Lubrication Degradation Mechanisms

A Complete Guide

Sanya Mathura

• Publisher: CRC Press
• Year: 2020
• Print ISBN: 9780367607760
• eBook ISBN: 9781000288551
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Industrial Design
• Index: Technology & Engineering

In industry, owners, engineers and workers have struggled with lubricant degradation and its effects on their equipment. The purpose of Lubrication Degradation Mechanisms: A Complete Guide is to help personnel to understand the reasons behind the degradation of their lubricant, determine methods to identify the onset of degradation and reduce or eliminate lubricant degradation within their equipment.

One of the most common forms of lubricant degradation is oxidation. However, this is not the only method by which a lubricant degrades. By understanding the differences between degradation patterns, personnel can employ specific tasks / tests to aid in their identification of the type of degradation and the factors responsible.

The aim of this book is to educate facility personnel on the methods of degradation and ways in which it can be reduced or eliminated while keeping an eye on the cost of operation.

1

What Is a Lubricant and What Are Its Functions?

Ideally, we must firstly define a lubricant and understand its functions before delving into its degradation mechanisms. It is of paramount importance that we understand the basics and get these right before we begin to look into the more complex side. This chapter will provide information of the basics. These are the fundamentals upon which the entire lubrication industry is based.

1.1 What Is a Lubricant?

A liquid lubricant is defined by Menezes, Reeves and Lovell (2013, 295) as a combination of base oil and additives. It is important to note that there are different types of base oils and additives, and each type has particular properties. As such, the combination of base oil and additives can exist in varying ratios depending on the application of the final lubricant and the lubricant manufacturer.

The classification of base oils is based on the refining method used on the crude oil. According to the Noria (2012b), there are five classifications of base oils as listed below:

• Group I: Solvent refined (Mineral)
• Group II: Hydrotreated (Mineral)
• Group III: Hydrocracked (Mineral)
• Group IV: PAO Synthetic Lubricants
• Group V: All other base oils not included in Groups I, II, III or IV.

Each base oil group has differing levels of solvency, sulphur, saturates and viscosity index which help to differentiate them. Globally, Group I base oils have been the most common since around 1970 because they were the easiest to refine and more readily available. However, as technology evolved, there has been a trend towards the Group II and higher groups as the value that their chemistry adds to the system has been realized. While these higher groups are costlier than the traditional Group I’s, their chemical structures offer more stability and better properties which make them very desirable for use in finished lubricants.

Similarly, each of the additives has its specific function but needs to be formulated to particular ratios when they are being designed for applications. According to Noria (2012a), additives can serve any of the three main purposes as follows:

1. Impart new properties (extreme pressure additives, detergents, metal deactivators and tackiness agents)
2. Suppress undesirable properties (pour point depressants and viscosity index improvers)
3. Enhance properties (antioxidants, corrosion inhibitors, anti-foam agents and demulsifying agents).

Each of the aforementioned purposes acts in tandem with the properties of the selected base oil group to provide the overall characteristics of a finished lubricant. As such, it is critical to fully understand both the properties of the base oil group and additives such that the final characteristics of the finished lubricant can be understood and applied accordingly.

We will cover more details on the functions of the most common additives in Chapter 3 of this book.

1.2 What Are the Functions of a Lubricant?

Essentially, a lubricant is any material that allows for one surface to slide over the other. For instance, if there was a banana peel on the floor and someone stepped on this, they would slide. In this case, the banana peel acts as a lubricant and allows the human to slide over the floor.

If we speak specifically towards equipment and components, banana peels will not work here. Within the industry, a lubricant must perform five main functions as per Menezes, Reeves and Lovell (2013, 295): reduction of friction, minimizing wear, distribution of heat, removal of contaminants and improvement of efficiency.

Let’s think about sharpening the blade of an axe (or any other sharp object). Ideally, we press the blade continuously over the grinding stone in a sliding manner. After some time, the blade starts to get hot, and any uneven surfaces that were present on the blade or the stone begin to get smooth. All of this is done without the presence of a lubricant.

Since the blade is sliding against the stone continuously, it produces friction, which in turn produces heat. At the same time, we have wear occurring between the two surfaces, and any of the filings that came off during this process would remain on the blade or the stone.

If a lubricant was present (such as a grease or an oil), there would be a film between the two surfaces. Now, this film would protect both the blade and the stone, reduce the friction (and by extension the heat being produced), minimize the wear and keep all the filings in suspension, thereby keeping both the axe and the stone clean.

Now, let’s take this same example and apply it to components in the facility. Can you imagine what a gearbox with the gears turning against each other (at much higher speeds!) would have to undergo? Therefore, the purpose of a lubricant is to essentially protect the equipment and aid in improving its efficiency during operation. If a lubricant no longer performs any of its five functions, then it has fundamentally failed. We will be exploring these concepts in the upcoming chapters and methods of identification and dealing with these failures.

1.3 Understanding Lubrication Regimes

According to Noria (2017), there are four different types of lubrication regimes which can be experienced by surfaces: boundary lubrication, mixed lubrication, hydrodynamic lubrication and elastohydrodynamic lubrication.

Essentially, the best type of lubrication regime is elastohydrodynamic as it provides the most ideal environment for both the lubricant and the surface. However, there are instances where the other types of lubrication can exist due to a lack of lubricant or contaminants. Let’s explore each type and typical situations in which we can find them.

In boundary lubrication, as the name suggests, the surfaces actually touch each other and the oil film does not act as a wedge between them. When we look at surfaces under a microscope, we can see tiny asperities (which we can liken to sharp or jagged edges) which are prevalent along the surface. Even though a surface may appear shiny and smooth, when we microscopically examine it the actual surface has a lot of asperities. Imagine if two rough surfaces are sliding against each other, like a piece of sand paper against a wall, eventually parts of the wall will be removed due to the asperities of both the sand paper and the wall. Let’s translate that on a microscopic level to the surfaces that are to be lubricated.

With an oil film present, this would act as a barrier between the two surfaces and allow them to slide over each other almost seamlessly without damage. However, in boundary lubrication, the oil film is not thick enough and the surfaces of the components come into contact with each other. This typically happens during the start-up or shutdown of equipment as the lubricant has not had the opportunity to be fully present to perform its function and can also occur when there are heavy loads at low speeds. According to Noria (2017), as much as 70% of wear occurs during start-up and shutdown of equipment!

Mixed lubrication occurs just after the start-up when the lubricant is now forming a larger film to protect the surfaces. However, it is not fully formed and there are some contact areas where the surfaces will still experience boundary lubrication as well as elastohydrodynamic or hydrodynamic lubrication. During this period, wear occurs due to the areas that are still experiencing boundary lubrication. However, it can be considered a transition phase as the surfaces move from one type of regime into another as the lubricant film gradually increases.

In hydrodynamic lubrication, the lubricant provides a film such that both surfaces are adequately separated and their asperities do not come into contact with each other. This is one of the most ideal forms of lubrication as it greatly reduces the wear between the two surfaces and the oil film safeguards that these can easily slide over each other, thus decreasing the friction between them. In this type of lubrication, the oil wedge is maintained in all operating conditions and guarantees that the asperities of both surfaces do not interact with each other.

Conversely, in elastohydrodynamic lubrication, the lubricant deforms the contact surface at which it has the highest contact pressure to ensure that the asperities do not touch while maintaining the oil wedge. This type of lubrication usually occurs when there is a rolling motion between two moving surfaces and the contact zone has a low degree of conformity (Noria 2017). Essentially, elastohydrodynamic lubrication occurs when the lubricant allows the contact surface to become elastically deformed while maintaining a healthy lubricant film between the two contact surfaces.

It is important to understand that different lubrication regimes exist. Depending on the regime that is prevalent, we can now recognize whether the lubricant is effectively carrying out its functions or if this can lead to its non-performance and eventual failure.

By understanding the various lubrication regimes, we can now apply this to our knowledge of lubricant degradation mechanisms. For instance, if we know that a surface experiences boundary lubrication, we can now assume that the material may undergo abrasive or fatigue wear. Thus, we can deduce that the lubricant will undergo high-temperature valves and may even become contaminated with the abrasive material. As such, by understanding the lubrication regime, we have more information on the environment in which the lubricant existed, and this will help us in determining the type of lubricant degradation mechanism.

2

Understanding the Types of Degradation Mechanisms

As per definition, a lubricant will fail if it does not effectively perform any of its functions as outlined in the previous chapter. Therefore, we need to examine methods in which the lubricant can fail or degrade to such an extent that it can no longer perform any of its five functions, namely, reducing friction, minimizing wear, distributing heat, removing contaminants and improving efficiency. When a lubricant fails, this can spell disaster for the surfaces which it should be protecting.

A partial function of the lubricant is to be sacrificial. In its sacrificial nature, it can release antioxidants to protect the equipment from oxidation or corrosion. Additionally, its Total Base Number (TBN) reserve can be depleted to reduce the acidity of the oil. Furthermore, the lubricant can have suspended contaminants that may either act as catalysts to reduce or increase its viscosity, thus making the lubricant unsuitable for the application.

Degradation occurs throughout the service life of the lubricant and can even occur due to non-ideal storage and handling practices. However, limits exist whereby the lubricant can be deemed unfit for service. When these limits are reached and the functions of the lubricant can no longer be carried out, it is said to be degraded.

There have been arguments within the industry that there are only three methods of lubricant degradation (as per Barnes (2003, 1536)). However, upon closer examination by Livingstone, Wooton and Thompson (2007, 36), there are actually six methods of degradation with slight variations in the process of degradation. These variations can be critical in determining the next steps forward for any facility personnel as it would identify the areas in which they need to improve or change.

The six methods of lubricant degradation as defined by Livingstone, Wooton and Thompson (2007, 36) are oxidation, thermal breakdown/degradation, microdieseling, additive depletion, electrostatic spark discharge and contamination.

Each of these mechanisms undergoes different environmental conditions and produces varying by-products which set them apart. However, if we class them according to their environmental triggers, we can get four main types. Firstly, a trigger of the presence of oxygen and increase in temperature can lead to oxidation. On the other hand, if there is a rapid increase in temperature (over 200°C), this can result in either of the following mechanisms: thermal breakdown/degradation, microdieseling or electrostatic spark discharge. Furthermore, if we categorize the trigger as the sacrificial nature of additives, then this can result in additive depletion. Lastly, by classifying the environmental trigger as ingress of foreign material, this can result in contamination.

Let’s dive a bit deeper into the identification of each of these mechanisms and how they can be told apart.

2.1 Oxidation

Ther...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgements
• Author
• 1 What Is a Lubricant and What Are Its Functions?
• 2 Understanding the Types of Degradation Mechanisms
• 3 Identification of Lubricant Degradation
• 4 Tests to Determine the Types of Lubricant Degradation
• 5 Dealing with Degradation
• 6 Summary
• 7 Case Studies
• References
• Index