Torque wrenches go out of calibration with use. Verifying the accuracy of a torque wrench is a key quality measurement that must be taken for a successful outcome. Two accepted methods may be used. Verification is a process step to validate if the wrench is in or out of calibration. This test can be conducted with a suitable torque tester/checker. Any operator or inspector may verify a wrench by connecting it to a suitable torque tester and pulling the wrench. The torque tester will display a result that must be compared to the allowable tolerance of the wrench. Some torque testers offer a tolerance-setting feature that provides a GO/NG result (RED or GREEN) when the wrench is pulled. If the verification result is OK then the wrench is deemed in tolerance. If it is NG then calibration or adjustment of the wrench is required.

Unless broken, calibrating a torque wrench is a fine-tuning of the wrench to bring it back within tolerance. A competently trained technician with equipment that is certified to the National Institute of Standards and Technology (NIST) must perform the work if a calibration certificate is required. The international standard for torque wrench calibration (ISO6789) mandates all conforming wrenches meet or exceed 5000 cycles before requiring calibration. It also defines the methodology for calibration. Most competent wrench manufacturers use ISO 6789 as the basis for establishing performance criteria or standards for their tools.

A regular scheduled verification and calibration program helps to eliminate errors based on a wrench being out of tolerance. Competent calibration labs in conformance with ISO17025 or best practices will issue “as found” readings for a calibration, allowing examination of the wrench condition at time of calibration. It is an industry practice to reduce the calibration interval by one half if a wrench is found to be out of tolerance at time of calibration. Failure to calibrate a torque wrench may lead to failure of a joined connection.

From tightening large industrial bolts used in windmill gearbox assemblies to bolting maintenance applications for oil & gas pipelines and rigs, the Mountz electric torque multipliers achieve precision torque in a quick, cost-efficient manner.

The electric torque multiplier shuts-off when pre-set torque is achieved. It’s a non-impacting and operates at a smooth, continuous rotation; the electric torque multiplier increases speed and productivity, as it is faster than a hydraulic wrench and is less expensive. The tool eliminates the cumbersome set up time and slow ratcheting process of hydraulic wrenches. The electric torque multiplier is also ergonomically safer than the harmful hammering of impact wrenches and eliminates the frequent costly repairs of impact wrenches.

When installing or removing heavy-duty fasteners that require power, precision and safety, the Mountz electric torque multipliers is the solution. Without the hassles of air hoses, the electric torque multipliers simply plugs into a power outlet. The speed, frequency and torque output of electric torque multiplier are correspondingly regulated and controlled with a built-in controller.

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Regular torque tool calibration and re-calibration guarantees the operator repeatable accuracy and adherence to international standards. Calibration also ensures torque equipment is operating to peak performance and can highlight potential tooling problems before they arise perhaps due to tool wear or broken components.
Torque calibration equipment can be found in many industry sectors such as:

Research & Development
As technology moves ever forward, with the implementation of new materials, processes and assembly techniques there is a demand for accurate torque application. The accurate calibration of torque tools is a vital part of this process.

Inspection & Quality Control
The accurate and repeatable application of torque is fundamental in any quality control process in order to save time and money. Torque equipment calibrated regularly and accurately can and will allow you to reach these goals:

Production
Torque tools calibrated correctly can provide repeatable accuracy saving time, money, materials and labor. Regularly calibrated and certified torque tools, reduce the risk of products failing in service, saving money from warranty and re-work costs.

Servicing
Engineers involved in servicing may have to apply various and accurate torque settings to a range of components. Accurate calibration of these tools is vital in achieving accurate torque applications.

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Generally, in the majority of applications, the reliability of the joint is dependent upon the bolts ability to clamp the parts together. Adequate clamping prevents relative motion between parts of the joint and leakage from joints containing gaskets. Measuring a bolts clamp force is difficult, especially under production assembly conditions.

The clamp force generated by a bolt can be indirectly controlled by regulating the applied torque. This method, known as torque control, is by far the most popular method of controlling a bolts clamp force. The initial clamp force generated by the bolt is frequently called preload.

There is a link between the torque applied to a bolt and the resulting preload. A problem exists in that friction has a large influence on how much torque is converted into preload. Besides the torque required to stretch the bolt, torque is also required to overcome friction in the threads and under the nut face. Typically, 10% to 15% of the torque is used to stretch the bolt. Of the remaining torque, typically 30% is dissipated in the threads and 50% to 55% under the nut face. Because friction is an important factor in the relationship between torque and preload, variations in friction have a significant influence on the bolts preload. Different bolt surface finishes generally have different friction values.

The torque required for a socket head screw will not be the same as that required the same size standard hexagon bolt. The larger bearing face of the standard bolt will result in an increased torque being required compared to a socket headed screw. This is because more torque is being dissipated between the nut face and the joint surface.

Stresses indicated into a Bolt: When a bolt is tightened the shank and thread sustain a direct (tensile) stress due to it being stretched. In addition, a torsional stress is induced due to the torque acting on the threads. These two stresses are combined into a single equivalent stress to allow a comparison to be made to the bolts yield strength. In order to effectively utilize the strength of the bolt, yet leave some margin for any loading the bolt would sustain in service, an equivalent stress of 90% of yield is commonly used. It is this approach that is used in this guide.

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