The gearbox hydraulic seawater pump is an important equipment for lubricating oil cooling in ship gearboxes, which is crucial for the safe and reliable operation of gearboxes. The variable displacement pump on the gearbox drives the hydraulic motor through pressure oil, and then drives the seawater pump to rotate, providing cooling seawater to the lubricating oil cooler.
During a dock repair, the ship dismantled the gearbox hydraulic seawater pump and found that the right seawater pump impeller, pump shaft, and impeller inner diameter had severe corrosion, resulting in excessive assembly clearance and could no longer be used. The repair unit replaced the pump shaft, impeller, bearings, and mechanical seals with new ones. After reassembly, there was a phenomenon of bearing temperature being too high and even burning during the navigation test.
1. Equipment and System Architecture
The hydraulic seawater pump model EVCG-80MC of the ship is a vertical centrifugal pump with a discharge flow rate of 45m ³/ h. The head is 36m and the speed is 2400r/min.
The seawater pump mainly consists of a shell, impeller, shaft, bearings, mechanical seals, and oil seals. This pump has no self suction capacity and is installed below the waterline of the ship. The bearing is arranged at one end of the shaft, and the bearing chamber is separated from the water chamber by a mechanical seal. The impeller is vertically fixed at the end of the shaft. The bearing is a rolling bearing, and the lubrication method of the bearing is closed self-lubricating.
The seawater pump shaft is connected to the hydraulic motor shaft through a coupling. When the hydraulic motor drives the seawater pump to rotate, the seawater pump continuously sucks in seawater from the hull, providing cooling water to various cooling parts.
2. Cause investigation and analysis
From the analysis of the composition and operating principle of the hydraulic seawater pump, the main reasons that may cause its bearing temperature to be too high are: bearing failure; Excessive imbalance of the impeller; Excessive radial runout of pump shaft; The assembly of bearings and pump shafts is too tight. We have investigated and analyzed these four possible reasons separately.
2.1 Bearing failure causing high temperature
After discovering that the bearing temperature was too high, the main engine stopped and the hydraulic seawater pump was disassembled and inspected. The results showed that the bearing cage and balls were intact, and the amount of yellow oil in the oil seal was normal. No abnormalities were found in the bearings. In addition, the bearing models 6308-2RS, C3 and 6307-2RS, C3 deep groove ball bearings, if the temperature rises due to bearing failure, will be accompanied by abnormal noise, while the hydraulic seawater pump operates without any abnormal noise. Combined with the above disassembly and inspection results, the possibility of bearing failure is ruled out.
2.2 Excessive imbalance of the impeller leads to high bearing temperature
The rated speed of the hydraulic seawater pump is 2400r/min. If the imbalance of the impeller is too large, it will cause the bearing to be overloaded during pump operation, causing the bearing to bear additional axial and radial forces. The irregular additional force will cause the bearing temperature to rise or even burn out. The impeller has undergone a dynamic balance test before leaving the factory, with an imbalance of 1gmm/kg (standard ≤ 9gmm/kg), and the dynamic balance is qualified; In addition, during the disassembly of the pump, it was found that the bearing cage and ball bearings were intact. From the factory data and inspection results, it can be ruled out that excessive imbalance of the impeller may cause the bearing temperature to be too high.
2.3 Excessive radial runout of the pump shaft causes excessive bearing temperature
The radial runout of the pump shaft has different requirements for different shaft diameters. If the radial runout of the pump shaft is too large, it will cause the bearing to be overloaded, resulting in an increase in bearing temperature or even burning. The radial runout of the pump shaft should not exceed 0.05mm. The measured radial runout of the pump shaft on the lathe is 0.01mm, which meets the standard. It can be ruled out that excessive radial runout of the pump shaft may cause high bearing temperature.
2.4 The assembly of bearings and pump shafts is too tight, resulting in excessive bearing temperature
Bearings 6308-2RS and C3, as well as 6307-2RS and C3, are enclosed bearings with oil seals. Butter has been added to the bearings before leaving the factory, and it acts as a lubricant during bearing operation. Due to the enclosed nature of the bearing, the heat generated by the bearing cannot be circulated and cooled like oil lubricated bearings. Therefore, only by minimizing the heat generated during bearing operation can higher requirements be placed on the assembly of the bearing. When the interference between the bearing and pump shaft assembly is too large, the bearing balls will bear excessive radial force during bearing operation, which not only affects the service life of the balls, but also intensifies bearing heating, causing the bearing temperature to be too high. During measurement, it was found that the assembly interference between the bearing and pump shaft was 0.035mm. According to the assembly clearance requirements for deep groove ball bearings in the FAG rolling bearing sample manual (sample number WL41520/3ChA), the assembly interference of bearings 6308-2RS, C3 and 6307-2RS, C3 should be between 0-0.018mm, and the possible interference of Z is 0.010mm. The above situation indicates that the tight assembly of the bearing and pump shaft is the reason for the high bearing temperature.
3. Solution measures and experimental assessment
Correct the interference amount according to the recommended range in the bearing manual. Due to the fact that the bearing is a standard component, the pump shaft is subjected to secondary processing. In order to select an appropriate value of interference to ensure that the bearing operates at a normal and stable temperature, the assembly interference of the bearing and pump shaft is reduced in three stages and tested and assessed. The assembly interference values for the three stages were 0.030mm, 0.020mm, and 0.010mm, respectively, with a test time of 60 minutes. The test was conducted every 5 minutes using a thermometer from the temperature measuring hole. The test results and comparison are shown in Tables 1, 2, 3, and 4.
The following conclusion can be drawn from the results of the three stages of testing: ① As the interference decreases, the final temperature of the bearing Z after 60 minutes of testing decreases. ② After 60 minutes of testing with interference amounts of 0.030mm and 0.020mm, the bearing temperature still shows an upward trend When the interference is 0.010mm, the bearing temperature tends to stabilize after 40 minutes of testing. After being assembled on board with an interference fit of 0.010mm, the seawater pump has been running for a total of 2000 hours, and the bearing temperature has stabilized at around 50 ℃, running well. The problem of high bearing temperature in seawater pumps has been solved, and engineering experience that can be used as a reference for subsequent ship maintenance has been provided.
2024 April 2nd Week VAFEM Product Recommendation:
VAFEM single bearings, manufactured to original equipment quality, are assembled in Langenbach, along with all the necessary components. There are four different models: grooved and angular roller bearings,tapered roller bearings,integrated wheel bearing units (hub units with built-in electronic sensors), and the bearings used for ABS, TCS, CPWM and navigation systems. In VAFEM catalogue you will also find images of each individual item. Our range of wheel hubs has also been considerably extended.
2024-04-10