Choosing the right bearing

Like plain bearings, rolling-element bearings reduce friction in a rotating guide between two parts. Depending on the direction of the force, axial or radial, there are different types of bearings. Bearings are composed of rolling elements, usually balls, cylindrical or tapered rollers or needles.

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What is a bearing?

TIMKEN bearings
TIMKEN bearings

A bearing is a mechanical component whose function is to guide a rotating assembly. The bearing therefore allows the rotation of one element relative to another.
Bearings are therefore high-precision parts that allow equipment to move at different speed levels by efficiently transporting notable loads. They must offer high precision and durability, as well as the possibility of working at high speeds with minimal noise and vibration.

This type of device is found in all applications such as, for example, the automotive industry, the aerospace sector, construction equipment, machine tools, etc.

There are several types of bearings, the four main types are: ball bearings, cylindrical roller bearings, tapered roller bearings and needle bearings.

While ball bearings are the most common mechanisms, each device has its advantages and disadvantages. That is why we recommend that you compare each of these bearings to determine which is the ideal system for your requirements.

Types of bearings

  • ball bearings
  • cylindrical roller bearings
  • tapered roller bearings
  • needle bearings

How to choose a bearing?

When choosing a bearing, you must consider several important factors. The first factor to consider is the load that the bearing can support.
There are two types of loads:
– axial load: parallel to the axis of rotation
– radial load: perpendicular to the axis

Each type of bearing is specifically designed to support an axial or radial load. Some bearings can support both loads: in this case we refer to a combined load. If you have to support a combined load, for example, we recommend that you choose a tapered roller bearing. If you need a bearing capable of withstanding a high radial load, then we recommend a cylindrical roller bearing. On the other hand, if your bearing needs to support lighter loads a ball bearing might be sufficient as they are often less expensive.

Rotation speed is another element that should be considered. Some bearings can withstand high speeds. The presence of a cage for cylindrical roller bearings and needle bearings allows for greater speeds than bearings without a cage. However, the choice of a higher speed is sometimes made at the expense of the load. You should also consider the possible presence of misalignment; some bearings are not suitable for this situation, such as double row ball bearings.

You will therefore need to pay attention to the bearing construction: insert and spherical bearings make it easy to support these misalignments. We suggest that you lean towards bearings with automatic alignment which automatically correct alignment defects caused by shaft bending or installation errors. Similarly, operating conditions are very important when choosing the ideal bearing. It is thus necessary to analyze the environment you will be using the bearing in. Your bearing may be subject to a number of contaminations. Some uses can lead to noise disturbance, impact and/or vibration.

Your bearing must therefore withstand this impact on the one hand and not be an inconvenience on the other. Another essential element to consider is the bearing life. Several factors, such as speed or repeated use, can impact the bearing life.

Choosing a sealing system is essential in order to ensure the correct and long-lasting operation of a bearing; it is therefore important to ensure that the bearing is always well-protected from any impurities and external agents, such as dust, water, corrosive fluids or even used lubricants. This choice depends on the type of lubricant, the ambient conditions (and therefore the type of contamination), the fluid pressure and the rotation speed. To give you a good place to start, the fluid pressure is a determining factor in the choice of sealing system. If the pressure is high (in the range of 2-3 bars for example) a mechanical seal is ideal. Otherwise, the choice will be directly linked to the type of lubricant, grease or oil. For example, for grease lubrication the most commonly used solutions are: deflectors or washers, narrow passages that are machined or with grooves; in the case of oil lubrication the sealing system is often accompanied by the presence of a groove for oil recovery.

Conditions of use can also influence your choice, especially when it comes to assembling the bearing. The rigidity and precision that your use requires must also be taken into account. In some cases, you can provide for the application of a preload in the assembly of your bearing to increase its rigidity. Additionally, the preload will have a positive impact on the bearing life and noise level of your system.

Be careful, in order to choose the preload (radial or axial) you must know the rigidity of all parts through software or experimentation. In your selection criteria, you must also consider the ideal material for your bearing. Bearings can be made out of metal, plastic or ceramic. The bearing material depends on its intended use. We recommend that you choose the most compression-resistant bearing. Keep in mind however that the material used affects the price of the bearing.

Selection criteria for choosing a bearing

  • load
  • sealing system
  • rotation speed
  • bearing life
  • rigidity
  • precision
  • vibration and impact

Why choose a ball bearing?

A NTN SNR ball bearing
A NTN SNR ball bearing

Ball bearings are often optimized for radial contact: this means that they will be very useful if the force you want to bring into contact with the bearing is perpendicular to its axis of rotation. However, double row ball bearings are optimized for angular contact. Additionally, if you want to use a ball bearing for an axial load, it should be noted that this device only supports moderate axial loads. However, this flaw can be avoided by using a double-row ball bearing construction. As well as being low-cost, ball bearings are also the most compact, making them the most widely used type of bearing.

We strongly recommend the use of ball bearings for small bearings that operate at high speeds.

Why choose a cylindrical roller bearing?

A NTN SNR cylindrical roller bearing
A NTN SNR cylindrical roller bearing

Cylindrical roller bearings are able to support significant radial loads, or even very significant ones. There are several types of cylindrical roller bearings. They vary according to the number of rows of rollers (one, two or four) and whether or not there is a cage. A cage allows the bearing to support significant radial loads and a high speed. The absence of a cage allows the bearing to have more rows of rollers and therefore to support even heavier radial loads. The only disadvantage is that cylindrical roller bearings without cages do not tolerate as high speeds as the ones with cages.

Cylindrical roller bearings are also very robust and have a long bearing life. Additionally, some bearings can also support an axial load, provided that they do not have a shoulder on the inner and outer rings. If this is the case, you will not be able to use your bearing to support an axial load.

Why choose a tapered roller bearing?

A TIMKEN tapered roller bearing
A TIMKEN tapered roller bearing

Tapered roller bearings can support radial, axial and combined loads (both at the same time). The loads can be very heavy, due to their high rigidity. If you are unsure between a ball bearing and a tapered roller bearing, be aware that a tapered roller bearing of equivalent dimensions can support heavier loads.

We therefore recommend this type of bearing for uses such as machine spindles, vehicle transmissions (from car to boat, helicopter), or the steering of car or truck wheels.

Why choose a needle bearing?

A SCHAEFFLER needle bearing
A SCHAEFFLER needle bearing

Like cylindrical roller bearings, needle bearings can come with or without a cage. If they have a cage then they can withstand a very high speed, while a needle bearing without a cage can support a very significant radial or axial load. Also note that these bearings are small in size, and as such have a small footprint. They are regularly used in gearboxes, for example.

What type of bearing assembly should I choose?

Each type of bearing can be deep-groove, spherical or insert. Deep-groove bearings are the most common. Insert bearings are deep-groove bearings which, like spherical bearings, effectively support misalignment.

They differ from spherical bearings in that they have a smaller swivel angle. The larger the swivel angle, the more the bearing will withstand misalignment.

Another factor to consider: depending on the use, you will need a rotating shaft (for example wheel drive bearings for cars) or a rotating hub (like for washing machine bearings).

Then you will need to pay attention to how the bearings are assembled. If two bearings are too close together, you should use the O-assembly method so that the bearings do not interfere with each other. If this is not the case then the X-assembly method is perfectly suitable.

Finally, each bearing has specific dimensions. These dimensions, expressed in mm, are subject to international standards. We therefore advise you to pay attention to the bore of the bearing, i.e. the inner diameter (always represented by the letter d), the outer diameter (represented by the letter D) and the width (represented by the letter B).

Bearing configurations and dimensions

  • deep-groove bearing
  • spherical bearing
  • insert bearing
  • X-assembly
  • O-assembly
  • inside diameter (mm)
  • outside diameter D (mm)
  • width B (mm)

How should I lubricate the bearing?

Example of a grease-lubricated bearing
Example of a grease-lubricated bearing

Choice of lubricant is an essential factor in ensuring the proper functioning and long service life of bearings. Lubricant does the following:

  • facilitates movement
  • avoids wear and premature fatigue
  • reduces internal friction
  • ensures that the bearings are sealed against external bodies
  • reduces the operating noise
  • protects the bearing against corrosion

You have 2 choices: grease or oil. Generally speaking, grease lubrication is definitely the simplest and most efficient choice because it provides all the functions mentioned above and also allows for operation with low maintenance. However, if your bearing is integrated in an oil-lubricated machine, we recommend that you use the same lubricant system. In any case, the choice of lubricant will depend on three factors: the load, the type of operation (continuous or intermittent), and the speed of rotation.

If the bearing is subject to high loads, the operation is continuous and the rotational speed high, then oil lubricant is the ideal solution to ensure the proper functioning of your rolling system. On the other hand, if the loads are low and the speed limited then grease will be more than sufficient.