Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1189076
By Roger Story, Owens Corning www . machinerylubrication.com | November - December 2019 | 11 Why do machine compo- nents fail? Is there anything you can do to prevent it? is article will explain eight of the most common failure mechanisms, the types of equipment to which each applies and non-intru- sive monitoring techniques to help determine when components are in various stages of progressive failure. FAILURE MECHANISMS Abrasion, corrosion, fatigue, adhesion, cavitation, erosion, elec- trical discharge and deposition are among the most frequent failure mechanisms in industr y. The defining characteristics for each of these mechanisms are detailed below and included in Table 1. Abrasion Abrasion af fects nearly a ll mechanical systems. It begins when silica dust particles are transported by the lubricant to a narrow clear- ance between moving surfaces. Hard particles that are too large to pass through embed into one surface and cut the other. e shear force between the lubricated hard particles and the moving surface cut a V-notch into the moving metal surface. is cutting process emits a spectrum of mechanical vibration from the point of abrasion and generates abrasive wear debris which is carried away by the lubricant. is mechanism generally is not self-propagating and is easily offset by contamination control. It can be triggered by a surge in the circulating system or by a defective breather. Corrosion 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 being strong, thermally and electrically conductive metal into soft, electrically and thermally resistive oxide. e resulting oxide is easily rubbed off by shear, which exposes fresh metal for sustaining oxidation. is 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 contam- ination, a coolant leak or defective desiccant breather. Fatigue Fatigue af fects mechanica l 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 Hertzian contact shear that eventually work- hardens the metal until microcracks form, grow, interconnect and then release metal debris. is generates stress waves from the impacts and releases debris into the lubricant. Fatigue can be offset by minimizing dynamic loading from imbalance, misalignment and resonance, as well as by static load reduction and other maintenance practices. It can be trig- gered by an improper fit or thermal growth. Cavitation can also cause cyclic subsurface shear resulting in material fatigue cracks and spalling. Adhesion (Boundary Wear) Adhesion impacts nearly all mechanical systems with loaded components. Adhesive wear and other boundary wear damage is progressive and self-propagating while also accelerating corrosion. Metal- to-metal contact occurs when the lubricant 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. The contact emits stress waves. Compression 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. Cavitation This failure mechanism typi- cally occurs on impellers, pumps, valves and other flow devices. Liquid cavitation is stimulated by pressure variations in the cyclic fluid flow near the surface. In a slow part of the pressure cycle, suction enables evac- uated 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 dislocations 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. Cavi- tation 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 variation, but can be offset by fluid flow design, control, speed and surface treatment. Erosion Erosion can affect valves, pipes, baffles, impellers and other elec- trical and mechanical components ML