Advances In Technology Improve Rack And Pinion Performance And Positioning

By: Chris Muldowney

When an engineer needs to convert rotary motion to linear motion, or develop axes for machining operations, they often choose ball screw drive mechanisms such as electric lifting actuators and electric ball screw actuators. Coming in third are rack and pinion drives, which have largely been overlooked thanks to their reputation for outdated technology with limited positioning accuracy.

With recent improvements, however, engineers are beginning to reconsider rack and pinion systems, which can be used as electric lifting actuators. The new generation of rack and pinion systems feature advances in grinding mounting surfaces to tight tolerances, wear-resistant surface treatments, individually deburred gear teeth and compact low-mass designs; meaning they can offer high dynamic performance and unlimited travel distance. For precise and long travel, linear-motion applications, engineers may also consider rack and pinion drive packages from a planetary gearheads manufacturer, which can offer increased torque.



Advances like these are giving rack and pinion drives new life; and they are now comparing favorably to linear motors as well as rolled or ground-thread ball screws.

Rack and pinion vs. ball screws

Compared to electric ball screw actuators, new rack and pinion systems have the power to reduce cumulative errors, boost performance and ensure stable system behavior.

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 from 300 to 1,700 µ. Even with ground-thread precision, ball screw deviation over four meters varies from 30 to 100 µ. With a paired set of alpha rack pinion systems, cumulative error for the same travel length measures only 12 to 40 µ.

For applications with long travel lengths, ball screws are limited by very high-mass movements of inertia, critical speed and axial load capacity. Such applications could notice 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 positioning 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.

Rack and pinion vs. linear systems

While linear motors offer high performance at a relatively high price, the new generation of rack and pinion systems provide similar performance at a fraction of the price.

For starters, rack and pinion systems are smaller, which makes for a more compact and less complex machine design. In addition, the absence of magnet forces vastly decreases the need for support structures to absorb high normal forces, so standard guide rails can be used.

Another limitation of the linear motor is its inherently low efficiency, which requires water cooling. With the rack and pinion system, however, there is no need to cover and protect the guidance system from metallic particles. 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.

Finally, in the case of ready-to-mount components, the high-end rack and pinion systems facilitate blind assembly, which often leads to greater cost savings. This can reduce the time needed to assemble to roughly 10 minutes per meter travel length.

Though rack and pinion systems are still considered a third alternative, they deserve to be evaluated more often, thanks to recent advancements in performance and accuracy. Some of the top-performing rack and pinion tools and accessories are available from motion-control device distributors and planetary gearheads manufacturers.

Article Source:
http://www.articlecity.com/articles/business_and_finance/article_10844.shtml