Calibration can change gradually over an extended period. It can also jump suddenly, often as a result of the tool being dropped. This change is called “calibration drift,” and it can quickly become a very serious problem.

Calibration prevents both overtightening, which could lead to deformation and fastener failure, and under-tightening, which might cause an assembly to come apart in service.

The concern with calibration drift in torque tools is that when it happens, one of the conditions above will apply, leading to product failure and possibly resulting in a warranty claim and/or a recall. Most seriously, someone might be injured, and there could be liability issues.

Quality Management Systems, like those overseen by ISO 9001:2015, require a calibration frequency to be established for measurement devices. Assembly tool manufacturers usually recommend annual calibration, adjusting this based on operating conditions. However, a more pragmatic and effective approach to calibration frequency is possible when the causes of calibration drift are understood.

Here are the most common causes of calibration drift:

Worn Components

Assembly tools, like nutrunners, are complex mechanical devices with many moving parts. Wear is inevitable, and the more the tool is used, the faster this will happen. Accordingly, a tool used continuously throughout three shifts will suffer more wear and is therefore at greater risk of calibration drift than one that’s used just once a week.

Temperature Fluctuations

Calibration should always be performed at a set temperature, usually 70 degrees Fahrenheit. Using an assembly tool in a hotter or colder environment can cause the internal components to expand or contract slightly, which will impact accuracy.

This cause of calibration drift can affect torque measurement accuracy within minutes. To address this, consider keeping the tool in a temperature-controlled environment or, if possible, calibrating it where it will be used.

Environmental Contamination

Many assembly tools are used in dirty, dusty, or wet conditions. Over time, particles can get into the internal mechanisms, increasing friction and causing wear. If oil, grease, and coolants penetrate the housing, they can change the viscosity of the lubricants used. High humidity can also lead to condensation inside the tool, resulting in corrosion and even short circuits.

Individually and collectively, these factors will cause calibration drift. Accordingly, calibration frequency should be increased when a tool is exposed to any unusual or extreme conditions.

Vibration & Impact

In some industrial settings, assembly tools can suffer sudden shocks that damage and deform internal components. Bent shafts and cracked circuit boards are two examples of what can happen. These, and other impact-related faults, can change the torque and/or angle of rotation applied, leading to over- or under-tightening.

As a matter of policy, whenever a tool is dropped or struck, it should be sent for recalibration. If used in an environment where it’s subjected to vibration, consider increasing calibration frequency to minimize the consequences of any measurement drift.

Identifying Drift Before It Gets Worse

Calibration drift isn’t immediately obvious: a fastener will usually appear correctly secured, even if it’s over- or under-tightened. So, how do you tell when drift is occurring?

  • Check the tool for error codes (this is generally only applicable to newer units).
  • Compare readings on a sample joint with those given by other tools.
  • Install a bench torque tester (but remember this is not a substitute for professional external calibration).
  • Look for trends in calibration results.
  • Monitor joint or product performance for inconsistencies or faults during final inspection/test.

Preventing Drift

Prevention is always preferable to detecting and correcting a problem. Some steps to address the causes of calibration drift are:

  • When not in use, store tools in a dry, stable environment. Use rolls or foam inserts to prevent tools from banging against each other.
  • Clean tools regularly to prevent ingress of dirt or moisture.
  • Ensure tools receive appropriate maintenance at the correct intervals. For more insight, please visit our DC Torque Tool Maintenance Guide, and always consult your manufacturer’s manual.
  • Track calibration results to detect any trends in measurements. This method also supports applying predictive maintenance strategies rather than relying solely on preventive maintenance.

Do Your Tools Need Recalibration?

Since our founding, Encore Systems has been committed to helping OEMs and Tier 1 suppliers maximize torque tool performance. We carry refurbished nutrunners, impact wrenches, screwdrivers, cables, and accessories from most major brands, including Atlas Copco, DSM, and Stanley.

We also offer an extensive range of services, including standard and expedited repairs and calibration in our ISO 17025:2017-accredited and ANSI-compliant laboratory. We even provide a tool management program to help you track and analyze calibration history. Contact us today to learn more about our capabilities or to request a free evaluation.