Applying Optical Measuring Technology

Tribologically loaded functional planes: here NSK ball bearings

The functional surfaces of an NSK E12 deep thrust ball bearing (12 x 32 x 7 mm) are to be analysed in order to find out information about the friction and wear behaviour of the material. For this purpose, the bearings, the bearing inner ring and the bearing outer ring are measured in accordance with DIN EN ISO 4287 and 13565 as well as DIN EN ISO 25178.

Ball separator with ball
Inner sliding surface
Outer sliding surface

Measuring system and methods

Characterisation of the surface topography is carried out using touchless and high-precision measuring technology by Confovis. The 3D measuring microscope ConfoSurf CLV150 - seen here in combination with the 2-in-1 Scan Module ConfoCam® C1+ provides the most exact and reliable measurement results. When in confocal measuring mode, it facilitates the analysis of the finest of micro-structures as well as roughness measurements - even of smooth surfaces. The focus variations mode can be used for evaluating steep sides.
This evaluation is carried out with the MountainsMap® analysis software.

1. Evaluating a ball bearing

Single ball with measurement area
A) Profile-based roughness measurement in accordance with DIN EN ISO 4287 and DIN EN ISO 13565

Amplitude parameters obtained from a profile section according to ISO 4287 only allow for limited surface analysis, since they are only scanned along one line. Structural components lying outside this line are therefore disregarded. This can lead to incorrect analyses. Accordingly, the analytical software MountainsMap® offers a function for automatically creating multiple profile sections distributed over the entire scanned surface segment.

Primary profile series consisting of 1828 horizontal profile section (green: selectited single profile; grey: envelope curve of all measured profiles
B) Areal-based roughness in accordance with DIN EN ISO 25178

According to DIN EN ISO 25178, height parameters are calculated based on all the measurement data from the optically scanned texture area thereby facilitating a laminar assessment of the micro-topography at the site of the measurement.

C) Evaluation of the SK values in accordance with DIN EN ISO 25178

In order to characterise various surface structures as regards their tribological behaviour, the surface material proportion curve is used. It describes the proportion of surface material Smr dependent on the depth of a section plane through the surface. The proportion of surface material accounts for 0% at the highest point of the structure and 100% at the deepest. Parameters can be derived from the Abbot curve that characterise the functional behaviour of the measured surface.

The core roughness depth Sk is the depth of the roughness core and indicates how resilient the surface is in its core area. The reduced peak height Spk is the average height of the peaks protruding from the core profile which influence the contact and friction behaviour to other surfaces.

The reduced groove depth Svk is the average depth of grooves protruding into the material from the core. It is characterised, for example, by its capacity to absorb fluids. The upper load area Sr1 is the proportion of the material above the reduced peak height. The lower load area Sr2 is the proportion of the material under the reduced groove depth.

2. Evaluating the inner sliding surface

A) Profile-based roughness measurements in accordance with DIN EN ISO 4287 and DIN EN ISO 13565
Primary profile series consisting of 1699 horizontal profile section (green: selectited single profile; grey: envelope curve of all measured profiles
B) Area-based roughness analysis according to DIN EN ISO 25178
S-L surface; Nesting index: 80 µm

Height parameters are not necessarily comparable in value with the corresponding amplitude parameters of the roughness profile, since they are often based on different calculation methods. For example, Sz is not comparable with Rz, rather corresponds more to Rt (maximum peak-to-valley value within the whole surface or measuring section).

C) Area-wide analysis (Sk values) according to DIN EN ISO 25178

For characterising various structures of the surface with regard to their tribological behaviour, the areal material ratio curve (Abbott curve) is used. This describes the areal material ratio Smr as a function of the depth of a section plane through the surface.

Parameters can be derived from the Abbott curve that characterise the functional behaviour of the scanned surface.

D) Determining the volume parameters according to DIN EN ISO 25178

Integrating the areal material ratio curve gives you the material volume Vm as well as the material-free volume Vv per unit area as well as other functional volume parameters.

The peak material volume Vmp is the material volume of the highest peaks in the structure. The core material volume Vmc is the material volume in the core region of the structure. The core void volume Vvc is the void volume in the core region. The valley void volume Vvv is the void volume in the valley region.

3. Analysing the outer ring of the bearing

Section of the outer running surface with scanned area
A) Roughness analysis of the profile series according to DIN EN ISO 4278 and ISO 13565
Primary profile series, produced by a series of 1787 horizontal profile sections. (green: selected single profile; grey: Envelope profile of all measured profiles)
B) Area-wide roughness analysis according to DIN EN ISO 25178
S-L surface; Nesting index: 80 µm

Height parameters are not necessarily comparable in value with the corresponding amplitude parameters of the roughness profile, since they are often based on different calculation methods. For example, Sz is not comparable with Rz, rather corresponds more to Rt (maximum peak-to-valley value within the whole surface or measuring section).

C) Area-wide analysis of the Sk values according to DIN EN ISO 25178

For characterising various structures of the surface with regard to their tribological behaviour, the areal material ratio curve (Abbott curve) is used. This describes the areal material ratio Smr as a function of the depth of a section plane through the surface. The aerial material ratio is 0% at the highest point in the structure and 100% at the lowest point.

Parameters that characterise the functional behaviour of the scanned surface can be derived from the Abbott curve.

D) Analysing the functional volume parameters according to DIN EN ISO 25178

Integrating the aerial material ratio curve gives you the material volume Vm as well as the material-free volume Vv per unit area as well as other functional volume parameters.