Classical Guiding Systems & Force Transmission

Leadscrews can provide very high resolutions and very smooth motion. A leadscrew drive consists of a motor-driven screw with a nut coupled to the sled of the stage. The nut can be spring preloaded to reduce backlash. They have higher friction than recirculating ball screws so that they are self-locking; on the other hand, however, this has an effect on velocity, motor power, and lifetime. Typical leadscrews have a pitch between 0.4 and 0.5 mm/revolution, up to 1 mm/revolution for longer travel ranges.

Recirculating ball screws have significantly less friction than leadscrews because they replace sliding friction with considerably lower rolling friction. A recirculating ball screw drive consists of a motor-driven screw with a nut coupled to the sled of the stage. Balls in a closed circuit are located between nut (ball case) and drive screw. Backlash can be minimized by selecting the proper ball-to-thread-diameter ratio. Recirculating ball screws are not self-locking but very efficient and offer high velocities and long service life in continuous operation. PI uses pitches of 0.5, 1 or 2 mm/revolution.

Threaded spindle drives use rollers instead of balls as rolling elements. This allows achieving higher payloads, higher speeds, and a considerably longer service life.

The balls run in a brass cage and are preloaded with regard to the hardened precision guiding shafts. Exact tolerances between guiding and bearing are necessary for zero backlash and low friction. Load capacity is limited.

In crossed roller bearings, the point contact of the balls in ball bearings is replaced by a line contact of the hardened rollers. Consequently, they are considerably stiffer and need less preload so that friction is reduced and a smooth run is possible. Crossed roller bearings are also characterized by high guiding accuracy and load capacity. Permanent guiding of the rolling body cages avoids migration of the roller bearings.

High-precision stages are equipped with precision double linear rails. Precision assembly allows these bearings to yield excellent results in terms of load capacity, service life, low maintenance, and guiding accuracy. The moving part of the stages is supported by four preloaded linear ball bearing units which run on two guide rails. Each bearing unit is made up of two independent rows of circulating ball bearings. They are also immune to the cage migration problems of crossed roller bearings which can be an issue where small ranges are scanned repeatedly.

Linear air bearing stages have incredibly straight and flat travels, measured in the 100’s or 10’s of nanometers and subarcsecond pitch, roll, and yaw errors. Rotary stages have tilt (wobble) errors less than 1 arcsecond. Additionally, the angular performance of an air bearing is remarkably repeatable. This guarantees optimal part quality and measurement reliability for applications such as optics inspection, semiconductor inspection, and medical device manufacturing.