Joe Galliera

Calculating preload for Bolt connectors

Blog Post created by Joe Galliera Employee on Oct 15, 2009
How do you calculate the bolt preload for Bolt connectors in SW Simulation?

I'm going  to try to make this super easy for you all.  Here is a link to two PDF  files, one for USA bolts and the other for Metric bolts, where the values of Ats (Tensile Stress Area, length^2) and Sp (Proof Load Strength, force/length^2) are  provided in one document.  These two parameters are even colored in the tables so that they can be found  more easily.

To calculate the  Axial Preload, Fi, use this equation:
For a permanent  connection: Fi = 0.90 * Ats * Sp
For a reusable  connection: Fi = 0.75 * Ats * Sp
For those  of you who want to input a torque, T, then it gets a little more complicated:
With a nut:  T = k * Fi * Dn
Without a nut: T = 1.2 * k * Fi * Dn
where k is the Friction Factor, and Dn is the nominal (major) diameter, which  can be found in the tables in the PDF files.

The friction factor, k, is difficult to estimate in a real world  application, so for practical usage recommended values are suggested  below:
Non-plated, k = 0.30
Zinc-plated, k = 0.20 - 0.28
Lubricated, k = 0.18
Cadmium-plated, k = 0.12 - 0.15
Default (No  information / not provided / unknown), k = 0.20
After  running the study, check the forces on the bolts by  right-clicking the Results folder and choosing the "List Pin/Bolt/Bearing Force"  item.  Typically the absolute value of the axial force on the bolt should be  about the value of the preload that was entered.  If the axial load on any of  the bolts is nearly zero, then that means that the bolt was not properly  preloaded and so the bolt is loose, which can cause real problems!

Here are two practical  examples:

(a) 1/4" -  20 UNC Bolt Grade 5

Ats = 0.031822 in^2 and Sp = 85000 psi

So, for a  permanent connection, the bolt preload is calculated be

  • Fi = 0.90 * 0.031822 in^2 * 85000 psi = 2430 lbf

(b) M10  x 1.25 x 40 - 9.8

Ats = 61.199 mm^2 and Sp = 650 MPa

So, for a reusable connection,  the bolt preload is calculated to be

  • Fi = 0.75 * 61.199 mm^2 * 650 MPa = 29800 N

Just one more thing before I finish this blog about Bolt Connectors...

A feature for the bolt connectors (for releases 2008 and greater) allows the bolt to go through more than two components.  If you have this case, then make sure that you open up the Advanced Option and turn on the "Bolt Series" flag and select the cylindrical faces of the holes for all the other parts that are sandwiched in between.