While equipment wear is inevitable, premature wear is not. It can be prevented or significantly slowed by proper maintenance. For example, contamination—a leading cause of wear—can be caught by oil analysis and mitigated before it does any damage.  

Wear is a particular issue for machinery that lacks filtration, operates at higher and/or varying speeds and loads, is subject to shock loads and/or misalignment, and/or is subject to contamination. In industrial plants, gearboxes fit these descriptions the best. (In the mobile industry they are commonly called final drives, differentials or transmissions.) 

Gearboxes and heavily loaded anti-friction bearings typically show high levels of wear metals—particularly when they move at slow speeds and utilize a gear lube with an extreme pressure additive.

What Causes Machinery Wear?

There are several mechanisms and circumstances that generate wear. Some of the most common are described below:

Adhesion. Metal surfaces are never perfectly smooth. Contact between asperities (rough surface edges) causes friction. Although these asperities would be completely separated under full-fluid film lubrication, during boundary and mixed lubrication they tend to come into contact with each other, causing wear called adhesion. This contact leads to high temperatures producing a welding or bonding effect. In fact, the bond can be stronger than the metal itself. The result is particles that are sheared from the friction surface. 

Adhesion can take place under normal circumstances, such as break-in or normal rubbing. In cases of overloading, or in the absence of the appropriate additives, adhesion can be very destructive and lead to premature failure and wear. The typical causes for adhesion include high loads, speeds or temperatures; insufficient lubrication; lack of antiwear additives; and break-in wear. Susceptible components for adhesion include piston rings and cylinders, rolling and sliding bearings, gears and cutting tools.

Abrasion. Abrasion, which is the most common industrial wear mechanism, can occur in all moving surfaces. Abrasion basically is the cutting and deformation of materials in a machine. Two types of abrasion can occur:

• Three-bodied: When oil becomes contaminated with abrasive particles, such as dirt, these particles become lodged into the softer of two opposing wear surfaces. The particles can then cut into the metal surface via a lathing effect, generating excessive wear and material loss. 
• Two-bodied: When components aren’t properly aligned, the harder of two opposing wear surfaces can penetrate the softer metal surface. This results in cutting away of the component and excessive, rapid wear. 

Corrosion. Excessive water contamination can cause corrosion of metal parts.  As a result, an oxide layer forms on the metal surfaces. Surface motion then rubs the oxide layer, introducing oxide particles into the oil. When these oxides are harder than the component materials and if loose particles are formed, corrosive wear occurs. Components susceptible to corrosion include all bearings, cylinder walls and the valve train.

Addressing these factors through proper maintenance, lubrication, alignment and material selection can help to mitigate machinery wear and prolong its operational lifespan.

How Oil Analysis Mitigates Wear

Wear issues are often foreshadowed by lubricant-related issues, for example, varnish precursors. By the time the lab starts seeing actual wear metals in the sample, damage to the machine has already taken place.  Oil analysis is largely about detecting deviations and/or faults early. It does not reduce the number of failures, but rather reduces the severity. It stands to reason that leveraging oil analysis data to direct and guide maintenance actions can appreciably extend the life of the machine. In addition to detection, oil analysis can assess the cause and severity of wear and suggest affected parts. 

The primary tests that are specific to wear include: 

• Elemental Spectroscopy 
• Analytical Ferrography
• Ferrous Wear Concentration (a.k.a., to a lesser degree, Particle Quantifier Index (PQI), a semi-quantitative method used by some labs)
• Patch Test 
• Energy Dispersive X-Ray Fluorescence (EDXRF). 

While virtually all routine oil analysis packages will include Elemental Spectroscopy, Ferrous Wear Concentration is recommended for higher-wear applications like gearboxes. These tests are useful for trending normal wear and identifying the onset of abnormal wear. Once abnormal wear occurs, Analytical Ferrography is the logical next test.

How Additives Protect Against Wear

Other than proper maintenance, oil additives offer the best insurance against wear. Common wear-reducing additives include: 

• Anti-wear (AW)
• Extreme pressure (EP) 
• Rust inhibitors
• Demulsifiers
• Detergents
• Corrosion inhibitors
• Foam inhibitors
• Oxidation inhibitors

Antiwear. Antiwear additives provide protection against friction and wear in modest boundary film conditions. They form a chemical film, or protective coating, on the metal surface that allows moving parts to move against each other with little resistance and metal loss. Three commonly used AW additives are:

• Tricresylphosphate (TCP)
• Zinc dithiophosphate (ZDP)
• Zinc dialkyldithiophosphate (ZDDP)

Extreme Pressure. EP additives diminish friction, control wear and prevent significant surface damage under heavy loads and at high temperatures. Under heavy loads, pitting and scoring of metal surfaces is a big issue. Often welding of contacting surfaces occurs at the very high local temperatures that develop when opposing bodies are rubbed together under a high enough load. However, the excessive temperature itself starts a chemical reaction between the metal surface and the extreme pressure additive to fight welding. EP additives include:  

• Zinc dialkyldithiophosphate (ZDDP)
• Sulphurized fats
• Molybdenum disulfide 
• Esters
• Chlorinated paraffins

Oxidation Inhibitors. Oxidation inhibitors slow down the rate of oxidation and prevent premature thickening of the lubricant. Oxidation happens when oil is heated and is in contact with air. This results in the formation of acid, sludge, varnish and a generally denser oil. Oxidation inhibitors can either react with peroxides to create inactive compounds or they can cause these materials to decompose into less reactive compounds. Oxidation inhibitors include:

• Aromatic amines
• Zinc dithiophosphate (ZDP)
• Hindered phenols 
• Aromatic sulfides
• Alkyl sulfides

Rust Inhibitors. Rust is surface damage that results from the attack of water and oxygen on iron and its alloys. Because they have high polar attraction to the metal surface, rust inhibitors prevent water from reaching that surface. Through chemical interaction, they form a protective layer on the metal and prevent rusting. Frequently used rust inhibitors include:

• Esters
• Alkaline compounds
• Amino-acid derivatives
• Organic acids

Detergents. Detergent additives, sometimes referred to as dispersants, attack dirt and solid contaminants to break them up and prevent sludge and varnishing. These additives then attach themselves to the contaminants to hold them in suspension in the oil, so that they can be filtered out. Phosphonates, sulphonates, and phenolates of alkaline/alkaline-earth elements are used as detergents in lubricants. These earth elements include calcium, magnesium, sodium, and barium.

Corrosion Inhibitors. Corrosion inhibiting additives have an alkaline property to neutralize acids formed through oxidation or from combustion in engine applications. Calcium-based azdditives are commonly used corrosion inhibitors. The list of common corrosion inhibitors (the same as rust inhibitors) include:

• Esters
• Alkaline compounds
• Amino-acid derivatives
• Organic acids

Foam Inhibitors. Foam inhibiting additives are formulated to dissipate foam more quickly. They encourage small bubbles to congeal into larger bubbles that rupture more readily. Dimethyl silicone (dimethylsiloxane) is commonly used as an anti-foaming agent in lubricants. 

Demulsifiers. Demulsibility is a measure of an oil’s ability to separate from water. Highly refined straight mineral oils are usually highly demulsible. This trait is key to the maintenance of circulating oil systems. Even after vigorous shaking of the oil-water mixture, a demulsified oil will separate and rise rapidly to the surface of the water. Most demulsifiers are proprietary blends using trade names only.

The bottom line is that additives play a crucial role in improving the performance and longevity of lubricants, reducing wear as a result.

About Particles

Particles can be very destructive and are a significant contributor to wear. They can also be accurate indicators of problems in a machine. These particles can be either large (such as a piece of metal) or small (such as dirt or dust). Oil additives help to break down and eliminate them. 

Following are some sources of particles in a lubricating system:
• Wear: As wear occurs, potentially abrasive particles are generated into the oil.
• Corrosion: Corrosion generates oxides that travel in the oil.
• External contamination: Dust, dirt and other airborne contaminants can enter a system through breathers, open ports and hatches.
• Scale and rust in reservoirs: Large reservoirs and piping can rust and corrode, generating debris.
• Lubricant degradation: As the base stock of the oil degrades, solid byproducts are produced.

Just as particles are generated during routine operation, they can also diminish. While filtration is the primary reason for the reduction of particles, it can also happen naturally due to the following: 
• Settling: Particles will settle out of the oil in the reservoir or sump.
• Grinding: Particles will be ground up as they pass through friction points. 
• Oxidation (chemical breakdown): Some particles will oxidize and break down naturally over time.
• Dissolution: Some particles will dissolve in the oil.

Higher amounts of particles are prevalent when a new machine is broken in. In midlife, particle levels should even out and decrease. Increases after that may indicate machine health issues. 

Maximizing Machine Life Through Wear Mitigation

With the primary goal of avoiding unnecessary wear in the first place, it makes sense to monitor the lubricant as a first line of defense. Machine wear issues that oil analysis can pick up early include:   

• Water/corrosion-accelerated lubricant degradation
• Lack of lubrication at friction surfaces
• Hard contaminants that can cause filter plugging, valve sticking and scoring of friction surfaces
• Changes in viscosity, which can result in inadequate lubrication and improper machine operation
• Varnish, which can clog oil passages and result in inadequate lubrication and sticking valves. 

Maximizing machine life by reducing wear involves several strategies such as using the best lubricant with wear-mitigating additives and regular oil analysis to catch issues early. Regardless of diligence, some wear will occur.  The goal is to prevent as much wear as possible and extend optimal functioning at least until the end of the machinery’s expected useful life—and hopefully beyond.  

Mary Messuti is the president of Eurofins TestOil, Inc. located in Strongsville, Ohio.  Her lab offers a full line of lubrication testing as well as fuel, coolant, grease and associated tribology services.  Mary enjoys over 25 years of experience in both laboratory management as well as heavy industrial and aerospace manufacturing environments.