Simply put, bearing clearance is the clearance (or interference) inside a single bearing or a system composed of several bearings. Clearance can be divided into axial clearance and radial clearance, depending on the bearing type and measurement method.
Why should bearing clearance be adjusted?
For example, too much or too little water when cooking rice will affect the taste of the 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.
Bearing types suitable for different adjustment methods
The method of clearance adjustment is determined by the bearing type, which can generally be divided into non-adjustable clearance bearings and adjustable bearings.
Bearings with non-adjustable clearance mean that the clearance of the bearing is determined after the bearing leaves the factory. The well-known deep groove ball bearings, aligning bearings and cylindrical bearings all fall into this category.
Adjustable clearance bearings can move the relative axial position of the bearing raceway to obtain the required clearance. These include tapered bearings, angular contact ball bearings and some thrust 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 dimensions of the shaft and bearing seat are known, the corresponding fit of the inner and outer rings is determined, and the clearance cannot be changed after installation. Since the matching amount is within a range in the design stage, the final clearance also has a range, which is not applicable in applications that require clearance accuracy.
Adjustable bearings solve this problem very well. By changing the relative axial position of the raceway, we can get a certain clearance value. As shown in the figure below, when moving the position of the inner ring, we can roughly get two kinds of clearance, positive and negative.
Factors affecting bearing clearance
The selection 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, we cannot directly adjust the working clearance, 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 after installation of non-adjustable bearings is mainly affected by the fit, so the following mainly introduces the clearance adjustment method of adjustable bearings, taking as an example a tapered roller bearing that is suitable for a wide speed range and can withstand both axial force and radial force. .
1 Push and 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 in one direction to the shaft or bearing seat. After pushing it to the bottom, set the dial indicator to zero as a reference, and then apply a force in the opposite direction. After pushing it to the bottom, the amount of rotation of the pointer on the dial indicator is the clearance value. When measuring, it is necessary to slowly vibrate the rotating roller to ensure that the roller is correctly positioned on the large rib of the inner ring.
2 Acro-SetTM method
The theoretical basis of Acro-Set is Hooke's law, which states that the deformation amount of an elastically deformed object is proportional to the external force it experiences. Under a certain installation force, measure the gasket or spacer gap to obtain the correct clearance. Read the correct gasket or spacer size required directly from a chart created during prior testing.
This method works for both positive clearance and preload, and operators need to be trained to create the charts.
3 Torque-SetTM method
The principle of Torque-Set is that under preload, the rotational torque of the bearing increases as a function of the bearing preload. The experimental results show that for a set of new bearings of the same model, under the condition of a given preload force, the rotational torque of the bearings changes very little. Therefore, the rotational torque can be used to estimate the preload amount.
The principle of this method is to establish a conversion relationship between the rotational torque of the bearing and the preload amount, which needs to be obtained through testing. Then during actual installation, the thickness of the gasket can be determined by measuring the rotational torque.
4 Projecta-SetTM method
Projecta-Set is to project or transform the thickness of gaskets or spacers that cannot be measured directly into a place that can be easily measured. This is achieved using a special gauge sleeve and spacer. When the inner and outer rings of the bearing are in tight-fitting conditions, the removal and adjustment of the bearing will be difficult and time-consuming. At this time, Projecta-Set shows its advantages.
This method requires separate gauges for different series of bearings, which is relatively costly. But when installed in large quantities, the average cost per time is very cost-effective. Especially in the field of automation, it has proven to be a very effective method.
5 Set-RightTM method
Set-Right uses probabilistic methods and controls the dimensional tolerances of related parts to ensure that 99.73% of bearing clearances in all assemblies fall within acceptable limits. This is a mathematical prediction based on a set of random variables, namely bearing tolerances and tolerances of mounting components such as shafts and bearing seats.
This method does not require installation and adjustment, and the application components can be simply assembled and clamped, so it is very convenient to install in large quantities. However, you will end up with a clearance range (about 0.25mm). Whether Set-Right can be used in some applications needs to be decided at the design stage. For many years, the Set-Right method has been successfully used in both industrial and automotive fields.
