A properly lubricated screw compressor — oil-injected or water-injected — has essentially no mechanical wear on its rotors. The rotors don't grind against each other or against the casing.
Modern oil- or water-injected screws keep their rotor profiles intact for many thousands of hours of operation, and a well-maintained one can run a long time without losing meaningful metal off its rotor lobes.
So what actually wears as the machine ages? And if not the rotors, what is driving the steady decline in efficiency that the original article describes? The distinction matters, because it's exactly where the engineering argument lives.
The wear that drives the efficiency loss is in the bearings. Specifically, the thrust bearings whose job is to resist the constant axial force that pressurised air exerts on the rotors, and the radial bearings that hold the rotors at the micrometric clearances the seal depends on.
The lubricant that protects the rotors does not eliminate bearing wear; no lubricant does.
Rolling-element bearings under load wear from the first hour, by physics, however slowly.
As they wear, even microscopically, the rotors shift in their casing. The clearances that were set on the test bench begin to open. The internal leakage path — the blowhole — grows. High-pressure air slips past the rotor tips, along the flanks, and across the end faces, back to the inlet.
At no point in this process has anyone lost a milligram of metal off a rotor. The compressor's physical components look almost identical to the day they were assembled. What has shifted is the sealing geometry — the precision that depends on the bearings holding the rotors exactly where they were positioned at zero hours.
That is a more honest and more interesting picture than "screw compressors wear out." A lubricated screw, in steady-state operation, is a remarkably durable machine. What it is not — by architecture — is a machine that holds its sealing tolerance over a long working life, because the components that hold that tolerance are themselves subject to wear, and no lubricant eliminates that wear.
For completeness: every fact laid out on this mechanism in the original article is taken from research published by, or with, screw-compressor manufacturers themselves. Three decades of academic and industry work by the people optimising the technology, characterising and trying to address this intrinsic feature.
Three related questions deserve their own treatment and will get it in future articles: oil-free architectures, water-injected screws, and the speed-turndown question on rotary vane.
The full bearing-wear mechanism → https://www.linkedin.com/pulse/why-screw-compressor-efficiency-declines-over-time-rotary-contaldi-usqkf/
