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Validation of DC Torque Tools

 

Testing is the method of determining to what extent a requirement, such as the function or accuracy of a measuring device, is fulfilled. Testing a tool means checking that it performs within prescribed tolerance limits.

Characteristics tested include, for example:

  • Capability to reach Applied torque
  • Accuracy (torque trueness and scatter)
  • Some of the test are carried out one time i.e. during the process of assembly of tools at the laboratory or factory.

However, a tool must also be tested regularly throughout its entire life. Some tests can be carried out by the tool user, others require equipment that is normally only found in the manufacturer’s laboratory.

 

It is important to test the tools as the tools are subjected to mechanical stress e.g., gears, bearings and motors are subject to wear. As a result, a tools may not deliver its expected torque or may not be as accurate as expected. The consequences are obvious. Nuts and screws may not be tightened to the prescribed torque. Joints may fail, some even cause injuries. To avoid this, tools need regualr testing. Test is often carried out according to the international standard ISO 5393.

 

In order to test we must measure, When we test tools we use various kinds of measuring equipment. To be reliable, all measuring devices need to be calibrated at regualr intervals. when we discuss tightening, the most important measuring device is the torque transducer.

 

Precision refers to the closeness between a series of test values. A large scatter leads to bad precision. The machine capability index Cm is often used as a measure of precision. Good precision implies a large value of Cm. Bad precision calls for tool service. 

 

Good precision does not necessarily mean good trueness. Trueness refers to the closeness of agreement between the mean of a large number of test values and the reference value. Bad trueness can be corrected by an adjustment.

Accuracy means the closeness between the individual test values and the reference value. Good accuracy means that all test values are close to the reference value. In the machine capability index Cmk both the precision and the trueness are taken into account. Good accuracy implies a large value of Cmk.

Different test methods are adopted to audit the tool. Figure below shows the torque tool under test with reference master.

 

Figure 1 - Torque tools under Test

Tolerance defines the range of acceptable values. It is defined as the difference between the upper and lower permissible value. The tolerance of power tools´ torque scatter is often stated in terms of the permissible standard deviation.