Solutions to common oil trapping of KCB gear pump

The causes of oil trapping of KCB gear pump can be eliminated as long as the oil trapping volume becomes smaller and communicates with the discharge chamber and the suction chamber when it increases without bypassing the suction and discharge chamber. There are many specific methods, which are introduced as follows:

Pressure relief tank method:

This method is recommended by KCB gear pump design in China, and it is also the most commonly used pressure relief method. On the inner side of the covers at both ends of the gear pump, two grooves are excavated along the common tangent direction of the two gear pitch circles corresponding to the position of the suction and discharge cavity, which are called pressure relief grooves. The two grooves are symmetrically arranged relative to the connecting line between the centers of the two gears, as shown by the dotted line in the figure. The size of the relief tank can be calculated as follows.

It can be seen that before or after the trapped oil volume reaches the minimum value, regardless of whether the trapped oil volume increases or decreases, the gear pump leads the oil in it to the discharge chamber through the pressure relief groove on the right, or leads the oil in the suction chamber into the trapped oil volume through the pressure relief groove on the left. This eliminates trapped oil compression and trapped oil expansion, so as to achieve the purpose of eliminating trapped oil.

Pressure relief hole method

The structure of pressure relief hole is shown in Figure 4. Each tooth top and root of the driven gear are drilled radially, two crescent grooves are cut on the driven shaft, and the driven gear is sliding sleeved on the driven shaft. When the trapped oil is compressed, the high-pressure oil in the trapped oil volume can be led to the discharge chamber through the corresponding drilling on the top or root of the driven gear and the groove on the driven shaft; When the trapped oil expands, the oil in the suction chamber can be introduced into the trapped oil volume through corresponding boreholes and grooves, so as to eliminate the trapped oil phenomenon. The center of the circular hole shall be located so that the circular hole can be tangent to the meshing point of the driven tooth profile, so its center shall be on the meshing line.

Modified tooth profile method

This method is to machine the pressure relief surface on the driven gear at the angle of A0 = 50B. In this way, the latter of the two meshing lines constituting the trapped oil volume is replaced by a small gap between the tooth profile of the driving gear and the inclined plane of the modified driven gear, so that the trapped oil volume is connected with the discharge space to achieve the purpose of pressure relief.

Helical gear or herringbone gear is adopted

Oil trapping only occurs in spur gear pump. When helical gear or herringbone gear is used, the gear meshing process does not suddenly make linear contact and sudden linear disengagement like spur gear, but makes point gradual meshing and gradual withdrawal, so oil trapping will not occur.

The pump with the above two tooth shapes has the advantages of more uniform displacement, stable operation and low noise. However, the use of helical gears will produce axial thrust, so thrust bearings need to be added; Although herringbone gear does not produce axial thrust, it is difficult to process and high cost. Therefore, herringbone gears composed of two groups of helical gears are often used in gear pumps with large displacement.