How To Choose Bearing Clearance, Which Is More Conducive To The Guarantee Of Motor Performance?

How to choose bearing clearance, which is more conducive to the guarantee of motor performance?

 

The choice of bearing clearance and configuration is an extremely important part of motor design, and the choice of scheme without knowing the performance of the bearing is likely to be a failed design. Different operating conditions, the requirements for bearings will be different, Ms believes that it is necessary to study and discuss the knowledge of bearings.

       The relationship between the various clearances of deep groove ball bearings

 

Bearing operating clearance calculation according to equation (1), the selection of bearing working clearance, theoretically, in the smooth operation of the bearing, when there is a slight negative operating clearance, the bearing service life is the longest.

 

△=△O-Δfi-dfO-dt+dw ..................... In equation (1):

△0 - original clearance

Δfi - the amount of clearance reduction caused by the fit of the inner ring with the shaft

df0 - the amount of clearance reduction caused by the mating of the outer ring with the housing

dt - the amount of clearance reduction caused by the temperature difference between the inner and outer rings

dw - the amount of clearance increase caused by the load

 

               About bearing grease

 

The purpose of bearing lubrication is to separate the rolling elements and the rolling surface with a thin oil film, and form a uniform lubricating oil film on the rolling surface during operation, thereby reducing the internal friction of the bearing and the wear of each component, and preventing sintering. Good lubrication is a necessary condition for bearing work, and analyzing the causes of bearing damage shows that about 40% of bearing damage is related to poor lubrication. The lubrication method is divided into grease lubrication and oil lubrication.

Grease lubrication has the advantage that it does not need to be replenished for a long time after filling the grease once, and the sealing structure is simple, so it is widely used. Lubricating grease is a semi-solid lubricant mixed with a solid thickener with strong lipophilic properties based on lubricating oil, and various additives are added to improve certain characteristics.

 

Oil lubrication, often circulating oil lubrication, jet lubrication, oil mist lubrication. The lubricating oil of the bearing is generally refined mineral oil with good oxidation stability and rust resistance, and high oil film strength, but various synthetic oils are also often used.

Picture of bearing grease high temperature failure characteristics

 

Motor bearing arrangement

 

The bearing arrangement of the rotating part of the motor (such as the spindle) usually needs to be supported by two sets of bearings, and the rotating part is positioned radially and axially relative to the fixed part of the machine (such as the bearing housing). Depending on the application conditions, such as load, required rotational accuracy and cost requirements, bearing arrangements can be made in the following ways:

 

1 Bearing arrangement of fixed and floating ends

2 pre-adjusted bearing arrangements (fixed at both ends)

3 "floating" fine load configuration (floating at both ends)

 

Bearing arrangement of fixed and floating ends

 

The fixed end bearing is radially supported at one end of the shaft and positioned axially in both directions at the same time. Fixed-end bearings must therefore be fixed to both the shaft and housing. Bearings suitable for use at fixed ends are radial bearings capable of withstanding combined loads, such as deep groove ball loads, double row or paired single row angular contact ball bearings, self-aligning ball bearings, spherical and roller bearings or matched tapered roller bearings.

Radial bearings that can only withstand pure radial loads, such as solid column roller bearings in which one ring is not flanged, can also be used at fixed ends when used in combination with another type of bearing (e.g. deep groove ball bearings, four-point contact ball bearings or bidirectional thrust bearings). In this arrangement, the other bearing is only used for axial positioning in both directions, and a certain degree of radial freedom must be left in the housing (i.e. clearance with the housing should be reserved).

 

The floating end bearing is only radially supported at the other end of the shaft, and the shaft must be allowed to have a certain axial displacement so that no mutual forces are generated between the bearings, for example, when the bearing expands due to heat, axial displacement can be achieved inside some types of bearings. Axial displacement can occur between one of the rings and the components to which it is connected, preferably between the outer ring and the housing bore.

There are many different combinations of bearing arrangements for fixed and floating ends, and here are some of the more commonly used combinations. For rigid bearing arrangements, a combination of axial displacement within the bearing should be used, e.g.

 

(1) Deep groove ball shaft/cylindrical roller bearings

(2) Double row angular contact ball bearings / cylindrical roller bearings

(3) Paired single row tapered roller shaft/cylindrical roller bearings

(4) NUP type cylindrical roller bearings / NU type cylindrical roller bearings

(5) NU type cylindrical roller bearings + four-point contact ball bearings / NU type cylindrical roller bearings

 

The above combination must minimize the angular error between the shaft and the housing. If the application does not allow, it is recommended to use a combination of self-aligning bearings that can withstand greater angular errors, such as:

(1) Self-aligning ball bearings / CARB toroidal roller bearings

(2) Spherical roller shaft/CARB toroidal roller bearing

These configurations can withstand certain angular errors and axial displacements and avoid internal axial forces in the bearing.