The thrust roller is an important auxiliary part of a rotary kiln. It is designed to control the axial vibration of the rotary kiln, so that the tyres and rollers can be evenly distributed on the full width. At the same time, it can ensure the linearity of the kiln body centerline, make the big and small gears mesh well, and reduce the power consumption of the kiln process.
Because traditional thrust rollers have defects in the structure, they can not meet today’s needs of large-scale cement rotary kiln and are gradually replaced by hydraulic thrust rollers.
The Mechanic Analysis of Thrust Roller
The weight of the kiln body cylinder is loaded on the thrust roller through the tyre, and generates radical force and axial force acting on the thrust roller. The radial force makes the cylinder slide downward, and the role of the thrust roller is to overcome the sliding force and push the cylinder to move upward.
Assuming that the total mass of the rotary kiln cylinder is G, the inclination of the rotary kiln is sin β, and the angle of the thrust roller is α, the radial thrust Fx and axial thrust Fy of the hydraulic roller are:
Through the analysis, we can calculate the force on the thrust roller and select the most proper type of roller structure according to the calculation results.
The Structure of Hydraulic Thrust Roller
The hydraulic thrust rollers used in rotary kilns nowadays all have a similar structure.
As the blueprint shows, the thrust roller is loaded on a support with two guide sleeves, through which the two guide shafts are connected to the support. On the support, there is also installed the hydraulic cylinder, and the thrust roller can move back and forth on the guide shafts through the control of the hydraulic oil.
Though hydraulic thrust rollers share the same working principle, there are still some differences in their structures. The thrust rollers are mainly divided into tri-bearing structure rollers and bi-bearing structure rollers.
In the tri-bearing structure, the main components include the thrust roller, the axle, the hollow shaft, and the bearing.
The thrust roller and the axle are fixed and assembled and loaded in the hollow shaft to make it a thrust roller body. Three bearings are installed between the hollow shaft and the axle. Two of the three bearings are self-aligning roller bearings, which mainly bear the radial force of the thrust roller. A thrust bearing is also installed at the bottom of the hollow shaft, which mainly bears the axial force of the retaining wheel. The three sets of bearings bear the resultant force of all external forces so that the thrust roller is in a stable state.
In the bi-bearing structure, the main components include the thrust roller, the hollow shaft, and the bearings. Different from the tri-bearing structure, there is no axle in the bi-bearing structure.
The thrust roller and the axle are an integral structure, which is connected with the hollow shaft through bearings to form a thrust roller main body. Two bearings are installed in the hollow shaft, one of which is a self-aligning roller bearing, which mainly bears the radial force of the thrust roller; a thrust bearing is also installed at the bottom of the hollow shaft, which mainly bears the axial force.
Comparison between Tri-Bearing Structure & Bi-Bearing Structure
The two structures share the same working principle, but each has its own features.
In the tri-bearing structure, the radial force of the thrust is borne by two self-aligning roller bearings. In the distribution of force, theoretically, two bearings can share the total thrust, but in the actual production process, it is difficult to accurately calculate which bearing bears how much force. So in the design process, to be on the safe side, the upper bearing needs to bear all the thrust to ensure the normal operation of the thrust roller; and the middle bearing can be selected within the design scope according to the structure requirements. This ensures that in extreme cases, the thrust roller can still operate normally.
In the bi-bearing structure, only one bearing bears the radial force, so we only need to consider its bearing capacity. In order to ensure the rationality of the design, the bi-bearing structure is simpler than the tri-bearing system. In the tri-bearing structure, the movement of the bearing adopts the form of tightening inside and loosening outside, which makes the thrust roller rotate through the locking force of the inner ring of the bearing, while the structure of the bi-bearing is opposite, because the thrust roller and the axle are integrated, when the thrust roller rotates, the bearing tightens outside and loosens inside.
Due to the large variation of rotary kiln operating conditions, the total thrust F under these two structures may be affected by many external factors and change greatly, such as the materials in the kiln, the impact of the spherical mixture on local parts, the change of the force of single thrust roller after the change of the centerline, the friction force between the thrust roller and the contact surface of the supporting ring, etc. The direction and size of the resultant force F change constantly when the rotary kiln cylinder rotates. Due to the insufficient self-locking force of the self-aligning roller bearing in the axial direction, the vertical shaft of the thrust roller moves up and down and sways.
Although, theoretically, there is only downward pressure in the axle of thrust roller, in actual production, when the axle moves up or sways, the lower thrust bearing bears not only axial force, but also certain radial force, which may cause the bearing roller to disengage from the track, destroy the bearing force balance, and result in excessive local force and damage. Therefore, at the beginning of the design, some extreme considerations must be taken into account to ensure the stable performance of the thrust roller.
As the core device in a cement plant, the rotary kiln must be able to work safely and stably even after a long time of continuous operation. And in mechanical designs, a simple structure is usually more stable than a complex one.
As to the two widely used thrust roller structures, the tri-bearing structure is small in size, but has more parts and a rather complex design; the bi-bearing structure is large in size, but has a simpler design and is more stable in operation.