Failures attributed to leaking seals cost the transportation and power industry millions of dollars a year in warranty costs, recalls and fines. Advances in seal and shaft materials and design have extended end product lifetimes and reliability. As a shaft rotates, fine machined grooves wick lubricant through the shaft-seal interface. The amount of lead on a shaft’s surface contributes to the amount and effectiveness of the lubricant at the sealing interfaces. Since the measurement of a shaft’s lead angle is so important to predicting the performance of the dynamic seal, tighter tolerances have been outlined in the industry specifications ISO 6194 and RMA-OS-1-1 rev. 2004. However, traditional measurement techniques have had difficulty in reliably verifying these specified parameters. QUALIFICATION VERSUS QUANTIFICATION Historically, shaft and seal manufacturers qualify their parts using a defined combination of stylus and string measurement methods, which depend heavily on part alignment, mounting, and operator experience. The contact nature of these techniques makes them susceptible to imperfections in shaft surfaces, leading to inconsistent results. The string method in particular has proven to be qualitative rather than quantitative, particularly from operator to operator, and lacks sensitivity to detect micro lead and lead angles near 0 degrees. Bruker has developed the NPFLEX-LA™ 3D Surface Metrology System, the first quantifiable surface measurement instrument specifically designed for lead angle measurement. Leveraging the intrinsic benefits of non-contact, interferometry-based optical profiling technology, the NPFLEX-LA predictably and quantifiably measures both shaft lead angle and surface texture of sealing surfaces in a single integrated solution. The system has been designed to be independent of surface finish and part mounting and setup inconsistencies, such as warp, level and concentricity. A reliably quantifiable technique enables manufacturers to make better decisions on machining processes. THE BENEFIT OF THREE-DIMENSIONAL PARAMETERS Though the NPFLEX-LA can calculate the two-dimensional Parameters (Ra, Rz, and Rpm) generally employed throughout the precision machining community, it is inherently a 3D measurement instrument that calculates the equivalent three-dimensional (3D) surface parameters (Sa, Sz, and Spm). Such calculations are highly stable over the entire shaft as they encompass a large area, and are also insensitive to alignment, as the full 3D images are used in the calculation. Both lateral X and Y directions are measured simultaneous by scanning in the vertical direction. The result is a fast 3D analysis of the entire surface area. SIMPLE STEPS TO RELIABLE MEASUREMENTS Operating the NPFLEX-LA is easy. An operator loads a shaft in a standard three jaw chuck, centers the shaft in the field of view of the microscope, and focuses on the shaft. Once mounting and focusing are complete, which takes less than 2 minutes, the user selects a pre-loaded, part-specific recipe and clicks the start button to begin the measurement routine. From here the system does all the work, scanning the surface at multiple locations. At each location the system calculates the best fit true cylinder to the surface and angle of that cylinder with respect to the system camera. The system also simultaneously conducts a Fourier transform of the data to determine an angular power spectral density of the angle of the machining marks on the shaft, with reference to the same system camera. These two Calculations are subtracted from each other at every measurement point, to eliminate any bias due to part alignment, wobble, or out-of-roundness errors. The result is a lead angle measurement independent of part alignment, and unaffected by surface imperfections. In addition, roughness is calculated at each location, and the global roughness of the part is simultaneously reported. COMPREHENSIVE RESULTS You can refine your data by simply setting pass/fail criteria to identify and/or eliminate scratches, defects, or contamination. These identified defects or anomalies are logged for future review, but are removed from the global lead angle calculation to maintain an accurate characterization of the part’s true lead. Combining both the measurement of surface roughness with that of lead angle on a single platform, helps to improve product yield while facilitating ease-of-use and overall throughput. In this fashion the NPFLEX-LA delivers on the promise of a comprehensive, quantifiable, gage-capable lead angle surface metrology solution. Employing this technology will enable the transportation and related industries to better control and reduce fluid leakage of rotary dynamic seals. For more information contact Bruker Nano Surfaces Business at +1.520.741.1044/800.366.9956 ext. 3, firstname.lastname@example.org, www.bruker-axs.com
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