Bolted Connectors - Cap Screw - Useful Length

[dberrie]

Hi there,

I am working on a project to determine how many cap screw will be required to hold a flange (20mm thick) on a pressure tube and I would like to calculate the axial stress generated in the screws.

I am using M12x 50mm long cap screws on a circular pitch with a 2mm thick washer 24mm in diameter and I am attempting to use the bolt connectors in Inventor Nastran 2022.

I have split the surface of the gland to account for the washer diameter (24mm) and I have also split the surfaces in the tapped hole to ensure correct length of the bolt (50mm in total)

I have a few questions regarding the inputs required: 

a) Diameter of the bolt - would this be the major diameter of 12mm or would I calculate a diameter based off the thread stress area (84.3mm^2 for M12)  and arrive at 10.36mm ?

b)    What value would I use for the Useful Length? Would this be 50mm as I am using a 50mm screw ?

I look forward to your assistance. Thanks

[John_Holtz · Accepted solution]

Hi @dberrie 

The bolt connector is attempting to duplicate the stiffness of the bolt. I would think that most of the bolt that is stretching is 12 mm diameter, so that would be the bolt size to enter. (The portion of the bolt in the threads is very short and does not contribute too much to the stiffness.)

For the useful length, please see this article. Nastran In-CAD: Useful Length of cap screw type bolt connector | Inventor Nastran | Autodesk Knowled...

Basically, you want to determine (by engineering judgement) how many threads into the hole are effective, meaning that the rest of the threads deep into the hole do not stretch. The threads that are effective along with the unthreaded length of the bolt determine the useful length. 

John

[dberrie]

Thank you John. Your explanation clarifies what I needed to know.

Thanks again !