Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1378657
6 | May - June 2021 | www . machinerylubrication.com AS I SEE IT Wear Full Speed Conditions Starts, stops, shockloads, direction changes Ideal Ideal Coecient of Friction Film Thickness Boundary Lubrication Mixed Film Lubrication Protection Hydrodynamic Lubrication High Moderate Mild Low No W it hou t EP o r AW With EP or AW No EP or AW add itive Wear controlled by AW and EP additives No film due to inadequate speed or viscosity Wear controlled by both viscosity and boundary lubrication film Z = Viscosity N = Shaft Rotational Speed P = Load Wear controlled by viscous separation X axis = ZN P X axis = ZN P Figure 1 The Stribeck Curve showing the states of protection against mechanical wear. additive film strength. For rolling contacts (e.g., ball bearings), specific film thickness (Lambda) is also influenced by composite surface roughness. Refer to the chart in Figure 1 and the article in the link below for more information on the Stribeck curve: https://w w w.machinerylubrication.com/ Read/27725/ Rooted in Tribology Very little of what happens between surfaces in relative motion should be viewed as simple or even predictable; this holds true for the formation of particles generated there. With this in mind, the 25 cells shown in the WPCD Table are coded with letters corre- sponding to likely wear modes — these are approximations based on experience and findings from controlled laboratory experi- ments. Let's get more specific about each of these likely wear modes. Normal Rubbing Wear. ese particles result from mild and slightly adhesive cyclic rubbing contact. ey are often described as traction-induced exfoliation or peeling of the shear-mixed layer (asperities). Small abrasive particles interposed between frictional surfaces can increase the concentration of rubbing wear. Most appear as thin flakes or platelets with polished surfaces on both faces; others may be somewhat more granular from roll- ing-sliding contact. Adhesive Wear. Caused by moderate to high traction forces from impaired lubrica- tion or high contact loads in sliding frictional zones. e wear modes are more pronounced where longer sliding planes/tracks exist (e.g., large gear teeth). High tangential forces and frictional heat transfer loosened metal from one surface to the other or released it into the oil as dislocated fragments. Particles have smooth, distressed or striated surfaces; striation is generally from two-body abrasion. Many exhibit the appearance of a momentary molten state (micro-welding). Pitting (Micro & Macro). ese parti- cles can be formed due to rubbing or adhesive contact, as well as rolling contact fatigue. Micropitting occurs where particles — usually less than 20 microns in size — dislodge. Macropitting is similar to micropitting, except the size of the pits and particles are larger than 20 microns, often as a result of an advanced state of micropitting. Rolling Contact Fatigue. ese are often due to contact (surface) fatigue aided by mild sliding contact (traction). Particles may appear peeled from delamination. Many have smooth