Polygon Scanners – ball bearing vs air bearing motors

Polygon scanners are most commonly running on Ball Bearings or self-generating Air Bearings.

Ball Bearings work well from zero to 10,000 RPM.  Ball bearings may be run at double that speed but the life will be reduced and grease misting may fog the polygon mirror facets.  Ball bearings are good for static applications as well as for dynamic applications in portable instruments, ground vehicles and aircraft.  However, dropping the scanner onto a hard surface while unpacking can damage the ball bearings.  The grease in the bearings limits the life.  Sitting on the shelf, unused, the grease can disassociate in a year.  This can be mitigated by running the scanner for a short time every few months.  In use, the grease has a life of up to 5 years depending on the RPM of the scanner.

Air bearings fly on a thin film of air between the shaft and sleeve.  To make the bearing fly, the motor must quickly accelerate to flying speed which is typically above 8000 RPM.  Once flying, there is no contact between bearing and shaft.  In this condition, the bearing has a theoretically unlimited life.  However, each time the motor starts there is shaft/sleeve contact for a brief period as it accelerates to flying speed.  When the motor is switched off, the shaft/sleeve makes contact at some point as it slows down.  The health of the air bearing may be determined by the touch down speed.  A slower touch down speed tends to be an indication of a healthier air bearing.  Air bearing life is measured in start/stop cycles.  Depending on the scanner, this may be tens of thousands of cycles or over one hundred thousand cycles.  A polygon spinning at high speed acts like a gyroscope.  The gyroscope will resist any tilting motion of the scanner.  If tilting occurs, the bearing shaft & sleeve will make contact (crash) at high speed.  For this reason, air bearing scanners are generally limited to static applications.  Another limitation of air bearings is the mirror weight.  Generally, air bearings are limited to light weight mirrors.

Table comparing Ball Bearing and Air Bearing Polygon Scanners