cage of the screw press e g typically the same as or slightly larger than the length of the main cage over the outer diameter of the flights of the working portion of the shaft e g typically the same as the diameter of the inner surface of the bars of the main barrel cage the u201c compression ratio u201d of a lengthwise portion of the shaft may be defined on a volumetric basis as the ratio of the expected material volume at the upstream end of the lengthwise portion of the shaft to the expected material volume at the downstream end of the lengthwise portion of the shaft in each case the expected volumes are calculated as the applicable flight pitch times the annular cross sectional material flow area at the particular location such area defined between the outer diameter of the flights on the working portion of the shaft e g typically the same as the diameter of the inner surface of the bars of the main cage and the diameter at the root of the shaft at the particular location for example and referring to fig8 where the first scroll flight section 104 is made up of flight member 106 spacer 108 flight member 110 and flight member 112 the compression ratio along this lengthwise portion of the shaft would be calculated as vol1 vol2 where r1 is the starting root diameter for the flight member 106 and r2 is the.