The rotary kiln body moves up and down in a regular manner to extend the service life of components such as wheels, supporting wheels, and large and small gears. The current effective technical measure for the movement of the kiln body is to use advanced hydraulic stop wheel devices. The hydraulic blocking wheel device is a laborious blocking wheel, and the upward movement of the large and heavy kiln body relies entirely on the push of the hydraulic blocking wheel. If the manufacturing, installation, debugging, and maintenance do not meet the requirements, the hydraulic gear wheel device under high stress will often experience some failures or even accidents, causing the rotary kiln to have to reduce production and speed or stop the kiln for treatment, resulting in a decrease in the operation rate of the rotary kiln and a loss of efficiency for cement enterprises. Common faults and accidents of hydraulic gear shifting devices include overheating and damage of the upper bearing, breakage of the main shaft, pulling up of the gear shifting wheel, rapid wear of the gear shifting wheel, oil leakage or internal leakage of the hydraulic cylinder, and excessive pressure or fluctuation of the hydraulic system. In order to reduce and avoid the occurrence of these problems, and minimize the losses caused by cement enterprises, this article briefly introduces the structure, composition, and working principle of advanced hydraulic gear wheel devices currently in use, analyzes common faults or accidents, and proposes corresponding solutions.
1.Composition of hydraulic gear wheel device
The hydraulic stop wheel device of the rotary kiln is composed of the mechanical part body, the stop wheel control system, and the stop wheel hydraulic system.
1.1 Mechanical part
Most rotary kilns are equipped with a set of hydraulic stop wheels, while a few large rotary kilns are equipped with two sets of hydraulic stop wheels. This is mainly because the downward force of the kiln increases, in order to further improve the reliability of the stop wheel operation.
The mechanical part of the hydraulic gear shifting device mainly includes: gear shifting wheel, gear shifting wheel bearing seat, bearing, gear shifting wheel spindle, guide rod support, bearing seat sliding guide rod, and hydraulic cylinder. Its common structure is shown in Figure 1.
Figure 1 Common mechanical structures of hydraulic gear wheels
1. Wheel stop; 2. Gear wheel bearing seat; 3. Upper bearing; 4. Gear wheel spindle; 5. Guide rod support; 6. Sliding guide rod of bearing seat; 7. Hydraulic cylinder
The blocking wheel is a component that directly contacts the wheel belt and pushes the kiln body up and down. It forms a composite motion of rolling and slight sliding with the wheel belt, and is a vulnerable part. The surface is lubricated by graphite blocks. The gear wheel bearing seat is equipped with gear wheel bearings, which are fixed on the sliding guide rods on both sides and can slide back and forth on the sliding guide rods. Spherical self-aligning roller bearings and thrust roller bearings are generally used for gear wheel bearings, which are installed at the upper, middle, and lower positions of the gear wheel spindle, respectively. There are also two bearing structures. Spherical self-aligning roller bearings bear the radial downward force of the gear wheel, while the bottom thrust roller bearings bear the axial force of the gear wheel. The bearings have two lubrication methods: dry oil and thin oil. Most of the bearing failures on site are related to stress and lubrication. The gear wheel spindle is a component that connects the gear wheel with bearings, gear wheel bearing seats, etc. It bears alternating bending moment loads, and the occurrence of breakage and bending during operation accounts for a certain proportion of accidents. The support of the gear guide rod is connected to the gear base with bolts, and its bottom surface is parallel to the centerline of the kiln, carrying all the loads of the gear. The sliding guide rod of the bearing seat carries the main body of the gear wheel, ensuring that the gear wheel slides on the required axis. It is fixed on the guide rod support in front and behind, and contacts the gear wheel bearing seat through a shaft sleeve. There are lubrication holes on the shaft sleeve that need to be regularly lubricated. Horseshoes are usually placed at both ends as mechanical limiters for the up and down movement of the kiln body; The hydraulic cylinder is a power component that drives the kiln body to move up and down, and its Z-pressure should generally be controlled at around 8MPa. The internal sealing ring is a vulnerable part, and sometimes problems such as internal leakage and oil leakage may occur.
1.2 Electrical Control Section
The electrical control system mainly consists of PLC control cabinet, on-site limit switch control device, and oil station monitoring system. The following mainly introduces the function and adjustment of the on-site limit switch control device.
The limit switch control box of the hydraulic gear wheel device is installed on the side of the gear wheel bearing seat. According to the position of the gear wheel moving up and down, a signal can be sent to the PLC control cabinet to control the hydraulic system of the gear wheel. Figure 2 shows a commonly used limit switch control device on site, consisting of 6 sensing blocks and corresponding 6 limit switches. The two limit switches and sensing blocks in the middle are used to control the normal up and down movement of the kiln, and the initial set stroke on site is generally ± 10mm. The two limit switches on the left are used to control the upper and lower limit alarms of the kiln, and the initial set stroke on site is generally ± 15mm. When this stroke is reached, an alarm signal should be sent to the central control. The two limit switches on the right are used to control the upper and lower limit positions of the kiln. The initial set stroke on site is generally ± 30mm, and it needs to be interlocked with the kiln host.
Figure 2 Limit switch control device for hydraulic gear wheel
1. Limit switch control box; 2. Induction block; 3. Limit switch
It is allowed to make appropriate adjustments to the stroke of the hydraulic gear wheel according to the actual situation on site, in order to ensure that each gear support wheel, wheel belt, large gear ring, and small gear are in contact and not separated throughout the entire length. At the same time, it should be noted that after changing the stroke, there should be no interference in the gear cover, kiln head seal, and kiln tail seal.
1.3 Hydraulic section
Figure 3 Working principle diagram of the gear wheel hydraulic system
1. Oil drain nut; 2. Temperature gauge; 3. Fuel tank; 4. Electric heater; 5. Exhaust valve; 6. Filter; 7. Electromagnetic directional valve; 8. Throttle valve; 9. Globe valve; 10. Piston pump; 11. One way valve; 12. Filter; 13. Safety valve; 14. Ball valve; 15. Pressure gauge; 16. Energy storage device; 17. Hydraulic cylinder; 18. Approaching the switch; 19. Close the proximity switch; 20. Wheel guard body
The hydraulic part of the hydraulic blocking wheel device is used to provide pressure oil to the hydraulic cylinder, forcing the kiln body to move upward and controlled downward. As shown in Figure 3, there is a flow control knob at the tail of plunger pump 10, which is used to adjust the output flow of the plunger pump and thus regulate the upward movement speed of the kiln. The upward movement speed of the kiln can generally be controlled at around 5mm/h. If the upward and downward movement speed of the kiln is too fast, it may cause scratches and rapid wear on the surface of the supporting wheel, wheel belt, large gear ring, and small gear. Safety valve 13 is used to regulate the Z-high overflow pressure of the system and prevent overload operation. The overflow pressure of the system can be set at around 10MPa. An accumulator 16 is installed at the inlet of the hydraulic cylinder to stabilize system pressure, filter peak values, and extend component life. During on-site installation, nitrogen gas needs to be filled into the accumulator, and the pressure of nitrogen gas in the accumulator should be about 70% of the working pressure of the gear wheel device.
Electromagnetic directional valve 7 is used to control when the oil in the hydraulic cylinder flows back to the oil tank. It is in a normally closed state. When the kiln reaches its upper limit, the electromagnetic valve opens the circuit from the hydraulic cylinder to the oil tank, and at the same time, the plunger pump stops working, and the kiln begins to move downwards. On the return oil pipeline, filter 12 is installed, mainly used to filter impurities and foreign objects in the oil circuit. When the filter experiences a pressure difference of 0.5MPa, the system will automatically alarm, indicating that the filter core is clogged and needs to be cleaned. Throttle valve 8 is used to adjust the hydraulic cylinder return oil flow rate and control the kiln down speed. When rotating clockwise, the kiln down speed slows down, and when rotating counterclockwise, the kiln down speed increases. The kiln down speed can generally be controlled at around 3mm/h.
2.Working principle of hydraulic gear wheel
1. Oil drain nut; 2. Temperature gauge; 3. Fuel tank; 4. Electric heater; 5. Exhaust valve; 6. Filter; 7. Electromagnetic directional valve; 8. Throttle valve; 9. Globe valve; 10. Piston pump; 11. One way valve; 12. Filter; 13. Safety valve; 14. Ball valve; 15. Pressure gauge; 16. Energy storage device; 17. Hydraulic cylinder; 18. Approaching the switch; 19. Close the proximity switch; 20. Wheel guard body
After the rotary kiln is running stably and the kiln speed is greater than 2.5r/min, the hydraulic station of the stop wheel can be opened, and some stop irons can be removed according to the stroke of the stop wheel. As shown in Figure 3, after the stop wheel hydraulic station is opened, the plunger pump motor starts to run, and the plunger pump is interlocked with the solenoid valve. The solenoid valve is in a normally closed state. At this time, the plunger pump absorbs oil from the oil tank 3 through the filter 6, enters the hydraulic cylinder 17 through the one-way valve 11 and the shut-off valve 9, and under the action of pressure oil, the hydraulic cylinder piston pushes the stop wheel to force the kiln body to move upward. When the kiln body reaches the upper limit position, the upper limit sensing block of the gear wheel touches the upper limit switch, and the solenoid valve is powered on to become connected, opening the circuit from the hydraulic cylinder to the oil tank. At the same time, the plunger pump motor stops supplying oil. At this time, the kiln body slowly moves downwards under its own downward force, and the hydraulic oil in the hydraulic cylinder flows back to the oil tank through the throttle valve and solenoid valve.
When the kiln body moves down to the lower limit of the stop wheel, the lower limit sensing block touches the lower limit switch, and the plunger pump is powered on again. At the same time, the solenoid of the electromagnetic reversing valve is powered off, and the valve body moves under the action of the spring, closing the passage from the hydraulic cylinder to the oil tank. The stop wheel then pushes the kiln body up again, and so on.
3. Common on-site faults, accidents, and cause analysis
The common faults and accidents that occur on site with hydraulic gears include: bearing damage, gear spindle fracture, gear wheel pulling up, gear wheel and belt contact surface not being round or worn too quickly, hydraulic cylinder oil leakage or internal leakage, high system pressure and large fluctuations, and other abnormal phenomena.
3.1 Bearing damage
Damage to the gear wheel bearing is a common form of damage to gear wheel Z, and the main reasons for bearing damage are:
(1) Poor lubrication. During use, if the lubricating oil is lost and cannot be replenished in a timely manner, especially when lubricated with thin oil, it is easy to cause oil leakage. In addition, the working environment temperature of the bearing is too high, the viscosity of the lubricating oil decreases, and the oil film is damaged, which can also cause poor lubrication.
(2) Excessive load. Due to the bending deformation of the kiln body and changes in the centerline, the bearings can bear excessive radial and axial forces. When the centerline of the supporting wheel is not parallel to the kiln centerline and additional downward axial force is generated on the kiln body, it can also cause excessive bearing load and high pressure.
(3) The nitrogen filling pressure of the energy storage device is too high or too low, causing peak impact force on the bearing load.
(4) Quality issues with the bearings themselves.
3.2 Gear wheel axle broken
Sometimes the main shaft of the gear wheel may break on site, as shown in Figure 4. It usually occurs at the root of the contact area between the gear wheel and the shaft. Possible reasons include:
Figure 4 The gear wheel axle is broken
(1) If the bearing capacity is too large, in addition to the downward force caused by inclined placement, the kiln also experiences an additional axial force due to the non parallel orientation of the supporting wheel axis with the kiln centerline. The magnitude of this force is related to the angle between the supporting wheel and the kiln centerline, and these two forces combine to form the downward force of the kiln. When the gear wheel is subjected to a large downward force for a long time, the gear wheel shaft also experiences an excessive alternating bending stress. Once this stress reaches the fatigue limit of the shaft, it will cause the shaft to fail.
(2) Under alternating impact loads, due to bending of the kiln centerline or local deformation of the simplified structure, the axial deflection of the wheel belt occurs, and the pressure of the stop wheel shows periodic and violent fluctuations, sometimes reaching N5MPa or more. If such severe pressure fluctuations occur frequently, it will cause the main shaft to bear excessive alternating bending loads, especially when the hydraulic system is turned off and the stop wheel is limited by a horseshoe. When used as a dead stop wheel, the hydraulic system can no longer provide cushioning and shock absorption, and the impact load of the wheel belt on the stop wheel becomes a rigid impact, which is more likely to cause bending fatigue fracture of the shaft.
(3) Due to inherent reasons such as casting defects or cracks on the surface and inside of the shaft, improper heat treatment resulting in residual stress inside, improper operation during welding repair of the shaft, and improper material selection, it will also lead to damage of the shaft Z due to mechanical performance not meeting actual requirements.
3.3 Pulling up the gear wheel
The blocking wheel and the wheel belt generally make a conical inclined contact. Under normal working conditions, the positive pressure perpendicular to the contact surface will generate a vertical downward force, and the wheel belt will exert a downward pressure on the blocking wheel device. During installation, it is required that the wheel stop device be biased towards the rotating side of the wheel belt relative to the centerline of the kiln, with an offset of 3mm to 5mm, as shown in Figure 5, to ensure the proper functioning of the wheel belt? The force of the blocking wheel is always downward. But when the installation position of the stop wheel is incorrect, the center line of the stop wheel device is biased towards the rotating side of the wheel belt, or the center line of the kiln is bent and deformed, the stop wheel will be lifted when the wheel belt rotates. In addition, uneven wear and grooves on the contact surface between the gear wheel and the tire belt, as well as the rotation of the tire belt, may cause the gear wheel to pull up due to the upward force on the gear wheel device.
Figure 5 Installation position of gear wheel device
3.4 The contact surface between the gear wheel and the tire belt is not round or worn too quickly
Is the contact surface between the tapered belt and the stop wheel theoretically pure rolling? The touch should have uniform wear and minimal wear. But when the offset between the center line of the stop wheel and the center line of the kiln is too large or the kiln center line is bent too much, the contact surface between the stop wheel and the wheel belt will not only roll purely, but also produce relative sliding, causing uneven or rapid wear of the contact surface, and initially, circular wear marks will appear on its surface. In addition, if the kiln centerline is not aligned, the axial deflection of the wheel belt is too large, or the base of the stop wheel is not parallel to the kiln centerline, it may cause local contact on the working surface of the stop wheel, or the stop wheel may not operate properly, the bearings may be abnormal, and periodic speed loss may occur. The material hardness of the stop wheel and wheel belt is also low, which may cause the contact surface between the stop wheel and wheel belt to be non-circular or worn too quickly.
3.5 Hydraulic cylinder oil leakage or internal leakage
Some gear wheel hydraulic cylinders have problems such as internal sealing failure, large internal leakage, severe wear of guide sleeves, easy to pull cylinders, and leakage at cylinder end caps, which prevent the hydraulic cylinder from working properly. As a result, the gear wheel device cannot push the kiln body up and down, causing uneven wear on the surface of the wheel belt, supporting wheel, large gear ring, and small gear, resulting in steps.
3.6 High system pressure and large fluctuations
The high pressure of the gear shifting hydraulic system is generally caused by the non parallelism between the supporting wheel and the centerline of the kiln, resulting in excessive axial force on the kiln body, or by the grinding of steps on the surface of the supporting wheel and small gear, resulting in an uneven busbar, after excluding the reasons of the hydraulic pipeline system itself. When the centerline of the kiln changes, excessive axial deflection of the wheel belt, or uneven working surface and wear of the stop wheel, as well as high or low pressure of nitrogen gas in the accumulator, can cause excessive fluctuations in the working pressure of the stop wheel, affecting its service life.
4. Solutions
(1) When installing the gear wheel, it is necessary to grasp the key dimensions and control the deviation range, such as the contact length between the gear wheel and the wheel belt, the slope of the gear wheel, and the lateral offset of the gear wheel. If it exceeds the required range, it must be corrected in a timely manner, otherwise there will be some problems in the subsequent operation.
(2) After the gear wheel is put into operation, it should be regularly measured and inspected, such as whether the deviation of the kiln centerline exceeds the standard, whether the offset between the gear wheel and the kiln centerline is within the required range, whether the inclination of the base is normal, and whether the connecting bolts at all places are loose.
(3) To control the downward movement force of the kiln, namely the hydraulic stop wheel pressure, it is generally between 3.5 and 6 MPa. If it exceeds the range, it should be adjusted in a timely manner. On site operation can generally achieve the required downward movement force by adjusting the angle between the center line of the supporting wheel and the center line of the kiln.
(4) The lubrication of the gear wheel bearing should be the responsibility of a dedicated person. If the gear wheel is lubricated with dry oil, it can be lubricated twice a week from the bottom, until old oil comes out from above each time. Lubricating oil should have high viscosity and extreme pressure resistance. If using lubricating oil, it is necessary to check the oil level every shift and raise the oil level as much as possible. The temperature should be controlled below 80 ℃, because if the temperature is too high, the viscosity of the oil will decrease, the bearing oil film will become thinner, and the bearing will be easily damaged.
(5) The temperature measurement signal of the bearing should be introduced into the central control, and temperature changes can be monitored at any time during operation. If there is a continuous upward trend in temperature, the cause should be identified and dealt with in a timely manner. Once the temperature rises to a certain value, the bearing oil film will become thinner or disappear, ultimately leading to bearing damage. The increase in bearing temperature is often a precursor to bearing damage. On site measures such as changing process operations, adding insulation boards, accelerating heat dissipation, and adding cold oil can be taken to deal with it.
(6) If there is serious wear or deformation on the working surface of the gear wheel and wheel belt, timely measures should be taken to deal with it, such as grinding, welding, replacement, alignment, etc., to restore the working surface to its initial state. Try not to run it haphazardly, and pay attention to improving its surface lubrication. If wear marks appear in the early stage, high-temperature lubricating grease can be sprayed regularly on the surface of the gear wheel to enhance the lubrication effect and effectively reduce the wear of the working surface.
(7) If there is a significant fluctuation in the working pressure of the gear wheel, the cause should be promptly identified and dealt with as soon as possible. If it is caused by uneven heating and deformation of the cylinder due to large temperature differences, resulting in belt deflection and pressure fluctuations, the calcination operation should be adjusted in a timely manner to bring the temperature of the cylinder body back to normal, reduce the deformation of the centerline of the cylinder, and restore the axial runout of the belt to normal. The accumulator should be checked every six months, and if the air pressure is insufficient, it should be promptly replenished.
(8) The centerline Z of the kiln should be regularly inspected by professional personnel within 1-2 years. If it exceeds the allowable range, it should be adjusted in a timely manner. Changes in the centerline will affect the force on the stop wheel. In addition, the surface of the kiln's supporting wheel, wheel belt, and gear should also be inspected regularly. If there are steps or uneven rotating busbars, they should be polished or turned in a timely manner, otherwise it will cause excessive upward force on the stop wheel, which poses a risk of damage to the stop wheel.
(9) Improve the structure of the hydraulic cylinder, select a new type of combination seal at the piston of the hydraulic cylinder, install an exhaust device on the hydraulic cylinder, and choose a hydraulic cylinder made by a professional manufacturer.
(10) Regularly inspect and maintain the vulnerable parts of the gear wheel, and avoid welding, baking, and other operations on the shaft surface during maintenance to avoid changing the internal organizational structure, stress concentration, and damaging the gear wheel spindle. During annual major repairs, the gear wheel device should be disassembled, the wear of the bearings should be checked, damaged parts should be replaced, and the interior of the bearings and gear wheel bearing seats should be thoroughly cleaned. In addition, when replacing standard parts such as bearings and bolts, it is necessary to purchase from reputable and reliable suppliers. A set of gear wheel bearings can be kept on hand for timely replacement in case of damage.
5. Conclusion
This article focuses on the common faults and accidents that occur in hydraulic gear wheel devices on site, and combines the experience of handling some on-site problems in the design, installation, commissioning, and maintenance processes to introduce some solutions and precautions on site.
The faults and accidents of the hydraulic gear wheel device mentioned above are often interrelated and interact with each other. The difference is that the order in which the problems occur is not the same. Therefore, when dealing with them, it is necessary to consider comprehensively and solve them comprehensively in order to effectively reduce the accident rate of the gear wheel.
If the pressure on the gear wheel remains high and fluctuates greatly due to the on-site working conditions, and the gear wheel bearings are frequently damaged, it is also possible to consider optimizing the structure of the gear wheel device to improve the bearing capacity of the gear wheel device and its adaptability to specific working conditions to solve the problem.
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