One of the many tools essential in a workshop is a torque wrench that allows tightening bolts and nuts with a precisely defined force. However, not everyone knows how to properly calibrate, use and maintain the wrench.
When buying in a hurry or even online, we do not always have the time or possibility to get all the answers from the seller. Lack of proper knowledge about such specific tools as torque wrenches can result in their damage or even complete destruction. When tightening bolted connections we rely on our own experience and feel, which often leads to over-tightening and damaging the bolt. It is worth familiarizing yourself with a few rules regarding the use, maintenance and calibration of torque wrenches.
Calibration certificate
When purchasing a torque wrench we should pay special attention to the calibration certificate or, alternatively, a declaration of conformity with the ISO 6789 standard (the latest version from 2017). Issuing a certificate or declaration of conformity is the obligation of every manufacturer of torque wrenches. We can also request a calibration certificate from an accredited laboratory, i.e. accreditation by the Polish Centre for Accreditation (PCA). Additional calibration by the PCA always requires extra financial outlay from the manufacturer, but it provides users with assurance that the product meets all the requirements resulting from the standard and that we can use it safely. Do not buy a torque wrench that does not have any certificate, declaration of conformity with the standard or calibration.
Measurement range of the wrench
The applicable ISO standard strictly specifies the calibration of mechanical wrenches in the range from 20 to 100%. Thus: a wrench up to 100 Nm should start measuring no lower than 20 Nm, a wrench up to 400 Nm no lower than 80 Nm, and up to 1000 Nm should start measuring no lower than 200 Nm, etc. Electronic wrenches, however, should be calibrated in the range from 10 to 100%, i.e. for example 10–100 Nm, 40–400 Nm, 100–1000 Nm, etc. The measurement range of a torque wrench should always comply with the standard's guidelines. Always pay attention to whether the manufacturer adheres to the specified standards.
Loosening the mechanism of a mechanical torque wrench
After finishing use of a mechanical torque wrench, you should always release its mechanism. Failure to perform this action after each use of the wrench is one of the most common causes leading to tool damage. The mechanism spring becomes defective, which leads to loss of the wrench's accuracy. You should never leave the wrench set to a torque value, because repeatedly doing so leads to the inability to correct the measurement error during the calibration process.
Loosening connections with a torque wrench
One of the most common mistakes is using torque wrenches not only to tighten connections but also to loosen them. If the wrench also works in the left (reverse) direction, when loosening a connection you should set the wrench to the maximum torque value. Do not apply more torque to the wrench if loosening is not possible after reaching the maximum torque value. Excessive use of torque wrenches to unfasten connections results in irreversible damage to the mechanism. To avoid serious damage to the tool, always remember to use the torque wrench correctly.
Regular calibration
An important aspect is the regularity of torque wrench calibration. This procedure should be performed on average every 12 months. For electronic wrenches there is the possibility to save a reminder after 5,000 cycles. In this case you should ask at the time of purchase about the communication software for the wrench and the service password. If a measurement error or deviation from the assumed standard is detected, without the software and password we will not be able to adjust the electronic torque wrench. Both mechanical and electronic wrenches should be calibrated regularly. This action ensures the long-term and error-free operation of the tool, without the risk of rapid damage.
In summary, it should also be added that perhaps the most important matter is the individual serial number of the wrench. When buying a wrench we should check whether this number is on the tool; it is usually embossed in a visible place. It is a unique number and gives us the guarantee that the calibration certificate was issued for that specific product. The same serial number should also be present on the certificate attached to the wrench. Remember to check the matching of the number on the torque wrench and the calibration certificate.
