Rack and Pinion Suggestions


Our team is considering to use a rack/pinion method for our cone/cube grabber. We’d have one stationary side, with the second moving side mounted to the rack. Unfortunately, we’re struggling to come up with solutions that aren’t massively bulky for the rack+slider. Do you guys have any suggestions, like possible parts to use? We’re initially planning to use a 10kg continuous servo to drive it. We’re pretty sure this stays below the 2.2A limit.

Edit: I’ve found the 15mm Linear Motion Kit V2 - REV Robotics that may be good for us.

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Suggestion: don’t use a rack and pinion.

Pneumatics are superior for almost all linear applications in FRC.


In 2019 we used rack gears from McMaster-Carr along with the appropriate pinion gear from a FRC vendor.

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This is absolutely NOT what you want to use. A pneumatic Cylinder is your best bet for < 2 ft linear motion.

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Although many comments are pointing out that pneumatics may be superior, there are plenty of reasons to avoid them you might be worrying about.

If you’re up to it, 971 in 2017 has a good motor driven mechanism for their intake extension that may be applicable for what you’re looking for. Spartan Series/ Overview of 971's 2017 Robot - Ginger Schmidt and Adam Snaider - YouTube

They also had a similar belt driven mechanism in 2019 for a simpler and more refined design.


I would argue there is no need for the absolute nature of the linked device being the wrong answer, as it does offer the ability that OP is looking for.

Pneumatics are great tools for solutions, howver they bring their own nature of problems and needs like any mechanism. Its up to OP and their program to decide whatbis the best solution that fits their needs and abilities.

Full disclosure: I am affiliated with a team that will nearly always select a mechanical device prior to pneumatic solutions as they add addition weight and complexity to our systems.


Another option is to use a simple linkage. With links between the attachment points of your parallel jaws (or mini 4 bar jaws to clamp) and the other end of the links attached to the servo horn, 180 degrees from each other.

If you do this with a strong externally powered large servo you may want to look into ways to isolate the servo such as having the gripper bars sprung open (surgical tubing, rubber bands, or springs, shown in red) and the servo linkages (shown in blue) compliant (very strong string or cord)


In regards to this, the rulebook states that the roborio is limited to just 6V 2.2A power output across all the servos. Would this board allow us to go beyond that and use more powerful servos?

Also, thanks for that linkage design! We’ve already been considering something similar and we’re starting to come to the same conclusion that this would be better.

Yes, you can use a PDP/PDH slot for the servos with that board.

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Yes, rule R505 c. (page 92)

No problem!

You may need to go sniffing around for the right servo (not my department) but there are lots of people here that should be able to help you pick if you want a sanity check. (remember there is a 75$ price cap (R501, page 88), so you may have to shop around)

Nice thing is there are a lot of servo options with ~110 deg of rotation and a lot of torque.

if you decide to spring it open be careful of using too powerful springs. you can adjust where the blue linkages are on the servo (say you have a ~180deg servo) so you have better mechanical advantage and grip strength. i.e.

You may want to look into other applications where a servo may help you, as that hub can handle multiple.

The rio only has 2.2 A available on that rail. So it is a limit regardless of the rule. Rev board in the link provides a max of 15A 6V power. The REV board gets its power from the power distribution board. It also has the benefit of isolating the Rio from the servos. A stalled servo will not pull down the 6V rail of the rio.

For your pinion, did you just use any 20DP gear, or was there a specific gear you use? I was looking at racks and the 20DP gears and the 20 degree pressure racks line up, but I haven’t found the right pitch and was wondering if just any 20DP gear would work.

So, we just used a VEX 20DP gear, not realizing it was a different pressure angle. We didn’t find out it was different until we posted some information about it and someone pointed it out.

It worked fine for us though. :man_shrugging:

Cool, thanks!

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