Abstract: The Jiuhua Mountain Huatai Cableway was officially put into operation in March 2010 and has been in operation for twelve years, with a cumulative safe operation of nearly 42000 hours. Hydraulic systems are widely used in passenger ropeways and play an important role. Although some components of the hydraulic system may have minor faults, it is not difficult to repair and handle them. However, if we leave them unattended for a long time, it will cause major failures and even cause the cableway to stop and not start up. I have analyzed some of the hydraulic system failures that have occurred in the Huatai Cableway since its operation, categorized into four main types: power components, actuating components, control components, and others, from the perspectives of fault symptoms, troubleshooting, causes, and preventive measures. I would like to communicate with my colleagues and apologize for any shortcomings.
Keywords: hydraulic system; Fault phenomenon; Troubleshooting; Analysis and processing; Reason for malfunction; preventive measure
1. Faults in power components
1.1 Hydraulic Drive System Variable Pump Control Proportional Valve
Fault phenomenon: On October 9, 2012, during the emergency drive test run, as soon as the diesel engine was started, a forward running high-pressure A display pressure of 20 bar appeared. At this time, we had not yet operated it and the emergency brake was also closed, so we had to stop the diesel engine and restart it according to the old situation.
Troubleshooting: Based on the diagnosis of the fault phenomenon and the analysis of the drawing (see Figure 1), it is possible that the variable displacement pump is not in the neutral position. Therefore, we have decided to order spare parts for the displacement control module of the variable displacement pump, as shown in Figure 2. After the spare parts arrive, they will be replaced. During installation, it is important to note that the mechanical control linkage between the displacement control module and the pump body must be installed correctly. If there is obvious resistance when separating the displacement control module, it is considered to be installed correctly. We will then try running it again to restore normal operation.
Reason for malfunction: Afterwards, we checked and maintained the displacement control module of the replaced variable pump, and found that the control damping hole of the displacement control module was blocked by impurities, causing the variable pump to not be in the neutral position. Specific cause analysis: Under normal circumstances, after the diesel engine is started, the supplementary oil pump on the variable pump supplies oil, as shown in Figure 1, which is connected through an overflow valve (part 300) with a set pressure of 24 bar. The control pressure oil passes through hose NW25 to the oil filter (part 235), and then through the flow limiting valve to the P port of the hydraulic control proportional valve (part 301) and the P1 and P2 ports of the electromagnetic proportional valve (part 302). When no driving operation is performed, the proportional valves (part 301 and part 302) are in the "zero position", and the hydraulic oil is connected to the oil tank through the flow limiting valve (part 302), controlling the pressure oil to flow back to the oil tank. Due to the blockage of the damping hole in the middle of the P1-A port flow limiting valve as shown in Figure 4, it is equivalent to the cutoff of P1 and A of the proportional valve (part 302), and the control pressure oil cannot flow back to the oil tank. This causes the pressure oil at P1 port to act on the lower port of the proportional valve (part 301), causing its valve core to move upward. The P and A of the proportional valve (part 301) are connected, and the pressure oil acts on the servo valve through the pipeline AM, causing the variable displacement pump's swash plate to not be in the center position and have a certain inclination angle, thereby generating a certain positive high-pressure pressure.
Preventive measures: After discovering the fault point, we cleaned the displacement control module of the variable displacement pump, unblocked the control damping hole, repaired it, coated it with hydraulic oil, and stored it as a spare part.
Figure 1 Schematic diagram of variable pump control oil circuit and fault location
Figure 2 Variable displacement pump control module
2. Malfunctions in execution components
2.1 Hydraulic cylinder for curved rail lifting system
Fault phenomenon: On February 8, 2020, during the post shift equipment inspection, it was found that the hydraulic cylinder for lifting the curved track at the bottom station (tire position 53) had obvious oil leakage during use, but there was no obvious oil leakage when not under pressure or stationary.
Troubleshooting: After inspection, there is no oil leakage at the pipe joint, and it is preliminarily judged that the hydraulic cylinder body is leaking oil. Considering that the bending rail lifting device at the upper station is used less, the hydraulic cylinder of the bending rail lifting device at the 53rd tire of the lower station and the hydraulic cylinder at the 53rd tire of the upper station will be swapped for use. After researching and measuring the relevant dimensions of the hydraulic cylinder for the curved rail elevator, we found that the model of the hydraulic cylinder we currently use for the curved rail elevator is basically the same as the parameters of the Enpak RCS101 hydraulic cylinder. The pipe joint thread is R3/8, so we ordered this model of hydraulic cylinder online. After replacing the newly ordered hydraulic cylinder, the trial operation was normal, as shown in Figure 3.
Figure 3 Hydraulic cylinder of curved rail elevator
Reason for malfunction: After disassembling and inspecting the replaced hydraulic cylinder, it was found that the sealing ring of the hydraulic rod was aging. After replacing the sealing ring again, the pressure was normal. The sealing ring is a combination type, commonly known as a grid ring, consisting of an O-ring and a wear-resistant ring, as shown in Figure 4.
Figure 4 Hydraulic cylinder sealing ring
3. Control component faults
3.1 Hydraulic tensioning system pressure sensor
Fault phenomenon: On August 24, 2010, the cableway area was shut down and lightning protection measures were taken due to thunderstorms. After the thunderstorm stopped, the control power was powered on for self inspection, and the pressure sensor of the hydraulic tensioning system on the upper station showed a fault and could not be reset.
Troubleshooting: After measuring the voltage output of its sensor, the output voltage of sensor 1 was only about 2.5V, which is lower than the normal state of about 4.2V, confirming that sensor 1 is damaged. Temporarily adopt the method of simultaneously sending the signal from sensor 2 to the input ports of pressure sensors 1 and 2 of the PLC to restore normal operation of the cableway.
Reason for malfunction: The pressure detection component of the hydraulic tensioning system uses two pressure sensors for redundant detection. When the detection deviation between sensor 1 and sensor 2 exceeds 8%, the control system will issue a "pressure sensor difference" fault.
3.2 Hydraulic tensioning system pressure relief valve
Fault phenomenon: During the hydraulic tensioning system test and pressure relief operation on July 10, 2015, the black circular cap of the pressure relief valve could not be relieved when rotated counterclockwise.
Troubleshooting: As I had not disassembled the pressure relief valve before, I did not understand the function of the two fastening screws on the round cap. I removed the round cap and used a wrench to relieve the pressure, as shown in Figure 5.
Figure 5: Pressure Relief Valve
Reason for malfunction: After disassembly, it was found that one of the hexagonal locking screws in the pressure relief valve serves to fix the round cap and prevent it from falling off, while the other one serves as a locking screw to prevent the round cap from loosening and releasing pressure. At that time, the pressure relief operation could not be performed when rotating due to the loosening of the hexagonal set screw and the displacement of the round cap inside the pressure relief valve.
3.3 Hydraulic brake system check valve and throttle valve
Fault phenomenon: After the completion and delivery of the cableway construction in 2010, the emergency brake was opened for too long when it was turned on in the morning. When the reset button was pressed, a brake valve fault alarm occurred.
Troubleshooting: After replacing the pressure switch (part 60) and multiple on-site debugging, it was initially determined that there was a problem with the one-way valve (part 42). As there were no spare parts available for replacement on site, the problem was not ultimately resolved. Later, spare parts were ordered to replace the hydraulic brake system's part 42 one-way valve and part 50 throttle valve, restoring normal operation.
Reason for malfunction: The valve core of part 42 is stuck and cannot be fully opened, and the fixed thread size of part 50 is too small to easily fall off, but the female thread is intact, resulting in the emergency brake opening time being too long.
3.4 Hydraulic brake system overflow valve
Fault phenomenon: On December 1, 2010, the hydraulic system was unable to maintain pressure when manually pumping.
Troubleshooting: The sealing ring of the overflow valve (part 36) has aged and broken, causing internal leakage in the system. After replacement, it was restored to normal, as shown in Figure 6.
Figure 6 Overflow valve
3.5 Hydraulic brake system ball valve
Fault phenomenon: On July 12, 2019, during the manual opening test of the working brake, it was found that the working brake could not maintain pressure when opened with a manual pump. After replacing the spare ball valve (part 80) with a new one, it was tested that the working brake could maintain pressure normally when opened with a manual pump.
Troubleshooting (see Figure 14):
(1) Determine the integrity of the working brake flushing valve
Remove the flushing valve pipeline of the working brake and block the outlet pipe joint on the working brake with a plug, as shown in Figure 7. Switch the 80 ball valve to the "open with manual pump" state, manually apply pressure to the working brake to 120 bar. After stopping the pressure, the pressure of the working brake quickly drops below 100 bar, forming a slow pressure reduction state around 65 bar. After about 4 minutes, the pressure of the working brake drops to around 20 bar. At this point, the working brake has been closed and no further testing is required. The pressure is manually released. This phenomenon indicates that the failure of the working brake to maintain pressure is not related to the flushing valve, and there is still internal leakage in other positions of the working brake circuit when the manual pump is opened.
Figure 7 Working Brake
(2) Determine the integrity of the emergency brake manual pump opening circuit
Switch the ball valve of item 81 to the "open with manual pump" state, and use the manual pump to open the emergency brake for pressure maintenance testing. The pressure can be maintained at 70 bar, as shown in Figure 8. This indicates that the inability of the working brake to maintain pressure when opened with a manual pump is not related to the overflow valve 36 and the manual pump circuit. After reviewing the drawings, it was analyzed that the inability of the manual pump to maintain pressure when the work brake is turned on may be related to ball valve 80.
Figure 8 Pressure gauge
(3) Judging the integrity of ball valve 80
Replace the 80 ball valve with a new spare part, and press the working brake to 120 bar. The pressure of the working brake quickly drops to below 100 bar and remains at around 70 bar (there is no significant decrease in pressure for about 10 minutes), as shown in Figure 9. This phenomenon indicates that there is internal leakage in the use of component 80 ball valve.
Figure 9 Ball valve and pressure gauge
(4) Adjust the pressure setting of overflow valve for component 36
Considering that sometimes the work brake needs to be opened to switch between the main and auxiliary drive handles when emergency driving is activated, we adjusted and tested the overflow valve of part 36, as shown in Figure 10. First, we used an open-end wrench 13 to lock the adjustment screw and loosen the cap. Then, we adjusted the pressure setting adjustment screw of the overflow valve clockwise (increase) starting from a straight line. We manually pressed the work brake to 120 bar, and the pressure slowly decreased to around 100 bar and was maintained (the pressure did not decrease significantly for about 10 minutes), as shown in Figure 10.
Figure 10 Overflow valve and pressure gauge
(5) Wash the valve pipe of the working brake? Restore and test the manual pump to open the circuit and maintain pressure. The hydraulic brake system was tested separately by manually opening the working brake and emergency brake with a pump, and the pressure was maintained normally without any significant decrease for 20 minutes.
Reason for malfunction: After disassembly, the sealing ring of the ball valve was found to be aged and cracked, as shown in Figure 11, causing internal leakage in the system and resulting in a decrease in the pressure of the working brake.
Figure 11 Ball valve and sealing ring
Preventive measures: Order spare parts for ball valves and sealing rings, conduct manual pressure tests on working brakes and emergency brakes every month, and if necessary, disassemble and inspect the ball valves.
3.6 Hydraulic system one-way throttle valve for curved rail lifting
Fault phenomenon: During a half month inspection on a certain day, it was found that the curved rail could not be lowered after being raised during the operation test of the curved rail elevator.
Troubleshooting: After reviewing the drawings, as shown in Figure 12, it was analyzed that the one-way throttle valve (part 19) may have been closed, causing the curved rail to not descend. The one-way throttle valve is now adjusted and the lifting operation is resumed to normal.
Reason for malfunction: The one-way throttle valve was mistakenly closed while cleaning the hydraulic station of the curved rail lifting system. The function of the one-way throttle valve is to control the flow rate of the hydraulic system, so that it can control the descending speed of the curved rail elevator, and the speed can be adjusted.
Preventive measures: After adjusting the speed control of the one-way throttle valve, tighten the set screw on the adjustment cap to prevent accidental closure of the one-way throttle valve.
Figure 12 Hydraulic diagram for curved rail lifting
4. Other types of faults
4.1 Brake pressure loss fault
Fault phenomenon: On July 23, 2010, during the shutdown period, there was a loss of brake pressure.
Troubleshooting: During the shutdown, it was discovered that the emergency brake had tripped, resulting in pressure loss and tripping. And the control system did not report any emergency brake circuit faults. Further observation of the system instruments revealed that the A1+O1 cabinet voltmeter was not displaying. Then, according to the drawing, check that the 20K1 (13-14) contact of the contactor is separated from the connection with the main contactor, resulting in the main contact not being engaged when it is engaged. The X4 input voltage is normal. Check that 20K1 is engaged, but 11K1 is not engaged. Check that 20K1 (13-14) contacts are not engaged and not engaged. Remove the auxiliary contact, turn off the control power, reinstall when 20K1 is released, turn on the control power, and restore normal operation.
Reason for malfunction: If the emergency brake does not open or falls off during operation within 5 seconds after the reset of the "emergency brake parking" circuit, the control system will experience an emergency brake malfunction parking. Due to the malfunction occurring during shutdown, the emergency brake pressure was lost and no fault was reported, so the system did not issue a fault warning. So when this fault occurs, you need to carefully observe the display of each instrument and see if there are any differences from the normal state, in order to provide reliable basis for you to solve the fault.
4.2 Brake valve malfunction
Fault phenomenon: After replacing the hydraulic hose and oil during the 2015 annual maintenance, the control power supply was turned on, but the system failed the self-test. The pressure of the working brake and emergency brake was zero, indicating a brake valve fault.
Troubleshooting: Open the nut plug of the oil pump outlet pipe on the top of the fuel tank, and there is no pressure oil overflowing. Manually pressurize the hydraulic motor fan blades and loosen the oil pipe nut plug. Use a thin hexagonal bolt to push the one-way valve ball valve inward to release air until there is no gas in the overflowing hydraulic oil. Release the one-way valve, close the nut plug, and clean the oil stains (as shown in Figure 13).
Reason for malfunction: Due to the replacement of a new hydraulic hose, there is air in the pipeline that needs to be vented.
Figure 13 Oil pump outlet pipe
Figure 14 Hydraulic brake system diagram
5. Conclusion
In summary, hydraulic transmission and control are widely used in cableway equipment. It can conveniently convert electrical energy into mechanical energy (pressure energy) and is easy to transmit and control. We use hydraulic braking and driving, hydraulic tensioning, and curved rail lifting hydraulic systems in our cable car, which requires us to use and maintain hydraulic equipment correctly and carefully to prevent premature wear and unnecessary damage to the components, thereby reducing the occurrence of failures and effectively extending the service life. In order to ensure that hydraulic equipment can achieve the expected working capacity and stable and reliable technical performance, we must be proficient in operating, adjusting reasonably, maintaining carefully, and planning maintenance for hydraulic equipment.
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