A screw compressor comprising: a housing having a suction port and a discharge port; a male rotor having: an axis; a lobed portion extending from a suction end to a discharge end; a suction end shaft portion; and a discharge end shaft portion; a female rotor having: an axis; a lobed portion extending from a suction end to a discharge end and enmeshed with the male rotor lobed portion; a suction end shaft portion; and a discharge end shaft portion; a male rotor suction end bearing mounting the male rotor suction end shaft portion to the case; a male rotor discharge end bearing (90-1, 90-2, 90-3) mounting the male rotor discharge end shaft portion to the case; a female rotor suction end bearing mounting the female rotor suction end shaft portion to the case; a female rotor discharge end bearing (92-1, 92-2) mounting the female rotor discharge end shaft portion to the case; a lubricant flowpath (181; 281; 381; 581; 681; 781); at least one valve (182; 282; 382-1, 382-2, 382-3; 82; 582-1, 582-2; 682-1, 682-2; 782-1, 782-2) along the lubricant flowpath and having an energized condition and a de-energized condition; and at least one restriction (184; 84-1, 84-2; 84-1, 84-2, 84-3; 484-1, 484-2, 84-3; 84-1, 84-2, 584; 84-1, 84-2, 684; 84-1, 84-2, 784) along the lubricant flowpath, wherein the at least one valve and the at least one restriction are positioned to create a lubricant pressure difference biasing the rotors away from the discharge end of the case, characterised in that at least one of said male rotor and said female rotor is supported without a bearing positioned to react thrust in a suction-to-discharge direction, and in that the at least one valve is a solenoid valve and is positioned to, in the de-energized condition, block lubricant flow to the suction end bearings (96, 98), wherein the at least one valve is positioned along the lubricant flowpath (181; 281; 381; 581; 681; 781) between the discharge end bearings (90-1, 90-2, 90-3, 92-1, 92-2) and the suction end bearings (96, 98). a housing having a suction port and a discharge port; a male rotor having: an axis; a lobed portion extending from a suction end to a discharge end; a suction end shaft portion; and a discharge end shaft portion; an axis; a lobed portion extending from a suction end to a discharge end; a suction end shaft portion; and a discharge end shaft portion; a female rotor having: an axis; a lobed portion extending from a suction end to a discharge end and enmeshed with the male rotor lobed portion; a suction end shaft portion; and a discharge end shaft portion; an axis; a lobed portion extending from a suction end to a discharge end and enmeshed with the male rotor lobed portion; a suction end shaft portion; and a discharge end shaft portion; a male rotor suction end bearing mounting the male rotor suction end shaft portion to the case; a male rotor discharge end bearing (90-1, 90-2, 90-3) mounting the male rotor discharge end shaft portion to the case; a female rotor suction end bearing mounting the female rotor suction end shaft portion to the case; a female rotor discharge end bearing (92-1, 92-2) mounting the female rotor discharge end shaft portion to the case; a lubricant flowpath (181; 281; 381; 581; 681; 781); at least one valve (182; 282; 382-1, 382-2, 382-3; 82; 582-1, 582-2; 682-1, 682-2; 782-1, 782-2) along the lubricant flowpath and having an energized condition and a de-energized condition; and at least one restriction (184; 84-1, 84-2; 84-1, 84-2, 84-3; 484-1, 484-2, 84-3; 84-1, 84-2, 584; 84-1, 84-2, 684; 84-1, 84-2, 784) along the lubricant flowpath, wherein the at least one valve and the at least one restriction are positioned to create a lubricant pressure difference biasing the rotors away from the discharge end of the case, characterised in that at least one of said male rotor and said female rotor is supported without a bearing positioned to react thrust in a suction-to-discharge direction, and in that the at least one valve is a solenoid valve and is positioned to, in the de-energized condition, block lubricant flow to the suction end bearings (96, 98), wherein the at least one valve is positioned along the lubricant flowpath (181; 281; 381; 581; 681; 781) between the discharge end bearings (90-1, 90-2, 90-3, 92-1, 92-2) and the suction end bearings (96, 98).