Machinery Lubrication July August 2008

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LUBRICATION 101 Abrasive Wear Other Names: plowing, cutting, gouging and broaching Two-Body Abrasion Three-Body Abrasion Hard Abrasive Abrasive wear occurs in sliding contacts, usually due to particle contamination. Machines/components affected by abrasive wear: piston/cylinders, swash plates, journal bearings, gears, cams, rolling element bearings Adhesive Wear “Soft” Surface “Hard” Surface “Hard” Surface “Soft” Surface Scuffing occurs on both sides of the pitch line. Changes tooth profile. Drives load density towards pitchline. Loss of involute profile increases gear noise. Adhesive wear occurs in highly-loaded, poorly lubricated slidingmachine contacts. Machine/components affectedby adhesive wear: • Pistons/cylinders • Swash plates • Gear contacts • Hypoid gears • Cams and followers • Rolling element bearings Corrosive Wear Fluid Uniform Attack Fluid Pitting Attack Imbedded Particle Scratch Marks Surface damage: scratch marking, scoring, furrows, grooves and polishing Influencing factors: • Surface hardness • Particle size/hardness • Alignment Juncture(cold weld) Fragment (material transfer) Fluid Intergranular Attack Fluid Subsurface Attack Corrosion wear is surface damage resulting from exposure to a reactive environment (atmosphere, moisture accumulation, bacteria, acids, electrolytes, process chemicals or lubricant by-products). • Film thickness (load, viscosity, speed) • Particle concentration Pitch Line Pitch Arc Scuffing and Adhesive Wear Machine surface damaged by adhesive wear Influencing Factors: • Similarity of mating surfaces • Antiscuff, EP, AW additives • Film thickness (load, viscosity, speed) • Gear tooth size • Surface roughness Corrosion rate (rust) typically doubles for every 10°C (18°F) increase in temperature. Techniques to Reduce Corrosion: • Corrosion-resistant Metallurgy • Fluid Contamination Control (heat, moisture, water, acids, bacteria) • Protective Barrier (coatings, surface treatments, etc.) • Corrosion-controlling Additives (rust inhibitors, metal deactivators, overbase additive, etc.) of a bearing is the average time (in hours or cycles) to fail 10 percent of a set of identical bearings under certain conditions. An estimate of the L10 life can be calculated, providing a rating life of a bearing. Surface-initiated Fatigue This begins with reduced lubrication regime and a loss of the normal lubricant film. The oil film is reduced to boundary or a mixed regime. Some metal-to-metal contact and sliding motion occurs. Surface damage occurs. The high points of the metal surface asperities are removed, which initially appear as a matted or frosted surface. This is not smearing, as in adhesion (discussed below). This type of surface damage is usually visible with a magnification of three to five times. The surface damage is coupled with the cyclic loading of the rollers rolling over the race. This creates asperity microcracks and microspalling. The cracks start at the surface and migrate down into the metal. An edge of metal is created at the surface which flexes at the edge of the surface crack. This creates a cold worked edge which is lighter in color. The cracks propagate and may intersect within the metal, and a piece of surface material is then removed. Flaking, mechanical pitting and micropitting are other names used to describe spalling. Surface fatigue can also occur as a result of plastic deformation (described below). Contaminant particles in the oil enter the high-load rolling contact area between rollers and the race, or between gear teeth, and cause some form of surface damage - a dent. Improper handling of bearings can cause similar surface damage. These round-bottomed dents often have a raised berm around their edges. The raised berm of metal acts as a point of increased load or stress, or creates a reduced lubrication regime (mixed or boundary), and leads to a lower surface fatigue life. Improved filtration reduces plastic deformation, and therefore indirectly reduces the occurrence of surface fatigue. Notice that the term “contact fatigue” is not used by ISO. This is a vague term sometimes used to describe both forms of fatigue. It does not specify whether metal flexing damage started in the subsurface or from some initial surface damage. It encompasses any change in the metal structure caused by repeated stresses concentrated at a microscopic scale in the contact zone between the rolling elements and raceways, and between gear teeth. Abrasive Wear Abrasive wear is estimated to be the most common form of wear in lubricated machinery. Particle contamination and 36 July - August 2008 machinerylubrication.com Machinery Lubrication
oughened surfaces cause cutting and damage to a mating surface that is in relative motion to the first. Three-body abrasion occurs when a relatively hard contaminant (particle of dirt or wear debris) of roughly the same size as the dynamic clearances (oil film thickness) becomes imbedded in one metal surface and is squeezed between the two surfaces, which are in relative motion. When the particle size is greater than the fluid film thickness, scratching, ploughing or gouging can occur. This creates parallel furrows in the direction of motion, like rough sanding. Mild abrasion by fine particles may cause polishing with a satiny, matte or lapped-in appearance. This can be prevented with improved filtration, flushing and sealing out small particles. Two-body abrasion occurs when metal asperities (surface roughness, peaks) on one surface cut directly into a second metal surface. A contaminant particle is not directly involved. The contact occurs in the boundary lubrication regime due to inadequate lubrication or excessive surface roughness which could have been caused by some other form of wear. Higher oil viscosity, increased metal hardness and even demagnetizing bearings after induction heating during installation may help to reduce two-body abrasion. Adhesive Wear Adhesive wear is the transfer of material from one contacting surface to another. It occurs when high loads, temperatures or pressures cause the asperities on two contacting metal surfaces, in relative motion, to spot-weld together then immediately tear apart, shearing the metal in small, discrete areas. The surface may be left rough and jagged or relatively smooth due to smearing/deformation of the metal. Metal is transferred from one surface to the other. Adhesion occurs in equipment operating in the mixed and boundary lubrication regimes due to insufficient lube supply, inadequate viscosity, incorrect internal clearances, incorrect installation or misalignment. This can occur in rings and cylinders, bearings and gears. Normal break-in is a form of mild adhesive wear, as is frosting. Scuffing usually refers to moderate adhesive wear, while galling, smearing and seizing result from severe adhesion. Adhesion can be prevented by lower loads, avoiding shock loading and ensuring that the correct oil viscosity grade is being used. If necessary, extreme pressure (EP) and antiwear (AW) additives are used to reduce the damage. Corrosion Moisture corrosion involves material removal or loss by oxidative chemical reaction of the metal surface in the presence of moisture (water). It is the dissolution of a metal in an electrically conductive liquid by low amperage and may involve hydrogen embrittlement. It is accelerated, like all chemical reactions, by increased temperatures. No metal-to-metal contact is needed. It will occur with a full oil fluid film. Corrosion is often caused by the contamination or degradation of lubricants in service. Most lubricants contain corrosion inhibitors that protect against this type of attack. When the lubricant additives become depleted due to extended service or excessive contamination by moisture, combustion or other gases or process fluids, the corrosion inhibitors are no longer capable of protecting against the acidic (or caustic) corrosive fluid and corrosion-induced pitting can occur. The pits will appear on the metal surface that was exposed to the corrosive environment. This may be the entire metal surface or just the lower portion of the metal that may have been submerged in water not drained from the oil sump or at the roller/race contact points. Generally, an even and uniform pattern of pits will result from this form of attack. Mild forms of moisture corrosion result in surface staining or etching. More severe forms are referred to as corrosive pitting, electro-corrosion, corrosive spalling or rust. Frictional corrosion is a general form of wear caused by loaded micromovements or Machinery Lubrication machinerylubrication.com July - August 2008 37
Page 3 and 4: CONTENTS July - August 2008 Feature
Page 5 and 6: temperatures flanked by metal parti
Page 8: VIEWPOINT When It Comes to CBM, Ins
Page 12: HYDRAULICS AT WORK BRENDAN CASEY Wh
Page 17 and 18: FROM THE FIELD How Effective Is You
Page 19 and 20: obtained from advances in the progr
Page 21 and 22: There is another solution: New addi
Page 24: CASE STUDY Storage and Dispensing S
Page 27 and 28: detail,” Kubera said. “Before w
Page 30 and 31: CONTAMINATION CONTROL forming a bar
Page 32 and 33: CONTAMINATION CONTROL Copper Passiv
Page 34 and 35: CONFERENCE REVIEW Lubrication Excel
Page 36: LUBRICATION 101 Basic Wear Modes in
Page 41 and 42: in the area and inadequate groundin
Page 44: PRODUCT IDEAS Absorbent Pads Oil Ea
Page 48 and 49: LUBRICANT APPLICATION On a microsco
Page 50: LUBRICANT APPLICATION Bakery At Mar
Page 53 and 54: Polyethylene bulk oil tanks. Comple
Page 56 and 57: NLGI UPDATE Introducing the Latest
Page 58 and 59: CERTIFICATION NEWS ICML Reaps Benef
Page 60 and 61: BACK PAGE BASICS What’s Contamina
Page 62: BACK PAGE BASICS but insufficient f

Machinery Lubrication July August 2008

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