Troubleshooting methods for forklift gear shift thrust faults

A CPCD5AⅡZ forklift experienced a strong impact and abnormal noise when shifting gears during operation. This vehicle is equipped with a hydraulic shift fixed-axis transmission, and there is a shock phenomenon when it is in reverse and forward gears. Therefore, we first suspect that it is caused by the looseness of the main reducer or differential.

Remove the transmission shaft of the main reducer and rotate its input shaft by hand. It is found that there is a free rotation clearance of nearly 30°, so it is determined that the meshing clearance of the main reducer is too large. The main reducer was removed and its meshing clearance was measured to be 0.65mm (the standard value is 0.30-0.50mm). Adjust the position of the bevel tooth so that the meshing clearance between it and the driving bevel gear is 0.35mm. After reassembly and test operation, there was still a gear shifting shock phenomenon and abnormal noises when shifting into various gears. The degree was only slightly less than before, but the fault persisted.

For this reason, the drive shaft was removed again to put the transmission in an unloaded state. The engine was started and the gear shifting, downshifting and upshifting tests were repeated. It was found that the abnormal noise generated at the moment of gear shifting still existed, and the transmission had relatively obvious vibration. Check the supporting and fixing bolts of the transmission to ensure they are not loose. Judging from this phenomenon, the fault should be inside the transmission. The transmission was disassembled, cleaned and inspected. It was found that the tapered roller bearings of the driving and driven shafts of the reverse and forward gear clutches were loose. By adjusting the bearing to an appropriate clearance by adding or removing gaskets and reassembling it for a test run, the phenomena of gear shifting impact and abnormal noise were somewhat reduced, but the fault still persisted.

After such a detailed inspection, the root cause of the fault still cannot be found. So, is there a defect in the design or manufacture of this type of forklift? So, we found forklifts of the same model produced by the same factory and conducted road tests on gear shifting, downshifting and upshifting. The results showed that there was a slight gear shifting shock phenomenon in all of them, but the severity was different. Therefore, the focus of identifying the cause of the fault should be placed on the hydraulic control part of the transmission. By referring to the control principle diagram and parts catalogue of the transmission of this machine, it is known that the return of the clutch piston is achieved by the preload force of the spring. Therefore, at the moment of gear shifting, there may be an excessive flow of pressure control oil in the main clutch, which quickly overcomes the preload force of the spring, causing the driving plate and the driven plate to engage too quickly and resulting in gear impact Just like the impact caused by the main clutch control pedal being released too quickly. To verify this assumption, certain modifications need to be made to the hydraulic control pipeline. An adjustable throttle valve should be connected in series between the directional control valve and the pistons of each clutch to control the flow rate, so as to achieve a smooth connection. The modified hydraulic control schematic diagram is shown in the attached figure.

After the above-mentioned modification, the engine was started, and the test drive of shifting gears, downshifting, and upshifting was conducted. The throttle valve was adjusted, and the gear shifting shock fault was eliminated. If the opening of the throttle valve is too large, the purpose of the renovation will not be achieved. If the opening of the throttle valve is too small, although the fault of gear shifting impact can be solved, new problems such as driving weakness and clutch slippage will occur. The throttle valve should be adjusted in such a way that the engine can just move when it is idling and in first gear. It was only then that the root cause of the fault was discovered. It was that the driving plate and the driven plate of the clutch engaged too quickly, causing an impact. Moreover, due to the excessive meshing clearance of the driving and driven gears of the main reducer and the loosening of the bearings of each gear clutch, the impact during gear shifting was exacerbated.

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