Patent GB-1189856-A (GB)
Title: Improvements in Screw Rotor Machines and Profiles
IPC: F01C1/08
Filed: 1967-06-01
Abstract: 1,189, 856. Screw rotor machines. ATLAS COPCO A. B. June 1, 1967 [June 22, 1966], No. 25352/67. Heading F1F. In a screw rotor machine comprising a housing having a pair of intersecting parallel bores and main and gate rotors mounted for rotation within the bores which have at least three inter-meshing helical threads and grooves with the gate rotor having at least one more thread and groove than the main rotor and with the threads of the main rotor having generally convex flanks and lying substantially outside the pitch circle and the threads of the gate rotor having generally concave flanks and lying substantially inside the pitch circle, the thread profiles are such that portions thereof have zero clearance therebetween to provide areas subjected to the pressure of the gaseous working fluid to produce an internal driving torque which in oil-flooded compressors or machines enables synchronizing gears to be dispensed with without any risk of wear of the rotor profiles. Also other portions of the threads have clearance therebetween large enough to ensure that contact will not occur during running. In Fig. 7, M and G indicate main and gate rotor axes, R 1  and R 2  main and gate rotor pitch circle radii and R M  and R G  main and gate rotor bores with the bore of the gate rotor equal to its pitch circle. A short straight radial portion a 2  -b 2  on the leading flank of the gate rotor forms a band seal portion which generates a first root portion a 1  -b 1  on the trailing flank of the main rotor to give a zero or minimum clearance Cl8, Figs. 10e. The point b 2  generates a second portion on the trailing flank of the main rotor between the root portion and crest C 1  to give a clearance Cl 6, Fig. 10d large enough to give a clearance under all operating conditions of pressure and temperature. Similarly a short radial portion e 2  -d 2  on the trailing flank of the gate rotor forms a band seal portion which generates a root portion e 1  -d 1  on the leading flank of the main rotor to give a zero or minimum clearance Cl 2, Fig.10c. Portion d 2  -c2 of the gate rotor and portion d 1  -C 1  of the main rotor are circular arcs with point P, the intersection point of the pitch circle as centre with a clearance Cl 4 therebetween again sufficient to ensure clearance in all operating conditions. Clearance Cl 1, Fig,10a, between the crest of the gate rotor and the trough of the main rotor is less than the clearance Cl 5 between crest c 1  of main rotor and b 2  -c 2  of gate rotor, the clearance Cl 7 between the crest C 1  and trough c 2  of the gate rotor and the clearance Cl 4. Clearances Cl 1, Cl 8, and Cl 2 are less than conventional clearances to the extent of being about 70% thereof whilst clearances Cl 6, Cl 4, Cl 5 and Cl 7 are larger than conventional clearances and about 130% thereof. Points 1, 2, 3, 4, 5, 6, Fig. 12 indicate the sealing line between the rotors and x, y, z radial distances of the sealing line from the gate rotor axis. Area A is acted upon by internal pressure to give a driving action and area B to give a braking action to the gate rotor with the influence of the area A greater than that of area B so that it is possible to drive the gate rotor without the use of synchronizing gears.