Boundary Lubrication vs. Hydrodynamic Lubrication:

The main purpose of any lubricant is simple: to reduce the friction between two surfaces. When a lubricant reduces friction the natural result is lower wear (longer life) and increased efficiency (less heat and energy wasted). Thus, for the most part, people are very clear on what they wish to accomplish when using a lubricant. What most design engineers quickly realize, however, is that HOW a lubricant functions varies widely across applications! One of the most important and misunderstood distinctions is between hydrodynamic and boundary lubrication in a bearing.

In an earlier blog post, I discussed two examples of the need for oil over grease in a high-speed bearing. The reason for this rule has to do with the difference in how a lubricant functions in high vs. low speed applications. Simply put, hydrodynamic lubrication occurs when the fluid film in front of a bearing builds enough force to lift the bearing off the shaft, overcoming the naturally thin fluid film created by oil.

The principle at work is the same as what takes place when at a certain speed your car begins to hyd

roplane:

As you can see in the diagram, as the rotational speed builds there is a build-up of fluid in front of the rotating wheel. When a sufficient rotational speed is achieved the fluid will form a “hydrodynamic wedge” in front of the wheel that actually lifts the wheel off the surface of the road.

A bearing will obviously function differently, but the same principle applies. The diagram below illustrates how the forces within the bearing are distributed when a bearing is lubricated using hydrodynamic lubrication. As you can tell from the diagram, a higher vertical load on the shaft will increase the necessary speed of the bearing and/or viscosity of the oil.

When the hydrodynamic wedge from the diagram is formed, the bearing will have phenomenally low levels of friction between the two surfaces. However, the key challenge when to hydrodynamic bearing lubrication is that friction builds up within the hydrodynamic wedge and begins to heat the oil. As the oil heats up, the viscosity of the oil will change, causing the wedge and film to weaken/break down over time. The industry has developed additives and cooling mechanisms to overcome the effects of oil temperature on performance, but it is always a key concern. The reason for concern is that there is a danger that the surfaces can be damaged if the fluid film created by the oil is too thin for the materials in question or the speed drops low enough for the wedge to fail. In some cases, using oil with higher viscosity (thicker film) will help to generate the wedge at a lower speed, but this also raises the amount of friction. In addition, as I mentioned above, vertical loading of the shaft will only exacerbate these challenges.

As a result of these technical issues, lubricants have been created that operate under principles of boundary lubrication. Essentially, boundary lubrication uses low-friction solids to create a layer of lubricating solid particles between surfaces that act like mini ball bearings. The key to an effective boundary lubricant is that its solid additives have the lowest trade-off between pressure resistance and low friction. This is because as you include more solids in a lubricant you are reducing that lubricants ability to reduce friction.

Examples of solid additives:

▪ Dark inactive sulphurized fat

▪ Dark active sulphurized fat

▪ Dark active sulphur hydrocarbon

▪ Short and medium chain chlorinated alkanes

(see chlorinated hydrocarbons and chlorinated paraffins)

▪ Esters of chlorendic acid

▪ Polymer esters

▪ Polysulfides

▪ Molybdenum compounds

▪ PTFE

Boundary lubricants that utilize these additives can still have an extremely low coefficient of friction, and they are essential when surfaces are loaded. In addition, boundary-lubricating greases have the added benefit of staying on a surface that is at rest, unlike oils, to avoid “dry-starts”. For more on the tradeoffs you can read our post on Grease vs. Oil.

Lubrication Selection: Cheap is Expensive

Price is always a major factor in purchasing any product, and lubricants are no exception. Luckily, lubricants are not as dependent upon commodity pricing as other petroleum products, such as gasoline. This is mainly due the large proportion of people who understand that proper lubrication has one of the highest returns on investment of any preventative maintenance or product design expense.

That being said, I often speak with people who simply do not care about anything other than buying the least expensive lubricant. Unfortunately for these people, they are often misled into thinking they are saving money when really they are improperly accounting for the cost of a lubricant. In essence, the upfront cost of any lubricant is only a fraction of its true cost because the majority of lubrication expense should be accounted for in the performance and life expectancy of your equipment!

For example, this past week a potential customer contacted me to see if he could replace his current lubricant priced at over $100 for 14 ounces with ours priced at just under $9 for the same size. The prospective customer asked me to compare the specs of his current grease with Magnalube-G for a high-end CNC lathe, operating at 8,000rpm with bearing diameters ranging from ½ inch to 8 inches. I explained that there were many major differences, such as thickener type, viscosity, base oil composition and some minor ones, as well. In fact, despite the high cost of his current lubricant, Magnalube-G outperformed the product on temperature, corrosion resistance, service life, and others. However, we failed to meet what may have been the most important specification, speed factor. Based on the speed of his bearings, it would be normal for the manufacturer to have specified an oil lubricant, but they had gone out of their way to specify grease with an abnormally high speed factor.

I was eager to land the customer, but I knew immediately that the manufacturer had probably done their homework. So, I called the manufacturer and spoke with someone in the company’s technical department about the reason for selecting the particular grease. The machine in questions was over 20 years old, so the current staff could not tell me why the grease was originally specified, but they did seem confident in the choice and said they sell the item. (As an aside, do not assume that a manufacturer has a conflict of interest if they make money by providing the lubricants they recommend. Not only does this make it easier for their customers to find a specific product, but it also ensures that the equipment is properly maintained! Thus, their customers get more out of their purchase.) In the end, I had to tell my prospect that he should stick with what they recommended even though the price was over 1000% higher. Ultimately, the prospect will be much better off paying for the right lubricant, even though we also sell a very high quality lube.

Making the right decision about your lubricant means that a customer’s short-term expenditure may be saving him big in the long-term. For those of you that want to see it in numbers, here is how you should think about lubrication:

(Cost of Re-Lubrication, Which Includes Labor and Downtime Expended + Cost of the Lubricant) * How Many Times You Need to Apply the Lubricant over the Life of the Equipment = Total Cost of Lubrication.

Therefore, even if your lubricant looks like it’s costing more, if it means that you will have to buy it fewer times and apply it fewer times, then you will have less downtime, and you will expend less man-power applying it. In addition, by using a higher quality lubricant that is optimized for your application, you can gain additional life out of your equipment, which can offer huge returns on your investment. Hopefully you can now see why price should be the last thought in your mind when choosing a lubricant that will result in the best performance.

Grease vs. Oil

I am often surprised by how much debate there is about what kind of lubricant to use where. In a way, this debate is a natural result of the wide range of applications and environments in which grease or oil can be used. Many of our customers, and potential customers, will test dozens of products before they find something that is just right for their new widget. You can read this entire post, but if you just want a good rule of thumb to follow, here it is:

If the system is self-contained, well-sealed, would be difficult to grease and does not have vertical loading, use oil. If the lubricant cannot be contained by the design, is not naturally dispersed in the application, has vertical loads, and benefits from have the lubricant be a barrier to the environment, use grease.

There are a few key indicators that you need grease:

1. Use grease for low speeds and high loads

a. Grease will prevent “Dry Starts,” which occur when the oil has settled out of spots during downtime on the equipment. Grease often contains extreme pressure additives that will prevent the surfaces from touching despite high vertical loading. It is true that oil can fill space quickly in high-speed applications, but in a low-speed application grease is the clear winner.

2. Use grease on areas that need lubricants to STAY PUT, such as semi-exposed surfaces or areas where oil would easily leak out.

a. Remember: Oil will move with gravity, the right grease will not.

b. People often think that grease will attract more contaminants than oil because it is stickier. The reality is that there is nothing about grease that attracts more contaminants than oil. The only reason you may have less contaminants is that the oil has evaporated or migrated from the surface and there isn’t anything for the contaminants to stick to. You are much better off using a grease lubricant that will maintain an even film and not migrate.

3. Use grease when you need certain performance enhancements. Oil often contains many performance-enhancing additives, but certain additives tend to settle out or not work in the first place.

a. Grease should be used in cases where extreme pressure, corrosion resistance, and longer-term lubricity are essential.

Keep in mind that this is not an exhaustive list. There are many times when application or product-specific benefits change these dynamics, but this narrative has been simplified for the sake of the comparison.

You may think that as a manufacturer of lubricating greases, I have an obvious bias in this debate. I am, however, the first person to admit that there are billions of surfaces that require lubrication and one solution will never work for every application. So, even though I have a predisposition to grease, I can still serve plenty of customers without claiming grease is superior to oil in every way.

In fact, oils will have some key advantages over grease:

1. Oils will act as a medium for heat transfer

2. Oils will perform better at extremely high speeds

3. Oils are easily inspected in the field

4. Oils can be filtered to remove contaminants

This is a fairly condensed list because there are a few advantages that are debatable. For instance, some people will claim that there are fewer risks of compatibility problems when mixing oils, and changing oil is easier, but this is not always true. Thus, we have left our advice fairly general, and encourage you to contact us or to comment on the blog. Then we can better address nuances in lubrication selection as they come up.

Fight Friction

This is the beginning of a campaign to limit the waste associated with friction.