Created on:2021-04-27 10:02

Working principle of screw pump

Working principle of screw pump

Figure B shows the working principle of the screw pump. There are three parallel double headed screws in the shell (or bushing) 2, which are the core working parts of the screw pump. The middle screw is a convex screw 3 (i.e. driving screw) which is also used as the transmission shaft, and two concave screws 4 (i.e. driven screw) are on both sides. The working volume of sealing is formed between the three mutually meshing screws and the shell. The length of each sealing space is approximately equal to the screw pitch. The oil port at the left end of the shell is a suction port, and the oil port at the right end is a pressure port. When the convex screw is driven by the prime mover to rotate clockwise (viewed from the extended end of the shaft head), the screw pump sucks the liquid from the oil suction port and holds it in the working volume. The working volume will move continuously from one end of the screw to the other end along the axial direction, so the liquid will be transmitted from the oil suction port to the oil pressure port, and then the liquid will be pressed to the hydraulic system through the oil pressure port, and the corresponding pressure will be established according to the resistance of the system The pressure of work. The follow-up spiral surface continuously forms a new sealing volume. With the prime mover driving the active screw to rotate continuously, the above process will go on again and again.

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On the working principle of screw pump should pay attention to the following points.

① The working principle of liquid nut screw pump pumping liquid is the same as that of ordinary screw nut. When the screw rod rotates, if the nut is fixed with a key, the nut will move axially. The liquid surrounding the screw groove is equivalent to a "liquid nut". Due to the fluidity of the liquid, it can not be fixed with an ordinary key, but a rack 2 (Fig. C) is used to cut off the screw groove. When the screw turns, the liquid cut off in the groove is lifted up like a nut. Because the key rack produces axial movement in motion, it must be infinitely long. However, this is impossible. In the screw pump, one or several conjugate driven screws have the function of infinite rack 2 as shown in Figure C. in the working process of these driven screws, as long as the spiral groove of the driving screw can be cut off (in fact, the cutting of the spiral groove is mutual), the liquid nut can be lifted up.

② The shell and bushing length of the shell and bushing screw pump should be able to cover at least one lead screw pair, and it can also be made into a multi-stage pump containing several leads connected from head to tail. When the working medium passes through these working volumes, it is gradually pressurized to obtain higher outlet pressure; the shell and bushing of the high-pressure screw pump are very long, and it needs to contain 6-12 leads (i.e. 6-12 stages). The inner wall of the shell and bushing and the tooth top circle of the screw form a radial clearance seal, while the axial seal is realized by the contact line on the meshing helical surface. Theoretically, a continuous meshing line with a "B" shape around the three screws can be formed at both ends of a lead by properly selecting the relevant parameters of the conjugate tooth profile. But because of the manufacturing error and the tooth tip trimming which is necessary to ensure the strength, it is difficult to achieve complete sealing, which is an important reason for the low volumetric efficiency of the screw pump.

The shell of screw pump is made of gray cast iron or nodular cast iron. The bushing of low-pressure screw pump can be made of nodular cast iron, wear-resistant cast iron or tin bronze. The bushing of high-pressure screw pump should be made of cast steel and inlaid with bearing alloy antifriction lining (Fig. d).

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③ In order to form a continuous and closed contact line in the meshing area, the screw tooth surface must be accurately machined and polished. The screw is mostly made of surface hardened carburized steel or nitrided steel.

④ The force acting on the screw and its compensation when the screw pump is working, the hydrostatic pressure and the force between teeth on the screw surface is the force acting on the screw. Including axial force and radial force.

a. Axial force and its balance in the screw pump, the pressure of working medium gradually increases along the axis. This pressure difference produces an axial thrust from the oil pressure chamber to the oil suction chamber on the screw pair, which will increase the friction between the screws, aggravate the wear of each matching surface and reduce the mechanical efficiency. The commonly used axial force balancing measures are as follows (Figure E).

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I. the shaft extension of the active screw is arranged at the side of the oil pressure port to reduce the action area of high-pressure liquid on the screw.

II. A balance disc (shoulder) with larger diameter is set at the shaft extension of the oil pressure port side, which forms a gap seal with the inner wall of the shell, and a unloading cavity communicated with the oil suction port is separated at the shaft extension end. In this way, the pressure difference acting on both sides of the balance plate can counteract most of the axial force on the driving screw. The unloading chamber also ensures that the working pressure difference of the rotating sealing element at the shaft extension is not too high.

III. high pressure liquid is led to the bearing chamber of the three screws at the oil suction port side through the channel set in the center of the two driven screws to form reverse thrust. A small part of axial force should be retained on the driven screw to ensure the compression seal on the meshing line. The final residual axial force on the driving screw is balanced by the thrust bearing on one side of the oil suction port.

IV. the thrust bearing is placed to bear the axial force. Its advantage is that the leakage is small, but the friction loss is increased.

V. for large screw pumps, double suction type is often used (Fig. f), i.e. oil suction at both ends and oil pressure in the middle. The axial force on the active screw can be almost completely balanced by using a pair of screw pairs with opposite screw directions.

b. When the three screw pump with symmetrical configuration of radial force and its compensation is in operation, the radial hydraulic pressure on the middle driving screw and the meshing reaction of the driven screw are balanced; while the driven screw only bears the driving force of the driving screw on one side, and the hydraulic pressure on both sides is not equal, so the radial force is unbalanced. By properly selecting the diameter ratio of the master screw and the slave screw and the concave screw section size of the driven screw, the torque generated by the hydraulic pressure can make the driven screw rotate and remove most of the mechanical driving force. The radial force of the driven screw can also be controlled in a reasonable range by appropriately limiting the pressure difference established by each lead stage.

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