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Some years ago, to compensate for loose manufacturing tolerances, compressor manufacturers drilled balancing holes in either end of the rotors to achieve a smoother running airend. We have actually seen an original factory rotor with lobes that varied in height by several thousandths of an inch from the centerline with numerous balancing holes drilled in it to compensate for this machining error.
To us, this seems a little like putting a nice paint job on a rusty car. While it allowed the airend to run a bit more smoothly, it remained inefficient due to poor rotor to stator sealing, bearings failed prematurely, and the customer was not getting what he paid for.
Most OEMs now find, due to greatly improved rotor cutting equipment and tighter quality control measures, that rotors no longer require matching in pairs or balancing. We seldom see rotors in newer airends that have been drilled. This all seems quite logical, since by design, the lobes of a rotor are cut on the same centerline as the shaft. They are concentric with the bearing surfaces on both ends of the rotor.
In our machine shop, we find that careful attention to concentricity of all machined surfaces goes a long way toward correcting vibration problems without drilling.
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Gardner Denver rotor on Dynamic Balancer
While we do offer dynamic rotor balancing if required, first we make every attempt to correct the cause of this vibration rather than compensate for it. With the possible exception of casting voids rarely found in rotors, we find that most properly machined rotors will run within ISO balancing specs without the holes.
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