Why do machine components fail? Is there anything you can do to prevent it? This article will explain eight of the most common failure mechanisms, the types of equipment to which each applies and non-intrusive monitoring techniques to help determine when components are in various stages of progressive failure.
Abrasion, corrosion, fatigue, adhesion, cavitation, erosion, electrical discharge and deposition are among the most frequent failure mechanisms in industry. The defining characteristics for each of these mechanisms are detailed below and included in Table 1.
Abrasion affects nearly all mechanical systems. It begins when silica dust particles are transported by the lubricant to a narrow clearance between moving surfaces. Hard particles that are too large to pass through embed into one surface and cut the other. The shear force between the lubricated hard particles and the moving surface cut a V-notch into the moving metal surface.
This 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. This 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 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.
The resulting oxide is easily rubbed off by shear, which exposes fresh metal for sustaining oxidation. This mild rubbing emits stress waves and spreads soft metal oxides into the lubricant, exposing metal to the oxidation process. This mechanism may be prevented by moisture contamination control. It can be triggered by process contamination, a coolant leak or defective desiccant breather.
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 Hertzian contact shear that eventually work-hardens the metal until microcracks form, grow, interconnect and then release metal debris.
This 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 triggered 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. The 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. This 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 viscosityor inadequate lubricant delivery.
This failure mechanism typically 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 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. The 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. Cavitation 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.
Table 1. Common failure mechanisms, equipment, contributing factors,
proactive measures and condition monitoring for each
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. This failure mechanism can be prevented by protecting surfaces with energy-absorbing coatings.
An electrical discharge can impact all statically charged and electrically powered equipment, including electrical switches, circuits, breakers, transformers, controllers, motors, variable-frequency drives, generators, filters, shaft bearings and housings. Electrons transported as sparks, partial discharges and arcs blast surfaces with intense local compression, causing deformation and shear. This results in a wide spectrum of mechanical and electrical energy.
The electrons pass through gaps at supersonic speeds, emitting radio waves and sonic booms. This leads to heat damage on the surfaces and various gaseous substances, such as hydrocarbons and ozone. An electrical discharge ionizes proximate matter to form a discharge or plasma track. It may be offset by maintaining clean and dry materials and compartments. This progressive mechanism can be triggered by moisture, deteriorated insulation, ground faults, looseness and corroded contacts.
Deposition results from a dysfunctional and progressive accumulation of foreign material on a critical component. Examples include precipitated varnish formation and accumulation on a control valve as well as fibrous material accumulation on a fan.
Any varnish accumulation on a control valve may lead to plugging and sticking, while fibrous material accumulations on a fan may cause imbalance and a potential fire risk. This failure mechanism can be prevented by detecting, interpreting and addressing the specific accumulation process. The corrective action plan should be specific to its characteristic process.
FAILURE MECHANISM MONITORING TECHNIQUES
X-ray fluorescence (XRF) elemental spectroscopy of filter patch specimens is preferred for large particle failure mechanisms including abrasion, fatigue and severe adhesion. Optical emission spectroscopy and XRF are both suitable for corrosion and mild adhesion mechanisms.
Particle Count and Shape Classification
Particle counts greater than 4, 6 and 14 microns enable condition monitoring for contamination control. Direct-imaging automatic particle shape classification or microscopic wear particle analysis can help distinguish the failure mechanism.
Radio-Wave Arc/Spark Detection
This new stress-wave analysis technique can detect arcing, sparking and partial discharge events in electrical and electromechanical systems.
Inspections and Special Tests
Electrical discharges in oil-filled compartments may benefit from dissolved gas analysis (DGA) looking for evidence of turn-to-turn arcing. Deposition and accumulation of matter on flow controls, filters, screens, valves, fans and oil compartments are failure mechanisms resulting from a variety of operational conditions. The inspection and testing protocol will depend on these factors. For example, membrane patch colorimetry (MPC) is a preferred testing technique to identify varnish precursors.
Wear mechanisms of abrasion, rubbing (associated with corrosion), fatigue, adhesion (boundary wear), cavitation, erosion and electrical discharge can be sensed using a suitable analog sensor. Accelerometers, microphones, radio-wave sensors, current probes and magnetic flux sensors are other examples of analog sensory-input devices employed in stress-wave analysis.
Analog to digital data is oversampled and selectively decimated to derive simultaneous sonic and ultrasonic peak-hold waveforms. A peak-hold stress-wave analysis waveform may be either maximum-rectified peak or maximum peak to peak. The data shown in figures 1-8 are the peak-to-peak type. The sensors utilized for these measurements were microphone or radio-wave sensors.
Electrical resistance and arcing as well as mechanical friction produce hot spots that can be detected with thermal imaging.
A magnetometer is preferred for determining the total ferrous concentration of all ferrous oxide and ferrous metal particles from the molecular to abrasive wear particle size ranges. This tool is useful for quantifying wear and severity for ferrous debris in lubricating fluids.
Mechanical vibrations below a maximum frequency of interest are monitored to characterize proactive root causes of failure mechanisms such as imbalance, misalignment, looseness, resonance and soft foot. They are also monitored in combination with stress-wave analysis techniques to distinguish incipient to catastrophic stages of failure.
Lubricant misapplication, e.g., wrong oil, is frequently identified by verifying the correct viscosity for in-service lubricants. This directly relates to monitoring for inadequate lubrication associated with adhesion.
Water, Coolant and Neutralization Number
A convenient onsite method for monitoring water, coolant and the acid or base number is transmission infrared spectroscopy of the in-service lubricant. Laboratory titration methods such as Karl Fischer are also effective. Water, coolant and acid are all related to corrosive wear mechanisms.
TENSION, COMPRESSION AND SHEAR
“Nothing – well almost nothing – fails in compression,” said John Googin, chief scientist for the Department of Energy’s Y-12 National Security Complex, when asked about failure mechanisms. Googin suggested that at times when he thought compression was a primary cause of failure, a closer study of the evidence would reveal either a tensile or shear mechanism was initiating the progressive failure sequence. Three decades later, I still have not found an exception to this statement. I also have discovered that shear force is nearly always a contributing factor from incipient to catastrophic failure mechanisms.
Lubricated load-bearing surfaces allow machines to work by way of compression through a lubrication film. Mechanical systems are designed to have a long life and perform work through applied tension and compression. The intended long life can be cut short by shear. Failure mechanisms of abrasion, corrosion, fatigue, adhesion, cavitation, erosion and electrical discharge each have a common failure element of shear.
Figures 1-8 illustrate compression and shear information using a collection of sonic and ultrasonic stress waves. These techniques apply to measurements from various sensors, including a piezoelectric accelerometer, microphone and radio-wave antenna. Each of the graphs includes an orange and blue plot. The abscissa (Y-axis) is the signal strength in millivolts (mV), and the ordinate (X-axis) is the time in seconds.
The area below the orange line is the total ultrasonic peak energy above 20 kilohertz (kHz), while the area between the blue line and the orange line represents the total sonic peak energy between 500 hertz (Hz) and 20 kHz. The ultrasonic energy below the orange line is related to friction and turbulence. The sonic energy between the blue and orange lines is associated with the compression energy transfer reflecting work done by force through distance or pressure through volume.
CAPITALIZING ON YOUR EFFORTS
Now that you have a better understanding of common failure mechanisms (abrasion, corrosion, fatigue, adhesion, erosion, cavitation, electrical discharge and deposition), as well as what can be done to prevent them, it is important to remember that each mechanism has contributing factors, affects different types of equipment and requires proactive measures. By employing non-intrusive monitoring techniques such as sonic and ultrasonic stress-wave analysis, you can begin capitalizing on your preventive efforts for greater reliability.