Thread Stripping Analysis using BOLTCALC

Case Study on a Thread Stripping Problem - Metric bolt

Partially stripped bolt thread Thread stripping or shearing of threads is a common problem. Thread stripping is gradual in nature and hence is not necessarily detectable at the time of assembly. For the threads to be fully sheared, a displacement of at least half a thread pitch is needed. If the threads fully shear on tightening, the bolt will detach from the nut. This is likely to be noticed before the product goes into service. If however the thread is only partially sheared, the nut will be retained on the bolt thread and the potential for failure may not be noticed. When subsequently a significant load is applied to the joint, the partially failed threads could shear, and the nut become detached from the bolt with potentially catastrophic results. A partially stripped bolt thread is shown in the image. Thread stripping can affect both internal and external threads, which thread will shear first depends upon their relative strengths.

Thread Engagement Length

Nuts, if appropriately selected to match the bolt, are designed so that the bolt can be expected to fail by tensile fracture before thread stripping occurs. In the case of bolt tensile fracture, the nut is detached from the bolt and is readily noticeable at the time of assembly. The problem for the Engineer is selecting the correct length of thread engagement when the bolt is secured into a tapped hole. Too short an engaged thread length and thread stripping could occur, too long, and thread pitch differences between the bolt and nut can arise causing thread binding to occur.

The thread binding issue is due to small differences in pitch between the internal and external threads. Too long an engaged length and the clearance between the threads is used up accommodating the thread pitch differences. In extreme cases, such binding can result in galling and complete seizure of the bolt in the tapped hole. Many Engineers do not appreciate that the standard thread tolerance classes, often specified as defaults on drawings, are not recommended for long lengths of thread engagement.

bolts in blind tapped hole

Joint Details

In a case study the BOLTCALC program is used to determine thread stripping strengths of tapped holes in two different materials. A bracket is secured into a blind tapped hole using M12 property class 10.9 bolts. The tapped hole is in a part made from spheroidal graphite cast iron with a thread engagement length of 15 mm, as per the image. As part of weight reduction proposals, aluminium instead of cast iron is being considered for the tapped hole material. The question the engineer is considering is whether 15 mm is a sufficient thread engagement length to prevent thread stripping from occurring.

Thread Stripping Analysis - Cast Iron

The BOLTCALC program will be used to complete the thread stripping analysis. The screen capture video below shows the program being used to investigate the potential for thread stripping occurring. This first video is for the thread going into the spheroidal cast iron. The video is just over 7 minutes long. To get an enlarged view of the video, click on the enlarge control on the right hand side of the video control.

Thread Stripping Analysis - Aluminium

So, based upon the above analysis, the tapped hole in the cast iron should not sustain thread stripping. Next, the data will be changed to check if the tapped hole in the aluminium is okay with an engaged thread length of 15 mm. The video for this analysis is shown below, it is just under 5 minutes long.

As shown in the presentation, a thread engagement length of 15 mm is insufficient for the tapped hole in aluminium. Based upon the analysis, a tapped hole providing a minimum engaged length of 25 mm is required.

There are various output options available that allow the data and the results to be printed and to create a PDF. The summary sheet for this analysis is shown below.

Thread Stripping Results Summary

Further Information

BOLTCALC pdf Datasheet Presented below are links to further information related to the BOLTCALC program:

Web pages giving further details about BOLTCALC

Training Presentations on the BOLTCALC program

Torque Tightening Analysis using BOLTCALC (pdf)

Thread Stripping Analysis using BOLTCALC (pdf)

Joint Analysis using BOLTCALC (pdf)

BOLTCALC Datasheet in Adobe PDF format

BOLTCALC Example in Adobe PDF format

BOLTCALC Demo Program

dBEditor - BOLTCALC's optional database editor

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