Home > Resources > Articles & Research > Benefits of Balanced Grease Formulations
Benefits of Balanced Grease Formulations

July 01, 2012
Paul Grives
Podcast

Audio Interview with Paul Grives, Global Industrial Marketing Advisor, ExxonMobil Lubricants & Specialties

Click here to download the audio, or stream the audio in your screen on STLE's blog, The STLE Compass)

INTERVIEW NOTES (Topics/Timings)

  • (1:30) Grease background
  • (2:30) Why use a grease and grease function
  • (3:55) Basic components of a grease including base oil, additive, thickener (and Grives’ fourth component – the grease’s ability to use solid load-carrying additives).
  • (6:05) What is a balanced formulation?
  • (7:40) NLGI Grades
  • (9:25) Most commonly used NLGI Grades
  • (11:00) Extreme environments – high temperature
  • (13:20) Extreme environments – low temperature
  • (16:10) Grease compatibility
  • (19:00) Over-greasing
  • (20:45) Filled-for-life applications
  • (22:20) 3 important conclusions/take-aways

INTERVIEW TRANSCRIPT

Sniegowski: Hello, I’m Kara Sniegowski. Welcome to the STLE Compass, brought to you by the Society of Tribologists and Lubrication Engineers. The STLE Compass is your convenient and reliable resource for the latest information and developments in the tribology and lubrication engineering fields.

In today’s episode we’ll be talking about balanced grease formulations and we’ll get some best practices and tips from an industry expert.

Our interviewee, Paul Grives, is the Global Industrial Marketing Advisor at ExxonMobil Lubricants & Specialties, based in Fairfax, VA. He holds a BS in Chemistry from Fordham University, a BS in Chemical Engineering from Columbia University, an MS in Polymer Science from Columbia University, and an MBA from the University of Houston.  He has over 22 years of experience in the field of lubricants, primarily focused on greases, since joining ExxonMobil as a grease researcher in 1989 at the Mobil Research and Development Center, located in Paulsboro NJ.  Over time, he has held numerous positions within ExxonMobil ranging from research and development, plant engineering, production & operations, and technical services before being appointed to his current marketing role in 2011.  Paul has been active in STLE since 1995, obtaining Certified Lubrication Specialist (CLS) Certification in 1996.  He has been active in the STLE Basic Lubrication Course since 1997 as a presenter and as Chairman of the section.  He has also been an active member of The National Lubricating Grease Institute (NLGI) since 1990 obtaining CLGS certification in 2006, and serving on the NLGI Board of Directors since 2009. NLGI awarded Paul the Clarence E. Earle Memorial award in 2000 for his contributions to the technical literature related to greases and the Shell Lubricants Award for Instructor Excellence in 2011.

Some background on grease: A balanced grease involves a good mix of thickener, oil and additive formulation. There are many considerations in the grease formulation process including: purpose/performance parameters of the grease, function and time in the machine, including operating conditions like speed, load, temperature and contamination that could occur and compatibilities with other grease in use.

Greases, as a category, fall into a classification of materials called non-Newtonian fluids. All fluids that experience a change in viscosity with change in shear stress are considered non-Newtonian. Mayonnaise, ketchup and hair gel are examples of common household non-Newtonian fluids. There are greases that exhibit shear-induced thinning and shear-induced thickening. For more information, you can check the June 2008 TLT article, Understanding Grease Construction and Function.

Sniegowski: Paul, welcome to the STLE Compass.

Grives: Thank you for having me.

Sniegowski: First, I want to start off with a little background – why would you use a grease and what is its function?

Grives: A grease’s primary function is the same as any other lubricant, and that is to reduce friction between mating parts. The difference between a grease and a fluid lubricant is that a grease is a semi-solid lubricant. That is, not only is it comprised of a lubricating oil and performance-enhancing additives, it actually has a third phase, a solid phase that is the thickener matrix. It is this thickener matrix which helps the grease stay in place to provide long-lasting lubrication in many, many severe applications. Typically one will use a grease in filled-for-life applications where it is very difficult to re-lubricate during the lifecycle or the lifetime of a piece of equipment, or where the lubricant not only is asked to provide friction reduction, but where the lubricant is also asked to provide an environmental seal or barrier. So, in applications such as steel mills or off-highway construction and mining, very wet, dirty environments where the bearings are exposed to some pretty harsh environmental conditions, from the perspective of water or dirt ingress, a grease will not only serve to lubricate that piece of equipment, it will also serve as its own seal, preventing the ingress of the environment into the lubrication zone.

Sniegowski: Okay. The three components of a grease are base oil, additives and thickeners. Can you tell us a little about each?

Grives:  There are three basic components to a grease and some people may say four, and I’ll take a moment to explain all of them. The simplest ones are the base oil – one would use the exact same base oils for a grease that one could use for a fluid lubricant – chosen for many of the exact same reasons: viscosity, low temperature, high temperature, oxidative stability, etc. And then there are the additives – performance enhancers. Greases use many of the same performance enhancers that are used for fluid lubricants for the exact same reasons: to enhance EP/anti-wear protection, rust, corrosion, etc. And then there is the grease thickener. The grease thickener is one of the unique components of a grease that sets it apart from a fluid lubricant. The grease thickener is a solid, three dimensional matrix that could be thought of as a sponge. Just like a sponge, that holds water inside of that matrix by a combination of surface tension and weak interactions between the water and the sponge or cellulose matrix. In the case of a thickener, for a grease, oil is held in that three dimensional matrix by a combination of surface tension as well as some mild interaction between the base oils used in that grease and the thickener material. Now, the fourth component that some would say are in a grease, are the grease’s ability to use solid load-carrying additives such as molybdenum disulfide, graphite or carbon black. These are solid, non-soluble materials that if I were to put them in a fluid lubricant, they would sink to the bottom and form a layer in the bottom of the package, thereby rendering them useless for preventing friction. In the case of a grease because of its semi-solid nature, we can actually use these solid load-carrying additives in a manner that provides extra benefit, longer protection for equipment, particularly under high loads, compared to a typical fluid lubricant.

Sniegowski: Okay, so we have these components – how do you know when they are all in balance in a grease?

Grives: Balanced formulation is just as important for greases as it is for any other material that one would buy, where you’re expecting performance. The best way to tell if a grease does indeed possess a balanced formulation is to evaluate it over a broad range of critical parameters that are pertinent to the application and the end use. I can’t emphasize how important it is to choose a grease that is developed with this balanced formulation approach to ensure that the customer, the user of that product, is able to maximize their productivity and reap the full benefits of a product. For example, there are some greases that may be formulated to have extremely good high EP/anti-wear. If we consider the function of providing EP/anti-wear, this is a surface-active characteristic where are an EP/anti-wear additive would physically bond to the metal surface, thereby lowering friction and preventing wear. Now, let’s consider rust inhibition. A rust inhibitor performs in exactly the same way. It is a surface-active component. So, when formulating a good overall balanced formulation product, the challenge is in balancing just the right amount of these additives that have a similar mechanism for providing their benefit, such that the customer is able to realize everything that the supplier of the product is promising.

Sniegowski: There are also NLGI grades for greases, can you talk a bit about those?

Grives: Sure. An NLGI grade is a measure of the thickness of a grease. This is not to be confused with viscosity. We should never confuse the NLGI grade, NLGI consistency number or NLGI number with viscosity. The NLGI grade is a rating from 000 up to an NLGI grade 6. What these do, the higher the number, the stiffer the grease. So, a 000 is typically called a semi-fluid grease and in a bucket, it would have the appearance of a very thick oil, if you were to have it in a jar, it would emulsify, it would slosh back and forth, just like a very heavy oil. An NLGI 6 grade grease or a block grease is solid. It would be very difficult to move. A very, very small number of greases are actually in the NGLI 5 and 6 range. Typically these are for an extremely specialized application, where you’re looking to the grease to provide much more than just lubrication. The NLGI number is determined by a common test called ASTM D-217 – this is the cone penetration of a lubricating grease. There is a scale – a rating scale within that method that allows the manufacturer or customer to validate the NLGI grade by measuring how many tenths of a millimeter a pre-weighed standardized cone at 25° C falls or sinks into a cup of grease in 10 seconds.

Sniegowski: So, what are the most commonly used NLGI grades?

Grives: If we look at the NLGI annual grease production survey, we’ll see that the most common NLGI grade of grease is an NLGI grade 2. This is commonly referred to as buttery – with the appearance of a softened stick of butter. It’s able to maintain its shape without being contained in a package, but when an external stress is applied, very easily conforms to whatever shape you want it to go ahead and take. Now, if we look at where different NGLI grades would be used – the 000 and 00 (semi-fluid greases) are typically used in very low temperature applications, where the inherent thickening of the base oil will add some body to the grease, or in long centralized grease distribution systems where a single pump would be asked to supply grease to many, many different lubrication points. A semi-fluid grease is much easier to pump effectively than a very heavy grease. When we look at NLGI 3 grade, so slightly harder than that NLGI 2 – these would typically find use in some specialized applications such as vertically mounted bearings or electric motors where the grease is required to stay exactly where you put it, typically without the benefit of an extensive sealing system.

Sniegowski: You talked about extreme environments. How can greases help maximize performance, within, let’s say high operating temperature environments like steel mills?

Grives: Ah, steel mills – one of my favorite industrial applications. Not only is a steel mill extremely hot, typically, where you look at greases being used, steel mills are also extremely wet and dirty or dusty. So you’re talking about a severe, extreme environment in which we’re asking a lubricant to not only provide friction reduction but also to protect equipment and enable that steel mill to maximize productivity of their operating facility. So, when we go ahead and look at a steel mill – greases, because they’re able to perform the function of a seal, are able to provide that lubrication and friction reduction while also preventing that environmental ingress. When we look at the kinds of greases that are typically used in a steel mill, we’re looking at polyurea or lithium complex greases – greases that have good high temperature stability. The other very important characteristic of a good steel mill grease, particularly when we’re talking about that high temperature environment, is the ability of the grease to have a very controlled, slow release of oil. When we look at a grease and the multiple components that are in it, the thickener has no lubricity at all. It is the lubricating oil that does the lubrication, but it is the grease thickener’s job or purpose to slowly wean out or release a little bit of oil – just enough to do what’s needed from a lubrication perspective, when the application needs it. This is truly a challenge when you have a very hot environment as typically a thickener matrix will want to release all of its oil very, very quickly when a very high temperature is applied to it. So, the grease formulator’s challenge is choosing the right combination of thickener(s), to not only provide the good matrix, that reservoir of lubrication that’s needed, but also to make sure that that reservoir of lubrication very slowly releases that oil and that also in that lubricating equipment, that that thickener matrix is very stable. What I’m trying to say is that that thickener matrix maintains its consistency, such that the grease stays in application, where it’s put.

Sniegowski: Got it. So, what about low temperatures?

Grives: When we look at low temperatures, the primary drivers from a grease perspective would be to look at the viscosity of the base fluid and then the NLGI grade. The rules for selecting a base fluid viscosity (ISO VG viscosity), for a grease to be used at very low temperatures are very similar to the rules one would use when trying to selecting the base oil viscosity of a fluid lubricant. Typically the lower the viscosity of the oil at operating temperature, the more easily that lubricant will flow. The more easily that lubricant flows, the less extraneous energy is needed to overcome the internal lack of inertia of the lubricant and actually allow the inputted energy to perform useful work. The NLGI grade, again, because at lower temperatures, materials tend to thicken, one would be looking for an NLGI 1, 0, 00 or 000 greases. These are greases that at room temperature are relatively thin, but at low temperatures like -40°C or -50°C, do not get so thick as to become solid. A wonderful example when we think of trying to marry both high and low temperature performance, would be an aircraft wheel bearing application. I’m sure many listeners have flown on an airplane, and when you’re onboard, you’ll go to see the air show and it will tell you all about the environmental conditions outside of that plane as you’re flying at 38,000 feet. Typically, it will say something like -75°C or -80°C, extremely cold. Let’s consider that those wheel bearings are exposed to that temperature and the grease in that wheel bearing becomes -75°C or -80°C. As that plane lands and touches down, and the wheels start to spin up at 140 or 150 miles an hour, the temperature very, very quickly spikes, but the grease must be fluid enough as that wheel first touches the tarmac to allow the wheel and the bearings to turn. So, by marrying both high and low temperature performance, one can meet these kinds of very critical applications. In low temperature regimes, there are many benefits and advantages that can be had from using a synthetic fluid over a conventional mineral oil. If I had to summarize overall the many different attributes that are needed at low temperature, I would highly recommend a semi-fluid grease for easy pumpability and flowability, a grease with a low ISO VG, and if I have the option, I would always recommend for low temperatures, a synthetic product over a conventional mineral product.

Sniegowski: Another question we get a lot is about grease compatibility – what do listeners need to know about and what is the key to determining if two greases are compatible?

Grives: Grease compatibility is one of the most common questions I’ve been asked in my many years at ExxonMobil. And it is also one of the most important items a customer must be aware of, particularly when changing from one lubricating grease to another. It’s during this critical interface, when moving from one product to another, where there is the greatest possible potential for damage or harm to the equipment that could arise from a grease incompatibility. One of the first manifestations of grease incompatibility would be a change in the appearance of the grease, that is, the grease will either become much thinner or much heavier. If we consider that the application was developed and specified – for instance, an NLGI 2 grade grease, if, when we’re combining two products, that NLGI grade, or that apparent thickness changes to a semi-fluid product, that grease will likely flow out of the bearing, causing the bearing or application to run dry, leading to equipment failure. So, it’s very important when changing from one product to another, that the thickener interactions governing any change – either thicker or thinner – of that grease are evaluated. Secondarily – and with a slightly longer time before it manifests – would be changes that are inherent to incompatibilities in additive chemistry. Typically incompatibilities in additive chemistry can lead to aggressive rust or corrosion, the formation of solids and at a much longer timeframe, or can lead to excessive hardening or excessive softening. So, all users of grease should be aware and should consider, that when working with a grease supplier and when moving from one supplier to another, that you are working with a trusted lubricant supplier that offers not only a broad range of product choices tailored for your application, but that the supplier has the application expertise and the good understanding of grease lubrication to be able to provide extremely good guidance to the customer regarding compatibility and incompatibility. It is very easy to have the misunderstanding that compatibility can be easily determined by looking something up on a chart. The only true way to measure and judge grease compatibility is to physically run a grease compatibility test. The ASTM has a grease compatibility protocol that initially is based upon the penetration test (ASTM D-217) and then expands as a second tier of evaluation to other more severe performance characteristics.

Sniegowski: Besides grease compatibility as a common issue, what are other problems you’ve encountered in the field and how have you solved them?

Grives: The second most frequent issue that I’ve come across in my 23 plus years is over-greasing. A common misconception is that “if some grease is good, more is better.” And in some applications, such as a plain bearing, we can think of the wrist pin or the bucket pin on a piece of construction equipment, yes, I could say that if we want to pump more grease in there, no real harm will come beyond wasting product and potentially environmental issues when the grease falls upon the ground. But when we’re considering things such as bearings – over-greasing a bearing is very bad. Bearings are designed for a very specific load of grease. By putting too much grease in them, you can have a couple of different issues occur. One would be an increase in temperature. As that excessive charge of grease is churned between the rolling elements, the cage and raceways, this will generate friction. We all know that as we generate friction, temperature increases. At higher temperatures, we increase the rate of oxidation and increased rate of oxidation can lead to a reduction in overall lubricant life as well as potentially the life of the equipment. The second item that happens from over-greasing, particularly if we look at the application of electric motors is contamination of motor windings with the excess grease. Even though some electric motors are indeed designed to be re-greased, over-greasing will force the excess grease out into the windings and as that grease hits the windings, the efficiency, if not the entire operation of the electric motor will be compromised.

Sniegowski: What impact has filled-for-bearings had on grease technology development?

Grives: Filled-for-life bearings are really a continuing emerging trend that has driven the technology of lubricating greases right to the very edges of what can be delivered by today’s thickener and base oil technology. When we consider a filled-for-life bearing, this is a bearing that on an initial small charge of grease, sometimes as small as two or three grams, is expected to provide flawless operation for upwards of ten plus years without ever having to replace the lubrication. What this requires a grease to do, is to have extremely good shear stability of the thickener system, the ability to maintain its NLGI consistency over many, many years of shearing as well as containing a lubricant, such as a synthetic lubricant or synthetic base oil that has extremely good oxidative and life stability. What filled-for-life bearings have enabled industry to do in many cases is to reduce the size of equipment by not having to put a grease insert on it, and by being able to develop a bearing, say as a sealed or shielded bearing – manufacturers of equipment that use these filled-for-life bearings have simplified their manufacturing process. So overall, filled-for-life bearings are an extremely important lubrication opportunity for lubricating greases and really enables a grease manufacturer to provide products that provide that step-out performance that customers demand these days.

Sniegowski: So, given all we’ve talked about today, what concepts or ideas do you want to leave listeners with?

Grives: There are several very important key items that I believe listeners should take away and be aware of as they continue to use greases in their everyday lives. The first is to always follow your original equipment manufacturers’ (OEM) or equipment builders’ (EB) recommendations for grease. Typically these OEMs/EBs have a long-standing relationship with the developers of the lubricants to both tailor their application as well as provide input to the development of the lubricant to provide that equipment with the maximum protection and enable customers to realize the maximum productivity and benefits from the combination of lubricant and equipment. The second is to be very aware of the application needs when selecting a grease. Not all greases are created equal. We’ve talked about those different components that a grease is comprised of (including base oil, thickener, additives, and the grease’s ability to use solid load-carrying additives). Selecting the right base oil for the temperature, speed and load of your application is critically important. Selecting the right thickener system is also important. There are many thickener systems in use in the grease industry today. The most common is lithium and lithium complex, but there are also thickener systems such as inorganic clays, polyureas, calcium sulfonate, simple lithiums and calciums. All of these thickeners by themselves, although they provide no lubrication, do impart certain performance characteristics to the grease which should be considered when selecting the right grease for your application. The next thing I believe all listeners should be very aware of is grease compatibility or incompatibility, particularly when moving from one grease to another, or moving from one grease supplier to another. Making sure your incumbent greases have been evaluated for compatibility is one way to have a flawless transition from one product to another. Lastly, when I look at some of the most important things listeners should be aware of when selecting greases is: are you working with a company that has a long track record as a technology leader? Are you working with a company that has been able to innovate and stay ahead of the curve, providing the best products for today’s ever-changing, ever more severe applications? Does that supplier not only have the products to provide, but do they have the application expertise to provide the in-depth guidance that is needed today to ensure users or operators are able to maintain equipment so it can continue to operate over a very broad operating temperature range, broad range of environmental conditions and different application severities.

Those are the three primary things I would recommend: make sure you’re dealing with a reputable supplier of greases with a long-standing experience; be very aware of grease compatibility and incompatibility; and make sure you select a grease based upon the application, not based upon whatever grease happens to be at hand.

Sniegowski: Perfect! Thank you so much Paul for your insight today.

Grives: Thank you very much Kara. It was a pleasure to speak with you all today.

Sniegowski: I’m Kara Sniegowski. For more information, articles, and resources on grease and formulations, please visit our website, www.stle.org. Thank you for joining us today. This has been another episode of The STLE Compass, pointing you in the right direction.

You might also like:

  • Podcast Interview with Chuck Coe regarding Grease 101, an Annual Meeting Education Course
 
©2008 STLE All rights reserved.