Analysis of Oil Leakage Phenomenon in Gear Pump Oil Seals of Loaders

"Gear pump oil leakage", that is, the hydraulic oil breaks through the skeleton oil seal and overflows. This phenomenon is widespread and strongly reflected by the main engine manufacturers. The oil leakage of gear pumps seriously affects the normal operation of loaders, the reliability of gear pump use and environmental pollution. To facilitate the resolution of the problem, the causes and control methods of the oil seal leakage fault of the gear pump are analyzed now.

1. The influence of the manufacturing quality of components.

(1) Quality of oil seals.

If the geometry of the oil seal lip is not up to standard or the compression spring is too loose, etc., it will cause air leakage during the air tightness test and oil leakage after the gear pump is installed in the main unit. At this point, the oil seal should be replaced and its material and geometric shape inspected (the quality gap between domestic oil seals and advanced foreign oil seals is relatively large).

(2) Processing and assembly of gear pumps.

If there are problems with the processing and assembly of the gear pump, causing the rotation center of the gear shaft to be out of alignment with the stop of the front cover, it will result in uneven wear of the oil seal. At this point, the symmetry and displacement of the front cover bearing hole to the pin hole, as well as the coaxiality of the skeleton oil seal to the bearing hole, should be checked.

(3) Material and processing quality of the sealing ring.

If this problem exists, it will cause cracks and scratches on the sealing ring, resulting in poor secondary sealing or even failure. The pressure oil will enter the skeleton oil seal (low-pressure channel), thus causing oil leakage from the oil seal. At this point, the material of the sealing ring and the processing quality should be inspected.

(4) The processing quality of variable-speed pumps.

The feedback information obtained from the main engine manufacturer indicates that the oil seal of the gear pump assembled with the variable speed pump has a relatively serious oil leakage problem. Therefore, the processing quality of the variable speed pump also has a significant impact on the oil leakage. The variable speed pump is installed on the output shaft of the transmission, and the gear pump is positioned on the output shaft of the transmission through the stop mouth of the variable speed pump. If the runout (perpendicularity) of the stop mouth end of the variable speed pump facing the rotation center of the gear exceeds the tolerance, it will also cause the rotation center of the gear shaft to not coincide with the center of the oil seal, thus affecting the sealing. During the processing and trial production of variable-speed pumps, the coaxiality of the rotation center with the stop mouth and the runout of the end face of the stop mouth should be checked.

(5) The oil return channel of the front cover between the skeleton oil seal and the sealing ring of the CBG gear pump is not unobstructed, causing the pressure here to rise and thus puncturing the skeleton oil seal. After the improvement in this area, the oil leakage phenomenon of the pump has been significantly improved. Its structural form is shown in Figure 1.

2. The influence of the installation quality of the gear pump and the main unit.

The installation requirement for the gear pump and the main unit is that the coaxiality should be less than 0.05.

Usually, the working pump is installed on the variable-speed pump, and the variable-speed pump is installed on the gearbox. If the runout of the splined shaft rotation center at the end of the gearbox or the transmission pump exceeds the tolerance, a cumulative error will be formed, causing the gear pump to bear radial force while rotating at high speed, resulting in oil leakage from the oil seal.

(2) Whether the installation clearances between components are reasonable.

Whether the clearance fit between the external stop of the gear pump and the internal stop of the variable speed pump, as well as between the external spline of the gear pump and the spline shaft of the gearbox is reasonable, all have an impact on the oil leakage of the gear pump. Since the inner and outer stop mouths belong to the positioning part, the fit clearance should not be too large. The internal and external splines belong to the transmission part, and the fit clearance should not be too small to eliminate interference.

(3) Oil leakage in gear pumps is also related to the spline rolling of keys.

Due to the short effective contact length between the spline extending outward from the gear pump shaft and the spline inside the output shaft of the gearbox, and the large torque transmitted by the gear pump during operation, the spline is subjected to high torque, resulting in compression wear or even key rolling, generating excessive heat. This leads to the burning and aging of the rubber lip of the skeleton oil seal, thereby causing oil leakage. It is recommended that when the main engine manufacturer selects gear pumps, it should check the spline strength of the gear pump shaft extension to ensure sufficient effective contact length.

3. The influence of hydraulic oil.

(1) The cleanliness of the hydraulic oil exceeds the tolerance, and the contamination particles are large. The adhering sand and welding slag in various hydraulic control valves and pipelines are also among the causes of contamination.

Because the gap between the gear shaft diameter and the inner hole of the sealing ring is very small, larger solid particles in the oil enter it, causing wear, scratches or rotation with the shaft on the inner hole of the sealing ring. As a result, the pressure oil of the secondary seal enters the low-pressure area (at the skeleton oil seal), causing the oil seal to break down. At this time, the oil should be filtered or replaced with new anti-wear hydraulic oil.

(2) When the viscosity of the hydraulic oil decreases and deteriorates, the oil becomes thinner. Under the high-pressure state of the gear pump, the leakage through the secondary seal gap increases. Due to the inability to return the oil in time, the pressure in the low-pressure area rises, thereby breaking through the oil seal. It is recommended to regularly test the oil and select anti-wear hydraulic oil.

(3) When the main unit operates under heavy load for too long and the oil level in the oil tank is low, the oil temperature can rise to 100℃, causing the oil to become thinner and the lip of the skeleton oil seal to age, thereby leading to oil leakage. The liquid level in the fuel tank should be checked regularly to prevent the oil temperature from being too high.