Fastener

Example Problem Drawing of the Example Joint

In this section an example analysis is presented, a file containing the input data for this example is included with the demo program in the directory that the program is installed into under the name example.fas.

In this example a M12 bolt (property class 10.9) is used to secure a bracket to a casting that is made from spheroidal graphite cast iron. The tolerance class of the bolt thread is 6g and that of the internal thread is 6H. A tapping drill of 10.2 mm diameter was used to form the internal thread and the hole is countersunk to aid starting the thread. There is also a chamfer at the start of the bolt thread. The ratio of shear strength to tensile strength for the SG cast iron is taken as 0.9 - this is based upon published test results. The output from the program is presented below.

 

 

      FASTENER - THREAD STRIPPING STRENGTH PROGRAM
 
Analysis of a M12 bolt secured into a SG cast iron block.
 
                       THREAD DETAILS
                      ______________

Fastener Diameter                          = 12.000 mm
Thread Pitch                               = 1.750 mm
Thread: M12 x 1.75 - 6g/6H Coarse Thread Series

EXTERNAL THREAD
Maximum Major Dia.                         = 11.966 mm
Minimum Major Dia.                         = 11.701 mm
Maximum Pitch Dia.                         = 10.829 mm
Minimum Pitch Dia.                         = 10.679 mm
Maximum Minor Dia.                         = 10.072 mm
Minimum Minor Dia.                         = 9.602 mm
Material: Property Class 10.9 (Diameter Range M5 - M100)
Minimum Tensile Strength                   = 1040.00 MPa
Maximum Tensile Strength                   = 1230.00 MPa
Ratio of the shear to tensile strength     = 0.580
Minimum Shear Strength                     = 603.00 MPa
A chamfer is present on the end of the thread.
Length of Chamfer                          = 2.000 mm

INTERNAL THREAD
Minimum Major Dia.                         = 12.000 mm
Maximum Pitch Dia.                         = 11.063 mm
Minimum Pitch Dia.                         = 10.863 mm
Maximum Minor Dia.                         = 10.441 mm
Minimum Minor Dia.                         = 10.106 mm
Note: The Tapping Drill Dia. has been used to determine the
shear area of the internal thread rather than the maximum
size of the Minor Diameter. However, using the maximum size
of the Minor Dia., if this is applicable, will give a lower
thread stripping strength!
Tapping Drill Diameter                     = 10.200 mm
Radial engagement with the external thread = 83.8%
Thread Engagement Length                   = 15.00 mm
Bellmouthing Ratio                         = 1.03 mm
Length of Bellmouthing                     = 6.00 mm
Material: Spheroidal Graphite Cast Iron
Minimum Tensile Strength                   = 500.00 MPa
Ratio of the shear to tensile strength     = 0.900
Minimum Shear Strength                     = 450.00 MPa
The hole is countersunk on one side only.
Countersink Diameter                       = 13.000 mm
Countersink angle                          = 90.00 degrees
Thread Friction Details
Black oxide steel external thread, internal thread in cast
iron, no lubricant.
Thread Friction Value                      = 0.120
 
                   DERIVED INFORMATION
                   ___________________
 
Basic Pitch Dia.   d2                      = 10.863 mm
Basic Minor Dia.   d1                      = 10.106 mm
Nominal Minor Dia. d3                      = 9.853 mm
Stress Diameter of the Thread              = 10.358 mm
Theoretical Stress Area - External Thread  = 84.267 mm˛
Minimum Stress Area - External Thread      = 80.762 mm˛
Maximum Stress Area - External Thread      = 85.776 mm˛
Nominal Root Area of the External Thread   = 76.247 mm˛

                        RESULTS
                        ________
 
Effective Length of Thread Engagement      = 12.932 mm
Shear Area of the Internal Thread per mm   = 26.117 mm˛
Shear Area of the Internal Thread          = 337.743 mm˛
Shear Area of the External Thread per mm   = 21.086 mm˛
Shear Area of the External Thread          = 264.728 mm˛
 
Internal to External Thread Strength ratio = 0.61337
Boss/Nut Dilation Factor C1                = 1.00000
External Thread Bending Factor C2          = 0.89700
Internal Thread Bending Factor C3          = 1.02258
  
Direct Forces to fail the Fastener:
Minimum Tensile Force to fail the Fastener = 83992.781 N
Maximum Tensile Force to fail the Fastener = 105504.050 N
 
Fastener Failure Forces Allowing for Combined Tension-Torsion
Loading:
Minimum Tension-Torsion Failure Load       = 74263.460 N
Maximum Tension-Torsion Failure Load       = 93883.693 N
When the bolt/screw is being tightened it experiences both
tension and torsion. The effect of this is that the bolt/screw
will fail at a lower force then if only a directly applied
force is applied. Higher the thread friction value, higher
will be the induced torsion and lower will be the direct
force that results in fastener failure.

Thread Stripping Forces:
Minimum External Thread Stripping Force    = 143188.827 N
Minimum Internal Thread Stripping Force    = 155416.303 N

Relative to Torque Tightening:
Factor of Safety - External Thread         = 1.525
Factor of Safety - Internal Thread         = 1.655
Critical Length of Thread Engagement       = 10.547 mm

NOTE
Because the upper limit of the force to cause tensile fracture
of the fastener of 105504 N is smaller than the external
thread stripping force of 143189 N and the internal thread
stripping force of 155416 N; the fastener will fail by
tensile fracture before either the internal or external thread
will strip. If bolt breaks on tightening, it is obvious that
a replacement is required. Thread stripping tends to be
gradual in nature. If the thread stripping mode can occur,
assemblies may enter into service which are partially failed,
this may have disastrous consequences. Hence, the potential
of thread stripping of both the internal and external threads
must be avoided if a reliable design is to be achieved.

Pricing and ordering details for the program

Details and explanation about the program

Datasheet for the program in Adobe pdf format

Download the FASTENER Demo Program