External Gear Pump

External Gear Pump

External gear pump refers to a gear pump composed of external gears. It is mainly composed of driving gear, driven gear, pump body, pump cover and safety valve. The sealed space formed by the pump body, the pump cover and the gear is the working chamber of the gear pump.

The axles of the two gears are respectively installed in the bearing holes on the two pump covers, and the driving gear shaft extends out of the pump body and is driven to rotate by the motor. The external gear pump has the advantages of simple structure, light weight, low cost, reliable operation and wide application range.

Working principle of external gear pump

Hydraulic pumps rely on the principle of seal volume change to work.

1) The housing, front and rear covers and the tooth valleys of the gears form many sealed working chambers;

2) When the gear rotates as shown in the figure, as the gear is disengaged, an oil suction chamber is formed on the right side, and as the gear meshes, an oil pressure chamber is formed on the left side;

3) The contact line of the tooth surface at the meshing point separates the gear into high and low pressure cavities, which play the role of oil distribution.

Structural features and advantages and disadvantages

Oil trapping, leakage and radial unbalanced force of external gear pump are the three major problems affecting the performance index and service life of gear pump.

  1. The phenomenon of trapped oil
    1) Causes of oil trapping
    a. In order to ensure the continuous and stable operation of the gears, and to separate the oil suction port and the oil pressure port, the coincidence degree (the number of meshes) when the gears are engaged must be greater than 1.
    b. Before a pair of gears is disengaged, the other pair of teeth starts to mesh again, so a closed volume is formed between the meshing of the two teeth.
    2) Adverse consequences caused by trapped oil
    The high-pressure oil is forcibly squeezed out from all possible leaking gaps, causing the shaft and bearing to be subjected to a large radial force, and at the same time, the reactive power loss increases, and the oil heats up.
    A partial vacuum is formed to separate the dissolved gas in the oil, resulting in cavitation, causing noise and cavitation.
    3) Methods to eliminate trapped oil
    Principles: a)→b) The closed volume is reduced, so that it can pass through the pressure oil port; b)→c) The closed volume is increased, so that it can pass through the oil suction port; b) The sealing volume is the smallest, and the suction pressure oil is separated.
    Elimination method: Open unloading grooves on the front and rear end covers (or bearing seats) to eliminate trapped oil.
  2. Radial unbalanced force
    Reason: The hydraulic pressure decreases from the oil pressure chamber to the oil suction chamber along the circumference, and decreases tooth by tooth along the outer circle of the gear
    Consequences: The greater the working pressure, the greater the radial unbalanced force. When it reaches a certain level, the shaft will be bent, and the tooth tip will contact the housing, and at the same time, the bearing wear will be accelerated and the bearing life will be reduced.
    Improvements:
    1) Reduce the pressure oil port to reduce the pressure oil action area;
    2) Increase the inner surface of the pump and the clearance of the tooth tip;
    3) Opening the pressure balance groove will reduce the volumetric efficiency.
  3. Leak
    There are three possible leakage parts in the gear pump: tooth side leakage (where the tooth surfaces of the two gears mesh) – accounting for about 5% of the total leakage of the gear pump; radial leakage (between the outer circle of the gear and the inner hole of the casing) ——About 15%~20% of the total leakage of the gear pump. End face leakage (between the gear end face and the end cover) – about 75% to 80% of the total leakage of the gear pump. In short, the higher the pump discharge pressure, the greater the leakage.
External Gear Pump
External Gear Pump

Advantages and disadvantages

Advantages: simple structure, small size and quality, convenient manufacture, low price, reliable operation, strong self-priming ability, insensitivity to oil pollution, and easy maintenance.

Disadvantages: Bearing unbalanced radial force, serious wear, large leakage, limited working pressure, large flow pulsation and noise, non-adjustable displacement

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