FASTENER - Thread Strength Analysis Program

Fastener

Thread Strength Analysis Program

Powerful yet easy to use PC Software which has been designed to assist you in solving problems, or potential problems, related to the failure by thread stripping of fasteners and threaded parts.

Fastener Failure

Fastener failure on a product can have potentially disastrous consequences. In an attempt to ensure that such consequences do not occur, rigorous and extensive testing of a product is frequently completed. However in many applications, extensive testing is neither practical nor economical. In such instances, the Engineer usually relies upon analytical analysis together with his experience and judgement to ensure that failure does not occur. Failure of a threaded fastener during assembly generally occurs in one of three modes:

1. Failure by tensile fracture through the shank or threaded section of the fastener.
2. Shear failure through the thread profile (thread stripping) of the external thread.
3. Shear failure through the thread profile (thread stripping) of the internally threaded part.

Thread Stripping
Thread stripping is a shear failure of an internal or external thread that results when the strength of the threaded material is exceeded by the applied forces acting on the thread. Thread stripping can be a problem in many designs where tapped holes are required in low tensile material. In general terms thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature and it may go unnoticed at the time of assembly. It starts at the first engaged thread, deformations causing this thread to carry the highest load; and successively shears off subsequent threads. This may take a number of hours to complete and so the product may appear fine at the time of assembly. The risk is therefore present that threads that are partially failed, and hence defective, may enter service. This may have disastrous consequences on product reliability.

Because of the more widespread use of angle control and yield control tightening methods, bolt preloads, for a given size and strength of a fastener can be greater than traditionally was the case. This coupled with the widespread use of automatic and semi-automatic tightening procedures increase the likelihood that thread stripping will occur.

The strength of a nut or bolt thread cannot be viewed in isolation without considering the inter-dependence that both elements have on the strength of the assembly. One of the problems in predicting thread stripping strength is that, without considering such effects as thread bending, nut dilation or bellmouthing, an optimistic result occurs. The actual stripping strength being lower than that calculated.

About the Program
FASTENER is a computer program that provides a means of determining the forces required to strip the internal and external threads of a fastener. It also calculates the force required to fracture an external threaded fastener across the threaded section.

To precisely predict the force and mode of failure of a threaded assembly demands consideration of a large number of factors. Thread stripping is a complex phenomenon. The program considers the following factors when determining failure mode:

1) The effect of variation in the dimensions of the thread, such as major, pitch and minor diameters, has on fastener failure mode of both the internal and external threads.
2) Tensile and shear strength variations in the material for both the internal and external threads.
3) The effect of radial displacement of the nut (generally known as nut dilation) in reducing the shear strength of the threads. The tensile force in the fastener acts on the threads and a wedging action generates a radial displacement.
4) The effect which the bending of the threads, caused by the action of the fasteners tensile force, has on both internal and external thread shear strength.
5) The effect which production variations in the threaded assembly, such as slight hole taper or bellmouthing can have on thread strength.
6) The effect that torque tightening a threaded fastener can have on the tensile strength capabilities of a fastener.
7) To assist and guide the Engineer, the program incorporates default values such as maximum and minimum thread dimensions based upon standard thread tolerances.

Using the program it is possible to allow at the design stage for anticipated variations in the forces required to cause failure. This variation is inherent due to dimensional and property differences between fasteners of the same size and property class.

Minimum system requirements
To use FASTENER on your computer, make sure that you have the following minimum system requirements:
i. Microsoft Windows 95, Windows 98 or Windows NT.
ii. 4 MB of RAM memory.
iii. A hard disk with 2MB free disk space.

Thread and Material Databases
The program accesses tables of standard values to allow the easy selection of the most appropriate fastener size and material. The program allows selection of metric fine as well as metric coarse threads, the table that the program accesses contains all the main metric thread sizes from 1mm to 100mm diameter. The inch based Unified thread table contains thread sizes from Number 0 (0.06" diameter) to 4 inch diameter. The user can enter sizes outside these values directly into the program.

The program also accesses material property tables for both metric and inch based fasteners; the tables contain the popular fastener materials defined in ISO, SAE, ASTM and BS standards. The program displays in metric units for metric fasteners and inches and lbs for unified threads.

Ease of Data Entry and Help File
The program uses spreadsheet style editing facilities, allowing each value to be individually changed without the need to re-enter all the input data. This gives the program a user-friendly feel and allows individual values to be specifically adjusted. The calculations can then be repeated to assess its effect. When possible the program will enter values automatically based upon the thread size of the fastener previously selected by the user; any value being able to be over-written.
An extensive help file is included that explains the main terms used and furnishes information relating to both the program and to threaded fasteners in general. The comprehensive user guide that is supplied that gives guidance on the use of the program and presents valuable background information on the failure of threaded fasteners.

Printing and Filing Facilities
The program has menu options to allow :
1. Printout of the results and the data to both the screen and printer.
2. The facility to store both input data and the results to disk.
3. The results to be directly loaded into Notepad for editing and inclusion in a user defined report.

Demonstration Program
A demonstration program is available for FASTENER by clicking on the link below.

Pricing and ordering details for the program

Example problem analyzed by the FASTENER program

Datasheet for the program in Adobe pdf format

Download the FASTENER Demo Program