When Rack and Pinion Beats Ball Screws and Linear Motors for Long-Travel Axes

A gearbox, pinion, and rack system provide the 7th axis, or lateral movement, for an articulated robot. (Image: GAM.)

Pull a tape measure out a few inches and the blade is rigid. Keep pulling and at some point it starts to flex and wobble, then eventually buckles. It is a problem engineers know well when designing long-travel axes.

There are three main ways to create precise straight-line motion in machines: rack and pinion, ball screws, and linear motors. In this article, we’ll explore when rack and pinion is the best choice—and when others might be better.

Understanding the three technologies

A rack and pinion works by engaging a rotating gear called the pinion against a toothed bar called the rack. Depending on the setup, either the rack moves while the pinion stays fixed, or the pinion assembly travels along a fixed rack.

A ball screw works on a similar idea. It is essentially a precise threaded rod with a nut that rides along it. Spin the rod and the nut travels back and forth linearly; hold the nut fixed and the rod travels instead.

A linear motor takes a different approach. Think of an electric motor opened up and laid out flat. One part holds permanent magnets and electromagnets, while the moving part has corresponding magnets. Through the input of a servo controller, the magnetic interaction moves the load in a straight line.

Rack and pinion vs. ball screws

Over shorter distances, rack and pinion and ball screws compete in the same space; in fact, ball screws can offer higher precision. Their orientation may also be easier to fit into certain machine designs, depending on how the machine is laid out.

However, like the tape measure, the ball screw has a length (around 2-3 meters) at which things go wrong, with the distance based on the size of the screw.

“As you get into longer distances with ball screws, there’s a phenomenon called whip,” says Matt Ruggles, senior design engineer at GAM, a U.S.-based manufacturer of servo gear reducers and other motion control components. “Basically, the ball screw turns into a jump rope, and that can cause all sorts of problems in your system: premature wear, vibration, sometimes catastrophic failure.”

Unlike ball screws, rack and pinion is very easy to customize for length.

“The advantage is that the rack and pinion can basically scale infinitely,” says Ruggles. “You can cut a piece of rack shorter or you can assemble multiple pieces of rack together to get a much longer travel.”

Rack and pinion vs. linear motors

Linear motors have two main advantages: speed and highly precise control. However, because a linear motor relies on magnetic flux, stiffness can be harder to manage. The magnets introduce other complications as well.

“I’ve had experiences where people doing installs, their keys in their pockets become magnetized,” says Ruggles. “It can be very awkward when all of a sudden the wristwatch starts collecting things. That can play into the environment they’re working in, where they can collect chips and other debris as they’re running along.”

Both technologies can scale to longer distances, but the per-meter cost of extending a linear motor is much higher—the permanent magnets and electromagnets involved are expensive, compared to adding another section of rack. Linear motors also need constant power just to hold position, and their power consumption is directly related to how much feed force they have.

Conversely, a rack and pinion is not magnetized. Back-drive forces mean the system holds position as soon as the motor stops, and the built-in motor brake handles the rest with no continuous power draw. The components tend to be less pricey too.

“You do have to buy the rack and pinion and usually a gearbox and a motor, but that combined cost is usually less than the linear motor implementation,” says Ruggles. “You can get a much stiffer system and much higher feed forces for the cost, package size and power consumption.”

Rack and pinion does have one environment where it falls short: clean rooms. The gear teeth need lubrication to work properly, which means exposed grease open to the environment. This can be mitigated with covers, seals and the right choice of lubricant, but it adds complexity.

Ball screws have the same issue, although they can be protected with bellows and similar solutions. Linear motors have no exposed moving parts so there is nothing to seal or cover, making them the best fit for clean room environments like medical or semiconductor manufacturing.

GAM sells rack in standard one-meter sticks that can be chained together, and can cut rack to shorter lengths as well. Longer sticks are available on special order. For engineers working through component selection, GAM’s team can help identify the right components and quantities to meet each application’s requirements.

“It’s one of those things where you get out of it what you put into it,” says Ruggles. “If you take your time and install precisely and carefully, you’ll get a very well operating rack and pinion, ball screw, etc. If you slap things together, you might have problems.”

To learn more about rack and pinion, visit GAM.