Machine element or mechanism – Gearing – Teeth
Utility Patent
1998-12-10
2001-01-02
Hannon, Thomas R. (Department: 3682)
Machine element or mechanism
Gearing
Teeth
Utility Patent
active
06167771
ABSTRACT:
BACKGROUND OF THE INVENTION
The profile design of a conjugate pair of screw rotors starts with zero clearance profiles which satisfy all conjugacy requirements and do not have any gap between the male and female rotors at any conjugate point. The design is then modified to include a clearance resulting in clearance profiles. The modifications are in the direction normal to the rotor profile at any given point and can vary from point-to-point.
The need to provide a clearance is the result of a number of factors including: thermal growth of the rotors as a result of gas being heated in the compression process; deflection of the rotors due to pressure loading resulting from the compression process; tolerances in the support bearing structure and machining tolerances on the rotors which may sometimes tend to locate the rotors too close to one another which can lead to interference; and machining tolerances on the rotor profiles themselves which can also lead to interference. Superimposed upon these factors is the existence of pressure and thermal gradients as the pressure and temperature increase in going from suction to discharge.
To accommodate these factors, the zero clearance profile coordinates are modified before manufacturing the rotors. If zero clearance profiles are manufactured and put in an operating compressor, it will result in interference between the rotors due to some or all of the foregoing reasons, causing excessive wear and high bearing loads, if the compressor can even operate at all. Clearance profiles introduce leakage as the price of reducing wear and bearing loads since a zero clearance profile has no leakage area through the seal line of the mesh zone of the rotors. The leakage through the clearance area of the seal line will flow directly from the compression chamber back to the compressor suction and thus tends to be a more severe leak than, for example, leakage across the tip clearance or through the blow hole, both of which tend to be between successive compression chambers.
SUMMARY OF THE INVENTION
In providing a clearance profile according to the teachings of the present invention, a number of factors are initially considered. As to thermal growth, it is estimated based upon the size and material of the rotors and the design operating temperature of the compressor. The associated clearances should be such that when maximum expected thermal growth occurs, there is no unwanted interference between the rotors. As to deflection, it is a function of the loading and stiffness characteristics of the rotors and of the support bearing structure. The pressure loading on the rotors is determined from the operating characteristics of the compressor application. Stiffness characteristics of the rotors are determined from their size and material as well as the supporting mechanism. The associated clearances are selected such that there is no unwanted contact at maximum deflections. Finally, the associated clearances of the rotors are determined by the capabilities of the respective manufacturing processes for the rotors, supporting mechanism, and locating features. The associated clearances are selected such that there is no unwanted contact at maximum tolerance deviations. All of these various factors are considered in determining the total amount of clearance to be introduced between the rotors. As a result the typically maximum clearance requirements can be determined for selected key points. At the same time, there are other key zones, such as the contact band and the backlash zone, where either zero or minimum clearance is desired. The typically minimum clearance requirements can be determined for these selected key points.
Knowing the clearance requirements, the next step according to the teachings of the present invention is to achieve a reduced leakage area while still having required clearances to produce functional screw rotor profiles.
Selecting two points on the zero clearance rotors such as a point at or near the tip and a point at or near the root of the rotor, the clearance requirement for functional screw rotor profiles at these points can be determined based on the requirements of manufacturing tolerances, deflection, thermal growth, etc.
There are certain sections of the rotor, such as the contact band, where zero clearance is maintained between the rotors. The segment of the rotor defining the contact band is the region where the required torque is transmitted between the rotors. These segments are positioned near the pitch circles of the rotors which is the location of equal rotational speed on the rotors resulting in rolling contact and thereby in less wear. As contact starts to move away from the pitch circle there is more sliding contact rather than pure rolling contact which would result in more wear if the contact band were to be located away from the pitch circle.
There are other sections of the rotor, such as the backlash zone, where a controlled clearance is maintained after allowing for the effects of tolerances, deflections, etc. The backlash zone is positioned near the pitch circle on the opposite side of the screw rotor lobes from the contact zone. The controlled clearance of the backlash zone prevents too tight of a fit between the two rotors which might otherwise cause binding and wear while at the same time limiting the space available for the rotors to rattle or impact each other through the backlash clearance which might otherwise result in objectionable noise and/or vibration.
Together, the four zones, namely the rotor tip, the root, the contact band and the backlash zone, constitute portions where a specific, well-defined clearance or clearance range is established separately for each portion. Conventionally there would be a linear distribution of the clearance between these positions. A quadradic, cubic or higher order distribution can be used to vary the clearances while reducing the leakage area defined by the clearances since such distributions initially reduce the clearances more rapidly than a linear distribution, leaving smaller clearances over the rest of the profile between the two points. It should be noted that distribution of clearance should be smooth to accommodate manufacturing processes with no steps allowed.
It is an object of this invention to provide functional screw rotor profiles with reduced leakage areas.
It is another object of this invention to reduce compressor noise/vibration.
It is a further object of this invention to more sharply define the contact band.
It is an additional object of this invention to provide clearance distribution such that the contact band is close to the pitch circle with sufficiently large clearances away from the contact band such that no sliding takes place even when tolerances and deflections are considered. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, zero clearance screw rotor profiles are modified by determining clearance requirements at spaced points on the profiles and varying the clearance distribution between adjacent points using a non-linear distribution. Additionally, when the clearances are put in the rotors, zero clearance is maintained between the rotors in the contact band which is maintained near the pitch circle.
REFERENCES:
patent: 4671751 (1987-06-01), Kasuya et al.
patent: 4673344 (1987-06-01), Ingalis
patent: 5044906 (1991-09-01), Tsuboi
patent: 5947713 (1999-09-01), Ohman et al.
Bush James W.
Kumar Keshava B.
Carrier Corporation
Hannon Thomas R.
Hansen Colby
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