I. Write down the words and word combinations below into your exercise book.
1.screw pump винтовой насос
2. propelled вводимый в действие, перемещаемый
3. thread резьба
4. enclosure камера, корпус, защитное ограждение
5. axially в осевом, продольном направление
6. radial bearing радиально-упорный подшипник
7. ball bearing подшипник качения
8. cartridge type закрытого типа
9. journal bearing опорный подшипник скольжения
10. psi фунт на квадратный дюйм; единица
давления, равная 6894,757ПА
11. journal bearing опорный подшипник
II. Read and translate Text 8 with a dictionary
TEXT 8
SCREW PUMP
Screw pumps for power transmission systems are generally used only on submarines. Although low in efficiency and expensive, the screw pump is suitable for high pressures (3000 psi), and delivers fluid with little noise or pressure pulsation. Screw pumps are available in several different designs; however, they all operate in a similar manner. In a fixed-displacement rotary-type screw pump fluid is propelled axially in a constant, uniform flow through the action of just three moving parts – a power rotor two idler rotors. The power rotor is the only driven element, extending outside the pump casing for power connections to an electrical motor. The idler rotors are turned by the power rotor through the action of the meshing threads. The fluid pumped between the meshing helical threads of the idler and power rotors provides a protective film to prevent metal-to metal contact. The idler rotors perform no work; they do not need to be connected by gears to transmit power. The enclosures formed by the meshing of the rotors inside the close clearance housing contain the fluid being pumped. As the rotors turn, these enclosures move axially, providing a continuous flow. Effective performance is based on the following factors: 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.
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III. Answer the following questions to the text:
1. Where are screw pumps generally used?
2. How do screw pumps operate?
3. What provides a protective film to prevent metal-to metal contact?
4. What is effective performance based on?
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