Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1449093
www.machinerylubrication.com | January - February 2022 | 31 ML ML wear damage is progressive and self-prop- agating while also accelerating corrosion. Metal-to-metal contact occurs when the lubri- cant film (designed to eliminate friction and separate a roller from a race or a journal from a shaft) fails due to inadequate lubrication. e increase in friction and shear causes mixed and boundary lubrication regimes. e contact emits stress waves. Compres- sion with mixed and boundary lubrication leads to shear and friction, which results in intense heating, melting and discoloration. Metal debris and oxides are released into the lubricant, and a spectrum of vibration is emitted. is mechanism can be prevented by maintaining the proper lubricant at the correct level and by operating at the designed speed and load. It may be triggered by a slow speed, high load, low viscosity or inadequate lubricant delivery. Rolling Fatigue (also called Hertzian fatigue) affects mechanical systems with loaded bearings and gears. Roller bearings and gears often fail due to the process of rolling contact, which eventually leads to material fatigue cracks and spalling. Compression between the rollers and races and between gear teeth produces subsurface Hert- zian contact shear that eventually w o r k - h a r d e n s the metal until microcracks form, grow, interconnect and then release metal debris. Rolling impacts at spall and other surface defects magnify stress waves and release more debris into the lubricant. Fatigue can be offset by minimizing dynamic loading from imbal- ance, misalignment and resonance, as well as by static load reduction and other maintenance practices; it can be triggered by an improper fit or thermal growth. Bending Fatigue affects shafts, joints, fasteners and structures. Cyclic bending produces fatigue cracks originating from surface defects in the vicinity of tensile stress concentrations. Contributing factors include sharp fillet radius, mate- rial f laws and geometric stress conc ent rat ion s it e s . M it i - gating factors m ay i nc lude FAILURE MECHANISM AFFECTING COMPONENTS CONTRIBUTING FACTORS MITIGATING FACTORS Abrasion Mechanical Dust contamination, System Sure, Defective breather Contamination control, clean, dry, fit for use Adhesion Mechanical Lubricant misapplication, slow speed, excess load, low viscosity Lubrication, speed, load, viscosity Fatigue Rotating, vibrating, flexing Elevated dynamic loading Minimize dynamic loading Rolling Fatigue Bearings, gears High dynamic load improper fit Minimize dynamic loading Bending Fatigue Shafts, joints, fasteners, structures Sharp fillet or other tensile stress concen- tration Pre-stressed compression, fracture-tough- ness, fillet radius Cavitation Fatigue Impellers, pumps, valves, piping Speed, pressure, and flow extremes Speed control, fluid dynamics, and surface treatment Electric Discharge Mechanical, electrical, inductive, static Shaft currents, moisture, round faults, deterioration, looseness, corrosion Grounding, contamination control clean, dry, fit for use Corrosion Mechanical, electrical Water or other corrosive contamination non-desiccating breather Contamination control, clean, dry, fit for use Stress Corrosion Shafts, joints, fasteners, structures Sharp fillet or other tensile stress concen- tration Pre-stressed compression, surface seal, frac- ture-toughness, fillet radius Galvanic Corrosion Structures, fasteners, electrical and mechanical Electrolyte between dissimilar metals such as carbon and stainless steel Insulate between dissimilar metals Erosion Corrosion Impellers, pumps, valves, piping Corrosive fluid with eroding flow Seal/protect surfaces Deposition Flow controls, filters, screens, valves, fans Temperature cycles, polar constituents, static charge Awareness & prevention Table 1. Common failure mechanisms, components, contributing factors and mitigating factors. ML ML