Preloading is a critical aspect of ensuring the performance, reliability, and longevity of rolling bearings. It plays a vital role in enhancing rigidity, reducing operational noise, and preventing premature wear under dynamic loads. This article author Maxwell explores the key preload methods for rolling bearings, providing insights into their mechanisms, applications, and benefits. Whether you’re optimizing machinery for precision or durability, understanding preload techniques will empower you to achieve superior bearing performance in various industrial settings.

The preload methods of rolling bearings are divided into two categories: radial preload method and axial preload method.

The radial preload method is also called radial top tightening method. It is mostly used in tapered bore bearings that bear radial loads. A typical example is a double-row precision short cylindrical roller bearing. The axial position of this bearing relative to the tapered journal is adjusted by a nut so that the inner ring has a suitable expansion amount and a radial negative clearance is obtained. This method is mostly used in machine tool spindles and jet engines.

The axial preload method is also called the axial preload method. It can be roughly divided into two types: positioning preload and constant pressure preload.

In positioning preload, the appropriate preload amount can be obtained by adjusting the size of the bushing or gasket; the appropriate preload amount can also be adjusted by measuring or controlling the starting friction torque; the paired duplex bearings with pre-adjusted preload amount can also be used directly to achieve the purpose of preload. At this time, the user generally does not need to adjust again. In short, the relative position of any bearing that has been axially preloaded will definitely not change during use.

Constant pressure preload is a method of using spiral springs, disc springs, etc. to obtain appropriate preload of the bearing. The rigidity of the preload spring is generally much smaller than that of the bearing, so the relative position of the bearing with constant pressure preload will change during use, but the preload amount remains roughly unchanged.

Comparison between fixed-pressure preload and fixed-pressure preload

(1) When the preload amount is equal, fixed-pressure preload has a greater effect on the increase of bearing rigidity, and the influence of rigidity change on bearing load during fixed-pressure preload is also much smaller.

(2) During the use of fixed-pressure preload, the preload amount will change due to the axial length difference caused by the temperature difference between the shaft and the bearing seat, the radial expansion caused by the temperature difference between the inner and outer rings, and the displacement caused by the load; while during the use of fixed-pressure preload, the change of preload can be ignored.

Torque, load and life of rolling bearings

• Starting torque The torque required to start the rotation of a bearing ring or washer relative to another fixed ring or washer.

• Rotational torque The torque required to prevent the movement of one bearing ring or washer when the other ring or washer rotates.

• Radial load The load acting in the direction perpendicular to the centerline of the bearing.

• Axial load is the load acting in the direction parallel to the centerline of the bearing.

• Static load is the load acting on the bearing when the relative rotational speed of the bearing rings or washers is zero or when the rolling element does not move in the rolling direction.

• Dynamic load is the load acting on the bearing when the bearing rings or washers rotate relative to each other or when the rolling element moves in the rolling direction.

• Equivalent load is a general term used to calculate the theoretical load. In certain cases, the bearing is subjected to the actual load under this theoretical load.

• Radial basic static load rating is the radial static load corresponding to the total permanent deformation of the rolling element and raceway. If the roller and raceway are or are assumed to be normal generatrix under zero load, the total permanent deformation produced at the contact between the rolling element and the raceway under high contact stress is 0.0001 times the diameter of the rolling element. For single-row angular contact bearings, the radial load rating is the radial component of the load that causes the bearing rings to be purely radially displaced relative to each other.

• Radial basic dynamic load rating The constant radial load under which a rolling bearing can theoretically withstand a basic rated life of one million revolutions. For single-row angular contact bearings, the radial load rating is the component of the load that causes a pure radial displacement of the bearing rings relative to one another.

• Life The number of revolutions of one ring or washer relative to another before fatigue growth first occurs in the material of one ring or washer or rolling element of a bearing. Life can also be expressed in hours of operation at a given constant speed.

• Reliability The percentage of a group of nearly identical rolling bearings operating under the same conditions that is expected to achieve or exceed a specified life. The reliability of a set of bearings is the probability that the bearings achieve or exceed the specified life.

• Rated life The predicted value of the life based on the radial basic dynamic load rating or the axial basic dynamic load rating.

• The rated life associated with a 90% reliability.

• Life factor The correction factor applied to the equivalent dynamic load in order to obtain the basic radial dynamic load rating or the basic axial dynamic load rating corresponding to a given rated life.

• Bearing with seat A component that combines a radial bearing and a seat, with a base plate for mounting screws on the support surface parallel to the axis of the bearing.

• Seat for vertical seated rolling bearings.

• Flange seat A seat with a radial flange and screw holes for mounting on the support surface perpendicular to the axis of the bearing.

• Adapter sleeve An axially open sleeve with a cylindrical inner hole, whose outer surface is conical and has external threads at the small end. Used to mount bearings with tapered holes on shafts with cylindrical outer surfaces.

• Withdrawal sleeve A sleeve with an axially open sleeve with a cylindrical inner hole, whose outer surface is conical and has external threads at the large end. Used to install or remove bearings with tapered holes on shafts with cylindrical outer surfaces.

• Locking nut A nut with a cylindrical outer surface and an axial groove, the nut is locked with an outer claw of a locking washer and a ring wrench. Used for axial positioning of rolling bearings.

• Locking washer A thin steel washer with many outer claws. One outer claw is used to lock the nut, and one inner claw is inserted into the axial groove of the adapter sleeve or shaft.

• Eccentric sleeve A steel ring with a groove at one end that is eccentric to the inner hole and is installed on the extended end of the inner ring of the spherical bearing with equal eccentricity. The eccentric sleeve is rotated relative to the inner ring to tighten the inner ring, and then the top screw is tightened to tighten it on the shaft.

• Concentric sleeve A steel ring installed on the wide inner ring of the spherical bearing, with a top screw screwed into the hole on the inner ring and in contact with the shaft.

conclusion

Properly preloading rolling bearings is key to balancing torque, load distribution, and service life. A well-applied preload minimizes excess torque, ensuring smooth operation and energy efficiency. It also improves load sharing among bearing elements, reducing stress concentrations that could lead to premature wear. Ultimately, this careful balance extends the bearing’s lifespan, offering reliable performance even under demanding conditions. By understanding and implementing the right preload, you can optimize the efficiency and durability of rolling bearings in practical applications.