Utilizing Oil Analysis for Machine Condition Monitoring

Oil analysis can go far beyond simply telling you the condition of the lubricant itself. Advanced oil analysis laboratory techniques are being used to monitor the condition of the equipment. By utilizing these advanced techniques, equipment reliability increases and unexpected failures and down time can be minimized.

There are many types of abnormal wear that can exist inside a piece of machinery.  Although there are many different types of wear, there are only a few primary sources of the wear.  Problems related to the oil itself may contribute to wear, in cases where the lubricant has degraded or become contaminated. The machine condition can also contribute to the generation of wear, if a component is misaligned or improperly balanced.  Improper use of the equipment such as overload or accelerated heating conditions can also generate wear.

Below are some examples of the different types of wear that can occur.

Ø      Abrasive Wear is the results of hard particles coming in contact with internal components.  Such particles include dirt and a variety of wear metals.  Introducing a filtration process can reduce abrasive wear.  It is also important to ensure vents, breathers, and seals are working properly.

Ø      Adhesive Wear is when two metal surfaces come in contact allowing particles to break away from the components. Insufficient lubrication or lubricant contamination normally causes this.  Ensuring the proper viscosity grade lubricant is used can reduce adhesive wear.  Reducing contamination in the oil will also help eliminate adhesive wear.

Ø      Cavitation occurs when entrained air or gas bubbles collapse.  When the collapse occurs against the surface of internal components, cracks and pits can be formed.  Controlling foaming characteristics of oil with an anti-foam additive can help reduce cavitation.

Ø      Corrosive Wear is caused by a chemical reaction that actually removes material from a component surface.  Corrosion can be a direct result of acidic oxidation.  A random electrical current can also cause corrosion.  Electrical current corrosion results in welding and pitting of the wear surface. The presence of water or combustion products can promote corrosive wear.

Ø      Cutting Wear can be caused when an abrasive particle has imbedded itself in a soft surface.  Equipment imbalance or misalignment can contribute to cutting wear. Proper filtration and equipment maintenance is imperative to reducing cutting wear.

Ø      Fatigue Wear results when cracks develop in the component surface allowing the generation and removal of particles.  Leading causes of fatigue wear include insufficient lubrication, lubricant contamination, and component fatigue. 

Ø      Sliding Wear is caused by equipment stress. Subjecting equipment to excessive speeds or loads can result in sliding wear. The excess heat in an overload situation weakens the lubricant and can result in metal-to-metal contact.  When a moving part comes in contact with a stationary part sliding wear becomes an issue.

By providing proper lubrication, filtration, and equipment maintenance, much of the wear that occurs inside of the equipment can be reduced.  By implementing predictive maintenance practices such as vibration, infrared, thermography, and oil analysis, you can identify potential problems. By monitoring the equipment’s condition with oil analysis, you can identify different types of wear and take proper corrective action before a failure occurs. In fact, there are many cases where oil analysis will identify problems with rotating equipment prior to even vibration analysis detecting it.

When implementing an oil analysis condition-monitoring program, it is important to select proper tests that will identify abnormal wear particles in the oil.  When components inside the equipment wear, debris is generated.  By identifying the wear debris present, you can establish the source of the problem.

Below are some examples of different laboratory tests that can help identify wear.  

Spectrometric Analysis is the technology that is most commonly used for trending the concentrations of wear metals.  The main focus of this technology is to trend the accumulation of small wear metals, elemental constituents of additives, and identification of possible introduction of contaminants.  The results are typically reported in parts per million (PPM).  It is important to remember that this technology only monitors the smaller particles present in the oil.  Any large wear metal particles present will not be detected or reported.

Particle Counting will track all ranges of particles that are found within the sample.  However, particle counting will not differentiate the composition of materials present.  The main focus of this technology is to identify the number of particles present in the sample.  The results are typically reported in certain size ranges per milliliter or per 100 milliliters of sample. 

Direct Reading Ferrography monitors and trends the relative concentration of ferrous wear particles and determines a ratio of large to small ferrous particles to provide insight into the wear rate of the lubricated component. This method can be used as a tracking and trending tool, especially in systems that generate a high rate of particles.

Analytical Ferrography is a technology that utilizes microscopic analysis to identify the composition of the material present.  This technology will differentiate the type of material contained within the sample and determine the wearing component from which it was generated.  This test method is used to determine characteristics of a machine by evaluating the particle type, size, concentration, distribution, and morphology.  This will assist in determining the source and resolution of the problem.

It is important to remember that each laboratory test has limitations.  It is essential that you select a well-balanced test package that will correctly identify potential problems within your equipment. Many of the laboratory tests actually compliment each other.

The purpose of an oil analysis program should not be to merely check the lubricant's condition. The real maintenance dollars saved by utilizing oil analysis are going to be when equipment problems are detected.  

Below are some examples of wear metals and their component origins.

Break-In Wear, Normal Wear, and Abnormal Wear are the three phases of wear that exist in equipment.  Break-In Wear occurs during the start-up stages of a new component.  This phase typically generates significant wear metal debris that will be removed during the first couple of oil changes.  Normal Wear occurs after the Break-In Wear stage.  During this stage the component becomes more stabilized.  Wear metals will increase with equipment usage and decrease when makeup oil is added or oil changes occur.  Abnormal Wear occurs as a result of some form of lubricant, machinery, or maintenance problem.  During this stage the wear metals increase significantly. 

By utilizing oil analysis on a routine basis, a base line for each piece of equipment can be established.  As the oil analysis data deviates from the established base line, abnormal wear modes can be identified.  Once abnormal wear modes are identified corrective action can be planned.

Implementation of an oil analysis program with analyses consistent with the goals of the program will significantly reduce maintenance costs and improve plant reliability and safety.  Lubricant analysis for the purpose of machinery conditioning monitoring is at its best with a significant amount of historical data.  It is important to establish a base line for each piece of equipment.  Certain analytical results may change with lube oxidation and degradation due to normal use, the major changes occur due to contamination from environmental factors and machinery wear debris.  The analytical costs of a properly implemented program should be covered by the extension of the lubricant change interval.  Increased reliability, availability, and the prevention of unanticipated failures and downtime are added benefits.

As seen in:
Maintenance Journal
May/June 2000

Lana Robin, PdMA Corporation