To improve the reliability of automobiles, rapid advances are made in the design and manufacture of dynamic sealing systems. Two properties that impact design of rotary dynamic seals are surface texture and lead of the shaft. Fluid leakage through a seal is determined by these properties, which affect the service life of automobiles.
For optimum performance of seals, the shaft surface texture must be optimal. A rough surface texture will cause the seal to wear out quickly, while a smooth texture will cause the seal to bed incorrectly. The shaft lead, commonly called machine lead angle, is formed during the manufacture of shafts and has to be ideally zero. Standards such as ISO 6194-1:2009 and RMA OS-1-1-Rev.2004 provide specifications for surface finish and lead angle.
Shaft Surface Texture and Lead Angle Measurement
Several measurement techniques can be used to determine shaft surface texture and shaft lead. A few are discussed below.
The String Method
The RMA standard suggests the String method for measuring shaft lead. In the String method, the lead angle is measured by using a weight suspended from a string. The String method is simple and economical to implement. However, it is not very reliable as it depends on factors such as surface finish and the type of string and lubricant used. Figure 1a-c illustrates a string method assembly in detail.
Figure 1. (a) Typical string method assembly with test shaft mounted. (b) Shaft is rotated both clockwise and counterclockwise to inspect string traverse. (c) Distance of string traveled is measured with verneers.
The RMA standard recommends a stylus profiler for determining surface texture. Stylus profilometers are also used at present for measuring lead angle. A stylus profilometer measures 2D surface parameters such as Ra, Rz and Rpm. Ambiguity in the measurement of these parameters and measurement errors caused by the stylus tool are a few limitations of this method. Shaft alignment with respect to the stylus is critical for accurate measurement of roughness. Moreover, 2D parameters may not thoroughly describe the functional nature of the shaft’s surface. While this method is a definite improvement when compared to the String method, it can be used only for the quantitative measurement of macro lead angle and not micro lead angle.
Figure 2. Surface textures can vary widely while their Ra values are similar (a) when using a 2D contact stylus technique (b).
White Light Interferometry
White Light Interferometry, also called optical profiling, is a non-contact technique ideal for 3D surface roughness measurements. 3D surface parameters (S-parameters) are determined instead of 2D roughness parameters (R-parameters) in this method. For instance, Sa, Sz and Spm are the equivalents of Ra, Rz and Rpm, respectively. Some of the advantages of this method are speed, resolution and repeatability.
Figure 3. Standard schematic of an optical interferometer.
3D Surface Metrology System for Measuring Lead Angle
The NPFLEX-LA measurement system developed by Bruker allows accurate, quantitative measurement of key shaft parameters. The system handles shafts with diameters ranging from 38 to 203 mm. This is a non-contact method of measuring lead angle and the accuracy of measurement is unaffected by the variation in the amount of lead. The system makes it possible for industries to get comprehensive 3D information, measure all critical shaft parameters using a single system and ensure high quality of shafts. It has a short set up time and produces repeatable measurement results. The NPFLEX-LA is designed such that it is possible to achieve consistent measurements regardless of how parts are aligned by different operators.
Figure 4. Test shaft mounted on NPFLEX-LA.
Figure 5. NPFLEX-LA is designed such that operators do not have to be concerned with perfect part level or wobble during mounting.
Traditional techniques used for measuring shaft properties such as the string method and the stylus based metrology are inadequate now, with smoother shafts being designed and seal interfaces more tightly controlled. New shaft specifications developed in line with current industry standards demand more robust methods that can quantitatively measure shaft properties. White light interferometry is a better technique for quantitatively measuring shaft surface texture and lead angle. Bruker's operator-independent NPFLEX-LA measurement system enables manufacturers to quantitatively and repeatably measure shaft lead.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.