Common Mechanisms of Failure & Preventive Methods

Common Mechanisms of Failure & Preventive Methods

Ray Garvey, Reliability Consultant, CRL, CLS, Spectro Scientific, AMETEK

Posted 01/31/2023

Why do components fail? – This article explains common failure mechanisms, component types to which each applies, and suggests non-intrusive monitoring techniques to discover components in various stages of progressive failure. 

What can we do to prevent it? – This article recommends periodic reliability assessments for continuous improvement in line with organizational objectives.  The initial or baseline assessment compares asset reliability with standard practice from an industry body of knowledge (BOK) and identifies near and long term areas for continuous improvement.  Each assessment considers asset criticality analysis, root cause analysis, asset condition information, defect elimination, risk management, asset lifecycle management, and organizational decision making.

Common proactive measures intended to exclude root causes from initiating failure mechanisms included in continuous improvement activities often include contamination control, misapplication elimination, and defect elimination.

Each periodic assessment concludes with a short list of aligned priority task assignments for completion during the next assessment interval.

Periodic Assessments of Lubrication and Oil Analysis

Periodic reliability assessments provide key milestones for continuous improvement of reliability.  The initial assessment establishes both a baseline and a gap.  The baseline is intentional comparison of actual with a desired or best practice level to achieve organizational objectives. Organizational objectives provide focus and aim for allocation of limited resources during each interval between periodic assessments.  Example organizational goals for reliability:

• Improved risk management with less reactive maintenance
• Improved decision making with better communication
• Improved reliability program including baseline assessment, goal, gap, improvement opportunities.

Excellent reliable industrial manufacturing requires alignment of effort.  The 36-element framework diagram in Figure 1 is used by Certified Reliability Leaders (CRL) to represent functions that must work collectively in an industrial plant or mill to achieve reliability.  Some or all of the blue outlined elements in Figure 1, for example, may be evaluated during periodic reliability assessments.

Each periodic assessment should follow consistent evaluation criteria based on industry body of knowledge (BOK) comprising the following published reference materials:

• Introduction to Nondestructive Testing, The American Society for Nondestructive Testing:
• Audit it. Improve it!, Reliabilityweb.com, 2015
• “Best Practices in Bulk Lubricant Storage and Handling”, Machinery Lubrication July 1999,
• “Identifying Root Causes of Failure with Condition Monitoring”, Machinery Lubrication Magazine, December 2012,
• ISO 14224
• ISO 55000
• Uptime® Elements Body of Knowledge, Reliabilityweb.com, 2019
• “Why Equipment Fails and What You Can Do to Prevent It”, Machinery Lubrication, Dec 2019.

Each reliability assessment is structured to identify common failure mechanisms, root causes, contributing factors, and proactive measures.  The above BOK references are selected in support of improved reliability through failure mechanism, root cause, and defect elimination.

Based on the organizational objectives, a periodic reliability assessment will comprise evaluating asset criticality analysis, root cause analysis, asset condition information, defect elimination, risk management, asset lifecycle management, and organizational decision making consistently using a prescribed list of assessment questions based on the relevant BOK.  A complete assessment including all categories is likely to cover many dozens of assessment questions and BOK references.  Table 1 shows a structure for consistent assessment questions and associated BOK references.

The following table lists 58 questions and relevant Body of Knowledge references for assessing effectiveness and measuring progress and improvement with industrial lubrication and fluid analysis programs.

Assessment findings and follow on actions to achieve continuous improvement documented as shown in Table 2.  For this example, organizational objectives are:  1) risk reduction and 2) improved decision making.  Note that the category, Asset Condition Information, is typically divided into several parts including vibration analysis, alignment and balance, fluid analysis, lubrication, infrared thermal imaging, and nondestructive testing, for example.

The interval between periodic assessments is used to accomplish continuous improvement.  A short list of three to five priority categories with three to five tasks and assigned responsibilities focuses teams on organizational aim and alignment.  For example, lubrication and fluid analysis categories for an interval between periodic assessments might include:

1. Machinery lubrication with tasks pertaining to contamination control and misapplication elimination.
2. Oil analysis with tasks pertaining to measuring ferrous density, particle counting including particle shape classification, and water contamination.

Failure Mechanisms

Abrasion, corrosion, fatigue, and adhesion, cavitation, erosion, electrical discharge and deposition are common failure mechanism from the referenced literature. Characteristics of these failure mechanisms are stated below and listed in the first column of Tables 3 and 4.

Abrasion

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

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

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 (or 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 viscosity or inadequate lubricant delivery.

Example Mechanism/Cause Condition Monitoring Techniques

Proactive and Predictive Condition monitoring

It is best to use a plurality of condition monitoring techniques when practical to assess a fault condition.  The following diagram from Machinery Lubrication, March 2003 article, “How Machinery Wear Rates Impact Maintenance Priorities”, demonstrates how oil analysis and vibration analysis are complementary proactively finding root cause issues and predictively finding failure mechanisms in progress.

Elemental Spectroscopy

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.[iv]

Inspect and Special Test

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.

Total Ferrous

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.

Viscosity

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.[v] 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.

Conclusion

This article attempts to describe how equipment fails and what may be done to improve overall reliability. The article identifies common mechanisms of failure: abrasion, corrosion, fatigue, adhesion, erosion, cavitation, electrical discharge, and deposition.  Each mechanism has contributing factors, proactive measures, and affects various kinds of equipment. Several preferred non-intrusive monitoring techniques are identified to enable proactive and preventive efforts for improved reliability.  Periodic reliability assessments based on industry standard criteria may be used reduce risk, improve decision making, eliminate defects, and achieve continuous improvement.

References

[i] A Reliability Framework for Asset Management System™ and Uptime® Elements™ trademarks of Reliabilityweb.com and the graphic image in Figure 1 is copyright of Reliabilityweb.com.

[ii] “Why Equipment Fails and What You Can Do to Prevent It“, Ray Garvey, Machinery Lubrication Magazine, December 2019

[iii] “How Machinery Wear Rates Impact Maintenance Priorities”, Ray Garvey, Machinery Lubrication, March 2003

[iv] “Converting Tribology Based Condition Monitoring into Measurable Maintenance Results”, by Ray Garvey and Grahame Fogel, Computational Systems Inc., 1998

[v] “Composite Hull for Full-Ocean Depth”, R. E. Garvey, Marine Technology Journal, Volume 24, Number 2, June 1990

Ray Garvey

Ray Garvey is an engineer and inventor named on more than  20 U.S. patents associated with oil analyzers, infrared thermography, machine monitoring, and composite structures. Ray has been an employee of Emerson Process Management for 22 years, and is known for his participation in the development of Emerson’s CSI 5100 and CSI 5200 Minilab analyzers. Ray received his BS degree from West Point and MS degree from the University of Tennessee. His professional certifications  include Professional Engineer (PE), Certified Lubrication Specialist (CLS), and U.S. Army Engineer ( Lt. Col. retired).  Ray worked for the U.S. Army Corps of Engineers (five years active, 20 years reserve), U.S. DOE Uranium Gas Centrifuge Program (11 years), and is currently employed with Emerson Process Management.

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