Follow me on: 
The Ultimate Guide to Bearing Lubrication
Bearing lubrication is critical to maintaining bearing performance and extending bearing life. Lubrication helps separate the rollers or balls and raceways from each other to prevent wear from metal contact between the rolling elements and raceways and reduce the coefficient of friction between the internal sliding surfaces. Proper lubrication can also prevent bearing corrosion and also take away some heat. Bearing lubrication can even determine the success or failure of your factory’s manufacturing process. To this end, Aubearing, a leading bearing manufacturer, has put together a guide to bearing lubrication based on its years of experience, discussing different types of lubricants, the best methods of lubrication and correct procedures for the reference of all engineers.
There are two types of lubricants on the market: oil and grease. Oil is easy to understand and is a free-flowing liquid, while grease is a little complicated. In people’s minds, grease is a relatively viscous oil. Both lubricants have their own advantages and disadvantages. Choosing the right grease depends on several factors: including “type of equipment, bearing type, size, temperature, load conditions, speed, operating conditions and external environment.” So let’s first look at the difference between oil and grease:
| Lubricant | Advantage | Disadvantage |
|---|---|---|
Oil | Easy to distribute, lubes other components, less drag, easier to drain out and change. Better for high temperature. | May leak (environmental concern), then no lubrication |
Grease | Remains in place, doesn’t leak out easily, improves sealing, and does not require monitoring. | Requires labor to clean out and replenish. High temperature grease is very expensive. |
In most cases, the best starting point for selecting the appropriate lubricant is to follow the bearing manufacturer’s recommendations.
lubricating oil
Lubricating oils consist of natural mineral oils (containing additives to prevent oxidation and rust) or synthetic oils. The four most common natural mineral oils include petroleum, diesters, silicones, and fluorocarbons. Synthetic oils are typically polyalpha olefins (PAO), polyalkylene glycols (PAG), esters and silicones and are suitable for cold and low torque conditions. However, these two types of oil have unique properties and are not interchangeable. Mineral oil is common than synthetic oil. The viscosity of an oil is related to the thickness of the oil film it creates, which is critical for bearing lubrication. From a performance perspective, oil is the best form of lubrication for bearings. It provides several types of lubrication for bearings. In its simplest form, it maintains a static oil level in the bearing housing. For example, on equipment with gears and/or connecting rods, oil supplied to these components can create a mist or spray that wets the bearing contact surfaces. This is sometimes called “splash lubrication.” Next in complexity is the oil mist and air/oil system, designed to provide the precise amount of oil needed for lubrication, without the bearing having to stir up excess oil, preventing increased drag and temperature. For high speed applications, injection of circulating oil is often required. The nozzle injects oil directly into the bearing, providing the dual functions of lubrication and heat dissipation. These systems are complex and expensive, and should only be selected when absolutely necessary.
Each lubricant manufacturer can provide a specification sheet for each of its products, and each specification sheet lists approximately 20 properties and their values relevant to that lubricant. The most important characteristic of any lubricant for rolling element bearings is its oil viscosity. If the spec sheet applies to an oil, the viscosity value applies to the oil. Typically, the four viscosity values are as follows:
cSt @ 40° C(104°F) SI Units
cSt @ 100°C (212°F) SI Units
SUS @ 100°F(38°C) Imperial units
SUS @ 210°F(99°C) Imperial units
It is important to select a lubricant that provides the lowest acceptable viscosity at the bearing operating temperature, which is usually between the minimum and maximum reference temperatures mentioned above. Generally, the viscosity value of oil decreases rapidly with increasing temperature.
Bearing grease:
Some bearings use oil as a lubricant, but grease is the preferred lubricant for 80 to 90 percent of bearings. Grease adheres to bearing surfaces better than oil, has a longer service life, and is less likely to run off or eject from rotating parts. It can also be pre-lubricated, eliminating the need for an external lubrication system, meaning less maintenance in the future. Grease consists of three ingredients: additives (usually rust inhibitors), base oils, and thickeners. When you select a grease, it is important to note that the viscosity of the base oil (called “base oil viscosity”) determines how the lubricating film forms. According to the National Lubricating Grease Institute (NLGI), a grease’s consistency grade indicates how the grease flows and disperses within the bearing. Typical grease systems are much simpler and less expensive than oil systems. Typically, the only features required are the grease supply hole and an external grease nipple for replenishment. When selecting a grease for an application, some of its properties need to be considered based on the expected operating conditions. The priority of these properties is:
Required oil viscosity at bearing temperature.
Operating temperature level.
The most suitable soap base.
Availability of EP (Extreme Pressure) additives.
It is important to remember that no matter which type of lubricant you choose, if it is not properly maintained with the help of an experienced lubrication service, it will naturally lose its lubricating properties over time, eventually leading to bearing failure. Fault. Aubearing has summarized common bearing lubricant types and lubricants based on years of experience.
Calcium: | Inherent EP ability, corrosion resistance, safe for food processing, low temperature use only. |
Sodium: | Lower cost, general purpose, medium-high temperature. |
Lithium: | Higher temperature, high speeds. |
BentoneClay: | Heavy loads at high temperature, water washout resistance. |
Synthetic: | Very high temperature. (High cost) |
The best way to lubricate bearings
As we mentioned earlier, bearing lubrication plays a vital role in bearing life and performance as it helps separate moving parts to minimize friction and prevent wear. In addition, lubricants also have the following functions to support bearings to maintain excellent performance for a long time:
Provides a barrier against moisture and contaminants
Compatible with adjacent components
Non-corrosive
Stable structure, extending bearing service life
Maintain stable viscosity within a certain temperature range
Good film strength to support load
There are many different methods for applying oil and grease, however, there are four standard methods commonly used to provide lubrication to bearings.
Oil drop lubrication
Oil splash lubrication
Oil pressure lubrication
Grease lubrication
Grease is typically deposited between the balls using special equipment, forcing it in and around the ball or roller raceway interface. Unlike oil, grease is usually expressed as a percentage (e.g. 20% fill), which represents the actual grease volume compared to the free internal space inside the bearing. Bearing manufacturers often use special equipment to apply oil but do not specify how much to add to the bearing.
Dripping oil lubrication
In short, drip lubrication is when the oil in the oil cup or manifold above the bearing flows into the bearing using its own gravity within a set time. Suitable for bearings in systems with low loads and low to medium speeds, usually in small quantities. Sometimes this method of lubrication is done by hand, but this actually creates the risk of over- or under-lubrication. Drip lubrication systems deliver the right amount of oil at the right intervals.
Splash lubrication
Splash lubrication is when moving parts that are regularly dipped in lubricating oil splash oil onto the bearings. This is the first option for bearing lubrication when rotation speed is not sufficient to stir up the oil. A common type of splash lubrication is the oil ring system. This approach reduces the operating temperature of the bearing and is ideal for applications operating at higher speeds and temperatures. Its only disadvantage is that it is only suitable for horizontal applications due to the dynamic nature of the oil ring.
Forced feed system
When operating equipment under heavy loads and at high speeds, you need to protect the equipment from high temperatures caused by friction by supplying high flow rates of oil. In a forced lubrication system, an oil pump pressurizes the oil and directs it to rotating parts. Examples of systems that use this method include boiler feed pumps, compressors, gearboxes, and turbine generators.
Grease lubrication
Since grease is a semi-solid lubricant, it is typically used when the lubricant needs to be kept in one place or adhere to a part and is an ideal bearing lubrication option since it requires less maintenance.Grease can also be used when parts are inaccessible during operation or cannot be lubricated regularly. Because grease is very viscous and does not leak as easily as oil, it cannot continuously take away heat from the bearing.
Correct application procedure
It’s no secret that lubrication affects bearing life. In fact, machinists generally agree that at least 80 percent of bearing failures are related to lubrication and contamination issues. Proper lubrication can solve common bearing problems such as corrosion, wear and overheating. So how to confirm whether the bearing lubrication is correct? You need to choose the right lubricant for your bearings and stick to a lubrication schedule that meets the requirements of your machinery’s operation. Let’s first look at the causes of bearing failure due to lubrication.
Loss of Lubricant – If bearings are not lubricated at appropriate intervals and with the right amount of grease, it can cause lubricant and lubrication failure and result in equipment damage.
Incorrect grease – make sure you use the correct grease.
Overlubrication – This failure usually only occurs in open bearings when too much grease causes the bearing temperature to rise.
Grease Degradation – The chemical breakdown of grease and the hardening of the grease due to excessive temperatures.
Grease incompatibility – Use the same grease (or a compatible alternative) throughout the bearing’s service life. Not all greases are compatible with each other.
The most important aspects of selecting the correct lubricant include bearing cleaning, lubricant fill quality and bearing break-in.
Step 1: Clean
First, you need to remove any existing oil, anti-corrosion coatings and grease from the bearings. This part is crucial as longevity and reliability become even important and helps eliminate any potential incompatibilities. Be sure to use residue-free solvents when cleaning bearing surfaces to provide optimal lubrication conditions.
Step 2: Ensure proper fill volume
Proper filling ensures a suitable lubricating film on all bearing element contact surfaces. This step is critical because, as we’ve discussed, both over- and under-lubrication can harm bearing life. Over-lubrication increases internal friction, which results in additional heat production, while under-lubrication causes wear or insufficient lubrication because the contact surfaces are not adequately lubricated. The correct amount of lubricant can be determined based on operating speed, design, tank volume, and the degree of sealing or shielding in the application.
Step 3: Determine the free movement space of the bearing
The correct filling amount of a grease lubricated bearing is usually specified as a percentage of the bearing’s free movement space, so it is important to correctly determine the bearing movement space.
The equation that determines the free motion hall space of the bearing:
Step 4: Break-in Procedure
Proper running-in procedures are critical to bearing and lubricant performance in applications where high speeds, fill volume and specific preload are critical.
Drain excess grease
Adjust the direction of the lubricating film on each contact surface
Creates a grease ring that delivers oil to the contact area
Establish low equilibrium operating temperature
Achieve lifelong sealed lubrication conditions
If a break-in procedure is not performed, overlubrication and/or excessive operating temperatures can occur. Now that we’ve covered the best practices for bearing lubrication, let’s identify three lubrication mistakes you may be making that can ruin your bearings.
3 Mistakes That Can Damage Bearings
Lubrication errors can have far-reaching consequences. Common side effects of improper lubrication include overheating or excessive wear, which can lead to bearing failure. This can result in unplanned downtime and lost revenue for your facility. Aubearing summarizes three common lubrication mistakes below and how to avoid (or fix) them so that you can be confident in the health of your bearings.
Mistake 1: Over and Under Lubrication
Adding too much or too little grease is one of the most common mistakes in our industry. As we’ve already discussed, excess grease builds up and ultimately leads to increased friction and pressure, which leads to excess heat. Too little grease can have the same life-shortening effect on bearings. How do you determine if you are adding the correct amount of grease? Start by using ultrasound to monitor the friction level of the bearings as you apply new grease, one injection at a time (slowly of course). When grease starts flowing into the bearing, you need to listen to the bearing and try to measure the drop in friction. When the decibel level is close to minimum and stable, note that adding single shots, you can stop if the decibel level starts to increase slightly.
Mistake 2: Lubricating by schedule rather than by condition
While lubricating your bearings weekly or daily may seem like a practical task, it won’t actually make your bearings run longer. One of the reasons bearings need grease is to prevent and reduce friction. If the lubricant is working well, there is no need to continue changing or adding lubricant. Instead of relubricating bearings on a schedule, you can use ultrasound to monitor, measure and trend friction levels so you know exactly when it’s the right time to grease.
Mistake 3: Using an Ultrasonic Instrument in “Listen-Only” Mode
In short, listening to bearings using an ultrasonic device that provides no measurement feedback sounds like a good idea in theory, but it will only hurt you in the long run. Auditory feedback alone won’t work because it’s too subjective to draw any real conclusions since no two people hear the same thing. It’s also too difficult to remember the sound of a bearing just from a few months ago. A simple solution is to use an ultrasound with digital decibel metering. You can use a device that provides multiple status indicators—if you have one.
There are clear benefits to optimizing bearing lubrication and avoiding these three mistakes. Doing so will extend the life of the bearing, reduce grease consumption, and reduce the time spent relubricating when unnecessary.