What is bearing clearance?
In simple terms, bearing clearance is the clearance (or interference) within a single bearing, or within a system consisting of several bearings. Clearance can be divided into axial clearance and radial clearance, depending on the bearing type and measurement method.
Why adjust bearing clearance?
For example, too much or too little water when cooking rice can affect the taste of rice. In the same way, if the bearing clearance is too large or too small, the working life of the bearing and even the stability of the entire equipment operation will be reduced.
Types of bearings that can be adjusted differently
The method of clearance adjustment is determined by the type of bearing, which can generally be divided into non-adjustable bearings and adjustable bearings.
The clearance non-adjustable bearing refers to the bearing after leaving the factory, the clearance of the bearing is determined, and the deep groove ball bearing, spherical bearing, and cylindrical bearing that we are familiar with belong to this category.
Adjustable clearance bearings refer to the relative axial position of the bearing raceway to obtain the required clearance, which belongs to this category of tapered bearings and angular contact ball bearings and some thrust bearings.
▲Tapered roller bearings ▲Angular contact bearings
Bearing clearance adjustment classification
For the clearance of non-adjustable bearings, the industry has corresponding standard values (CN, C3, C4, etc.), and specific clearance ranges can also be customized. When the size of the shaft and bearing seat is known, the corresponding inner and outer ring matching amount is determined, and the clearance after installation cannot be changed. Since the amount of mating is a range in the design stage, there is also a range in the final clearance, which is not applicable in applications where clearance accuracy is required.
The adjustable bearing solves this problem very well, and by changing the relative axial position of the raceway, we can obtain a definite clearance value. As shown in the figure below, when moving the position of the inner ring, we can roughly get two kinds of clearances: positive and negative.
Factors that affect bearing clearance
The choice of the optimal operating clearance is determined by the application conditions (load, speed, design parameters) and the desired operating conditions (maximum life, best stiffness, low heat generation, ease of maintenance, etc.). However, in most applications, it is not possible to adjust the operating clearance directly, which requires us to calculate the corresponding post-installation clearance value based on the analysis and experience of the application.
How to adjust the clearance of tapered roller bearings
The clearance of the non-adjustable bearing after installation is mainly affected by the fit, so the following mainly introduces the clearance adjustment method of the adjustable bearing, taking the tapered roller bearing that is suitable for a wide range of speeds and can bear both axial and radial forces as an example.
1 Push-pull method
The push-pull method is generally used for positive clearance, and the axial clearance between the bearing raceway and the rolling elements can be measured. Apply a force to the shaft or bearing seat in one direction, set the dial gauge to zero for reference after pushing to the bottom, and then apply a force in the opposite direction, and the rotation of the pointer on the dial gauge after pushing to the end is the clearance value. During the measurement, the roller should be slowly vibrated to ensure that the roller is correctly positioned on the large flange of the inner ring.
2 Acre-SetTM法
The theoretical basis of ACRO-SET is Hooke's law, which states that the deformation of an object that undergoes elastic deformation is proportional to the external force exerted on it. Under a certain installation force, the spacer or spacer gap is measured to obtain the correct clearance. Follow a diagram created during a pre-test to read out the correct gasket or spacer size directly.
This method is suitable for both clearance and preload, and operators need to be trained to create charts.
3 Torque-SetTM法
The principle of Torque-Set is that the increase in the rotational torque of the bearing under preload is a function of the bearing preload. The experimental results show that the rotational torque of a new group of bearings of the same type changes very little under the condition of given preload. Therefore, the amount of preload can be estimated using the rotational torque.
The principle of this method is to establish a conversion relationship between the rotational torque and the preload of the bearing, which needs to be obtained through testing. Then, when it comes to actual installation, the thickness of the gasket can be determined by measuring the rotational torque.
4 Projecta-SetTM法
Projecta-Set is the projection or conversion of gasket or spacer thickness that cannot be directly measured into a place where it can be easily measured. This can be achieved using a special gauge sleeve and spacer. Projecta-Set is an advantage when the inner and outer rings of a bearing are tightly fitted, and the removal and adjustment of the bearing can be difficult and time-consuming.
This method requires separate gauges for different series of bearings, which is relatively costly. But when it comes to high-volume installations, the average cost per session is very cost-effective. Especially in the field of automation, it has proven to be an effective approach.
5 Set-RightTM法
Set-Right uses a probabilistic approach and controls the dimensional tolerances of the relevant parts to ensure that 99.73% of the bearing clearance in all assemblies falls within acceptable limits. This is a mathematical prediction of a combination of random variables, such as bearing tolerances and mounting components such as shafts and housings.
This method does not require installation adjustments, and the application components can be easily assembled and clamped, so it is very convenient to install in large quantities. However, in the end, a clearance range (about 0.25mm) is obtained, and the use of Set-Right in some applications needs to be decided during the design phase. For many years, Set-Right's approach has been used successfully in both the industrial and automotive sectors.