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Tham khảo tài liệu 'volume 1 mechanical science handbooks doe-hdbk part 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | POSITIVE DISPLACEMENT PUMPS DOE-HDBK-1018 1-93 Pumps Lobe Type Pump The lobe type pump shown in Figure 16 is another variation of the simple gear pump. It is considered as a simple gear pump having only two or three teeth per rotor otherwise its operation or the explanation of the function of its parts is no different. Some designs of lobe pumps are fitted with replaceable gibs that is thin plates carried in grooves at the extremity of each lobe where they make contact with the casing. The gib promotes tightness and absorbs radial wear. Figure 16 Lobe Type Pump Screw-Type Positive Displacement Rotary Pump There are many variations in the design of the screw type positive displacement rotary pump. The primary differences consist of the number of intermeshing screws involved the pitch of the screws and the general direction of fluid flow. Two common designs are the two-screw low-pitch double-flow pump and the three-screw high-pitch double-flow pump. Two-Screw Low-Pitch Screw Pump The two-screw low-pitch screw pump consists of two screws that mesh with close clearances mounted on two parallel shafts. One screw has a right-handed thread and the other screw has a left-handed thread. One shaft is the driving shaft and drives the other shaft through a set of herringbone timing gears. The gears serve to maintain clearances between the screws as they turn and to promote quiet operation. The screws rotate in closely fitting duplex cylinders that have overlapping bores. All clearances are small but there is no actual contact between the two screws or between the screws and the cylinder walls. ME-03 Page 24 Rev. 0 Pumps DOE-HDBK-1018 1-93 POSITIVE DISPLACEMENT PUMPS The complete assembly and the usual flow path are shown in Figure 17. Liquid is trapped at the outer end of each pair of screws. As the first space between the screw threads rotates away from the opposite screw a one-turn spiral-shaped quantity of liquid is enclosed when the end of the screw again meshes with the .