How To Choose Bearing
There are many different types of bearings available today, but little information is available about the differences between them. Perhaps you have asked yourself “Which bearing is best for your application?” “Or” How do I choose bearings? This article will help you answer these questions.
First, you need to know that most bearings with rolling bodies fall into two main categories:
Ball bearing
Roller bearing
Within these groups, there are subcategories of bearings with unique features or optimized designs to improve performance.
In this article, we’ll look at four things you need to know in your application to choose the right type of bearing.
Find bearing load and load capacity
Bearing load is usually defined as the reaction force exerted on the bearing by the component when it is in use.
Axial load animation of GMN Bearing USA
When choosing the right bearing for your application, the bearing capacity should first be determined. Load capacity is the load that a bearing can bear, and it is one of the most important factors in the selection of bearings.
Bearing loads can be axial (thrust), radial or combined.
Axial (or thrust) bearing load is force parallel to the axis of the axis.
Radial bearing load is when the force is perpendicular to the axis.
Then, the combined bearing loads are parallel and vertical forces generated relative to the shaft when the angular force is exerted.
How do ball bearings distribute load
Here is a quick reference for bearing load types and the best ball bearings:
Axial (thrust) (parallel to shaft) loads: Select thrust ball bearings
Combined radial and axial loads: angular contact bearings are selected. Ball bearings contact the raceway at angles that better support the combined load.
Roller bearings and bearing loads
Roller bearings use cylindrical roller design, which can distribute the load over a larger surface area than ball bearings. They tend to be better suited for heavy load applications.
Here is a quick reference for bearing load types and the best roller bearings:
Radial (perpendicular to axis) loads: Choose standard cylindrical roller bearings
Axial (thrust) (parallel to shaft) loads: Select cylindrical thrust bearings
Combined radial and axial loads: Choose tapered roller bearings
Speed
The speed of the application is the next factor to consider when selecting bearings.
If your application will operate at high RPM, then ball bearings are usually preferred. They perform better at higher speeds and provide a higher speed range than roller bearings.
One reason is that the contact between the rolling element and the raceway in ball bearings is a point rather than a contact line, as in roller bearings. Because rolling elements press into raceways when rolling on the surface, much less surface deformation occurs during point loading of ball bearings.
Centrifugal force and bearings
Another reason why ball bearings are more suitable for high-speed applications is centrifugal force. Centrifugal force is defined as the force that pushes outward on an object moving around a center and is generated by the inertia of the object.
Centrifugal force is the main limiting factor of bearing speed, because it will be converted into radial and axial loads on the bearing. Because the mass of roller bearings is greater than that of ball bearings, roller bearings will produce higher centrifugal forces than ball bearings of the same size.
Use ceramic ball material to reduce centrifugal force
Sometimes the speed of the application is higher than the rated speed of the ball bearing.
If this happens, a simple and common solution is to change the ball bearing material from steel to ceramic. This keeps the bearing size the same, but increases the speed rating by approximately 25%. Because ceramic materials are lighter than steel, the centrifugal force generated by ceramic balls at any given speed is smaller.
Angular contact bearings are best for high speed applications
Angular contact bearings are the best bearing choice for high speed applications. One reason is that the ball is smaller, and smaller balls weigh less and create less centrifugal force when they spin. Angular contact bearings also have a built-in preload on the bearing, which works with centrifugal force to correctly roll the ball in the bearing.
If you are designing for high speed applications, then you will need high precision bearings, usually within the ABEC 7 accuracy class.
Compared with high-precision bearings, bearings with lower precision have larger size “wiggle space” when manufactured. Therefore, when the bearing is used at high speed, the ball rolling quickly in the bearing raceway reduces the reliability, which leads to bearing failure.
High precision bearings are manufactured in accordance with strict standards, and the deviation from the specifications is very small. High precision bearings are reliable for fast running applications because they ensure good ball and raceway interaction.
Bearing runout and rigidity
Bearing runout is the amount by which the shaft rotates from its geometric center. Some applications, such as tool spindles, allow only small deviations in their rotating parts.
If you are designing such an application, then choose high precision bearings, because bearings are manufactured with very small tolerances and it will produce even smaller system run-out.
Bearing stiffness is resistance to forces that cause the shaft to deviate from its axis and plays a key role in minimizing shaft runout. Bearing rigidity comes from the interaction between the rolling body and the raceway. The more the rolling body is pressed into the raceway, the more elastic deformation is produced and the higher the rigidity is.
Bearing stiffness is usually classified as follows:
The axial stiffness
The radial stiffness
The higher the bearing stiffness, the greater the force required to move the shaft when in use.
Let’s see how it works with precision angular contact bearings. These bearings usually have a manufacturing offset between the inner and outer rings. When the angular contact bearing is installed, the offset is eliminated, which causes the ball to be pressed into the raceway without any external applied force. This is called preloading, and the process can improve bearing stiffness even before the bearing is subjected to any applied force.
Bearing lubrication
Understanding your bearing lubrication requirements is important for selecting the right bearings and needs to be considered early in application design. Improper lubrication is one of the most common causes of bearing failure.
Lubrication forms an oil film between the rolling element and the bearing raceway, which helps prevent friction and overheating.
The most common type of lubrication is grease, which consists of oil with a thickening agent. The thickener keeps the oil in place, so it does not leave the bearing. When the ball (ball bearing) or roller (roller bearing) rolls on the grease, the thickener will separate, leaving only an oil film between the rolling body and the bearing raceway. After the rolling body passes, the oil and thickener recombine.
For high speed applications, it is important to know the speed at which oil and thickener can be separated and readded. This is called the application or bearing n*dm value.
Before choosing grease, you need to find your application n dm value. To do this, your application RPM is multiplied by the diameter (dm) of the ball center in the bearing. Compare your n dm value with the maximum speed value of the grease on the datasheet.
If your n*dm value is higher than the grease maximum speed value on the datasheet, the grease will not provide adequate lubrication and premature failure will occur.
Another lubrication option for high-speed applications is the oil mist system, which mixes oil with compressed air and then injects it into the bearing raceway at metering intervals. This option is more costly than grease lubrication because it requires an external mixing and metering system and filtered compressed air. However, oil mist systems allow bearings to operate at higher speeds while producing lower heat than lubricated bearings.
For low speed applications, oil baths are common. An oil bath is when a part of the bearing is immersed in oil. For bearings that will operate in extreme environments, dry lubricants can be used instead of petroleum-based lubricants, but due to the nature of lubricant film decomposition over time, the service life of bearings is usually shortened. There are several other factors to consider when choosing a lubricant for your application, see our in-depth article “Everything You Need to Know about bearing Lubrication.”