A pump formed by meshing two or three screws together is called a screw pump. For twin-screw pumps, the flow rate is: Q = 3Ω / 16 (D²-d²) tn where D - the outer diameter of the screw; d - the inner diameter of the screw; t - pitch; n - the number of revolutions. Ω ≈ 3.14 This formula can be understood as: Ω / 4 (D²-d²) is the annular area between the inner and outer diameters of the screw. When the screw rotates a week, the end position does not change, but the liquid advances in the screw by a pitch. Therefore, when the twin-screw forwards the liquid to be conveyed forward by one revolution, the volume of the liquid to be fed forward is / / 4 (D²-d²), and then accounts for 1/4 of the volume of the engagement. Ω / 4 (D²-d²) t. Multiply the number of revolutions n is the flow rate per minute. For the three screw pump, the same understanding can be used to calculate, but must be clear about the form of the screw, and some of the screw outer diameter are the same as the active follower, and some (most) is driven rod is smaller than the active rod, and therefore The actual flow is different. As with the gear pump, the flow of the screw pump is a fixed value. Its characteristic curve is a vertical line. In order to prevent the outlet pressure from rising excessively, the outlet should also be provided with a safety valve (which is constructed like a pressure reducing valve). Screw pump at work, the rotor to withstand axial thrust, so some of the screw pump suction chamber is located at both ends of the screw, and the pressure chamber is located in the middle of the screw; screw on both sides of each thread is reversed , So when turning, the oil enters from both ends of the screw, and flows out from the middle of the screw, so that the axial thrust is balanced. In order to eliminate the radial load acting on the driving screw during the operation of the twin screw pump, the pump can be designed in the form of a triple screw, and the active screw is installed between the two driven screws. Screw pump is characterized by self-priming performance, noise-free work, long life, slightly higher efficiency than the gear pump (0.70 ~ 0.95). In the power plant as an oil pump, such as the main steam turbine pump and some of the main feed pump steam pump, is the use of screw pump. Screw pump during maintenance measurements of the gap is: 1, the radial clearance between the screw and the housing, generally take 0.10 to 0.15 mm. Measure with feeler gauge. 2, screw and screw mesh at the top, side of the gap, usually 0.10 to 0.15 mm. 3, driven screw shaft head and shell axial clearance, generally take 2 ± 0.5 mm. 4, bearing clearance, 0.08 ~ 0.12 mm.
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