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Driving motor assembly calibration

The difference between a calibrated screw and an screw assembly error can cost billions of dollars, writes German air motors engineer and staffer at a new DKD-accredited facility in the region, Dagmar Hierl.

Screw assembly error – 15,000 cars recalled to the factory! Headlines such as these shatter car owners’ confidence and damage the image of an automobile manufacturer. A car contains around 1,200 to 1,400 screws and various assembly tools are employed in the automotive sub supply industry as well as by the OEM manufacturer.

Diverse safety requirements set high demands on quality management, so international screwdriving technology and automation supplier, DEPRAG SCHULZ GmbH & CO, built a calibration laboratory in which torque testing instruments can be checked and recalibrated with the highest precision.

Heavy weight

Roughly 12 kilos of screws are used in the construction of a medium-sized vehicle. Fifteen per cent of these are subject to the safety hazard marking: “Danger to life and limb”. Precision and stability is vital for these components!

If brakes fail, a resulting accident could have catastrophic effects. If panelling constantly rattles, it affects customer satisfaction. Therefore modern assembly tools are equipped with torque transducers which are responsible for control, documentation and recording of torque values during assembly.

The precision of a measurement transducer must be checked regularly, for instance by a production employee at shift change. This check is conducted in a testing station. Testing stations meet the highest standards of accuracy, but they in turn must also undergo checks at regular intervals – this is known as recalibration. The measurement parameter for the tightness of a screw is ‘torque’. This service is now offered in-house by DEPRAG SCHULZ GmbH & CO in Germany.

Test laboratory

The test laboratory at DEPRAG, one of only four DKD-accredited calibration laboratories (the German Calibration Service) in Bavaria, is run by specially trained personnel who carry out high-precision repeatable measurements. It is possible to determine calibration parameters in a torque range from 0.01 Nm (1 Ncm) to 500 Nm in the new DKD-accredited test lab. The best measurement capability is 0.1 per cent of the target measurement value. Calibration should be repeated at certain intervals. The frequency varies from case to case.

If a measurement station is often used during assembly then correspondingly, it must be tested often.

The electric and pneumatic screwdrivers manufactured at DEPRAG must also of course be tested before delivery.

As screwdriving specialists, DEPRAG offers a wide spectrum of products including screwdrivers with shut-off clutch which must also be inspected with a machine capability test. These tests are carried out using in-house control measurement platforms which are externally checked and regularly recalibrated.

Assembly Technologies P/L

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