Rotary, positive displacement machine

Abstract: A rotary positive-displacement machine of the type having intermeshing lobed rotors, comprising first and second two-lobed rotors (22,24) mounted respectively in the two housing bores (14,16) for synchronous rotation. The first, valve rotor (22) has a hub portion which periodically occludes an outlet port (20) to control the generation and discharge of high pressure fluid from the housing (12). Each lobe (32) of the valve rotor (22) has a leading tip portion (34) which is radiussed so that it does not define a sharp edge. Each lobe (32) also has an outer flank, a major portion (1-2) of which is a convex curve, which is generated to correspond to the form of the tip (8-9) of the second, displacement rotor (24) and which merges with a convex arcuate portion (2-3) whose centre (38) is offset from the valve rotor axis (26). Furthermore, each lobe (32) has a trailing flank formed by a convex curve (4-5), generated to correspond to the form of the tip (8-9) of the displacement rotor (24), which merges with a convex arcuate portion (5-6), whose centre (42) is offset from the valve rotor axis (26), followed directly by a concave arcuate portion (6-7) whose centre (44) is also offset from the valve rotor axis (26).

Description: The present invention relates to rotary, positive displacement machines of the type having two intermeshing lobed rotors operating within a pair of parallel intersecting cylindrical bores in a chamber. A large variety of such machines are already known, see for example UK 2113767B, US 4 324 538 and US 4 224 016. Machines of this type have the advantage that the lobed rotors mesh without contact so that no lubrication is required in the compression chamber and compressed gas is delivered oil and contaminant free. These machines are therefore useful for application as gas compressors, expanders, pumps and the like. It is an object of the present invention to improve on the efficiency of known machines of this type. In particular, it is required to find a means of (a) increasing the displacement volume of the machine for a given size of overall chamber envelope; (b) to enable sharp points on the rotors to be eliminated and (c) to enable inlet and outlet port sizes to be maximised for a given rotor spacing. In accordance with the present invention, there is provided a rotary positive-displacement machine of the type having intermeshing lobed rotors, comprising: a housing having two parallel cylindrical intersecting bores defined therewithin; an inlet port communicating with said two bores for the introduction of low pressure fluid to the housing; an outlet port formed in one or both end walls of the housing for the discharging of high pressure fluid from the housing; first and second two-lobed rotors mounted respectively in the two bores for synchronous rotation; said first rotor has a hub portion which periodically occludes said outlet port to control the generation and discharge of high pressure fluid from the housing; each lobe of said first rotor having a leading tip portion which is radiussed so that it does not define a sharp edge; each lobe of said first rotor having a leading outer flank portion in the form of a first convex curve which is generated by a first portion of the other rotor having a concave arcuate form and each lobe of the first rotor also having a trailing outer flank portion in the form of a second convex curve which is generated by a further flank portion of the other rotor which is also of concave arcuate form, characterised in that the first convex curve portion of the first rotor merges with a first convex arcuate portion whose centre is offset from the first rotor axis; the second convex curve portion of the first rotor merges with a second convex arcuate portion, whose centre is offset from the first rotor axis, followed directly by a concave arcuate portion whose centre is also offset from the first rotor axis; and the first convex arcuate portion of the first rotor merges directly with a third convex arcuate portion which itself merges directly with said second convexly curved portion. The benefit of increasing the displacement volume of the machine for a given size of overall chamber envelope and a given set of clearances between rotary and stationary components is that the percentage of displaced fluid which returns as leakage from the high pressure side to the low pressure side of the machine reduces, and this gives a corresponding increase in efficiency and hence reduced operating fluid temperature. Increasing the displacement volume of the machine for a given size of overall chamber envelope also reduces the space occupied and weight of the machine which for road transport applications can be used for additional payload on the vehicle. The benefit of eliminating the sharp edges of the rotor tips is that erosion effects will not result in a reduction of performance over a period of time. With sharp rotor tips which have not suffered erosion or other damage, there is little or no unsealing between the two rotors. However, if tip erosion takes place, then excess leakage will rapidly occur at a part of the compression cycle where there is high pressure in the valve rotor (Fig.3; 9-11) area. Rotors having a defined tip radius unseal when new but do so at a part of the compression cycle where the two rotor chambers combine the charge of fluid at a relatively low pressure, momentarily and therefore without undue losses. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig.1 is a diagrammatic end view of one embodiment of a rotary, positive displacement machine in accordance with the present invention, showing the displacement and valve rotors and the housing which defines the compression chamber; Fig.2 is a line drawing showing the profile of the displacement rotor of the machine of Fig.1; Fig.3 is a line drawing showing the profile of the valve rotor of the machine of Fig.1; Figs. 4a to 4f are diagrammatic end views illustrating the operational co-operation between the displacement and valve rotors through a cycle of relative positions; Fig.5 is a diagram illustrating certain dimensions referred to in the description; and Fig.6 is a series of diagrams comparing certain characteristics of the present machine with those of the prior art. Referring first to Fig.1, the machine 10 has an outer housing 12 in which are formed a pair of parallel, cylindrical bores 14,16 which partially overlap one another in the axial direction to form an internal cavity of generally "figure 8" peripheral profile. An inlet, low pressure port 18 is formed in the peripheral side wall of the housing 12 and an outlet, high pressure port or ports 20 is/are formed in the end wall(s) of the housing bore 14. A first, valve rotor 22 is rotatably mounted in the bore 14 for periodically opening and closing the high-pressure outlet port 20 as it rotates. A second, displacement rotor 24 is mounted in the bore 16 for synchronous rotation with the gate rotor 22. The special constructional and performance characteristics of the present machine arise from the details of the complex, interdependent profiles of the valve and displacement rotors 22,24 and these will now be described and defined with reference to Figures 2, 3, 4 and 5. As illustrated in Figure 5, the centre to centre spacing of the valve and displacement rotors 22,24 is designated C, the maximum diameter of the rotors 22,24 (corresponding substantially to the internal diameters of the bores 14,16) is designated D and the radius of the valve rotor (which slightly exceeds the maximum radial extent of the high pressure outlet port(s) 20) is designated R. Considering first the valve rotor 22, see Fig.3 in particular, this has an axis of rotation 26 about which it is rotated in the direction shown by the arrow A. The rotor 22 is symmetrical about any diameter and has two identical hub portions 28, two identical recessed portions 30 and two identical tip portions 32 disposed symmetrically about a diameter D 1 . Each tip portion 32 has a radiussed tip 34 and does not define a sharp edge in the manner adopted in prior art machines. By omitting such sharp edges, the tips 34 are more resistant to damage and wear and are therefore longer lasting. As explained further hereinafter, in order to enable radiussed tips to be incorporated whilst retaining satisfactory mating of the valve and displacement rotors, it is necessary for other corresponding surfaces on the co-operating rotor (in this case on the displacement rotor) to be generated using the locus of motion of these radiussed tips. Extending rearwardly from the tips 34, the valve rotor has a first portion (0-1) extending over an angle a which is a true arc about the rotational axis 26. Merging smoothly with arcuate portion (0-1) is a second portion (1-2) which is a non-arcuate, generated convex curve. At the junction of the portion (0-1) with the portion (1-2), the tangents to the respective curves is identical so as to obtain a smooth transference. The generation shape of the portion (1-2) is determined to achieve effective rolling (non-touching) co-operation with an arcuate portion (1-2) on the displacement rotor described further hereinafter. Merging smoothly with the portion (1-2) of the valve rotor is an arcuate portion (2-3) of angle b whose centre of generation is disposed remote from the rotor axis 26 at a position 38. There is no discontinuity at the joint between the curves (1-2) and (2-3), the tangents to these curves being identical at the junction. The provision of the convex generated curve (1-2) followed directly by the arcuate curve (2-3) enables the ratio between rotor centres (C) and housing diameter (D) to be reduced beyond that of the prior art. The off-axis arcuate portion (2-3) merges smoothly with a portion (3-4) which is a true arc about the rotor axis 26 of angle c . Again, the tangents to the curves (2-3) and (3-4) are identical at their junction 40. The provision of the convex generated curve (1-2) followed directly by the off-axis arcuate curve (2-3) and then by the arcuate curve (3-4) enables the ratio between rotor centres (C) and housing diameter (D) to be reduced beyond that of the prior art. In the prior art exemplified by UK 2113767, the corresponding part of the valve rotor has a concavity connecting the tip portion to the main arcuate hub portion. The latter construction imposes a limitation of continuity of rotor profile (see Fig.6c) as centres (C) are reduced for a given housing diameter (D). Referring further to Figure 3, the arcuate portion (3-4) of the valve rotor merges smoothly with a convex generated portion (4-5), followed by a convex arc (5-6) of angle d and centre 42, and then a concave arc (6-7) of angle e and centre 44. The generation shape of the portion (4-5) is determined to achieve effective rolling (non-touching) cooperation with an arcuate portion (4-5) on the displacement rotor 24 described further hereinafter. The corresponding portion of the known machine of UK 2113767 consists of two generated curves of opposite hand. Compared to the latter structure, the present arrangement enables closer spacing C of the rotor axes and therefore greater displacement volume for a given size of the overall envelope of the compression chamber. The concave arcuate portion (6-7) is followed by a convex arcuate portion (7-8) of angle f which in turn is followed by a generated portion (8-10) coresponding to the locus of the tip (8-9) of the displacement rotor. The generated portion (8-10) is followed by the radiussed tip (10-11) of the valve rotor. Thus the valve rotor 22 is constructed such that each lobe (32) has a leading flank, a portion (1-2) of which is a convex curve, and which merges with a convex arcuate portion (2-3) whose centre (38) is offset from the first rotor axis (26); and such that each lobe (32) has a trailing flank formed by a convex curve (4-5), which merges with a convex arcuate portion (5-6), whose centre (42) is offset from the first rotor axis (26), followed directly by a concave arcuate portion (6-7) whose centre (44) is also offset from the first rotor axis (26). The convex arcuate portion (2-3) merges directly with a convex arcuate portion (3-4) which itself merges directly with the convexly curved portion (4-5). The convexly curved portion (1-2) merges directly with a convex arcuate portion (0-1) which itself merges directly with the radiussed tip portion (34). The concave arcuate portion (6-7) merges directly with a convex arcuate portion (7-8) which itself merges with a complex curved portion (8-10) generated to correspond to the form of the tip (8-9) of the second rotor (24). Thus, all portions of the valve rotor are true arcs except portions (1-2), (4-5) and (8-10). Turning now to the displacement rotor 24 (see Fig.2), this has a first portion (0-1) in the form of a true convex arc of angle g leading to a second portion (1-2) in the form of a true concave arc of angle h and centre at 46. Arcuate portion (1-2) merges smoothly with a convex generated curve (2-3) whose shape is determined by the convex arcuate portion of the valve rotor which merges with the outer flank of the valve rotor 22. The tangents to the curves (2-3) and (1-2) at their junction 48 are identical to achieve a smooth changeover. In the corresponding region of the displacement rotor in the prior art, the sharp change in rotor form is due to the loss of arc space caused by accommodating a concave form at (2-3) on the valve rotor. Generated convex portion (2-3) merges smoothly with a portion (3-4) which is a true convex arc of angle i about the rotor axis. This is followed by a true concave arc (4-5) of angle j whose centre is off-axis at 50. The arcuate portion (4-5) is followed by generated convex portions (5-6) and (6-7), and then by a true arc (7-8) of angle l about the rotor axis. The latter portion leads to a radiussed tip portion (8-9). Finally, the tip portion is coupled to a concave generated portion (9-11) whose shape follows the locus of the tip (10-11) of the valve rotor. Referring now to Figures 5 and 6, in order to achieve the requirement that displacement volume is to be increased for a given size of overall compression chamber envelope, two conditions are being sought. Firstly, the ratio C D = rotor centres rotor diameter is to be reduced as far as possible. Secondly, the ratio R D = rotor hub radius rotor diameter , which is a function of air flow restriction during the compression cycle, is to be optimised. The restriction arc between rotor radius R and housing radius D/2 must not be too small as fluid must transfer from one rotor/bore pocket to another (Figs. 4a-4e) with minimum pressure loss. In conflict with this requirement, the ratio R/D should be maximised to increase port opening area as rotor radius R governs the outer radius of the ports. Figs. 6a and 6b show the prior art and the present machine in the case where the ratios are C D = 0.76 and R D = 0.4136 Both profiles are mathematically correct at this C/D ratio, and also at higher values. Figures 6c and 6d show the situation at a location X on the displacement rotor corresponding to the generated portion (2-3) in Figure 2, when the ratio C/D has been reduced to 0.72. The ratio R/D remains at 0.4136. Although both profiles are still mathematically correct in the magnified region, the C/D ratio is near to its mathematical limit in the prior art machine. Figures 6e and 6f show the situation at the location X when the C/D ratio has been reduced to 0.68, the ratio R/D remaining at 0.4136. It can be seen from Fig.6e that the profile of the prior art machine has become disjointed and is no longer a smoothly continuous curve. This would result in practice in the rotors clashing or unsealing. It will be noted that the profile of the present machine (Fig.6f) remains correct at this, and lower, C/D ratios. A complete cycle of operation of the present valve and displacement rotors is illustrated in Figs. 4a to 4f. A detailed description of these Figures is not deemed necessary. The features described above contribute to achieving the stated objects of increasing displacement volume for a given chamber envelope, enabling sharp edges on the rotor tips to be eliminated and inlet and outlet port size to be optimised for a given rotor spacing. Furthermore, the large internal radii in the rotor profiles requires only the use of long edge spiral flute milling cutters of substantial diameter on a machining centre to produce rotors accurately of a substantial length. The relatively large internal radii defined on both rotors generate correspondingly large external curves on the flanks of the meshing rotor. This reduces internal gas throttling losses between the edge of the rotor and bore in which it rotates. The use of only large curves on the rotor flanks also serves to reduce gas slip from the high pressure chamber to the low pressure chamber, particularly at (2-3), (4-5) and (6-7). Finally, large curves on the rotor flanks suffer less from erosion when running at high speeds than sharp edges so that the useful life of the machine is increased.

Claims: A rotary positive-displacement machine of the type having intermeshing lobed rotors, comprising: a housing (12) having two parallel cylindrical intersecting bores (14, 16) defined therewithin; an inlet port (18) communicating with said two bores (14, 16) for the introduction of low pressure fluid to the housing (12); an outlet port 20 formed in one or both end walls of the housing (12) for the discharging of high pressure fluid from the housing; first and second two-lobed rotors (22, 24) mounted respectively in the two bores (14, 16) for synchronous rotation; said first rotor (22) has a hub portion which periodically occludes said outlet port (20) to control the generation and discharge of high pressure fluid from the housing (12); each lobe (32) of said first rotor (22) having a leading tip portion (34) which is radiussed so that it does not define a sharp edge; each lobe (32) of said first rotor (22) having a leading outer flank portion (1-2) in the form of a first convex curve which is generated by a flank portion (1-2) of the other rotor (24) having a concave arcuate form and each lobe (32) of the first rotor (22) also having a trailing outer flank portion (4-5) in the form of a second convex curve which is generated by a further flank portion (4-5) of the other rotor (24) which is also of concave arcuate form, characterised in that the convex curve portion (1-2) of the first rotor (22) merges with a convex arcuate portion (2-3) whose centre (38) is offset from the first rotor axis (26); the convex curve portion (4-5) of the first rotor (22) merges with a convex arcuate portion (5-6), whose centre (42) is offset from the first rotor axis (26), followed directly by a concave arcuate portion (6-7) whose centre (44) is also offset from the first rotor axis (26); and the convex arcuate portion (2-3) of the first rotor (22) merges directly with a convex arcuate portion (3-4) which itself merges directly with the convexly curved portion (4-5). A machine as claimed in claim 1, wherein the convexly curved portion (1-2) of the first rotor (22) merges directly with a convex arcuate portion (0-1) which itself merges directly with the radiussed tip portion (34). A machine as claimed in claim 1 or 2, wherein said concave arcuate portion (6-7) of the first rotor (22) merges directly with a convex arcuate portion (7-8), which itself merges with a complex curved portion (8-10) generated to correspond to the form of the tip (8-9) of the second rotor (24).

Inventors: SCHMITZ LOTHER PETER, SCHMITZ, LOTHER PETER
Applicants: DRUM INT LTD, DRUM INTERNATIONAL LIMITED
Application year: 1991
Main classification: F01C1
IPC codes: F01C1