Rexroth hydraulic pump - gear pump common problems and solutions

1. Trapped oil

(1) Causes

The trapped oil of the gear pump is caused by the overlap coefficient of the gear design, as shown in the figure below, assuming that the length of the meshing line of the gear in the actual meshing is l and the tooth pitch is t0, then the overlap coefficient of the gear meshing is ε=l/t0. The overlap coefficient has the following physical significance: when ε=1, the moment the first pair of teeth is out of mesh, the next pair of teeth can enter the mesh, that is, the beginning of engagement and the disengagement are carried out at the same time. In order to make the gear transmission smooth, reduce the impact, or avoid oil leakage, when designing the gear transmission mechanism, it is usually ε> 1 (generally taken ε=1.05-1.3). That is, before the first pair of teeth is out of mesh, the next pair of teeth enters mesh. However, when the two pairs of teeth are meshed at the same time, a closed dead volume filled with oil is formed between the two meshing points A and B and is not connected with the suction and pressure cavities. During the rotation of the gear from Figure 2-3 (a) to Figure 2-3 (b), this dead volume gradually decreases, and a lot of pressure is generated because the oil cannot flow out. The trapped oil pressure is much greater than the working pressure of the pump, so that the parts of the pump are impacted, vibration and noise are generated, and a part of the high-pressure oil is forcibly squeezed out through various gaps.

Rexroth hydraulic pump - gear pump common problems and solutions

　　When rotating from the position of Figure 2-3 (b) to the position of Figure 2-3 (c), the closed dead volume begins to change from small to large, and the pressure decreases due to the lack of oil absorption, and it is easy to form a vacuum, causing cavitation due to the escape of gas in the oil, or forming a negative pressure impact, which also produces vibration and noise. This kind of closed dead volume undergoes the process of "volume change in the sealed state" in the transition, and the phenomenon that the internal pressure will increase or decrease sharply is called trapped oil.

(2) Elimination measures

In order to eliminate the phenomenon of trapped oil, a pair of unloading grooves are usually opened on the covers at both ends, as shown in Figure 2-3 (c). In this way, when the dead volume is reduced, it is communicated with the oil pressure chamber through the right unloading groove, so that a part of the oil can be discharged; When the dead volume increases, it is communicated with the oil suction chamber, and a part of the low-pressure oil can be sucked in to supplement the increased space.

The principle of opening the unloading groove is to ensure that the suction and pressure chambers can not be communicated through the unloading groove at any time, otherwise the volumetric efficiency of the gear pump will be reduced. In addition, the distance between the two unloading grooves should not be too large, in case the elimination of trapped oil is not complete. Usually make b = 0.8m, the width of the unloading groove c>2.5m, the depth h≥0.8m (m is the gear modulus).

2. Leakage

There are three main ways of internal leakage in an external gear pump: face leakage, radial leakage and meshing line leakage.

(i) End face leakage

Face leakage refers to the leakage between the gear end face and the inner surface of the pump end cover, accounting for more than 75% of the total leakage. Since there must be a certain gap between the rotating gear end face and the inner surface of the pump end cover, a slight increase in the amount of clearance will lead to a significant decrease in the volumetric efficiency of the gear pump. Generally, the inner end clearance of medium and small gear pumps is controlled between 0.02 and 0.05mm.

In addition, end face clearance compensation measures such as floating bushings and elastic side plates can be used to reduce and eliminate face leakage.

(ii) Radial leakage

Radial leakage refers to the leakage between the top circle of the gear tooth and the surface of the pump cavity, which accounts for about 15% of the total leakage. In order to ensure that there is no "scraping" phenomenon in the process of gear transmission, and at the same time consider the manufacturing error, the radial clearance of medium and small gear pumps can reach 0.1mm. Despite the long radial leakage channel, this leakage has little effect on the volumetric efficiency of the gear pump. Sometimes the method of adding a compensating side plate on the side of the oil pressure chamber can be adopted, and the compensating side plate floats on the top of the gear tooth by using the action of high-pressure oil, so as to prevent the low-pressure oil from entering the high-pressure oil cavity.

(3) Leakage of the engagement line

Due to tooth direction error, it is not possible to mesh the entire width of the gear, and the resulting leakage is called meshing line leakage, which accounts for less than 5% of the total leakage. In addition, as the gear pump operating pressure increases, the contact at the meshing point becomes tighter, and the leakage through the meshing line is small, so the meshing line leakage is often ignored compared to the above two types of leakage.

3. Radial force

The force on the main and driven gears in the gear pump is shown in Figure 2-4. Due to the difference of oil pressure in the high and low pressure oil chambers, the oil pressure gradually transitions from low pressure pd to high pressure pg, and the gear on one side of the oil pressure chamber is subjected to a larger oil pressure, and the resultant force of the circumferential fluid pressure is Fp. The action direction of the two meshing forces with equal magnitude and opposite directions coincides with the meshing line, and the meshing force FT acting at the meshing point is simplified to the center O1 and O2 points of the main and driven gears respectively, and the simplified radial force is synthesized to see that the force F1 of the driving gear is less than that of the driven gear F2. Therefore, in general, driven gears wear out quickly. In the actual design, F1 = 0.75 Δ pBDe and F2 = 0.85 Δ pBDe are usually taken, where Δp is the pressure difference between the inlet and outlet of the pump, and B and De are the tooth width and the diameter of the tooth top circle respectively.

　　The unbalanced radial force of the gear not only causes uneven wear of the gear, but also deforms the bearings and gear shafts, reducing the service life of the gear pump. In order to reduce and eliminate the phenomenon of radial force imbalance, the method of reducing the diameter of the pressure port and reducing the unbalanced effect of the high-pressure oil on the radial direction of the gear is usually adopted. Sometimes, two balance grooves are set up on the opposite side of the original oil suction chamber on the pump end cover to communicate with the suction and pressure chambers respectively to balance the radial unbalanced force. However, this practice increases the leakage volume and reduces the volumetric efficiency, which is only suitable for low-pressure gear pumps.

Rexroth hydraulic pump - gear pump common problems and solutions

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