Torque wrench
A torque wrench is used where the tightness of screws and bolts is a crucial parameter of assembly or adjustment.
It allows the operator to set the torque applied to the fastener to meet the specification for a particular application.
[1] In 1935, Conrad Bahr and George Pfefferle patented an adjustable ratcheting torque wrench.
The tool featured audible feedback and restriction of back-ratcheting movement when the desired torque was reached.
When the wrench is used to apply torque, the lever bends and the indicating beam stays straight.
Paul Allen Sturtevant—a sales representative for the Cedar Rapids Engineering Company at that time—was licensed by Chrysler to manufacture his invention.
[5][6] The dual-signal deflecting beam torque wrench was patented by the Australian Warren and Brown company in 1948.
The operator can both hear the signal click and see (and feel) a physical indicator when the desired torque is reached.
The load carrying beam is straight and runs from head to handle, it deflects when torque is applied.
This firing makes a loud click and gives a visual and tactile indication that the desired torque has been met.
[8][9] A slipper type torque wrench consists of a roller and cam (or similar) mechanism.
These are specialized torque wrenches used by plumbers to tighten the clamping bands on hubless soil pipe couplings.
They are preset to a fixed torque designed to secure the coupling adequately but insufficient to damage it.
[13]With electronic (indicating) torque wrenches, measurement is by means of a strain gauge attached to the torsion rod.
The signal generated by the transducer is converted to the required unit of torque (e.g. N·m or lbf·ft) and shown on the digital display.
At the same time, this generation of torque wrenches can store all the measurements made in an internal readings memory.
A popular application of this kind of torque wrench is for in-process documentation or quality assurance purposes.
They typically have a standard mounting interface that allows for quick changeover from one wrench head to another while ensuring that the torque applied remains accurate.
In 1995, Saltus-Werk Max Forst GmbH applied for an international patent for the first electronic torque wrench with angle measurement which did not require a reference arm.
[14] The ISO standard also states that even when overloaded by 25% of the maximum rating, the tool should remain reliably usable after being re-calibrated.
Re-calibration for tools used within their specified limits should occur after 5000 cycles of torquing or 12 months, whichever is soonest.
In cases where the tool is in use in an organization which has its own quality control procedures, then the calibration schedule can be arranged according to company standards.
[15] Click type torque wrenches are precise when properly calibrated—however the more complex mechanism can result in loss of calibration sooner than the beam type, where there is little to no malfunction, (however the thin indicator rod can be accidentally bent out of true).
There is also the issue of increased user error with the beam type—the torque has to be read at every use and the operator must use caution to apply loads only at the floating handle's pivot point.
The use of cheater bars that extend from the handle end can damage the wrench, so only manufacturer specified equipment should be used.
Never set a micrometer style torque wrench to zero as the internal mechanism requires a small amount of tension in order to prevent components shifting and reduction of accuracy.
As previously stated, according to ISO standards calibration should happen every 5000 operations or every year, whichever comes first.
[16] Calibration, when performed by a specialist service which follows ISO standards, follows a specific process and constraints.
The tool is then connected to the tester and force is applied to the handle (at no more than 10° from perpendicular) for values of 20%, 60% and 100% of the maximum torque and repeated according to their class.
