Guide: How to choose the right lubricant for your bearing
Guide: How to choose the right lubricant for your bearing
5 min.
For a bearing to work correctly, it must be properly lubricated. In practice, "properly" means that the bearing must be relubricated at the right time, with the right lubricant, and in the correct quantity. If we succeed in doing this, we significantly extend the bearing's service life and prevent wear and failure. In this article, you will learn how, when, and with what to properly lubricate bearings to prevent unnecessary failures, downtimes, and ultimately time and financial losses.
Most common reasons why a bearing fails
The most common reasons for bearing failure include:
incorrectly selected type of lubricant,
insufficient quantity of lubricant,
excessive quantity of lubricant,
incorrect relubrication intervals,
lubricant not reaching the bearing due to poor bearing housing design.
Both an excessive amount of lubricant and a lack of it are harmful to the bearing. In both cases, the bearing's fate will be the same: premature bearing failure, resulting in the shutdown of the entire equipment. This ultimately leads to operational downtime and both time and financial losses. According to SKF statistics, up to 40% of bearings are shut down due to insufficient or incorrect lubrication.
3 main types of lubricants
So, what should you use to lubricate bearings? In the vast majority of cases, three basic types of lubricants are used:
Greases account for about 80% of all applications in rolling bearings. It can be said that grease is more versatile, suits the vast majority of operations, and is usually more cost-effective. Oil is used at very high speeds and in places where heat dissipation is necessary. In very special cases and at very high temperatures, solid lubricants are used, but they are limited by low speeds. Therefore, it is absolutely crucial to choose a lubricant that corresponds to the given application and operating needs. You can see the main differences between greases and oils in the table below.
Grease vs. oil
Grease lubrication
Oil lubrication
easy mounting and sealing method
can be complex - requires careful installation and maintenance
suitable for lower temperatures
high temperatures or extremely low temperatures
low speeds (approx. 85% of the speed limit achieved by oil)
high speeds
protection against ingress of contaminants, moisture and water
sealing against oil leakage
long-term operation and easy maintenance
bearings are lubricated from a central source
poor bearing cooling
heat dissipation due to oil circulation
impossible to remove particles from the lubricant
possibility of particle removal thanks to oil filtration
Composition of greases
Greases consist of three main components. The ratio of individual components in the grease depends on the specific type and application.
Base oil 70-90%
The main lubricating component of greases is the base oil. Mineral and synthetic oils are commonly used as a base. It is the type of oil used that determines the main properties of the lubricant.
Thickeners 5 - 30%
The consistency of the grease (primarily its density) is mainly determined by the type and amount of thickener the grease contains. The thickener also largely affects the final water resistance of the grease. Metal soaps, bentonite, or polyureas are most commonly used for thickening.
Additives 1-5%
The last, least represented component in the composition of greases is additives. These are added to the lubricant to improve certain properties of the grease – e.g., corrosion resistance, extending service life, capability for higher loads, etc.
Let's look at the 5 main principles that will help you choose the right grease, but also with determining the relubrication interval and quantity or the method for relubricating bearings.
1. Generally, do not mix different greases together
We do not recommend mixing different types of greases too much. This is because the structure of the individual lubricants can be disrupted. This can subsequently lead to grease leaking out of the bearing or loss of the properties that the original lubricant had. Ultimately, you increase the probability of a bearing failure. Exceptions are greases made on the basis of the same thickener. Thus, two greases made on the basis of metal soap can usually be mixed without undesirable effects.
2. Choose grease stiffness and softness based on the following factors
Operating position
Stiffer greases are more suitable for vertical mounting
Centrifugal load
Stiffer greases are more suitable
Sealing
Stiffer greases seal better
Pumpability
Softer greases are easier to push and pump
Friction
Softer greases are better for low friction
Noise
Softer greases reduce bearing noise
Speed (RPM)
Softer greases are suitable for higher speeds
The amount of lubricant depends on the design of the bearing housing, the amount of free space, the properties of the grease used, and the operating temperature. A large amount of grease in the housing leads to an increase in the operating temperature. Typically, the bearing itself is filled with grease, and the free space in the bearing housing is only partially filled. The amount of grease in the free space of the housing can be determined depending on the speed:
for very low speeds (up to 30% of the bearing's limiting speed), the space in the housing is filled by 2/3 or more,
for medium speeds (up to 50% of the bearing's limiting speed), 1/2 to 2/3 of the space is filled,
for higher speeds (above 50% of the bearing's limiting speed), 1/3 to 1/2 of the space is filled.
For bearings with a very low rotational frequency, it is advisable to fill both the bearing and the entire free space of the bearing housing with grease to protect the bearing from corrosion and contamination.
4. Relubrication interval is often key
Small bearings, especially &single-row ball bearings&, have relubrication intervals significantly longer than the service life of the bearing itself, so they are usually not relubricated. For most bearings, however, the lubrication interval is influenced by the type and size of the bearing, operating temperature, type of rotation, and the type of lubricant used.
Influence of temperature on relubrication intervals:
at higher temperatures, the intervals must be halved for every 15 °C increase,
at temperatures below 40 °C, the relubrication interval can be doubled,
shortening of the intervals is necessary under demanding conditions – vibration, shocks, ingress of dust and moisture.
Calculation of the appropriate grease quantity for relubrication
Quantity in g = D x B x 0.005
D = bearing outer diameter in mm B = bearing width in mm
The shorter the relubrication interval, the more the quantity can be reduced, using a coefficient of 0.003 (for monthly) and 0.002 (for weekly intervals).
5. You can relubricate manually or automatically
As a helper, you can use, for example, the battery-driven grease gun SKF TLGB 20. It is suitable for lubricating bearings in machinery in industrial and manufacturing plants. It can also be advantageously used for lubricating bearings in agricultural and construction machinery. Thanks to the LED flashlight, the TLGB 20 is also suitable for spaces with poor lighting conditions. With the help of the built-in flow meter, lubrication is a breeze, and you will never over-grease or under-grease a bearing again.
Automatically
With automatic relubrication, for example, the SKF SYSTEM 24 automatic lubricator can help. It is designed for relubricating a single lubrication point. Lubrication points can therefore be easily lubricated with grease from the reservoir of the automatic lubricator. SKF SYSTEM 24 lubricators can be set to deliver the correct quantity of lubricant throughout the entire lubrication cycle at pre-determined time intervals. Compared to conventional relubrication methods, they allow for more precise control of lubricant dosing. SKF SYSTEM 24 is optimal for relubricating points that are difficult to access with a grease gun, or for relubricating a large number of lubrication points where manual lubrication is less efficient.
Examples of use for SKF SYSTEM 24 automatic lubricators:
For a bearing to work correctly, it must be properly lubricated. In practice, "properly" means that the bearing must be relubricated at the right time, with the right lubricant, and in the correct quantity. If we succeed in doing this, we significantly extend the bearing's service life and prevent wear and failure. In this article, you will learn how, when, and with what to properly lubricate bearings to prevent unnecessary failures, downtimes, and ultimately time and financial losses.
Designation of rolling bearings might seem like a science, but it's not that bad. Even though at first glance it might not be completely clear. The designation consists of a combination of numbers and letters.
