Calibration of Torque Transducers

Calibration of Torque Transducers

Measuring torque is crucial for testing the performance of various types of rotating machinery used in the industry. Some examples are engines, pumps, gears, and electric motors, etc. Another need for torque measurement is in torque tools used in the field, from simple torque wrenches to much more complex rotary tools which may be manual, electric, or pneumatic. Torque tool transducers are given many names such as torque transducers, torque sensors, and torque meters.

Torque transducers convert torsion or surface shearing stress corresponding to a torque of the shaft to an electric quantity which is normally voltage and then sends this output signal through a brush, slip ring, rotary transformer, or transmitter to some display or recording device. These transducers ensure accurate and easy measurement of the torque of the device under the state of standstill to full speed rotation.

All these transducers use strain gauges as the sensing element which is robust enough to allow for accurate measurement even for continuous long-term operation even under severe conditions. Therefore they are widely used not only in laboratories for experiments and research but also in harsh industrial environments.

Torque transducers are often used to evaluate torque wrench accuracy. Like any other measuring equipment, torque transducers require regular calibration to continue to deliver accurate results. Continued use of un-calibrated transducers can not only compromise the quality but may also be unsafe in the operation and use of the equipment.

Traceable calibration of testing equipment is a requirement of every quality management system. Particularly as it is an essential part of any standard of the ISO 9000 series. In some types of industry, the requirement for a traceable calibration is even stricter. Although if it is known that calibration is required, it is for you to choose the kind of calibration best suited for your application and equipment.

There is also a significant difference between the accuracies in torque wrenches and torque transducers. Wrenches normally have an accuracy of around 2% to 5%, depending on the model but transducers, have an accuracy of 0.25% to 1%.

Since the torque transducer verifies the accuracy of your torque wrench, it is very important that you have your transducers calibrated regularly. Already many of our customers now get their transducers calibrated every six months instead of getting is done yearly.

 

Calibration of the Torque Transducer

The torque transducer calibration procedure starts with weights on a lever-arm to ensure the verification of torque. To ensure accuracy the weights and dimensions of the arm are themselves calibrated to very high tolerance at regular intervals. All these measurements take place in a shop like e2b calibration with controlled ambient conditions. The actual torque transducer calibration procedure is performed by a skilled technician who has been trained and certified to perform transducer calibration.

Calibration provides traceability of the torque transducer to national standards. This means that there must be an unbroken chain of comparisons to stated references to the national standard. Calibration by an accredited laboratory applying accredited procedures provides a good option. Usually, the procedure follows an approved standard or regulation. The calibration certificates issued by accredited laboratories have a good reputation as a document of traceability since the calibration laboratory itself has been certified by the official accreditation body.

Since calibrations according to the regulations used for an accredited certificate are expensive, most industrial users of torque transducers prefer the working standard calibration. The responsibility for determining the procedures and evaluating algorithms of the process rests with the laboratory performing the calibration. Users who order a working standard calibration from accredited calibration laboratories like e2b calibration laboratory usually benefit from the experience and expertise of the accredited laboratory.

It is recommended that prior to the first calibration, the torque transducer, including its mechanical couplings, is subjected to two overload tests in the course of which the nominal torque is exceeded by 8% to 12% of the nominal torque and maintained for over 1 minute. This may be done to avoid unexpected failure of the torque transducer during the calibration procedure.

 

Procedure for Calibration

  1. Adjust the indicating device according to the manufacturer’s instructions and in accordance with the customer’s requirements. All parameter values should be recorded before and after the calibration.
  2. Keep the torque measuring device with the supply power turned for a sufficient time so that its temperature stabilizes. Carry on calibration at a temperature stable to ±1°C. This temperature range should be between 18°C and 28°C and record it.
  3. Measure the zero-signal of the unloaded torque transducer and record it before the transducer is installed in the calibration equipment.
  4. This calibration is a static procedure by applying and measuring equally spaced torque values. It can be carried out for clockwise and/or anti-clockwise torque.
  5. Preload the transducer three times in the direction to be calibrated, applying the maximum torque value of the measuring range of the device and additionally once after each change of the mounting position. The preload duration should be approximately 30 seconds. Record readings after each preload have been removed for approximately 30 seconds.
  6. Check the stability of the zero signal as it provides an indication of the performance of the device during its calibration.
  7. Calibrate the torque transducer in at least three different mounting positions. Rotate the transducer or its mechanical coupling part each time through 120° about the measurement axis. For a square drive, four relative mounting positions should be used.
  8. Use a minimum of 5 equally spaced values from 20% to 100 % of the maximum torque value of the measuring range. If calibration points below 20% of the maximum torque value are required, then use calibration steps of 10%, 5%, 2% of the maximum torque value of the measuring range.
  9. The time interval between two successive calibration steps should be similar. Record the measured value only after the value has stabilized.
  10. The indicated value is defined as the difference between an indication in loaded condition and an indication in unloaded condition. Zero the indication at the beginning of each measurement series or adjust it manually by computation after the measurement.

All critical industries such as aerospace, nuclear, and medical regularly calibrate their torque measuring equipment to meet government regulations. If you need to make sure that your torque transducers are working perfectly, e2b calibration can help you. Be sure that your calibration procedure is performed by a certified shop to ensure that your tools meet the highest standards. e2b calibration is an ISO 17025: 2017 accredited facility. We can calibrate any type of transducers to the highest industry standards. Our services are unmatched in the industry. e2b calibration can also provide on-site services. Please contact us for all issues related to transducer calibration.

IN-HOUSE VS. OUTSOURCED CALIBRATION

Should you be calibrating your instruments in-house or outsourced? Read our guide to find out.

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