Hey all, here’s a little project I was working on yesterday. Our team has never really had the chance to work with any sort of linear actuator before (with the exception of pistions, we love those), so I thought it would be cool to try and prototype one. It’s a rack and pinion driven by a 775pro on a 7.3:1 gear ratio, using a 10-inch rack with a 20dp tooth profile and 44t pinion gear. The rack (because of my team’s machining capability) would have to be either outsourced or laser cut from delrin or some other plastic. Does anyone have feedback on how I can improve this design?
Also, I was wondering how to do calculations for a rack and pinion mechanism. Given the gear ratio and the 775pro’s specs, how much could I expect something like this to lift (assuming the delrin rack doesn’t shear)? Could I have chosen a better motor for this task? Just looking for general feedback about any part of this design.
Have a look at the Linear Motion tab in the JVN calculator, it’ll do what you need. The pitch diameter of the gear you are using ( number of teeth divided by 20) is the pulley diameter.
To figure out how much linear force the thing would output, calculate the torque of the shaft with the pinion gear, and then divide by the radius of the gear.
Fancy-looking design. Do you have an encoder anywhere on there? Also, is the center plate really necessary?
McMaster-Carr sells both plastic racks and steel racks:
that should be compatible with other McMaster, AM, VexPro, etc gears. Just make sure you match the pressure angle and pitch and you should be good to go. And check to make sure your materials are strong enough, Google for gear teeth and material strengths and JVN calculator for the calculations mentioned above should do the trick.
Never used them before, but that would be my first place to try not knowing any better.
If all 3 plates have bearings for your shafts, I would recommend getting rid of the middle plate or at least the bearings if there some on the shafts. 3 bearings too close together on the same shaft overconstrains your geometry and can lead to inefficiencies and unnecessary additional loads on your bearings. So you get a lighter weight and possible more efficient solution? That seems like a win-win.
I see you have bearings running on the bottom edge of the box tube to prevent the gear from moving away from the rack. Do you have anything to prevent the gears from moving closer to the rack, like a bearing on the top surface of the box tube as pictured?
First, you can replace all those shaft collars with a screw and washer. You’re already using ThunderHex, which has a hole specifically designed to be tapped for a 1/4"-20 bolt. Tap the ends of that hole, and put a bolt and washer on each side, and you will be able to constrain everything on the shaft without the large, heavy, and cumbersome shaft collar.
Second, if you’re looking to optimize for a certain speed or current draw, you should take a look at my design spreadsheet here. Specifically, the MechanismRatio tab. By selecting the motor (775pro), voltage applied (12 probably), radius (last gear pitch diameter = #teeth*dp), and load you want to lift, it will tell you immediately a few things. If you want to run the motor at max power or max efficiency, it will tell you the gear ratios needed for that. More likely you want to be somewhere between them. So you can specify the adjusted linear speed or current draw that you want, and it will also tell you the gear ratios for those. This should help you actually select the proper gear ratio for your application, rather than just selecting gears and then seeing its speed and how much weight it can hold.
Third, if you are planning on using plastic racks with metal pinions, you may find that your racks are degrading faster than normal. With such a big difference in hardness, the pinion will probably start to wear into the rack over time. You should look into buying metal racks (20dp, 14.5pa) from McMaster if you want a more sturdy solution.
Also, I’m a little confused on how the side bearings are being attached. Can you provide a description or a better of that so we can give feedback.
I think you’re likely to have better luck with a chain loop / belt loop / string-and-pulley loop instead of a rack and pinion. While rack and pinion systems are Really Cool, they have a few annoying constraints. The load they can take is limited by the strength of a few gear teeth, which can be constraining in situations where you lift heavy things (e.g. 2015 totes, the entire robot, etc).
The biggest constraint though is that you have to power your rack and pinion system at the “top” of the mechanism in the retracted position. This usually ends up raising your CG on something like an elevator. You could run a belt / chain loop to lower the motor / gearbox into the robot, but then you could just use that same loop to actually raise or lower your mechanism.
The other application that rack and pinion gears could be useful is as you’ve described, a sort of motorized linear actuator to raise / lower / deploy / retract a small mechanism, such as a gate or a wrist joint. People tend to gravitate toward leadscrews / ballscrews for this application since they can more easily have low backlash and can package nicely. But if you can deal with the backlash, you could certainly make a rack and pinion work there.
That said, the off-season is certainly the time to explore concepts like this, so I don’t mean to discourage that! Just be sure to figure out what applications this mechanism will perform better than simpler solutions.
Thanks! I assumed that would be it, but wasn’t sure. I was thinking, though. From my understanding, it’s easier to turn a smaller lever arm than a longer one, meaning you can get more torque out of a smaller gear to a bigger gear than a bigger gear to smaller one (I hope I got that right). Does this apply to rack+ pinion? In other words, would it be able to lift more (but move more slowly) by using a smaller diameter pinion?
Thanks! There’s no encoder (I was too lazy to add one) and no, the center plate isn’t super necessary. It was just there for a bit of extra support, since I figured there would be a bit of flex between the two outer plates. It could be gotten rid of, though.
I honestly can’t think of anything! Anything up to 10 inches would be covered by a piston, and anything longer would easily be accomplished by a length of chain. This was pretty much just done so I could figure out what an FRC rack+pinion would look like
That’s a really neat solution to constraining things to shafts! I figure it probably only works when things are on the end of the shaft (if something was in the middle, you’d till need to have a collar or C-Clip).
I aso, didn’t know that people sold metal racks, I’ll be sure to check those out.
The side bearings are attached via bolts through them to a piece of L-stock riveted to the side of the side-plate. they have ABS or Delrin (some other material may be better, idk) “wheels” around their outer races, in order to get their OD’s big wide enough to contact the ABS buffer on the side of the tube.
A rack and pinion operates in the same way that your robots wheel does on the floor( just with no slip). With a robot when you decrease the size of the wheel without adjusting gearing your robot you robot will have a lower maximium speed but more maximum torque.
So yes a smaller diameter pinion would allow you to lift more weight at the cost of speed. For something like a rack and pinion I would try and use either a 3rd stage of reduction or Versaplanetary gearbox so that you can easily alter your speed and torque as required.
Don’t put a clip in the middle of the shaft! More specifically, don’t put a groove in the shaft in a place where torque is being transmitted. This is just a “shear here” line.
This seems like an application where you would want to at least consider a leadscrew. It could end up being lighter and with lower backlash.
In any case, if you go the rack and pinion route, I would forego the normal center adder, particularly with an aluminum pinion on a steel rack, and instead break in the mechanism so it wears down to a smooth, low-backlash motion.
You might consider using only one rack/gear set. It is hard for me to imagine a FRC application that you would need 2 sets from a strength stand point.
Adding onto this, an imported ballscrew bought in the USA can be quite cheap if you are worried about efficiency or speed. From Amazon for $97 you can get a ~12" ballscrew with nut and bearings. Without the bearings the price goes down to $37.
From ebay a 250mm screw is available for $19 without bearing blocks, which run around $20 for the fixed and floating bearing block with all hardware.
PS sorry for making huge images, I couldn’t find an easy way to resize them.
