Pumps – With condition responsive pumped fluid control – Bypass or relief valve controls venting by movable pump...
Reexamination Certificate
2000-06-08
2001-07-10
Vrablik, John J. (Department: 3748)
Pumps
With condition responsive pumped fluid control
Bypass or relief valve controls venting by movable pump...
C418S014000, C418S107000, C418S194000
Reexamination Certificate
active
06257839
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to fluid machinery for controlling a fluid pressure, and more particularly to a double screw rotor assembly that can reduce starting power, start electric current, or adjust operating power. The double screw rotor assembly of the invention can be used in vacuum pumps, air compressors, water or oil pumps, or other fluid media.
FIG. 1
shows a double screw rotor assembly manufactured by KASHIYAMA INDUSTRIES, LTD., and designed for use in a vacuum pump. This structure of double screw rotor comprises two screw rotors
81
and
82
meshed together. Because the screw rotors
81
and
82
have a constant pitch P′ and constant height of tooth H′, the volume of air chamber
810
or
820
does not change while air is transferred from the inlet to the output end
80
, a high pressure difference occurs, thereby causing a reverse flow of air, high noises, and waste of energy.
U.S. Pat. No. 5,667,370 discloses another structure of double screw rotor assembly. According to this design, as illustrated in
FIG. 2
, the meshed screw rotors
83
and
84
have same height of tooth H″, and the pitch is gradually reduced in direction from the input side toward the output side
801
(P
1
>P
2
). Because of P
1
>P
2
, the volume of air chamber
830
or
840
is reduced during transmission, and the pressure in these chambers would be increased gradually. Therefore, when the air chambers were compressed and transmitted to the output end
801
, less pressure difference occurs, the reverse flow of air would be reduced and so as to the high noise. However, because of different pitches and pressure angles are defined at different rotor sections, the fabrication process of the screw rotors
83
and
84
are complicated, resulting in a high manufacturing cost.
FIG. 3
shows still another structure of double screw rotor assembly, which was filed to USPTO for a patent by the present applicant under application Ser. No. 09/372,674. According to this design, two screw rotors are meshed together and mounted in a compression chamber inside a casing, each comprising a spiral thread around the periphery. The thread has a height H made gradually reduced from the input side toward the output side
90
. The threads of the screw rotors define a constant pitch P in order to be manufactured easily. The volumes of the air chambers
910
and
920
reduce gradually from the input side toward the output side, so the pressure can be increased gradually during transmission of air, the consumption of operation power and noise can be reduced. Because a constant pitch P is provided and the height H is made gradually reduced from the input side toward the output side
90
, the outer diameter D has the shape of an invertedly disposed cone, and the inner diameter d has the shape of a regular cone.
According to the aforesaid second and third prior art designs, much starting power is required when starting the double screw rotor assembly. As illustrated in
FIG. 3
, the pressure (i.e. the atmospheric pressure) in all air chambers
910
and
920
, pressure Pi at the input side, and pressure P
0
at the output side, at the initial stage are the same. Because the volumes of the air chambers
910
and
920
are gradually reduced during rotary motion of the screw rotors, the pressure Pmax near the output side surpasses the pressure P
0
(=the atmospheric pressure) at the output side when starting the double screw rotor assembly. Therefore, much more power and electric current are required to drive the rotors
91
and
92
to conquer the flow pressure of all air chambers
910
and
920
. A certain period of time after starting, the flow pressure at the input side
901
is gradually reduced (for example, being drawn into a vacuum state), causing the flow pressure in the air chambers
910
and
920
near the input side
901
to be gradually reduced, and hence the electric power consumed is gradually reduced to the level of the rated working power. Because high working power is required when starting the double screw rotor assembly, high current, noise and vibration occur at the initial state when starting the screw rotors, resulting in an unstable operation.
FIG. 4
shows another prior art design constructed according to U.S. Pat. No. 5,533,887. According to this design, a movable case is sliding in a fixed case, however the spring at the top of the movable case is not adjustable, and the presence of the gap
22
C which is left between the movable case and the fixed case for enabling the movable case to slide in the fixed case which may cause air leakage directly from the high pressure area to the low pressure area, thereby causing a low working efficiency. Further, if the process gas condensed in the gap between movable and fixed cases, the movable case maybe jammed at some position, and the bypass mechanism failed.
In view of the drawbacks of the aforesaid prior art designs, there is a strong demand for a high performance double screw rotor assembly that requires low starting power, and can be conveniently adjusted to fit different manufacturing requirement.
SUMMARY OF THE INVENTION
The present invention has been accomplished to provide a double screw rotor assembly, which eliminates the aforesaid drawbacks. It is one object of the present invention to provide a double screw rotor assembly, which reduces starting power and starting current by adjusting its internal pressure difference, allows the user to adjust operating power for a stable operation, and can be freely adjusted to change the clearance between the rotors and the casing subject to different manufacturing requirements. It is another object of the present invention to provide a double screw rotor assembly, which prevent a leakage to achieve a high performance. According to one aspect of the present invention, the double screw rotor assembly comprises a casing having a receiving chamber; an inlet and an outlet; a bushing axially movably mounted in the receiving chamber inside the casing, the bushing having an inside wall defining a receiving chamber, and an outside wall fitting with the inside wall of the casing; guide means provided to guide axial movement of the bushing relative to the casing; two screw rotors meshed together and mounted in the receiving chamber inside the bushing; and an electric driving means controller to move the bushing axially in the casing relative to the screw rotors to adjust inside pressure in the bushing. According to another aspect of the present invention, the electric driving means comprises at least one rack fixedly mounted in the bushing, and a driving gear meshed with the at least one rack and rotated by, for example, a motor or other electric driving apparatus to move the at least one rack. Chain transmission means, belt transmission means, electromagnetic induction type transmission means may be used instead of the gear and rack transmission mechanism. According to still another aspect of the present invention, the double screw rotor assembly further comprises an O-ring mounted on the top wall of the busing and facing the casing. The O-ring can be made of rubber or any suitable equivalent material. According to still another aspect of the present invention, the double screw rotor assembly further comprises control means controlling the operation of the electric driving means, the control means comprising pressure sensor means, temperature sensor means, wattmeter, or voltmeter which detects the pressure or temperature in the receiving chamber near the outlet, or the value of electric current or voltage at the motor, for enabling the control means to control the electric driving means in moving the bushing subject to the value detected by the pressure sensor means. According to still another aspect of the present invention, the guide means comprises at least one sliding groove formed on the outside wall of the bushing, and at least one guide rib respectively formed integral with the inside wall of the casing and coupled to the at least one sliding groove on
Chen Jiun-Hung
Fang Hong-Sheng
Liu Ming-Hsin
Industrial Technology Research Institute
Rabin & Champagne, P.C.
Vrablik John J.
LandOfFree
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