Seymour, PE
Seymour Engineering & Consulting Group

http://www.seymourecg.com

 

[jvian]

Steve,

Thank you for that, it is amazing how hard it is to pinpoint information from ptc. That link definitely answers my questions and again thank you for the reply.

 

[jvian]

Steve (or anyone else who may have some insight),

I do have another question that has risen since running more fastener test cases. What I have been trying are a few test cases with different fastener properties and mesh conditions to get a feel for the effects of various parameters of both mesh and fastener properties. What has come up is I am running a two piece assembly with (4) non symmetric blind screw type fasteners (.164" dia) with an included preload (both 760lbf & 980lbf with same results) and fix separation off with defined contacts at the appropriate locations of the fasteners and a rather well defined tight mesh around the fastener separation test diameters currently 1.5x bolt diameter. The assembly is two aluminum .250" thick flanges with roughly 0.5" diameter material thickness around the fastener but only .250" wide away from fasteners. The fasteners are roughly in an 8" square (not easy to describe nor picture I apologize) for an idea of size and scale.

The problem I am running into is with NO internal or external loads applied to the assembly I am measuring a positive separation force in all of the fastener test diameters indicating joint separation. This does not make sense to me as how can there be a separation without a separating force of some kind? I would expect a bit of contact pressure but not a separation within the separation test diameter.

I have ran several fastener test cases to get an idea of how the fastener idealization works and responed to various inputs but have never seen this reaction in any of the cases once the contact interfaces and mesh were correctly done (i.e too coarse of a mesh resulted in similar results) but I am confident this is not the case now as I have done three iterations of mesh refinement in the area to eliminate the issue.

Any advice or suggestions would be greatly appreciated,

-J-