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About pump Screws #2
Screw Pumps
A screw pump, consists of two or more screws. The screws mesh with each other, letting the liquid flow through the pofiles of the screws. The screws are eather both driven by external timing gears, or the idler screw is driven by liquid pressure.
The most common screw pump design is with 3 rotating screws with different profiles. The center screw is driven by an external gear, and the two outer screws are driven by the liquid pressure. The screw helixes provides sealing elements, when the pump rotates. The pump relies on the viscosity of the liquid for sealing and for driving the idler screws.
The screw pump with 2 screws are constructed with both screws opposite constructed on each shaft. The liquid is pumped into the centre of the shaft en flows towards the outlet side. Triple screw pumps are often used with liquids like oil and other lubricating liquids. The pumps are self-priming, but shouldn't be run dry. This can cause damage to the screws and pump-casing.
1. The rolling action obtained with the thread design of the rotors is responsible for the very quiet pump operation. The symmetrical pressure loading around the power rotor eliminates the need for radial bearings because there are no radial loads. The cartridge-type ball bearing in the pump positions the power rotor for proper seal operation. The axial loads on the rotors created by discharge pressure are hydraulically balanced.
2. The key to screw pump performance is the operation of the idler rotors in their housing bores. The idler rotors generate a hydrodynamic film to support themselves in their bores like journal bearings. Since this film is self-generated, it depends on three operating characteristics of the pump-speed, discharge pressure, and fluid viscosity. The strength of the film is increased by increasing the operating speed, by decreasing pressure, or by increasing the fluid viscosity. This is why screw pump performance capabilities are based on pump speed, discharge pressure, and fluid viscosity.
The supply line is connected at the center of the pump housing in some pumps. Fluid enters into the pump’s suction port, which opens into chambers at the ends of the screw assembly. As the screws turn, the fluid flows between the threads at each end of the assembly. The threads carry the fluid along within the housing toward the center of the pump to the discharge port.
Three Screw pumps employ just three screw elements - one power screw and two idler screws running in the three precision bores in the housing or liner. The screws are so profiled that they form a liquid tight seal between the thread. As the screws rotate, pockets/ cavities are formed where liquid gets trapped and conveyed from one end of the infinite stroke. This results in smooth, pulsation free flow with extremely low noise and vibration levels, almost impossible to be achieved by any other design of positive displacement or other types.
The power rotor does not drive the idlers as commonly believed. The hydraulic forces acting on screw flanks turn the idlers torquelessly thus reducing the friction. The idlers simply roll over the root diameter of idler dia.reduce the unit pressure, centralize the power rotor and absorb the radial loads. A balance piston integrally machined on power rotor takes care of axial thrusts.
While in operation, the screws are in hydrodynamic balance on film lubrication and do not require any additional bearings. The one ball bearing often used is only for axial positioning of rotor and safer operation of mechanical seals. Small rotor dimensions enable the pump to be operated at high speeds directly coupled 3000 or 3600 RPM motors or other prime movers. Such unique design and features has demonstrated excellent reliability with pumps in operation continuously non-stop for decades together without replacement of any components or maintenance on clean fluids.
