Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1449093
32 | January - February 2022 | www . machinerylubrication.com pre-stressed compression, use of materials with high fracture toughness and smooth fillet radius. Cavitation Fatigue failure mechanisms typically occur on impellers, pumps, valves and other flow devices. Cavitation fatigue involves cyclic void c ol l ap s e s h o c k w a v e s that cause sub- surface fatigue cracking, pitting a nd spa l l i n g. C o n t r i b u t i n g factors include extreme variations in speed, pressure and flow. Mitigating factors include speed control, fluid dynamics and surface treatments. Liquid cavitation is stimulated by pres- sure variations in the cyclic fluid flow near the surface. In a slow part of the pressure cycle, suction enables evacuated micelle nucleation originating from solid surface irregularities. Highly saturated dissolved gas from the surrounding liquid may diffuse into expanding bubbles. Later in the pressure cycle, suction is released, and the bubbles implode back toward the nucleation surface irregularities. e implosion causes a supersonic surface impulse and transfers compression and shear stress waves. Shear from the stress wave dislocates subsurface material morphology. Eventually, these dislo- cations lead to fatigue cracks and spalling. Note that when the bubbles contain partial pressure gases diffused from the surrounding liquid, there is also intense heating from the compressed gases. Cavita- tion damage, which normally is progressive and self-propagating, often results in fatigue cracking and stress corrosion cracking. It is triggered by pressure, flow and speed varia- tion but can be offset by fluid flow design, control, speed and surface treatment. Erosion can affect valves, pipes, baffles, impellers and other electrical and mechanical components exposed to streaming particulate matter. It occurs when high-velocity liquid or solid matter impacts a solid surface, causing intense points of compression and resulting in deformation and shear. Stress waves are emitted from the impact points, and debris is dislodged from the damaged surface. is failure mech- anism can be prevented by protecting surfaces with energy-absorbing coatings. Electric discharge affects electrical components and inductive or static charged mechanical components with voltage. e p o t e n t i a l to ground applies an electric field through a m e d i u m . E l e c t r o n s and ions accelerate via spark, track, arc plasma. Arc-spark events yield a wide spectrum of mechanical and electrical energy. Surface morphologies undergo surface erosion leading to heat damage. An electric discharge ionizes proximate matter to form a discharge or plasma track. Contributing factors include moisture, degraded insulation, ground faults, looseness, corroded contacts and contamination. Miti- gating factors include clean and dry materials and compartments. Corrosion impacts almost all electrical and mechanical systems and is synergistic with all other failure mechanisms. It occurs when a corrosive substance attacks metal and changes the surface from strong, thermally and electri- cally conductive metal into soft, electrically and thermally resistive oxide. e resulting oxide is easily rubbed off by shear, which exposes fresh metal for sustained oxidation. Mild rubbing emits stress waves and spreads soft metal oxides into the lubricant, exposing metal to the oxidation process. is mechanism may be prevented by moisture contamination control. It can be triggered by process contamination, a coolant leak or a defective desiccant breather. ree synergistic combinations of corrosion with tensile stress, galvanic current and erosion are identified below. Stress corrosion affects shafts, joints, fasteners and structures. is synergistic combi- nation of tensile stress and corrosion produces intergranular corrosion accompanied by tensile fracturing, which progressively self-propagates. MECHANICAL FAILURE Corrosion Abrasion Deposition Electric Circuit Discharge Galvanic Corrosion Erosion Corrosion Stress Corrosion Bending Fatigue Rolling Fatigue Adhesion Shaft Current Dishcharge Cavitation Fatigue Figure 1.