Production-related optical twist testing on radial shaft sealings


Optical Inspection of the absence of twist


To avoid leakage on radial shaft sealings the sealing surface has to be manufactured without twist. Optical diffraction analysis can visualize twist structures with amplitudes far below the amplitude of the surface roughness. Applying this method, the quality control can be realized quickly and reliable within the production process. 



Testing times of a few seconds are making the device applicable in many cases, for example at the reception control, for optimization of the manufacturing process and even for the 100%-control in the serial production. Depiction of twist structures was possible up to now only with very costly and expensive tactile form measuring technology. A check close to manufacturing could not occur on account of the high measuring time and the restrictive measuring terms. Alternatively in addition the thread method which dependents on different edge terms (e.g., thread material, thread strength, right weight or viscosity of the lubricant) is applied. Nevertheless, an essential disadvantage of this procedure is that it fails provably with very small or very big twist structures.




Visual testing - at a glance. These images show the behaviour on the sealing seat surface
without twist (1) or with twist (2)


 STT Family



STT - shaft twist tester NO

  • Production-oriented twist test
  • Laser technology
  • Integrated magnifying glass
  • High reproducibility

STT - shaft twist tester NK

  • Adapted digital camera
  • Production-oriented twist test
  • Evaluation and archiving software
  • Laser technology

STT - shaft twist tester NV

  • Integrated video camera
  • Construction as a twist test rig
  • Twist test on the monitor
  • Ideal for 100% inspection


Application Background:

The sealing property of a shaft seal is determined substantially by the twist. As soon as twist exists, a critical micro pumping effect which causes liquid escape or wear of the component begins with rotation of the shaft sealings. These twist structures can be visualised exactly with the Shaft Twist Tester. An optical filtering indicates the excistance of twist on the sealng´s surface and allows the evaluation of the twist parametres twist depth and period length. The number of the illustrated declension stripes is a measure of the twist depth, the distance between is a measure of the period length.



  • Quality control, quality assurance and manufacturing optimization of shafts and cylindrical components (general, STT - shaft twist tester NO)
  • Quality control and assurance of shafts and cylindrical components, recordable for quality management systems (STT - shaft twist tester NK)
  • Quality control on the test stand for twist detection on shafts and other cylindrical components (STT - shaft twist tester NV)


Technical Features

  • Range of measurement Twist period length: DP 20 - 200 μm
  • Depth of twist: Dt >= 200 nm* (information depending on the roughness and twist period, admissible roughness: Ra 0.1 - 0.5 μm)
  • Axial length of contact area: 15 mm
  • Shaft diameter: 5 - 200 mm (STT R150 N up to 300 mm)
  • Illumination source: Laser diode component laser class 2
  • Power requirement: Battery 6 V



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