Principles of Lubrication: The Causes and Effects of Wear
Check out this brief and insightful description of the causes and effects of equipment wear, as well as ways you can avoid equipment damage by using oil analysis.
If you are looking for a concise, accurate, and insightful description of the basics of lubrication – you’re at the right place.
In this article, I want to describe the mechanics of wear, the different types of lubrication regimes, and the steps you need to take to ensure your fluid does its job.
Recent estimates state that equipment wear could cost the US economy as much as $300B per year! This should provide plenty of justification for every operator to pay close attention to their equipment through oil condition monitoring.
Any time two metal surfaces move against each other, there will be friction. Friction is primarily caused by microscopic irregularities present on the two surfaces, which are called asperities.
Without effective lubrication, these asperities will contact each other, causing mechanical wear as well as decreasing the efficiency of the rotating equipment by energy loss.
This metal-to-metal contact can be avoided by using proper lubrication.
There are four different lubrication regimes:
1. Boundary Lubrication (BL) – This regime is characterized by an insufficiently thick lubricating film between the two moving surfaces, causing metal-to-metal contact via the asperities. This metal-to-metal contact causes high friction, which leads to energy loss and wear. BL is most common during start-up, shut-down, low speeds, and high loads. Anti-wear additives often play a critical role during these scenarios, as they can coat the metal surfaces and prevent welding and tearing of asperities. However, even with anti-wear additives, this lubrication regime is to be avoided as much as possible. One technique to avoid metal-to-metal contact during start-up is via hydrostatic lubrication, where a bearing may be supported by oil pressure supplied by a hydraulic pump. After the bearing has come up to speed, the external pressure can be removed, and full film lubrication will continue due to rotational forces.
2. Mixed Film Lubrication (MFL) – This is an intermediate condition between boundary lubrication and full film lubricating conditions. In MFL, a large amount of the load still rests on the asperities, causing significant metal-to-metal contact. The goal of the lubrication engineer is to successfully transition from MFL to a full fluid film.
3. Hydrodynamic Lubrication (HDL) – This regime occurs when there is a full oil film keeping the asperities of the two metal surfaces from contacting each other. This type of lubrication is typically achieved after the rotating equipment has come up to its operating speed. During HDL, direct metal-to-metal contact is entirely avoided, and friction/wear is at a minimum.
4. Elastohydrodynamic Lubrication (EHD) – This is a unique kind of lubrication that typically occurs in rolling-element bearings. During operation, the raceway and rolling element will elastically deform from the force of the oil wedge between them, creating a very fine point of contact. This very fine point results in high pressure, which increases the viscosity and load carrying capability of the oil. EHD is typically considered a form of full fluid film, where metal-to-metal contact is avoided and, therefore, most friction/wear is eliminated.
Now you’re probably wondering how you can ensure that you are achieving full film lubrication, and avoiding boundary conditions, as much as possible.
Well, other than avoiding unnecessary instances start-up and shutdown (where boundary conditions are certain to occur), you need check on the health of your fluid.
Lubricating oils don’t just prevent wear - they also help to clean, cool, and protect the equipment from corrosion.
To do these jobs effectively, lubricating oils need to have:
1. The Right Viscosity – The measure of a fluid’s tendency to flow is referred to as viscosity. Viscosity is typically considered to be the most important of the key fluid properties typically measured with oil condition monitoring. If the viscosity is too low, the fluid will not be able to support the load, and metal-to-metal contact will occur via the asperities. If the viscosity is too high, energy loss will increase due to fluid drag, decreasing equipment efficiency. However, most experts agree that if the machine operator is forced to choose between a fluid with an excessively high viscosity, or an excessively low viscosity, they should always go with the higher viscosity. Typically, applications with high speeds and low loads (such as turbines) will use a lower viscosity oil, while applications with low speeds and high loads (such as gearboxes) will use a higher viscosity oil. Viscosity can be measured using a viscometer, and we typically recommend most reliability specialists, with normal equipment conditions, to check their fluid viscosity every 3 months.
2. Low Water Content – The moisture content of a lubricating oil can be easily checked via Karl Fischer Titration (ASTM D6304). Although it depends on the base oil type, you are typically looking to have less than 100 ppm of water (with certain synthetic oils being more tolerant). Excessive water contamination harms equipment by preventing the oil from forming a full lubricating film, by promoting rust and corrosion, and by damaging or destroying the additives. Without Karl Fischer titration, water can be easily estimated by visual inspection, with any haziness or cloudiness in the oil typically representing abnormal water levels. Another test that can be extremely useful is ASTM D1401 Water Separability, which checks the fluid’s ability to readily shed entrained water.
3. Functioning Additives – Most modern lubricating oils contain additives (chemical compounds designed to improve fluid performance). In the case of engine oils, additives can make up as much as 25% of the total fluid. These chemical compounds can fulfill a variety of roles, such as protecting the equipment from wear, maintaining viscosity with temperature, and preventing oil oxidation. It is very common and effective to check the antioxidant health of the oil. The best way to measure oxidative resistance is through RULER, which allows direct measurement of the antioxidants in the fluid. Other test methods exist to check on the anti-wear capabilities of the additive, such as Nuclear Magnetic Resonance (NMR).
4. Minimal Contamination by Particulates - The majority of serious equipment damage occurs through abrasive wear, where foreign material such as dirt or metal debris (particulates) causes scratches and scars in the associated metal surfaces. Abrasive wear can be divided into two categories, two-body abrasion and three-body abrasion. In two-body abrasive wear, one surface rubs against another, with the harder of the two surfaces often scraping or scratching the softer surface. In three-body abrasive wear, hard particulates such as sand or hardened steel become trapped between two surfaces, causing gouges to one or both surfaces. The most dangerous particulate is one that is close to the clearance size of the two surfaces. Fluid cleanliness is commonly measured using ISO 4406 Particle Count. The test result is reported in the form of an ISO Code, which appears something like this: 17/16/14. Each number corresponds to the number of particles counted at a certain size range (>4 microns/ >6 microns/ >14 microns). Particle Count has quickly become one of the most important tests in oil analysis, for good reason – a recent study found that for each increase of a fluid’s ISO Code, the equipment life is halved!
5. Limited Oxidation/Degradation – It is inevitable for lubricating oils to “spoil” over time, this process is referred to as oxidation or degradation. It is most common for mineral oils to oxidize, while many synthetic oils will oxidize and/or degrade, depending on the base oil. As lubricants oxidize or degrade, they tend to lose key fluid properties such as viscosity. Also, mineral oils and certain synthetic oils can produce harmful organic acids as they degrade, which can greatly contribute to corrosion on metal surfaces. These organic acids can be detected through testing the Acid Number (ASTM D974) of the fluid. Of particular note, in today’s day and age, is the problem of varnish. Varnish is created from the byproducts of oil degradation. These varnish particles can eventually agglomerate and attach themselves to metal surfaces, where they can cause a laundry list of equipment problems. One of the best ways to detect these varnish particles (soft contaminants) is through Membrane Patch Colorimetry (ASTM D7843). We typically recommend our customers to run an MPC every 6 months, even when there are no obvious signs of varnishing. MPC can be paired with RULER to provide a truly predictive assessment of fluid health.
With these five properties satisfied, equipment operators can rest assured that their lubricating oil will perform its desired functions: cleaning, cooling, and protecting the equipment from wear. Those who take the time to set up a quality oil condition monitoring program can feel confident that they have maximized their fluid life and equipment run-time.
That concludes this brief summary of the fundamentals of lubrication. Thanks for sticking all the way through.
Please feel free to reach out to MRT at any time if you have further questions. We’re real oil nerds and we love to get into the nitty gritty of this material.
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