STT-shaft twist tester

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 diffractionanalysis can visualize twist structures with amplitudesfar 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.

STT Product pictures

Application background

Measurement method & advantages

The sealing function of a shaft seal is largely determined by the twist. As soon as there is twist, a critical micro-pumping effect begins with the rotation of the shaft, which causes liquid to escape or wear on the component. These twist structures can be precisely visualized with the shaft twist tester. Optical filtering shows the presence of twist on the test object surface and enables the twist parameters twist depth and period length to be estimated. The number of diffraction strips shown is a measure of the twist depth, their spacing a measure of the period length.

Measurement method

The mapping of twist structures was previously only possible with very complex and cost-intensive tactile form measurement technology. A production-related test could not be carried out due to the long measurement time and the restrictive measurement conditions. Alternatively, the thread method is used, which, however, depends on various boundary conditions (e.g. thread material, thread thickness, attached weight or viscosity of the lubricant). The main disadvantage of this method, however, is that the process demonstrably fails in the case of very small or very large twist structures.

*Figure: Visual inspection at a glance. The pictures show the behavior on sealing seat surfaces without twist (1) and with twist (2)*

Advantages of the STT-shaft twist tester

As standard, twist is currently tested using the thread method or a profilometric measuring device. Both methods are not suitable for quick use in the production process or 100% control. With the STT, a test device has been developed that meets these requirements and combines reliable testing for freedom from twist with high user-friendliness. The STT hand-held device is placed on the test item. The tester can immediately recognize the presence of twist structures on the surface of the manufactured component using a system of strips in the field of vision.