A pair of giant rotors with a girth to match a mature tree have been made by Lancashire machine tool company Holroyd for a compressor that treats gas at a large oil refinery in the US. Among the largest ever produced, each rotor measures 630mm across. With a combined weight of two tons, these dry-running rotors spin at around 1,000rpm with a clearance of just 75microns. Timing gears mounted on the ends of the shafts maintain the clearance between male and female rotor driven by a 5MW motor or a steam turbine. Holroyd is one of four companies capable of making compressor screws this big. The others, in Germany, Japan and the US, use Holroyd machines.