Designing long-travel axes: When rack and pinion outperforms ball screws and linear motors

Sponsored by GAM.

Pull a tape measure out a number of inches and the blade is inflexible. Maintain pulling and in some unspecified time in the future it begins to flex and wobble, then ultimately buckles. It’s a downside engineers know properly when designing long-travel axes.

There are three foremost methods to create exact straight-line movement in machines: rack and pinion, ball screws, and linear motors. On this article, we’ll discover when rack and pinion is the only option—and when others is perhaps higher.

Understanding the three applied sciences

A rack and pinion works by partaking a rotating gear referred to as the pinion towards a toothed bar referred to as the rack. Relying on the setup, both the rack strikes whereas the pinion stays fastened, or the pinion meeting travels alongside a set rack.

A ball screw works on the same thought. It’s basically a exact threaded rod with a nut that rides alongside it. Spin the rod and the nut travels forwards and backwards linearly; maintain the nut fastened and the rod travels as a substitute.

A linear motor takes a distinct method. Consider an electrical motor opened up and laid out flat. One half holds everlasting magnets and electromagnets, whereas the shifting half has corresponding magnets. Via the enter of a servo controller, the magnetic interplay strikes the load in a straight line.

Rack and pinion vs. ball screws

Over shorter distances, rack and pinion and ball screws compete in the identical house; in truth, ball screws can provide increased precision. Their orientation might also be simpler to suit into sure machine designs, relying on how the machine is laid out.

Nevertheless, just like the tape measure, the ball screw has a size (round 2-3 meters) at which issues go improper, with the space based mostly on the scale of the screw.

“As you get into longer distances with ball screws, there’s a phenomenon referred to as whip,” says Matt Ruggles, senior design engineer at GAM, a U.S.-based producer of servo gear reducers and different movement management parts. “Mainly, the ball screw turns right into a leap rope, and that may trigger all types of issues in your system: untimely put on, vibration, typically catastrophic failure.”

Not like ball screws, rack and pinion could be very straightforward to customise for size.

“The benefit is that the rack and pinion can mainly scale infinitely,” says Ruggles. “You’ll be able to reduce a bit of rack shorter or you may assemble a number of items of rack collectively to get a for much longer journey.”

Rack and pinion vs. linear motors

Linear motors have two foremost benefits: pace and extremely exact management. Nevertheless, as a result of a linear motor depends on magnetic flux, stiffness will be tougher to handle. The magnets introduce different issues as properly.

“I’ve had experiences the place individuals doing installs, their keys of their pockets turn out to be magnetized,” says Ruggles. “It may be very awkward when unexpectedly the wristwatch begins gathering issues. That may play into the setting they’re working in, the place they’ll acquire chips and different particles as they’re working alongside.”

Each applied sciences can scale to longer distances, however the per-meter price of extending a linear motor is far increased—the everlasting magnets and electromagnets concerned are costly, in comparison with including one other part of rack. Linear motors additionally want fixed energy simply to carry place, and their energy consumption is instantly associated to how a lot feed drive they’ve.

Conversely, a rack and pinion just isn’t magnetized. Again-drive forces imply the system holds place as quickly because the motor stops, and the built-in motor brake handles the remaining with no steady energy draw. The parts are typically much less expensive too.

“You do have to purchase the rack and pinion and often a gearbox and a motor, however that mixed price is often lower than the linear motor implementation,” says Ruggles. “You will get a a lot stiffer system and far increased feed forces for the associated fee, package deal dimension and energy consumption.”

Rack and pinion does have one setting the place it falls quick: clear rooms. The gear tooth want lubrication to work correctly, which suggests uncovered grease open to the setting. This may be mitigated with covers, seals and the appropriate alternative of lubricant, however it provides complexity.

Ball screws have the identical concern, though they are often protected with bellows and comparable options. Linear motors haven’t any uncovered shifting elements so there’s nothing to seal or cowl, making them the perfect match for clear room environments like medical or semiconductor manufacturing.

GAM sells rack in customary one-meter sticks that may be chained collectively, and may reduce rack to shorter lengths as properly. Longer sticks can be found on particular order. For engineers working by element choice, GAM’s workforce might help establish the appropriate parts and portions to satisfy every utility’s necessities.

“It’s a kind of issues the place you get out of it what you set into it,” says Ruggles. “Should you take your time and set up exactly and thoroughly, you’ll get a really properly working rack and pinion, ball screw, and so forth. Should you slap issues collectively, you might need issues.”

To study extra about rack and pinion, visit GAM.

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