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Why Do Machines Go Bump In The Night?

This article presents common failure mechanisms and associated root causes of machine component damage. It outlines how to use oil and vibration analyses to detect those mechanisms and their causes. It also makes reference to ASTM D7684-10, which is the relatively new Standard Guide for Microscopic Characterization of Particles from In-Service Lubricants. This standard establishes the terms for describing what is going on inside a machine while damage is progressing from incipient to catastrophic. The new standard guide and the following table will help connect the dots that lead to those bumps in the night.

Armed with that cause and effect understanding, there is good reason to believe you can detect and correct the important root causes that lead to many of the mechanical failures experienced in a plant.

Causes: Force, Reactive Agents, Environment, Temperature And Time

Reliability-focused engineer, Heinz Bloch, points out that root causes of failure typically involve one or more of these root cause elements: force, reactive agents, environment, temperature and time (FRETT)1. Table 1 applies FRETT to eight different failure mechanisms that are well-known for producing industrial machinery component damage.

Mechanisms: Abrasion, Corrosion, Fatigue And Boundary Lubrication (Adhesion)

These are the most frequently experienced machinery component failure mechanisms, and are responsible for damaging healthy machines and taking them out of service.

Abrasion is commonly caused by dust or other hard particles that get trapped in the nip of moving machine parts. The particles embed into a softer metal and then cut a groove into the harder one. Sandpaper is abrasive! To minimize abrasion, keep the air and oil clean with low particulate contamination.

Corrosion is typically caused by water or another corrosive substance that contacts metal surfaces. Moisture is the most common cause of corrosion. An important step to minimize corrosive wear is to keep the moisture content in lubricants well below saturation limits.

Fatigue wear is a result of subsurface cracks produced by cyclic loading, such as a highly loaded rolling element or at the pitch line of a gear tooth. Vibration analysis and correction of vibration problems, such as misalignment, imbalance and resonance, will help reduce fatigue wear and greatly extend machine life.

Boundary lubrication or adhesion is a lubrication regime where loads are transmitted between machine components by metal-to-metal sliding contact. Load bearing surfaces are generally designed to carry loads on a fluid film based on hydrodynamic or hydrostatic lubrication, such as in journal or thrust pad bearings. Or they may be designed to carry loads on very thin elastohydrodynamic (EHD) lubricant film in roller element bearings and pitch lines of gear teeth. Four reasons why lubricant film gets squeezed out and causes metal parts to drag over one another are: 1) no oil, 2) low oil, 3) excessive loading, and 4) slow speed. These are all related to inadequate lubrication. When you find evidence of sliding contact wear debris in oil, look for a machine component that is not adequately lubricated.

Mechanisms: Deposition, Erosion, Cavitation And Electrical Discharge

Besides the four most commonly experienced machinery component failure mechanisms, four other mechanisms sometimes contribute to component failures in industrial machinery. They are deposition, erosion, cavitation and electrical discharge.

Deposition of materials onto surfaces can lead to improper machine component operation. Two common examples of problematic deposition on machine surfaces are varnish formation and particulate debris accumulation. These lead to plugging, imbalance, improper fit and the like.

Erosion is a sandblasting type of damage where high velocity particles impinge on surfaces and erode the material away.

Cavitation is a result of void collapse on the backside of impellers.

Electrical discharge is an effect of current passing through motor shafts and bearings, causing fluting damage to bearing surfaces.

Condition Monitoring: Oil & Vibration Analyses Are Proactive & Predictive

The Venn diagram in Figure 1 represents complementary synergism between oil analysis and vibration analysis for industrial machinery condition monitoring.

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Figure 1: Oil and vibration analyses are combined for effective machine condition monitoring.

Oil and vibration analyses separately find the listed proactive issues where circles are not overlapping. For example, oil analysis monitoring for contamination control and moisture in oil are directly related to the abrasion and corrosion mechanisms, while vibration monitoring for imbalance and structural resonance are directly related to the fatigue mechanism.

Oil and vibration analyses supportively find the listed predictive issues where circles are overlapping. For example, experienced vibration analysts recognize the four-stage wear progression and PeakVue™ indications as bearing defects advance from incipient to catastrophic. Independently, oil analysts observe trends for ferrous density (such as Model 5200 Ferrous Index) supported by detailed microscopic wear particle analysis to discern the root cause and severity of a developing component damage problem.2

The strengths for vibration and oil analyses techniques are highly complementary and well suited for independently finding root causes and non-intrusively predicting component failures.3

Definitions from ASTM D7684-1044


abrasion, n-wear by displacement of material caused by hard particles or hard protuberances.

abrasivewear particles, n-long, wire-like particles in the form of loops or spirals that are generated due to hard, abrasive particles present between wearing surfaces of unequal hardness; sometimes called cutting wear particles or ribbons.

corrosivewear debris, n-usually extremely fine partially oxidized particles caused by corrosive attack. Particles can become quite large in cases of extreme corrosion.

debris, n-in tribology, solid or semi-solid particulate matter introduced to lubricant through contamination or detached from a surface due to a wear, corrosion, or erosion process.

fatigue wear, n-wear of a solid surface caused by a fracture arising from material fatigue.

rolling contact fatigue, n-damage process in a triboelement subjected to repeated rolling contact loads involving the initiation and propagation of fatigue cracks in or under the contact surface, eventually culminating in surface pits or spalls.

rolling contact fatigue wear, n-in tribology, fatigue wear caused by loaded rolling contact typically between roller and race in bearings or between gear teeth in the vicinity of the pitch line that typically forms spall-type pitting and releases rolling contact fatigue particles; also called rolling fatigue wear or subsurface spalling.

rubbing wear particles, n-particles generated as a result of sliding wear in a machine, sometimes called mild adhesive wear. Rubbing wear particles are free metal platelets with smooth surfaces from approximately 0.5 to 15 μm in major dimension and with major dimension-to-thickness ratios from about 10:1 for larger particles to about 3:1 for smaller particles. Any free metal particle <5 μm is classified as a rubbing wear particle regardless of shape factor, unless it is a sphere.

spheres, n-in tribology, metal spheres may be the result of incipient rolling contact fatigue, or they may be contaminant particles from welding, grinding, coal burning, or steel manufacturing. Spheres also may be caused by electro-pitting.

three-body abrasive wear, n-form of abrasive wear in which wear is produced by loose particles introduced or generated between the contacting surfaces.

References:

  1. Bloch, H. P. Successful Failure Analysis Strategies. Reliability Advantage Training Bulletin, Volume 3, April 2005. (http://www.heinzbloch.com/docs/ReliabilityAdvantage/Reliability_Advantage_Volume_3.pdf)
  2. Garvey, R.E. Cost Justification for Industrial Oil Analysis. http://www.docstoc.com/docs/44514639/Cost-Justification-for-Industrial-Oil-Analysis.
  3. Berry, James E. Good Vibes About Oil Analysis. Practicing Oil Analysis Magazine, November 1999. (http://www.machinerylubrication.com/Read/36/oil-analysis-vibes)
  4. ASTM International. ASTM D7684-11 Standard Guide for Microscopic Characterization of Particles from In-Service Lubricants. http://www.astm.org/Standards/D7684.htm.
  5. Garvey, R. E., and Henning, P. , “Identifying Root Causes of Machinery Damage with Condition Monitoring”, Machinery Lubrication Magazine, Dec. 2012.
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