The accuracy of any tightening system to deliver the required torque to a threaded fastener in an assembly cannot be specified as a percentage of the target torque that will universally apply in all situations. The reason for this is that the accuracy of the applied torque is not only dependent on the accuracy of the tightening equipment, but it is also dependent on the characteristics of the joint and its components.
In a transducerized DC electric tool, torque is measured by a strain gauged transducer which consists of a Wheatstone bridge circuit that converts one engineering parameter (strain) into another engineering parameter (voltage change by changing resistance). This is a well established technology and strain gauges are normally guaranteed to ±1% of full scale torque capacity.
However, in order to adequately address the accuracy issue, the industry has adopted a set of test standards and procedures to classify tools by their ability to deliver accurate torques under controlled conditions. The
one adopted in the USA by the APTP (Alliance for Power Tool performance) is the International Specification ISO 5393. As an international specification, it is widely recognized and used throughout the world.
This specification classifies the tool’s accuracy by a factor called ‘Capability.’ Capability is derived by conducting tests under controlled conditions on two types of joint: one that reaches its target torque in 30º (hard joint) and one that reaches its target torque in 720º (soft joint). The scatter in torque is measured by an independent, external torque transducer. The scatter from both joints is evaluated statistically and the standard deviation (a calculation of the torque variation) is determined. The capability is then calculated from the following formula:
Capability = (6 x standard deviation) / (Mean Torque Value)
Tools with a low value of capability produce less scatter, or are considered more accurate, than those with a high value. For example, a DC Electric Tool would typically have a value in the ≤10%, compared to a
pneumatic clutch tool which would have a capability in the 20% region.
Ingersoll Rand DC Electric tools have, on average, capabilities in the 5% range.
The use of DC Electric tools is growing for many reasons, not only because they are more capable. They can be ‘tuned in’ or adjusted for a particular joint to get the best capability or minimum torque scatter for the particular application on which they are being used. They have many features that can be adjusted to provide maximum flexibility and performance. These include the run down speed, final tightening speed, acceleration and torque overshoot compensation. In addition, the angle can be monitored during the tightening cycle and used to detect many tightening problems.
Other algorithms are available on the tool controllers which can also detect fastening problems. Examples of these problems include crossed threads and ‘stick-slip’ (severe frictional conditions) that make accurate
torque control, or more importantly, accurate clamp load control, extremely difficult.