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The primary requirement of an objective evaluation of the performance of lobe compressors is an indication of their efficiency of compression. This paper presents criteria for comparing different forms and methods of operation of lobed rotors.The evaluation of the performance of lobes is best made by comparing the sizes of the most effective types with those of standard rotors for given test conditions and center-to-center distances for close gearing. The standard rotors used were of simple forms (Table 1) but were comparable with the rotors of various types of compressors tested.The lobes of the standard rotors had a symmetrical profile. The driving rotor had four lobes of 288 ~ curvature and formed by a head without base. The driven rotor had six lobes with bases. The circles of the lobe tips are comparable for the driving and driven rotors. The length of the rotors was 1.5 times the diameter of lobe tip circle.In lobe compressors the gas pressure on the rotor lobes produces a torque acting about the axes of the lobes and a sag of the rotor. In the simplest analysis the rotor can be considered to be suspended ) \\i" F 2,o 8.4 I Di/Dis; De/Des; L/Ls Fig. 1 Fig. 2 tz Fig. 1. Plane end projections of the side surfaces of the lobes of the driven rotor bounding the cavity: ~ line of engagements of lobes; ---original contour of lobes of driving rotor; -, .-original contour of lobes of driven rotor; 1,2 base circle of driving and driven rotors.Fig. 2. Ratio of sags of rotors of cavity circles of diameter D i, of projections De, and length L differing fromthe standard rotors to the sag of standard rotor at Pn-Pv = const: i) f = ~ (Di/Dis); 2) f = q~ (De/Des); 3) f = ~ (L/Ls),