Bearings are an important part of rotating equipment. The final load, whether it is an axial load or a radial load, is ultimately grounded with the help of the bearing. In the case of a circuit, the bearing can be considered similar to a ground connection. Bearings can be classified according to various standards, such as design, load conditions, layout, lubrication type, etc. The correct selection of bearings is important, but not as important as correct installation. Following the correct assembly procedure when installing the bearing is essential to achieve the expected life of the bearing.

It is important to understand the dual bearing configuration during the selection and assembly process. Incorrect installation and ignorance of the content of the application may cause problems when operating the device, such as slipping or overheating.

This article aims to introduce the basic concepts and what actually happens when the bearing is preloaded.

Double bearing is a set of two bearings. When arranged on the shaft and the inner ring and outer ring are clamped together by pre-tightening force, greater radial and axial rigidity can be obtained. Single row angular contact ball bearings and tapered roller bearings are usually preloaded axially by installing them back to back (the load line diverges) or face to face (the load line converges) on a second bearing of the same type and size).

The bearing is a paired bearing, and the end face of the bearing is precisely machined and grounded to provide pre-tightening force during installation. As mentioned above, these different types require different surface contact of the bearing to generate preload. These arrangements are usually combined with angular contact ball bearings and tapered roller bearings. Depending on the type of arrangement, preloading is achieved by closing the gap between the outer ring surface or the inner ring surface.

The description of the arrangement includes:

It is important to understand why this arrangement provides more rigidity and stability. Observe carefully, what actually happens is that the outer ring is completely restricted in the radial and axial directions. So now, when the inner rings of the two bearings are in contact, the internal clearance of the bearings is closed. This is called preloading. When preloaded, the inner rings of the bearings are pushed towards each other.

The inner ring obviously has a rolling element with a cage on it. Pushing the inner ring will in turn push these rolling elements in the direction of the force. Now the rolling elements are restricted by the outer ring. Therefore, the outer ring will have a reaction force to the applied load, which reaction force will be towards the shaft centerline. Now, according to the contact angle, the reactions from the two bearings will diverge, and the distance between the two reactions is L.

Since this constant reaction force from the external stationary body (ground) acts on the shaft, and L is greater than the width of the bearing, the shaft will be tightly fixed at the position of the bearing position length L. This tightness is achieved through higher stiffness and shaft stability. This is why the back-to-back arrangement has better resistance to moments in the shaft.

Face-to-face arrangement: also known as X arrangement. It is well known that this arrangement can tolerate misalignment and cannot support moment loads as effectively as the back-to-back arrangement. If misalignment between bearing positions cannot be avoided, a face-to-face bearing arrangement is recommended (see Figure 3).

In this arrangement, compared with the back-to-back arrangement, it can be seen that the reaction load lines converge inside the reduced distance L. However, it is worth noting here that the inner ring is in contact with each other on the end faces, and the gap initially appears on the outer ring. Therefore, pretension is achieved by closing the gap between the outer rings. In the same way as the back-to-back arrangement, one can imagine that while pushing the outer ring closer, it will move the rolling elements and cage together.

Since the movement of the inner ring is completely restricted, it will produce a reaction force from the inside out, that is, the inner ring will resist the applied force and produce a reaction force. This is because the load line will be closer to reducing the length L. The reaction force from the shaft (inner ring) reduces the stiffness of the arrangement, and then it can tolerate some misalignment.

After understanding these arrangements, the advantages that are usually listed in the manufacturer's catalog will become clear.

The main benefits of preloading include but are not limited to:

Abhijeet Keer is a design engineer who has worked in the field of centrifugal pumps for more than six years. Relying on his advantages in mechanical structure and materials, he has been engaged in design work in companies such as KSB and Kirloskar Brothers. He received a bachelor's degree in mechanical engineering from the University of Mumbai, India. His professional experience covers new product design and development, material selection and application engineering, and complete mechanical structures. You can contact him at keer.abhijeet98@gmail.com.