对于外置轴承的双螺杆泵，通过轴承定位两根螺杆在衬套中互不接触，齿侧之间保持恒定的间隙(其间隙值由工况及泵本身规格决定)，螺杆外圆与衬套内圆面也保持恒定的间隙不变。两根螺杆的传动由同步齿轮完成。齿轮箱中有独立的润滑，与泵工作腔隔开。这种结构的优点大大拓宽了双螺杆泵的使用范围，即除了输送润滑性良好的介质外,还可输送大量的非润滑性介质,各种黏度(最高黏度可达3x 10° mm2/s)的介质以及具有腐蚀性(酸、碱等性质)、液计磨蚀性的液体。双螺杆泵由于其恒定间隙的存在以及型线上的特点,其属于非密封型容积泵，因此除了输送纯液体外，还可输送气体和液体的混合物，即气液混输,这也是双螺杆泵非常独特的优点之一。 双螺杆泵由于结构的独特设计，可以自吸而无需专门的自吸装置，而且由于轴向输送轴流速度较小而具备很强的吸上能力，即很小的NPSH,值。双螺杆泵还可干转。由于运动部件在工作时互不接触，因此短时的干转不会破坏泵元件，这种特点给自动控制的流程提供了极大的方便。但干运转时间受多种因素限制，一般很短。
For the twin-screw pump with external bearing, the two screws are not in contact with each other in the bushing through the bearing positioning, and a constant clearance is maintained between the tooth sides (the clearance value is determined by the working conditions and the pump specifications), and the clearance between the screw outer circle and the inner circle of the bushing is also kept constant. The transmission of two screws is completed by synchronous gear. There is independent lubrication in the gearbox, which is separated from the pump working chamber. The advantages of this structure greatly broaden the scope of application of the twin-screw pump, that is, in addition to conveying the medium with good lubricity, a large number of non lubricating media, media with various viscosities (up to 3x10 ° mm2 / s) and corrosive (acid, alkali, etc.) and abrasive liquids of the liquid meter can be transported. Due to the existence of constant clearance and the characteristics of profile line, twin-screw pump belongs to non sealed positive displacement pump. Therefore, in addition to conveying pure liquid, it can also transport gas and liquid mixture, that is, gas-liquid mixed transportation, which is one of the unique advantages of twin-screw pump. Due to the unique design of the structure, the twin-screw pump can self-priming without special self-priming device. Moreover, due to the small axial flow speed of axial transportation, it has strong suction capacity, that is, very small NPSH. The twin screw pump can also run dry. Because the moving parts do not contact each other during working, the short-term dry running will not damage the pump components, which provides great convenience for the automatic control process. But the dry running time is limited by many factors, which is usually very short.
2) Three screw pump
The inner part of the three screw pump consists of a driving screw, two driven screws and a bushing containing three screws. The liquid in the cavity moves axially with the rotation of the screw to achieve the purpose of conveying liquid. The special structure of the pump ensures that the master-slave rod is not affected by radial force. The axial force is balanced by hydraulic pressure. The bearing only bears the dead weight of the screw and a small residual axial force. The driven rod is inserted into the balance sleeve (the balance sleeve plays the role of balancing the axial force and is also used as a sliding bearing). The geometric size of the screw rod ensures that no torque is transmitted from the main rod to the slave rod. The driven rod is rotated by the pressure of conveying liquid, so as to ensure that the three screw has a long service life and high mechanical efficiency. The perfect linear design and processing technology make each sealing cavity completely separated, thus having high volumetric efficiency.
Three screw pumps are often used as auxiliary pumps to transport lubricating oil and medium viscosity crude oil in oil depots and pumping stations. YZ submerged slurry pump is used for oil-gas mixed transportation