Fastener Math

[looslib]

Does anyone know what math formulas are being used by Mecahnica to calcualte the function of a fastener? The PTC web site has a How2 paper, but it only says they are a spring element. We have some people who want to know the math formulas so we can varify the calculated results with actual test results. Also, we can take the fornulas into Excel and calculate results in a few seconds instead of the 2 hours it takes to do the setup and calculations in Mechanica.

 

[TrailBarge]

Wow... that is a rather large order.


Mechanica is a FINITE ELEMENT ANALYSIS environment. To oversimplify, that means that there is a generated spring/damper matrix between nodes. The spring/damper matrix is solved as a system of equations..... a very large system of equations.... one that excel could not even contain, much less solve "in a few seconds".


The whole idea of finite elements is to break down whatever structure is under consideration to small enough parts that relatively simple equations (spring/damper) will approximate reality closely enough to be useable. In order for the analysis to be useable, there has to be many parts... hence, the use of computer.


There is no function of a fastener, any more than there is a function for a bridge.


I recommend taking a course in finite element theory. Short of that, you might start with the wiki on the subject. ANSYS used to have quite a bit of information publicly available... perhaps it is still there.


After a few hours, perhaps you will begin to understand what you just asked for.


A man somewhat smarter than me once said "All analyses are wrong, it is the engineer's job to understand how wrong". (Or something like that) The idea of a hand calc is not to duplicate the work that FEA just did. It is an alternate method to arrive at a simplified answer for the purpose of checking the validity of your model.


Enjoy your lab.

 

[Mloew]

TrailBarge,

Poster looslib is a pretty smart person, he was looking for the numerical formulation for the fastener elements specifically, not encouragement to educate himself on the Finite Element Method. These equations are very simple and typically captured in closed form and easily represented in spreadsheets. In fact, I recently visited a customer that is using a spreadsheet to validate the stresses and pre-loads in a FEA simulation. I'll try to get a copy of the spreadsheet in question and post the equations.

 

[TrailBarge]

Mloew;

See what a dialog can get you? I have not used
mechanica for a few years, but I did use it rather
intensively for a while.

All this time, I had no idea whatsoever that there was
ever such a thing a "fastener element".

Sure, the spreadsheet might be interesting to look at...
but if there is a help file describing the element
(nodes, p vs h, presumed geometry, outputs)... I would
definitely love to learn about a new element.

I can accept that I'm out of touch, you see... because
my sister works for a major tire manufacturer and she
writes (if you can suspend your disbelief long enough)
tire elements. Anyhoo.... thanks for the heads up. I
guess I'm the one that needs the refresher course.

 

[JRay]

Looslib,



Assuming you're refering to a threaded fastener...



A great reference that will cover all equations and tables on fasteners
is included in Chapter 15 of Fundementals of Machine Elements by
Hamrock, Jacobson and Schmid (ISBN 0-07-228933-3). I have the
first edition and I know it has been updated at least once. The
books sells for ~$100 on Amazon and would be the best way to go if you
wanted to create a simple spreadsheet that produces accurate results.



-Josh

 

[looslib]

We have the spreadsheet formulas. What we want is the math formulas that Mechanica uses to verify the results from the different tools.