I sometimes get asked what I do in daily life. If you are a lawyer, librarian, lumberjack or a locksmith the one word name pretty much gives a good picture of what the person does. Lab owner is probably the most accurate description for me, which is strange though as almost every other lab I come across (at least in OCM) are large multi billion dollar organisations, so the owner in that respect would be a CEO role and likely never do anything lab related and be more handling shareholder meetings, acquisitions etc. They would certainly not be with an Allen key and a torch under a lab instrument trying to find a blockage stopping solvent flowing.
The problem with lab instruments is they are extremely temperamental so you learn a lot of plumbing skills in the role cleaning things out as those pesky customers keep sending very contaminated samples the instruments were never designed to handle. After all we are every single day operating instruments and taking them to twice the surface temperature of the sun in places, so these high end instruments need a lot of TLC.
Why under the instrument?
So lab person might be more accurate, or scientist, or if I want to confuse people with a fancy title I might even use the word ‘tribologist’. Now this is something most ordinary people have likely never heard of but probably half of my knowledgeable lube enthusiast readers have.
Tribologists tend to rub you up the wrong way
Why? Are they obnoxious know-it-alls about lubrication? Well probably yes, but that’s not the reason for the name.
It actually has Greek origins. The word “tribology” comes from the Greek word “tribos,” which means ‘to rub’. The ology bit basically means to study or study of, so it’s the study of how things rub together. This quirky subject field was first Coined its name in the 1960s, by a pioneering scientist Peter Jost, where he highlighted the cost of ignoring the implications of friction, wear, and lubrication. His paper first mentioning the word is often considered a foundational moment for the formal study of tribology as a distinct scientific discipline.
Tribology is the science of wear, friction, and lubrication, or in layman’s terms, the study of how materials complain when they’re forced to rub elbows without getting properly acquainted first. It’s the reason your car engine doesn’t throw a tantrum and the secret behind that non-stick frying pan’s slick performance. It’s not just about the smooth moves; it’s about making sure those moves last long enough to be worth the effort.
The field is to be honest far larger than my little bit of oil analysis, but covers everything from how car tires work on the road to how an orthopaedic surgeon decides how to fit a replacement hip joint.
In the field of oil analysis though it is important to understand that this area although admittedly niche is part of a global defence against friction.
Why does friction matter?
Well friction losses (energy loss and repairs and replacement’s) accounts for a loss between 2% and 7% of global GDP every year. When put into comparison the entire covid pandemic put a 3.4% impact on GDP in 2020 and the whole world knew about it. The global financial crash between 2008 to the mid twenty teens was not much greater in cumulative GDP impact over the years. So if you use, operate or lubricate machinery you might want to keep reading and brush up on friction effects.
How does friction happen?
When we talk about friction we often use the phrase coefficient of friction.
The coefficient of friction is a dimensionless scalar value that describes the ratio of the force of friction between two bodies and the force pressing them together. It is used to calculate how much force is required to slide one object over another and is a critical parameter in the study of tribology.
There are two types of coefficients of friction:
- Static Coefficient of Friction: This is the friction between two or more solid objects that are not moving relative to each other. It must be overcome to start moving an object at rest. This is important at machinery startups or acceleration.
- Dynamic (or Kinetic) Coefficient of Friction: This is the friction occurring between two objects that are already in motion relative to each other. It is usually lower than the static coefficient of friction. This is the main day to day friction we refer to in most cases.
The coefficient of friction depends on the materials used; for example, ice on steel has a much lower coefficient than rubber on tarmac. It is a crucial factor in the design of machines and structures and in the analysis of any system where objects interact through a contact surface. It is often represented in a variation of friction vs speed, load and viscosity often called a Stribeck curve and this is often on the syllabus of many of the USA based lubrication analysis programmes, although for some reason is not covered too much in European versions.
Understanding the relationships of friction
The key parameters of interest in our industry are viscosity, speed and load. High viscosity or speed, or low load reduce friction.
The graph represents a Stribeck curve, which is fundamental in the field of tribology. It illustrates the relationship between the coefficient of friction and the lubrication condition of a surface, depicted by the Hersey number—a dimensionless quantity that combines viscosity, speed, and load.
At the start there is no dynamic friction as the object is stationary and the friction rapidly rises as the inertia of static friction is overcome. Continuing from the left, the curve begins in the “Boundary Lubrication” regime, where the coefficient of friction is high. This is due to the asperities (high points) of surface textures coming into direct contact, which happens under heavy loads, slow speeds, or with low-viscosity lubricants.
As we move rightward, representing an increase in the Hersey number (either through higher speed, lower load, or greater lubricant viscosity), we enter the “Mixed Lubrication” regime. Here, the friction coefficient decreases and levels out because the surfaces are partially separated by a thin lubricant film, reducing metal-to-metal contact.
Further right, the curve slightly rises again into the “Hydrodynamic Lubrication” regime. In this phase, the surfaces are fully separated by a thick film of lubricant. The slight increase in friction coefficient is due to the increased fluid resistance as the speed or viscosity increases, which requires more energy to maintain movement.
The Stribeck curve is critical in designing and maintaining mechanical systems, as it helps engineers understand and predict the lubrication needs for different operating conditions to minimise wear and energy consumption.
What are boundary, mixed and hydrodynamic lubrication.
Normally when you google these words you see some awful windows 95 MS paint images to depict the contacting surfaces which look the same since 1960s textbooks. In fact I’m guilty and use them myself. Here are my amazing efforts of boundary lubrication and full fluid film hydrodynamic lubrication I made in paint a few years back. Mixed is somewhere in between.
However I think with more modern artistic software you can get something better, so below I try to look side on what the film might look like with a yellow river of oil between two rough surfaces that at normal human eye scales look very smooth and polished surfaces.
Boundary lubrication occurs when the moving surfaces are separated by a thin film of lubricant, usually only a few molecules thick, insufficient to completely prevent asperities on the surfaces from coming into contact. Here, the lubricant film’s strength relies heavily on the chemical properties of the additives within the oil, which can form a protective layer on the surface to prevent excessive wear. Additives like anti-wear agents or extreme pressure agents are designed to react with the surface to form a sacrificial layer that shears in preference to the surface itself, thus protecting the machine parts from wear. Shear mixing refers to the dynamic process where additives in the lubricant are mixed due to the relative motion of the lubricant layers, enhancing the protective quality of the lubricant film. In this context, “shear” means the sliding of one layer of fluid over another. It is this layer that contributes to normal wear.
Mixed lubrication is the transitional phase between boundary and hydrodynamic lubrication, where a thicker film of lubricant is present, but surface asperities may still occasionally break through the lubricant film. During this regime, both the physical and chemical characteristics of the lubricant are critical. The film is not yet thick enough to keep the surfaces completely apart, hence both the surface asperities and the lubricant film share the load.
Hydrodynamic lubrication is when a full film of lubricant is thick enough to completely separate the surfaces, preventing direct contact between them. The lubricant film is maintained by the relative motion of the surfaces, which drags the lubricant into the contact area and builds up pressure within the lubricant. This regime is characterized by shear within the fluid which results in much lower friction and wear than in the boundary or mixed regimes.
Effective lubricants need to balance low viscosity for better shear in hydrodynamic conditions and high viscosity for maintaining a protective layer in boundary conditions. The aim is to reduce normal rubbing wear which occurs due to asperities (pointy rough bits) on the contact surfaces breaking and forming debris in mixed films and removal of surface of the boundary lubrication layer, causing deterioration of the machine components over time. Here is another awful picture to show what an asperity is.
How to use this information practically?
It’s nice to understand the theory but ultimately you need something practical to apply these types of lubrication stages.
Well firstly if you are involved in lubricating machinery you will find if you increase the speed, increase viscosity or lower the load the friction goes down – up to a point. You notice the graph at the end does start rising up gradually and that’s because if you really thrash the machine speed or increase the viscosity very high the lubricant itself will be causing more friction than it is preventing. This is why in most my training materials I emphasise it’s all a case of moderation and there is always a Goldilocks sweet spot you can aim for.
Secondly, It is also important to understand the switch between these processes can be very fast as illustrated below.
If you think about an engine piston as illustrated in the image above it has all 3 lubrication processes within a second. This slides up bore accelerating at the mid point at max speed and then for a fraction of a second at top dead centre is completely stationary so speed is zero before reversing down the bore again. So in this type of application and indeed anything reciprocating like a piston compressor too you have boundary lubrication to mixed to hydrodynamic lubrication multiple times a second. Hence why oil analysis of these types of machines is so important.
How does oil analysis help me on this area?
Well first of all there will always be a run in phase in lubricated systems. So you will see wear as that initial inertia is overcome and the boundary lubrication layer is formed. This will then be the fallback wear position for startup and shutdown of equipment and variations in load, speed and temperature (as temperature will be the largest effect on viscosity daily).
So if you have a wide temperature range, variable loads and speeds, or repeated stop starts you may choose your lubricant differently to when it’s a constant steady system.
If something is out of balance you will start to see abnormal wear usually. Now ordinary oil analysis will only pick up a fraction of this but LubeWear acid digestion will allow you to identify what’s normal and abnormal. In the small graph case study below you can see the abnormal wear increasing (purple) between the ICP (less than 5 micron – normal wear is <15 micron) and LubeWear (total wear detecting all normal and abnormal wear). Ordinarily I wouldn’t get a nice graph like this as we would tell the customer to do something around the 6th sample but if you read the case study below you will see why the system was allowed to carry on past where we would flag the system.
So when you see abnormal wear what happens next?
Well it’s all well and good knowing the system has abnormal wear but the cause is often the illusive. Sometimes there is obvious things like dirt and water present, but not always. Sometimes there is just wear and everything else looks fine on the analysis. This would generally be an issue for most labs in they say something is wrong and then it takes ages to work out what. Especially for elements like iron that are in everything it’s hard to identify even a component. I insist in adding microscopic particle analysis as part of our LubeWear vision service to identify what those wear particles are. In the example below this is a sliding wear particle. Notice the lines called striations on the main surface of the flat wear particle. These are caused by sliding wear meaning the boundary lubrication layer has not only been reached but breached leading to wear of the underlying surfaces coming into contact. Using our formula we know the speed, viscosity and temperature, or loading will be having an impact on this and by understanding the scientific concepts we can start to picture how to fix the problem.
So what was the point of this article?
Well hopefully you feel more enlightened on basic lubrication concepts but more importantly you can now apply this information in a practical sense to make maintenance decisions when combined with oil analysis results including LubeWear and when needed microscopic analysis too.
If you want to learn more about improving your lubrication of your machinery or analysing your oils click the contact us button below to get in touch.