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.