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GAM rack, pinion, and gearbox systems are optimized for high performance in all the components. (Image: GAM.)
Picture a scissor car jack — the one in your trunk for changing a tire. The spinning motion as you turn the crank translates to the linear motion as the jack rises.
This is how a rack and pinion system works. The spinning part is the pinion, a gear that receives the rotational input. The toothed part being pushed along is the rack, a long bar with gear teeth that the pinion rides against. Depending on the application, either the rack moves back and forth while the pinion stays in place, or the rack stays fixed and whatever the pinion is attached to travels back and forth (or up and down).
It is important to select the right rack and pinion system, as a wrong decision can impact both performance and cost.
“If you go too small with your rack and pinion, it can break,” says Matt Ruggles, senior design engineer at GAM, a U.S.-based manufacturer of servo gear reducers and other motion control components. “If you go too large, you might run into space constraints, and you’ll be paying for more rack and pinion than you need.”
The pinion size relative to the rack has to be matched so the system can reach the required speed and deliver the necessary feed force. The overall size of the rack and pinion also plays into inertia matching between the motor and load, which affects how smoothly the system moves.
So, how does the selection process work?
“Usually when people are putting together a rack and pinion system, their primary concern is either speed or feed force, and so you start with that requirement and kind of work backwards,” says Ruggles.
When feed force is the priority, that means making sure the rack is big enough to transmit the required force, and that the pinion is sized to match the motor or gearbox driving it. Similarly, when speed takes precedence, the rack and pinion are selected to allow the system to reach the target speed based on the motor or gearbox input.
Sometimes the process runs in the other direction. If a customer already has a motor selected, sizing can start from that motor’s speed and the positioning accuracy needed. Applications that need better positional accuracy may call for a higher-precision rack.
From there, tooth size and shape come into play. “The larger the rack, the bigger and stronger the teeth,” says Ruggles. “A higher tooth quality is going to run quieter usually and will offer better linear positioning, be more accurate.”
Another consideration is whether the teeth are straight or helical. Helical teeth are cut at an angle so they engage progressively, producing smoother, quieter motion and slightly more strength. Straight teeth engage all at once, making the motion rougher and noisier by comparison. While there’s very little cost difference between the two, helical teeth do introduce an axial force on the pinion and rack, perpendicular to the direction of motion.
A gearbox, pinion, and rack system provide the 7th axis, or lateral movement, for an articulated robot. Credit: GAM
Pinion size plays a significant role as well. Smaller pinions are preferred for high-precision applications; for any rotational error or backlash in the gearbox or motor, the impact on linear position is a lot smaller.
“With a smaller pinion, the lost motion through your system has less of an effect on how that overall positioning works in a linear fashion, versus a large pinion which can almost amplify that slop in the system,” says Ruggles.
A smaller pinion also transmits torque more effectively and requires less of it from the motor and gearbox to deliver the same feed force, allowing for smaller, less expensive components. The higher ratio further improves inertia matching between the motor and load.
The downside is that the maximum speed the system can achieve is going to be a lot lower, or the motor and gearbox will have to spin much faster to compensate. When speed matters more than precision, a larger pinion covers more distance per rotation.
Once the rack and pinion are selected, that drives gearbox choice, which then determines motor sizing.
One consistent mistake Ruggles sees engineers making is sizing the system around only one variable — feed force, for example — and not accounting for others until it’s too late.
“All of a sudden, they need to go up a rack size, get a bigger gearbox, and suddenly everything’s doubled in cost,” says Ruggles.
The same happens with inertia: correcting a mismatch by increasing the gearbox ratio reduces output speed, which cascades back through the whole system.
GAM’s engineers can help with the selection of various components in rack and pinion systems.
“We have sizing software where the customer can give us their feed force, speed, moving mass, and we can calculate the gearbox size, motor size, inertia matching,” says Ruggles. “As we’re sizing all those things, we have different size racks that we can offer, different size pinions to go with those racks, and then a variety of different gearboxes that we can match with those pinions, to really optimize the system.”
To learn more, visit GAM.
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