Twist measurement

Twist refers to periodic structures which are comparable with the course of a thread. Magnified many times, it is visible as a mirco-grinding structure. Twist structures at the sealing surfaces of shafts occur during grinding and impair the sealing function between the shaft and the sealing ring.

Twist measurement according to Daimler standard MBN 31 007-7

The Daimler standard MBN 31 007-7 describes a twist analysis method and parameters for ensuring the sealing function. With the current version from 2009 the analysis methods have been improved in order to improve the repeatability of the measuring results as well as the comparabilité of measurements carried out with different measuring systems.

TwistLive® analysis method from Hommel-Etamic

The TwistLive® analysis method is based on this standard and works with a specifically developped method on order to reduce the normal measurement time by up to 75 %.

Depending on workpiece and measuring task, twist measurement is possible either with one of our roughness or form measuring systems. Thus, twist measurement can both be combined with roughness or with form measurement.

Twist parameters

Twist angle Dγ (°)
Angle between the peripheral direction and a twist structure. The angle is oriented to the right (+ sign) when the direction of the structure is from bottom right to top left. The angle is oriented to the left (- sign) when the direction of the structure is from bottom left to top right.

Twist depth Dt (µm)
Maximum vertical dimension between the wave peak and the wave valley of the calculated twist surface.

Number of threads DG
Number of periods in peripheral direction referenced to 360°.

Theoretical supply cross section DF (µm²/r)
Cross-sectional area of a period length in an axial section of the twist surface.

Theoretical supply cross section DF per turn DFu (µm²/r)
Cross-sectional area of a period length in an axial section of the twist surface, multiplied by the number of threads.

Period length DP (mm)
Distance between two successive wave peaks or wave valleys in axial direction.

Contact length in percent DLu (%)
A measure for the theoretical enclosure of the shaft surface in peripheral direction by the sealing lip contact surface as a ratio of the total circumference. The calculation is made in a cutting depth corresponding to a material ratio of the twist surface of 80 %.