Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1378657
8 | May - June 2021 | www . machinerylubrication.com AS I SEE IT which alters appearance, producing a more laminar shape. e particle's face may have striations or the appearance of adhesion (micro welding). Heat-tinting from friction/adhesion may exhibit colors from straw to brown to blue. Sliding Wear (Abrasion/Adhesion). Often occurs in high slide-to-roll or slide-only frictional zones. Boundary lubrication condi- tions from highly-loaded, low-speed conditions where only anti-scuff/AW additives play a role in mitigating friction and wear. More pronounced from longer sliding planes/tracks (e.g., large gear teeth), high traction forces due to boundary conditions and sliding serve to lift and dislodge fragments producing surface spalls. In gearing, the condition may be more pronounced across the dedendum zone of the gear tooth flank (below pitch line towards the root). Large particles with long straight edges and rectangular shapes are common. Otherwise, particles may have highly irregular shapes, and their edges may appear sharp or jagged. e base surface (opposite particle face) will appear torn or fractured. Evidence of abrasion, scoring or adhesion on the particle face is common. Deep striations may crisscross, travel the length of the particle (in the direction of elongation) or converge to the central region. Heat-tinting from friction may leave colors from straw to brown to blue. Black Oxides. Black iron oxides are typi- cally clusters of small pebble-like particles. Dark-metallo oxides may be larger, possessing the shape and size of rolling traction fatigue or adhesive wear particles. Most will have a free metal core with an oxide shell. Particles are very dark or black in appearance but may have small blue and orange dots near the lower limit of an optical microscope. Edges will transmit some light. Black oxides are associated with poor lubrication and steel-on-steel contact. Particle Size and Count e WCPD Table in Table 3 has been re-casted to show images of example particles corresponding to the cells' failure modes. Note, there is no reference (or scale) related to particle size. e purpose of the table is to emphasize and distinguish characteristic visual features of particle shape and texture. ese features provide important clues related to wear modes and wear location. It is widely known that particle size gener- ally correlates to failure stage or severity. For instance, particles on the left side of the table are distinctly larger (>50 microns) than those on the right side of the table (<15 microns). Large particles are even more pronounced in the cells in the upper-left zone of the table. Fatigue particles frequently produce larger particles (at advanced wear stages) than those produced from two-body abrasion, scuffing and adhesive wear. e population of wear particles at different sizes and shapes is important too and can help estimate the particle generation rate related to the precipitous state of the failure. However, particle concentrations are influenced by other factors too, including: • Where and how samples are taken • Fluid turbulence and sedimentation • Filtration (micron size and capture efficiency) • Age of the oil Because of this added complexity, it is often better to view the particle size distribution in terms of rate-of-change and size ratios (or percent large particles). If both increase signifi- cantly, the machine could be approaching an end-of-life condition. Work Backwards e best approach is to use the ASTM D7684 Classification Grid and the WPCD Table together. A careful examination of particle size, shape, edge detail and texture reveals much about contact dynamics and surface protection (position on the Stri- beck curve). From there, you can work backward to better understand and report condition monitoring information and prescriptive response: • What is happening: wear modes Table 2 The Wear Particle Contact Dynamics (WPCD) Table. The letters under the Wear Protection columns are described in the legend at the bottom of the table. CONTACT DYNAMIC WEAR PROTECTION Rolling Contact Sliding Contact Film Strength Influence by ZN/P and Additives None Low Mild Moderate High 100% 0% K-K K F-F B,D,F A 75% 25% E,H D,E C,D B,D A 50% 50% H-H E,H C-C,D,G B-B A 25% 75% H,J,L H,I E,G C,D B 0% 100% J-J, L J,L I C B A. No wear/no contact B. Normal rubbing wear B-B = Normal to moderate rubbing C. Mild Adhesive Wear C-C = Moderate D. Micropitting E. Macropitting F. Fatigue platelets F-F = Severe platelets G. Mild rolling traction fatigue H. Severe rolling traction fatigue H-H = Extremely severe I. Sliding wear/J-body abrasion J. Severe sliding/adhesive wear J-J = Extremely severe K. Severe rolling contact fatigue K-K = Extremely severe L. Black Iron Oxides START YOUR FREE SUBSCRIPTION www.machinerylubrication.com