HK1103115B - Screw machine - Google Patents

Description

Screw machine

This application is a divisional application entitled "screw machine" entitled application number 00819694.X filed on Appl. No. 12/21, 2000.

Technical Field

The present invention relates to a screw machine comprising a rotor housing having a pair of parallel, partially overlapping bores; a pair of conjugated intermeshing rotors located in the bores.

Background

In a conventional screw machine, a male rotor and a female rotor are each positioned in parallel in partially overlapping bores defined in a rotor housing and cooperate to gather and compress gas volumes. While two rotors are the most common design, three or more rotors may act in pairs. The tooth profiles and the number of teeth and tooth slots of the male and female rotors are different. For example, a female rotor may have six teeth separated by six tooth slots, while a male rotor conjugated therewith may have five teeth separated by five tooth slots. Accordingly, the interaction between the rotors for each possible combination of teeth and slots occurs on a base circle. The common effect between the conjugated rotor pairs is a combination of sliding and rolling contact, which results in different wear rates. In addition to the paired interaction, the rotor interacts with the housing. Because all combinations of rotor contact occur between conjugate pairs, the seal/leakage between different bonds may differ due to machining tolerances and wear conditions. This may occur despite machining tolerances being closely held due to the attendant machining costs and providing adequate lubrication and other fluid injection to seal.

The profile design of the conjugate pair of the screw rotors must be provided with clearance in most sections. A number of factors contribute to the need to set clearances, including: thermal expansion of the rotor due to the gas being heated during compression; deflection of the rotor due to pressure loading from the compression process; tolerances of the support bearing structure and machining tolerances of the rotors may sometimes tend to bring the rotors too close to each other, which will cause interference; and machining tolerances on the rotor profile can itself cause disturbances. Superimposed on the above factors are pressure and temperature gradients that accompany the increase in pressure and temperature from intake to exhaust.

The pressure gradient is generally in one direction during operation, so that the fluid pressure tends to force the rotor toward the suction side. The rotor is typically mounted on bearings at each end, providing both radial and axial constraint. The endplay of the rotor on the exhaust side is critical to sealing and fluid pressure tends to open the endplay.

There are certain sections on the rotors, such as contact bands, where zero clearance is maintained between the rotors. The section of the rotor defining the contact zone is the region where the required torque is transmitted between the rotors. The load between the rotors is different for a male rotor drive and for a female rotor drive. In a male rotor drive, the load between the rotors may be equal to about 10% of the overall compressor torque, while in the case of a female rotor drive, the load between the rotors may be equal to about 90% of the overall compressor torque. These sections are typically located near the pitch circle of the rotor, which is the equal speed position that results in rolling contact on the rotor, thereby reducing or eliminating sliding contact and thus reducing wear.

A large amount of end running clearance must be maintained at the discharge end of the screw compressor to prevent failure due to rotor seizure. Galling may be caused by thermal expansion of the rotor or by intermittent contact between the rotor and the end shell due to pressure pulsations during compression.

Disclosure of Invention

It is an object of the present invention to reduce leakage in screw machines.

It is another object of the present invention to relax machining tolerances without increasing leakage.

It is a further object of the present invention to reduce the oil seals required in screw machines.

It is a further object of the invention to minimize power losses due to friction and to prevent wear. These and other objects will become apparent upon completion of the present invention.

In accordance with the present invention, a coating is applied to one or more portions of the inner bore surface of the screw rotor and/or housing.

Specifically, the present invention provides a screw machine comprising a rotor housing having a pair of parallel, partially overlapping bores; a pair of conjugated intermeshing rotors located in the bores, each of the rotors having helical teeth and intervening tooth spaces; an outlet end housing at the discharge end of said rotor housing, each of said rotors having a discharge end facing said outlet end housing; wherein a low friction wear resistant coating is located between said exhaust end of said rotor and said outlet end housing.

In one aspect of the invention, a low friction wear resistant material may be located at the rotor tips where the rotors may have normal contact with the housing and with each other. The rotors co-operate in pairs with each other and also with the housing. While close machining tolerances reduce leakage due to interaction between the rotors themselves and the housing, other things can be accomplished by incorporating or following close tolerances. Examples of suitable low friction wear resistant materials include multi-layer Diamond Like Carbon (DLC) coatings, titanium nitride and other single materials, single nitride coatings, and cemented carbide and ceramic coatings, which have high wear resistance and low coefficients of friction.

In another aspect of the invention, a suitable coating may be located on the inner bore surface of the housing and/or the rotor cogging. Examples of suitable coatings include: such as iron phosphate coatings, magnesium phosphate coatings, nickel mercury polymers, and other materials that yield elasticity when force is applied. Proper coating location on the inner bore surface of the housing and/or rotor cogging may reduce the need for leakage and oil seals, while relaxing machining tolerances.

A surface coated with a low friction wear resistant material or by an equivalent treatment is more tolerant of sliding contact than an untreated surface. There is a synergistic effect in connection with the treatment that the coated surface has a greater tolerance for sliding contact. In accordance with yet another aspect of the invention, this allows the contact strip to move farther away from the pitch circle, further reducing the contact force and reducing the potential for total wear on the treated rotor with the relocated contact strip. In conventional practice, the contact band is placed close to the pitch circle of the rotor, which, as noted, represents a need to approach pure rolling contact.

The location of the contact strip is a design feature and can be removed from the pitch circle or where you want to locate. By removing the contact band from the pitch circle, the load between the rotors can be reduced, which is particularly important for female rotor drives. As the contact begins to move away from the pitch circle, there is more sliding contact than pure rolling contact. The cavitation zone, which relates to the leakage zone defined by the edges of the intersection between the tips of the meshing rotor teeth and the adjacent holes of the screw machine, can only be reduced to zero if the respective pitch circles coincide with the root circle of the male rotor and the tip circle of the female rotor. This necessarily requires that the contact band be located away from the pitch circle in response to a coordination between drive angle, contact pressure, machinability of the root radius of the male rotor, and the amount of slippage that will occur.

The penalty for maintaining such a large end tooth backlash is an increased leakage from the high pressure region to the low pressure region. In accordance with another aspect of the invention, the end tooth backlash is reduced by at least 50% by applying a wear resistant coating having a low coefficient of friction to the end surface of the rotor or to the surface of the end housing, or by interposing a coated member between the rotor end and the end housing. The performance of the compressor is improved due to the reduced leakage at the discharge end.

Drawings

For a more complete understanding of the present invention, reference is now made to the following detailed description of various embodiments, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view through a screw machine;

FIG. 2 is a partial cross-sectional view of the screw machine of FIG. 1;

FIG. 3 is an enlarged view of a portion of the discharge end of the screw machine of FIG. 1;

FIG. 4 is an enlarged portion of FIG. 1 with various coatings shown in the present invention;

FIG. 5 is a partial cross-sectional view showing the DLC coated on the rotor end;

FIG. 6 is a partial cross-sectional view showing the DLC coated on the exhaust end housing; and

FIG. 7 is a partial cross-sectional view showing a DLC coated disk;

FIG. 8 is an enlarged view of the DLC coating; and

fig. 9 is a perspective view of an axial segment of the rotor pair of fig. 1.

Detailed Description

In FIG. 1, a screw machine 10, such as a screw compressor, is shown having a rotor housing or shell 12 with partially overlapping bores 12-1 and 12-2 located therein. Having a pitch circle P_FIs located in the bore 12-1. Having a pitch circle P_MIs located in the bore 12-2. The parallel axes indicated by points A and B are perpendicular to the plane of FIG. 1 and are separated by a pitch circle P equal to the female rotor 14_FRadius R of_FAnd pitch circle P of the male rotor 16_MRadius R of_MDistance of the sum. The axis shown by point A is the axis of rotation of the female rotor 14 and is generally the center of the bore 12-1, the diameter of the bore 12-1 generally corresponding to the addendum circle T of the female rotor 14_FDiameter ofAnd (4) the same. Similarly, the axis indicated by point B is the axis of rotation of the male rotor 16 and is generally the center of the bore 12-2, the diameter of the bore 12-2 generally corresponding to the addendum circle T of the male rotor 16_MAre consistent in diameter. Typically, the centerlines of the rotor and bore are offset by a very small amount to compensate for clearances and deviations. Ignoring operational clearances, the extension of the bore 12-1 through the portion partially overlapping the bore 12-2 is in the root circle R with the male rotor 16_MRThe tangent point intersects line a-. Similarly, the extension of the hole 12-2 through the portion partially overlapping the hole 12-1 is in the root circle R of the female rotor 14_FRIntersects line a-B. The common point is represented by F₁、M₁Is marked, wherein F₁Relative to the female rotor 14, M₁Relative to the male rotor 16.

In the illustrated embodiment, female rotor 14 has six blades or tips 14-1 separated by six female slots or slots 14-2, while male rotor 16 has five blades or tips 16-1 separated by five female slots or slots 16-2. Accordingly, the rotational speed of the rotor 16 is 6/5 or 120% of the rotational speed of the rotor 14. Either the female rotor 14 or the male rotor 16 may be connected to a motive force (not shown) and act as a drive rotor. Other numbers of combinations of concave and convex tooth edges and gullets may be used.

Referring now to fig. 2 and 3, the rotor 14 has a shaft portion 14-3 with a shoulder 14-4 formed between the shaft portion 14-3 and the rotor 14. The shaft portion 14-3 of the rotor 14 is supported in the outlet or exhaust casing 13 on one or more bearings 30. Similarly, the rotor 16 has a shaft portion 16-3 with a shoulder 16-4 formed between the shaft portion 16-3 and the rotor 16. The shaft portion 16-3 of the rotor 16 is supported in the outlet or exhaust casing 13 on one or more bearings 31. The suction side shaft portions 14-5 and 16-5 of the rotors 14 and 16 are each supported by roller bearings 32 and 33, respectively, and accommodated in the rotor case 12.

In operation, as a refrigerant compressor, assuming the male rotor 16 as the driving rotor, the rotor 16 rotates and causes the meshed rotor 14 to rotate. The combined action of the rotating rotors 16 and 14 in the respective bores 12-1 and 12-2 draws refrigerant gas into the tooth slots of the rotors 16 and 14 through the suction port 18, the rotors 16 and 14 intermesh to gather and compress the gas volume and deliver the hot compressed gas to the discharge port 19. This trapped gas acting on rotors 14 and 16 is movable, tending to separate discharge end 14-4 from outlet housing surface 13-1 to create/increase a leakage path. The movement of rotors 14 and 16 away from outlet housing surface 13-1 causes rotors 14 and 16 to move toward or into engagement with surface 12-3 of rotor housing 12 via shoulders 14-6 and 16-6, respectively. In addition to the leakage path between the rotor shoulders 14-4, 16-4 and the outlet casing surface 13-1, leakage may also occur across the line contact between the rotors 14 and 16 and the line contact between the land tips 14-1 and 16-1 and the bores 12-1 and 12-2. Leakage across the land/line contact can be reduced by the use of sealing oil, but the oil creates viscous drag losses between moving parts and must be removed from the exhaust gas.

As noted herein, the contact zone is defined by zero clearance, not by location. Fig. 4 shows an enlarged portion of fig. 1 to illustrate the repositioning of the contact strip in accordance with an aspect of the present invention. The contact zone is located in the region of the concave cusp 14-1 at the pitch circle P of the concave rotor 14_FAnd the pitch circle P of the male rotor 16 in the region of the male roots 16-2_MOutside of (a).

For oil-free compressors, the rotor tips must be as close as possible to the rotor case bores 12-1 and 12-2 to reduce leakage due to sealing without oil. If contact occurs between the rotor and the housing, the wear and power loss due to friction between the rotor tips and the housing will be significant. Even if the rotor is lubricated, there are leaks across the oil seal and oil must be removed from the refrigerant to minimize its circulation through the refrigeration system which deteriorates heat transfer efficiency, as well as to retain the oil needed to lubricate the compressor.

In accordance with one aspect of the invention, the tips or slots 14-1 and 16-1 of the rotors 14 and 16, respectively, are coated with a low friction, wear resistant coating. One suitable low-friction wear resistant coating is a low-friction diamond-like carbon (DLC) coating, which is used locally on the tip surface of the blades of the rotary compressor disclosed in commonly assigned US patent 5672054. Such a DLC coating is used to overcome the lubrication difficulties associated with the use of new oil and refrigerant combinations. The DLC coating is both lubricious and wear resistant in that it is composed of a layer of hard material, such as tungsten carbide and amorphous carbon, as detailed in US patent 5672054, the complete disclosure of which is incorporated herein by reference.

Examples of other suitable low friction wear resistant coatings include titanium nitride and other single materials, single nitride coatings, and cemented carbide and ceramic coatings that have both high wear resistance and a low coefficient of friction. The presence of a low-friction wear-resistant coating in the tooth tips or tooth-blade tooth slots of each rotor provides several advantages. First, operation without or with reduced lubrication relative to the rotor may be possible without excessive wear or friction. Second, machining tolerances are relaxed because a degree of contact with the rotor bore can be tolerated. Third, because it is possible to operate with less clearance between the rotor tips or edges 14-1 and 16-1 and the rotor bores 12-1 and 12-2, respectively, the need for oil sealing between the rotor and the rotor bores may be reduced or eliminated.

Because the contact strips on the female rotor 14 are located close to the tips of the teeth, a single DLC coating can be used to cover two important areas on the female rotor depending on the rotor profile, due to their narrow spaces or partial overlap, a single DLC coating 40 on the female rotor is preferably easy to install, as shown in fig. 4. Portion 40-1 of coating 40 corresponds to the contact zone and portion 40-2 corresponds to the tooth tip or gullet portion 14-2 that will be closest to aperture 12-1. The corresponding coating DLC on the male rotor 16 is more widely separated by coating 60 on the rotor tips and coating 61, with coating 60 being located on the portions of the rotor tips and coating 61 being located adjacent the portions of the roots corresponding to the contact bands.

Like the rotor tooth tips, the rotor ends run with a clearance that forms a leakage path. In another aspect of the invention, the DLC coating may be applied to the exhaust end surface of the rotor, which surface faces the surface of the exhaust end housing 13, or to a coated liner located between the rotor and the exhaust end housing 13, to reduce backlash and thereby reduce leakage paths. Referring now to fig. 5, a DLC coating is applied to the exhaust ends of rotors 14 and 16. Specifically, DLC coating 42 is applied to the exhaust end of female rotor 14 and DLC coating 62 is applied to the exhaust end of male rotor 16. Because DLC coatings 42 and 62 can accommodate a degree of contact with outlet housing surface 31, reduced end tooth backlash with reduced leakage can be used. Referring now to FIG. 6, a DLC coating 82 is applied to the end housing surface 13-1, rather than to the exhaust ends of the rotors 14 and 16 as in the embodiment of FIG. 5. In the embodiment of fig. 7, a spacer 90 is located between the ends of rotors 14 and 16 and end housing surface 13-1. Because the partition 90 conforms to the cross-section of the apertures 12-1 and 12-2, there is no rotational or relative movement between the partition 90 and the discharge ends of the rotors 14 and 16. Accordingly, only the surfaces of spacers 90 facing rotors 14 and 16 need be provided with DLC coating 92. In the embodiment of fig. 5-7, the DLC coating is located between the ends of rotors 14 and 16 and surface 13-1 so that the lubricity of the coating will protect the rotors and casing from wear in incidental contact, allowing for close end tooth backlash and narrow leakage paths.

Referring now to fig. 8, a more exaggerated cross-section of exemplary coatings 40, 42, 60, 61, 82, and 92 is shown, although it is designated herein only at 40. The DLC coating 40 is composed of a hard bilayer 40' and a lubricious bilayer 40 ". The bilayer thickness is in the range of 1-20nm, preferably in the range of 5-10 nm.

In another aspect of the invention, a suitable coating that may be milled or extruded may be applied to the rotors 14 and 16 and/or the bores 12-1 and 12-2. While the entire rotor and bore may be coated, as shown in fig. 9, the partial coatings in the rotor slots or grooves 14-2 and 16-2 provide virtually all of the benefits with respect to the co-action between the rotors. Although the contact band is a zero clearance area and requires precise machining, tolerances may be relaxed relative to the co-action between the remaining portions of the rotor blade profile. In addition, proper coating of the bores 12-1 and 12-2 accommodates deflection of the rotors 14 and 16 during actual operation, thereby maintaining the sealing function. Referring to fig. 4 and 9, the female rotor slots may be provided with a suitable coating 44 and the male rotor slots may be provided with a suitable coating 64. Additionally, the apertures 12-1 and 12-2 may be provided with a suitable coating 84.

Various plastic suitable coatings may also be used, including: such as iron phosphate, magnesium phosphate, nickel mercury polymer, nickel zinc alloy, aluminum silicon alloy with polyester, and aluminum silicon alloy with Polymethylmethacrylate (PMMA). Of course, conventional coating means, including, for example, thermal spraying, Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), or any suitable water deposition, may be used to treat the surfaces of the screw machine of the present invention.

Although the present invention has been particularly shown and described in the form of a twin rotor screw machine, it may also be applied to a screw machine using three or more rotors. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims (7)

1. A screw machine comprising a rotor housing having a pair of parallel partially overlapping bores; a pair of conjugated intermeshing rotors located in the bores, each of the rotors having helical teeth and intervening tooth spaces; an outlet end housing at the discharge end of said rotor housing, each of said rotors having a discharge end facing said outlet end housing; wherein a low friction wear resistant coating is located between said exhaust end of said rotor and said outlet end housing.

2. The screw machine of claim 1 wherein said low friction wear resistant coating comprises a low friction wear resistant coating on said discharge end of said rotor.

3. The screw machine of claim 2 wherein said low friction wear resistant coating comprises a diamond-like carbon coating comprising a series of optional hard and lubricious layers.

4. The screw machine of claim 1 wherein said low friction wear resistant coating comprises a wear resistant coating on said outlet end housing.

5. The screw machine of claim 4, wherein said low friction wear resistant coating comprises a diamond-like carbon coating comprising a series of optional hard and lubricious layers.

6. The screw machine of claim 1 further characterized by a member located between said discharge end of said rotor and said outlet end housing, said member having a surface facing said discharge end of said rotor, said low friction wear resistant coating comprising a low friction wear resistant coating on the surface of said member.

7. The screw machine of claim 6 wherein said low friction wear resistant coating comprises a diamond-like carbon coating comprising a series of optional hard and lubricious layers.