Accuracy in the Screwdriving Technology
An intensive discussion is ongoing about a required, necessary, meaningful and attainable accuracy in the assembly process. We try to specify some concepts:
What does Accuracy mean?
First, it is necessary to differentiate between a qualifying accuracy and the accuracy definition according to DIN 55350. In the practical discussion however, accuracy is always meant as a precision concept, especially when assembly is made to torque. Other values, such as angle, distance or time, or even the total process may be termed accuracy.
If concrete values are named, then is always recommended to indicate the relative standard deviation of a measuring series.
For example: Torque shut-off value of 9 Nm with a precision (inaccuracy) from 3 % means, that the tool was subjected to a measuring series with an average of 9 Nm and a standard deviation of +/- 3 % in reference to the average of the measuring result. According to the DIN (ISO) norm, only measuring values may be used that were achieved under perfect test conditions.
The common description of the concept – repeatability – is defined as precision in accordance to DIN 55350 as qualitative name for the value of the equal approach of independent evaluation results with multiple applications of a fixed evaluation procedure under pre-determined conditions.
Basically, the statement of accuracy- according to the above explanation – is always Repeatability, since an average value (set-value) is determined using an accuracy (inaccuracy) of 3 % standard deviation from a series of independent measuring values. In the practice, the verified repeatability is again influenced by different parameters.
Therefore, a cyclic testing of the achieved torque value is practical.
The kinetic energy during an assembly depends primarily on the speed of the used screwdriver.
A slower screwdriver with a mechanical shut-off clutch, causes the movements in the clutch to slow down and therefore it also reduces the dynamic effect. According to our experience, slower screwdrivers have a lower standard deviation than faster screwdrivers.
With electronically controlled screwdriving-systems, the influence of the inertia is reduced because the driver slows down when reaching the end of the tightening process.
The measuring technology used for the dynamic process of the screw-assembly must be well suited for this task, which means it needs a tough rigidity to vibrations, incorporates filters as well as a suitable algorithm to display the accurate measuring value. Additionally, the drift of the measuring instrument has to be taken into account.
Particular attention has to be given to the sampling rates of the measuring instrument, because highly dynamic screwdriving processes require measuring frequencies of > 10,000 Hz.
Many readily available instruments however have sampling rates well below this value!
Because of insufficient sampling rates, the deviation may be 10 % and more.
The absolute precision of the screwdriver as well as the measuring instrument is, independent from the repeatability, vitally important. Generally it is only possible to obtain an absolute accuracy for measuring instruments, if they are traceable to national norms. Mainly the screw joint can influence the absolute accuracy of the screwdriver. Because of the above-described effect of the kinetic energy, the same screwdriver may obtain different absolute accuracies.
According to the construction of the clutch, the design of the screwdriving-station (moving masses after the clutch) and the rise of the torque curve, rather large and changing absolute accuracies may occur, because the speed changes at the clutch shut-off. Because of the special construction of our NANOMAT,MICROMAT, and MINIMAT-clutches, the screwdriver itself eliminates these influences, so that there are practically no fluctuations.
Also, when directly comparing two measuring systems, there will practically always be a difference in values. Assemblies with hard joints, high speeds, different measuring filters, electronic and mechanical dampening devices, will influence the accuracy especially during assemblies with high frequency cycles. Generally, the point of reference of an absolute accuracy is given by the tractability of individual measuring systems. However in the practice, the accurate test conditions of these calibration procedures are insufficient. Often, only static calibrations (DIN 51309) are performed and documented and the influence of a highly dynamic screwdriving process will not be considered.
Because of the need for absolute accuracy, the calibration certificate indicates the measuring uncertainty and points out where the measuring values are located in the tolerance field.
What torque-values are determined?
Of higher importance is the layout of the different measuring transducer and its influence of the absolute values of the screwdriver. Primarily, it is important to observe which value has to be measured and compared. Integrated screwdrivers transducers can always measure only the torque supplied by the screwdriver. If this value is being transmitted as pre-load or friction, cannot be determined by the sensor.
For clarification, it is necessary to re-check the screw-connection. An exact re-checking is only possible using a measuring electronic, which give true information about the transition of slip- and slide friction. The breakaway torque will never give a reliable statement in regards to the achieved pre-load. Also, during re-checking, the actually determined value is changed once again, which worsens the accuracy even more.
With making comparison measurements, there will always be deviations. Settling conditions of the screw-connection are the main cause. Furthermore, there are always different dynamic influences when comparing re-checked values with the original values.