Difficulties In Selecting Gearbox Bearings

Bearings are one of the key components of gearboxes, and they are also frequently damaged parts in gearboxes. At the same time, compared with the general shaft system, the selection, application, installation and maintenance of the bearings in the gearbox are very complicated. So what are the difficulties in the application of gearbox bearings, and what should we pay attention to?

Complex force

When the gearbox is running, the torque and speed are transmitted between the input shaft and the output shaft. The torque transmission is carried out in the gear meshing process, and the meshing force will be generated during the gear meshing process.

If the gears are spur gears, then this meshing force is a pure circumferential force. Seen from the angle perpendicular to this circumferential force, it is a radial load of the entire shaft system.

If the gear is a helical gear or a bevel gear, etc., then the gear meshing force has an axial component in addition to a circumferential component. The force of this axial component is the axial force of the shaft system.

On some gear shafts, sometimes several gears mesh with gears on other shafts, so there will be multiple axial and radial loads. The direction of these loads in space is related to the meshing. When calculating the bearing force, it is necessary to decompose and synthesize, and finally calculate the actual load on the bearing.

In addition, when the gearbox is running, if the external torque fluctuates, the meshing force of the gears will fluctuate, which will cause fluctuations in the bearing force. All of these make the force analysis of gearbox bearings more complicated. Equivalent treatment is required for actual life calibration.

Complex speed

Different shafts in the gearbox have different speeds, from low speed shafts to high speed shafts, the bearing speed on each shaft is the same, and the speed on different shafts is different. This makes the conversion time different when calculating the bearing life check.

Different speed requirements lead to differences in the selection of bearing speed capabilities when selecting bearings.

For example, for high-speed shafts, bearings with better speed capability will be selected, such as deep groove ball bearings, angular contact ball bearings, tapered roller bearings and other types.

For low-speed shafts, bearings with large load capacity will be used, such as roller bearings, even full roller bearings, tapered bearings, etc.

complex lubrication

It is not difficult to know from the basic principle of lubrication that the temperature, speed and load of the bearing have a direct impact on the lubrication of the bearing.

In terms of speed, the speed of the high-speed shaft and the low-speed shaft of the gearbox bearing are different, and the size of the bearing is also different. Therefore, it is sometimes very difficult to select a lubricant to meet a wide range of speed requirements. In the actual choice, we are often in a dilemma and have to make some compromises.

In terms of load, the heavy load of the low-speed shaft and the light load of the high-speed shaft are a pair of contradictions. If the viscosity is low, it is difficult for low-speed shaft bearings to form an oil film; if the viscosity is high, high-speed shafts will affect heat generation.

In terms of temperature, the lubrication inside the gearbox itself has a certain heat dissipation function, and the meshing of the gears is the source of heat generation in the gearbox. The temperature difference between the different shafts of the gearbox is related to the meshing, and the bearing temperature is related to the heat transfer distance between the gear and the gear. The influence of different temperatures on the viscosity of lubrication exists objectively, but due to the flow of lubricating oil, the heat is transferred along with the flow.

Shaft system design complex

There are often load-related axial forces in gearbox bearing systems, so bearings with axial load capacity are selected for pairing. At the same time, the axial position accuracy of the entire shaft system not only affects the bearings, but also affects the meshing accuracy of the gears, which is crucial to the gearbox design. In the design of the shaft system, it is necessary that the shaft not only has a certain rigidity and positioning accuracy in the radial direction, but also in the axial direction. Therefore, the design of the gearbox bearing system often uses a cross-locating structure.

In the cross positioning structure of the gearbox bearing system, not only the positioning, but also the positioning accuracy are considered. When the gear is subjected to an axial force, the movement of the entire shaft in the axial direction should be within an acceptable range.

At the same time, when the gearbox is working, the whole system will generate heat, and the degree of heat and heat dissipation of different parts is different, resulting in the temperature of the housing, shaft, gear, and bearing chamber being different from that in the cold state. This will affect the residual clearance inside the bearing when the structure is cross-located. Therefore, when designing the gearbox shaft system, it is necessary to consider the influence of different temperatures on the bearing clearance in the shaft system.

In fact, when the gearbox is running, the influence of the load and the influence of the temperature occur at the same time, so the corresponding check calculation should be carried out comprehensively.