Simply put, bearing clearance is the clearance (or interference) inside a single bearing or a system composed of several bearings. The clearance can be divided into axial clearance and radial clearance, which depends on the bearing type and measurement method.
Why should the bearing clearance be adjusted?
For example, too much or too little water when cooking rice will 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.
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.
The non-adjustable clearance bearing means that the clearance of the bearing is determined after the bearing leaves the factory. The well-known deep groove ball bearings, self-aligning bearings, and cylindrical bearings all belong to this category.
Adjustable clearance bearing means that the relative axial position of the bearing raceway can be moved to obtain the required clearance. In this category, there are tapered bearings, angular contact ball bearings and some thrust bearings.
Bearing clearance adjustment classification
For non-adjustable bearing clearance, 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 the bearing seat are known, the corresponding inner and outer ring fits are determined, and the clearance after installation cannot be changed. Since the fit amount is a range in the design stage, the final clearance also has a range, which is not suitable for applications that require clearance accuracy.
Adjustable bearings solve this problem well. By changing the relative axial position of the raceway, we can get a definite clearance value. As shown in the figure below, when moving the position of the inner ring, we can roughly get the positive and negative clearances.
Factors affecting bearing clearance
The selection of the best working clearance is determined by the application conditions (load, speed, design parameters) and expected working 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 clearance value after installation based on the analysis and experience of the application.
Adjusting method of tapered roller bearing clearance
After installation, the clearance of non-adjustable bearings is mainly affected by the fit, so the following mainly introduces the clearance adjustment method of adjustable bearings, 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 .
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. After pushing it to the end, set the dial indicator to zero as a reference, and then apply a force in the opposite direction. After pushing it to the end, the rotation amount of the pointer on the dial indicator is the clearance value. Slowly oscillate the rotating roller during measurement 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, the deformation of an elastically deformed object is proportional to the external force it receives. Under a certain installation force, measure the gasket or spacer gap to obtain the correct clearance. Directly read out the correct gasket or spacer size required according to a chart created during the pre-test.
This method is suitable for positive clearance and preload, and the operator needs to be trained to create the chart.
The principle of Torque-Set is that under preload, the increase in the rotational torque of the bearing is a function of the bearing preload. The experimental results show that a set of new bearings of the same model, under the condition of a given preload, the amount of change in the rotational torque of the bearing is very small. Therefore, the torque can be used to estimate the amount of preload.
The principle of this method is to establish a conversion relationship between the rotating torque of the bearing and the preload, which needs to be obtained through testing. Then when it is actually installed, the thickness of the gasket can be determined by measuring the rotating torque.
Projecta-Set is to project or transform the thickness of gaskets or spacers that cannot be directly measured to a place that is easy to measure. This effect can be achieved by using a special gauge sleeve and spacer. When the inner ring and outer ring of the bearing are in tight fit conditions, the removal and adjustment of the bearing will be difficult and time-consuming. At this time, Projecta-Set has its advantages.
This method requires separate gauges for different series of bearings, which is relatively costly. But when installing 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.
Set-Right uses probabilistic methods and controls the dimensional tolerances of related parts to ensure that 99.73% of the bearing clearances in all assembly assemblies fall within an acceptable range. This is a mathematical prediction after a combination of a set of random variables. The variables are 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 mass installation is very convenient. But in the end, a clearance range (approximately 0.25mm) will be obtained, and 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.