When developing axes for machining operations, engineers demand several features: high axial velocity and acceleration; high axial forces versus structural volume; long service life; high static and dynamic rigidity; high positioning accuracy; low noise generation; and low heat generation. Typical candidates evaluated for the job are linear motors and ball-screw drives. Rack-and-pinion drives are often overlooked as past-generation technology with limited positioning accuracy.
This assumption, however, is no longer valid thanks to
As travel distance increases, so does the
cost discrepancy between linear motors and rack-and-pinio systems due to
the extra expense of linear motor magnets and linear encoders.
improvements in grinding mounting surfaces to tight tolerances, wear-resistant surface treatments, individually deburred gear teeth, and compact low-mass designs. In fact, these rack-and-pinion drives compare favorably to linear motors as well as roller or ground-thread ball screws.
New-generation rack-and-pinion systems offer low mass moments of inertia, high dynamic performance, and unlimited travel distance. Rack-and-pinion systems from alpha include high-end servo gears and actuators offering a backlash less than 1 arcminute, efficiency to 98.5%, and a significantly more compact size than a standard servo motor gear combination. With 100% exit testing and a guaranteed true running of 10 microns (TP-version), the pre-assembled gear-pinion unit is designed to run safely and smoothly.
Rack-and-pinion versus ball screws
Ball screws can run up significant cumulative errors over total travel length. For example, deviation over four meters of travel length for a rolled screw drive may vary between 300 and 1,700µ. Even with ground-thread precision, ball screw deviation over four meters varies between 30 and 110µ. With a paired-set of alpha Rack & Pinion systems, cumulative error for the same travel length measures only 12 to 40µ. This makes rack-and-pinion an ideal solution for gantry drives.
For applications with long travel lengths, ball screws are limited by very high mass moments of inertia, critical speed, and axial load capacity. Such applications would therefore see a large boost in performance by switching to a high-end rack-and-pinion solution.
Ball screw rigidity is influenced by adjoining parts such as bearings, housing bores, or nut housings, making it difficult to ensure stable system behavior with high dynamic performance. Deviation of spindle stiffness depending on nut position over the spindle length compounds this problem. In contrast, rack-and-pinion drives offer constant stiffness over the complete travel length plus good system behavior and therefore a superior control system behavior.
Unlike rack-and-pinion systems, ball screws only allow one carrier per linear axis and are not suitable for short stroke applications—due to greasing demand, only a part of the balls will circulate in the nut.
Rack-and-pinion versus linear motor
Compared to linear motors, rack and pinion systems can offer similar performance but at far less cost. They are smaller, allowing a more compact, less complex machine design. The absence of magnetic forces vastly decreases the need for support structures to absorb high normal forces, so standard guide rails can be used.
Because of its inherently low efficiency, the linear motor often needs water cooling.
With the rack-and-pinion solution, there is no need to cover and protect the guidance system from metallic particles, and safety restrictions are minimal.
A high-end rack-and-pinion system eliminates the need for an expensive linear scale and external brakes; the standard motor feedback device and brake is enough.
Another effect of using linear motors is the need for a complete redesign of the whole machine. The huge normal forces, resulting from the attraction force between the primary and secondary part, thus have far-reaching consequences.
Finally, in case of ready-to-mount components, such as those from alpha, the high-end rack-and-pinion system facilitates blind assembly for additional cost savings and cuts the assembly time to roughly 10 minutes per meter travel length.
Applications for next-generation rack-and-pinion systems include woodworking machines; high-speed cutting of light metals; and automatic assembly machines.
Two new technologies from Stratasys, created in partnership with Boeing, Ford, and Siemens, will bring accurate, repeatable manufacturing of very large thermoplastic end products, and much bigger composite parts, onto the factory floor for industries including automotive and aerospace.
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