Manufacturing,Industrial Engineering & Maintenance Engineering : Wear and Debris Analysis

• No matter how wear-resistant the surfaces are, with time, particles from the surface of the mating machine members will be dislodged by a wear phenomenon and will be deposited in the lubricating oil.
• The concentration of such wear particles, their size, shape, and composition provide ideas to the maintenance engineer regarding the health of the machine. Further, due to the deposition of wear particles in the lubricant (which may be oil or grease), the physical and chemical properties of the lubricant change.
• Thus, by measuring these properties of the lubricating oil, an indication of the machine’s health can be obtained.

Types of lubricant analysis

The lubricant analysis can be grouped into following three categories.

(i). Fluid properties analysis:

Information on a lubricant's physical and chemical properties such as viscosity, total acid number, demulsibility, additives are considered during this analysis.

(ii). Fluid contamination analysis:

The presence of contaminations such as dirt, glycol, soot, fuel, water etc., in the oil are analyzed to monitor condition.

(iii). Wear debris analysis:

In this analysis, the concentration and characterization of wear metals and other contaminants are suspended in used oil mainly from the machine components through which the oil interfaces and generates some wear metals and wear particles are analyzed.

TESTS ON LUBRICATING OIL PROPERTIES

The wear debris from mating machine components is deposited in the lubricating oil. The physical and chemical properties of the lubricating oil also change due to the presence of debris and the temperature cycles they undergo due to machine operation, including intermediate shutdowns. The oil that is collected from the machine as per the sampling procedure described in the earlier section, is tested for its physical and chemical properties.

Viscosity:

• Due to internal friction, every fluid has a resistance to flow called viscosity.
• A lubricant's viscosity is measured at either 40°C or 100°C Viscosity Index (VI) improvers can be added as a base stock to help the lubricant maintain certain characteristics at given temperature.

Oxidation:

• Oxidation is caused by a lubricant's natural tendency to bond with oxygen. Oxidation is a chemical change that prevents the, oil from performing its job.
• It naturally results when lubricant is repeatedly heated up and cooled down.

Nitration/sulfation:

• Sulfation/Nitration occurs as a portion of the engine exhaust gets ingested back into the crankcase and the lubricant bonds with gases, forming sulfates (sulfur compounds) and nitrates (nitrogen compounds) in the oil.
• The compounds attack metal surfaces and cause metal corrosion.

Fuel soot:

• Soot or unburned carbon is a physical contaminant. Soot in the oil is a natural occurrence for diesel engines. However, too much soot causes the lubricant to become too viscous and thus it does not lubricate well. Also, soot can build up or group together and it causes significant deposits. Additives such as calcium are added to lubricants to prevent it.

Fuel dilution:

• Fuel dilution is an index of how much fuel is in the lubricant and it indicates the condition of the position rings and fuel injection system.
• Fuel dilution is considered when it reaches a level of 2.5 to 5%.

Glycol:

• Glycol is an index of the amount of antifreeze/coolant in the lubricant. An increase in this number indicates a leak between lubricant and coolant systems such as a blown head gasket or a cracked head or block.

Total Base Number (TEN):

• Total Base Number (TBN) is a measure of the reserve alkalinity in engine oils. Because acids form in the oil (sulfates and nitrates), lubricants are manufactured with a high alkaline count to counteract the acids.
• TBN should never be a problem if the proper lubricant change interval is followed. If the change intervals are lengthened too much or skipped, then acids that from will consume the alkaline reserve of the lubricant and advanced corrosion of the engine parts will ensure.

Total Acid Number (TAN):

• Total Acid Number (TAN) is a measure of the acid build-up in the oil. It is opposite to TBN. Oil sample results rarely provide both numbers.

Oil contamination:

• Oil contamination by water can cause major problems in a lubrication system.
• Water can change a lubricant's properties through accelerated oxidation, reaction with additives and emulsification. Water will also accelerate corrosion reactions.

Solids content:

• It is a general test. All solid materials in the oil are measured as a percentage of the sample volume or weight.
• The presence of solids in a lubricating system can significantly increase the wear on lubricated parts.

WEAR DEBRIS ANALYSIS

   Mechanisms of Wear:

When machine components rub against each other, there may be relative sliding or rolling motion between the parts. These forces and the surface condition of these parts are responsible for creating wear particles. Wear that occurs between mating machine components is classified into four categories:

1. Adhesion wear
2. Abrasive wear

iii. Corrosive wear

1. Fatigue wear

 Adhesive Wear:

• This type of wear is thus also known as scouring, gaffing, or galling wear.
• This type of wear occurs when two machine surfaces rub against each other, with or without any lubricant between them.
• Adhesive wear occurs because no matter how smooth the surfaces are, there will be asperities on the surface at the submicron level. This is usually quantified by the surface roughness value. Under compressive loads at the two mating surfaces, these asperities lock into each other and may become welded due to high pressures and be sheared off during the sliding motion. Thus, material is scoured out at the surface, which may leave behind larger pits.

 Abrasive Wear:

• This type of wear occurs when two materials of different hardness rub against each other. The harder material under load ploughs or cuts into the softer material and pulls out wear particles.
• Abrasion also occurs if a third hard material (dirt or impurity) is trapped in between two rubbing surfaces. This type of wear is also known as cutting wear.

Example of abrasive wear is a journal bearing with a Babbitt lining, which is softer than the hard steel shaft that is supported by it.

The hard impurity between the two surfaces could be due to hard dirt particles suspended in the air, for example, the coal dust in a material-handling system at a thermal power plant.

 Corrosion Wear:

• Corrosive wear occurs due to the presence of hard particles, usually oxides or hydrates, between two surfaces that slide against each other. These particles are responsible for the surface abrasion.

 Trapped foreign particle

• A layer of lubricant on the surface protects the material from exposure to air or moisture so that oxides or hydrates are not formed. However, if the proper amount and type of lubricant is not used, there may be oxide formation, which can be responsible for corrosion wear. Further, when two dissimilar metals rub against each other, the electrochemical reaction may also lead to the formation of salts that can rub against the surfaces during the sliding motion and cause surface abrasion.

 Fatigue Wear:

• When two surfaces rotate against each other, the loads on them also change, creating a fatigue load at the contact points of the two mating surfaces. The stress at the contact points can be estimated by Hertzian stress theory.
• Due to fatigue load, the stress that can be withstood by the material greatly reduces, and the shear stress at small depths below the surface is less. Thus, during rotation, particles suddenly break out from under the surface and create pits.
• Fatigue wear may be sudden and catastrophic, unlike the other three wear types which are gradual, and their wear rate is proportional with time.

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