When the gearbox is running, torque and rotational 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 gear is a spur gear, then this meshing force is a pure circumferential force. This circumferential force is a radial load of the entire shaft system when viewed from a perpendicular angle.
If the gear is a helical gear or a bevel gear, etc., 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, there are sometimes several gears meshing with gears on other shafts, so there will be multiple axial and radial loads. The direction of these loads in space is related to 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 gear meshing force will fluctuate, which will cause the bearing force to fluctuate. All of these make the force analysis of gearbox bearings more complicated. Equivalent processing needs to be performed when the actual life check is performed.
Different shafts in the gearbox have different rotational speeds, from low-speed shafts to high-speed shafts, the bearings on each shaft rotate at the same speed, and the rotational speeds on different shafts are different. This makes the conversion time different when calculating the bearing life check.
Different speed requirements lead to differences in the choice of bearing speed capability during bearing selection.
For example, for high-speed shafts, bearings with better rotational speed capabilities may be selected, such as deep groove ball bearings, angular contact ball bearings, and tapered roller bearings.
For low-speed shafts, bearings with large carrying capacity, such as roller bearings, even full-length roller bearings, tapered bearings, etc., will be selected.
Lubrication is complicated
From the basic principle of lubrication, it is not difficult to know that the temperature, speed and load of the bearing have a direct impact on the lubrication of the bearing.
In terms of rotational speed, the rotational speed of the high-speed shaft and the low-speed shaft of the gearbox bearing is different, and the bearing size is also different. As a result, it can sometimes be difficult to select a lubricant that meets a wide range of rotational speed requirements. In practical choices, there is often a dilemma and some compromises have to be made.
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. When the viscosity is low, it is difficult for the low-speed shaft bearing to form an oil film, and when the viscosity is high, the high-speed shaft will affect the 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 the heat generation of the gearbox. The temperature difference of the different shafts of the gearbox is related to the meshing, and the bearing temperature is related to the heat transfer distance from 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.
Complex shaft system design
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 bearing, but also affects the meshing accuracy of the gear, which is very important for the design of the gearbox. In the design of the shaft system, 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 gearbox bearing systems often uses a cross-located structure.
In the cross positioning structure of the gearbox bearing system, not only the positioning, but also the positioning accuracy is considered. When the gear is subjected to axial force, the movement of the entire shaft in the axial direction should be within an acceptable range.
At the same time, when the gear box is working, the whole system will heat up, and the degree of heat generation and heat dissipation of different parts is different, resulting in different temperatures of the housing, shaft, gear, and bearing chamber from the cold state. This will affect the residual clearance inside the bearing in the cross-located configuration. 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 load and the influence of temperature occur at the same time, so it is necessary to comprehensively carry out the corresponding check calculation.
The result of the actual check calculation in the above problem is the axial, radial tolerance and accuracy of the bearing chamber and shaft.
Customers can ask DKB’s sales and technology for details during the specific inquiry process, thank you.