Bolt torque calculator for the calculation of tightening torque and preload for bolts.
In bolted joints, clamping force is usually generated by applying torque to bolt
head or nut. During the bolt tightening process, the length of the bolt
increases and it behaves like an extension spring. Bolt tries to turn back to
its original length and exerts a force on the joint. This force is called
tension force or preload. Determination of tightening torque and preload
requirements for bolts and nuts is important in joint design. Fastener suppliers
often give information to determine torque requirements of bolts including
torque coefficient, bolt material properties and other effects.
Development of the bolt torque calculator has been done by using the theory given in the Machinery’s Handbook.
Some of the key features of the calculator are summarized as follows:
- Valid for Unified inch screws threads in the size range from ¼ in to 1 ½ in and covers standard series of UNC, UNF and UNEF
- Material database for steel
bolts and screws is available in the calculator. In case user wants to define
different strength properties for the bolt, there is an option to not use the calculator’s database. Database is
prepared by using the steel bolt properties given in the Machinery’s Handbook.
- Material database consists of ASTM and SAE steels with different grades.
- Calculates estimated required bolt tightening torque, minimum
length of engagement to prevent stripping of threads and dimensional properties of
bolts and nuts such
as shear area of external and internal thread and tensile stress area of bolt.
Note: 1 Information about this condition is needed for the
calculations of thread stripping. If nut is being used as an internal thread,
it's preferred to select a nut with a grade equal to or greater than the grade
of the bolt being used. [Ref-2]
Note: 2 This value is required if internal thread strength < external
thread strength. Otherwise not needed.
Note: 3 Recommended values: 75 % for nonpermanent connections and
reused fasteners. 90 % for permanent connections.
Note: 4 Use values given by your fastener supplier and manufacturer. If these values are not available, see Torque coefficient k factor table
Note: 1 Tolerance class 2A is used for external threads (bolts) and class 2B is used for internal threads (nuts)
Note: 2 Preload value is a calculated nominal value. Due to the different factors such as surface finish, presence of lubrication and bolt preload application method, it's possible to experience variations in this value.
Bolt Elongation Measurements with Micrometer:
A technique to control preload by measuring elongation with a micrometer. If there is an access to the head and the end of the bolt, bolt elongation can be measured by using micrometers or dial indicators. By using the elongation value of the bolt and bolt material properties, the bolt preload can be calculated.
Major Diameter: The maximum
diameter of a thread which is diameter of the crest of a male thread or root of
a female thread.
Minor Diameter: The minimum
diameter of a thread which is diameter of the root of a male thread or crest of
a female thread.
Proof load: The maximum load (force) that a bolt can withstand without
acquiring a permanent set.
Threads per inch: Number of full
thread per an inch length.
Torque Wrench: Torque wrench is a tool to tighten bolts, screws and nuts with a specific torque value.
During tightening of a fastener with a torque wrench, torque value is measured and used as an indicator of preload of a fastener tension.
Tightening fasteners with torque wrench is generally less accurate with respect to bolt tension (or elongation) measurement due to the uncertainty of the
friction in threads and mating surfaces but more practical to use. There are different type of torque wrenches such as slipper type, click type, beam type,
electronic and programmable type, hydraulic type and pneumatic type.
Ultrasonic Bolt Measurement:
According to data given in the Machinery’s Handbook, one of the most accurate
tightening techniques is controlling bolt elongation with ultrasonic elongation.
Ultrasonic bolt measurement is done by sending sonic wave from the one end of
the bolt and measuring the time of the wave to travel forth and back through the bolt.
By using material properties of the bolt and travel time of the wave, the elongation and preload of the bolt can be accurately calculated.
Unified Screw Thread: A thread
form used by the United Kingdom, Canada, and the United States to obtain screw
thread interchangeability among these three nations. It has 60° thread angle and
dimensions are stated in inch units.
UNC (Unified National Coarse):
Most commonly used type for general engineering applications. This thread form
is used in materials with low tensile strength which makes threads more
resistance against stripping (internal threads) .UNC give possibility for quick assembly.
UNF (Unified National Fine):
External threads of this Fine Series have greater tensile stress area than
comparable sizes of the coarse series. The Fine series is suitable when the resistance to stripping of both external
and mating internal threads equals or exceeds the tensile load carrying capacity of the
externally threaded member. It is also used where the length of engagement
is short, where a smaller lead angle is desired, where the wall thickness demands a fine
pitch, or where finer adjustment is needed.
UNEF (Unified National Extra Fine):
Used when finer
pitches than UNF are needed (Ex: Short engagement length).