OK so my team takes part in a local competition called crc and we have a strange rule which does not allow us to use pneumatics. To simplify the creation of our robot I looked into the vex versa drop system and it looked great. (I’m thinking of using butterfly or octocanum) but there is 1 problem. It relies on pneumatics. So, I was wondering if there was any way of getting rid of the pneumatics system and replacing it with something like a rack and pinion system. Could this work?
You could use linear actuators! I havent used them since 2015 (DART actuators I think), but they basically resemble the linear movement of a pneumatic cylinder, but it uses a motor instead.
We used the DART linear actuators in 2016 and we had so many issues with them. It got to the point where as soon as it got into the pit, we would take them off and replace them with another just because we knew without testing something was failing. In 2017 we created our own linear actuators, working so much better. You can look on McMaster for lead screws, we used 3/8" ACME, but you may want to go larger if it will be supporting the weight of a robot (depending on size).
I think the simplest solution would be a motor and a cam.
If you use any sort of electrical solution to actuating wheels, you will almost certainly want to have some sort of “lock” that does not require a constant current flow in order to maintain the force against the wheels normally off the floor.
There are lots of ways to do this; one of the more elegant is to use the “air brake” method in which a brake is engaged UNLESS force is applied to release it. Use this not on your wheels, but on your vertical travel. You may want to supplement this with some sort of pawl/ratchet or other high-grip system.
Thank you all for your wonderful responses, where exactly could I find linear actuators or parts to build some?
Certainly many others out there, but those are the first two I thought of.
The advantage of pneumatics is that you have a small distributed force at each module. You now to recreate adequate force at each module in the simplest and most reliable method possible.
If your not using FIRST rules, there are solenoids with more than adequate power. (The R32 - 10 Watt limit on solenoids makes them effectively not very useful)
The other option mentioned is a CAM. One of the AndyMark PG188 series & 775 with the built in encoder would be absolutely overkill with 33 foot*pounds of stall torque.
An other option is to have one central motor gearbox to actuate the modules, and use something like bicycle brake cable to position each module. The typical tension adjusters would allow you to individually adjust the position of each module.
And maybe in a way you didn’t intend to say! The gearing would make you think it was capable of about 44 ft-lb of torque, but the output shaft shears at about 33. Unless you’re going to monitor torque (or current as a fair proxy) you would be better off using the PG71 and (if you really did want that much torque) a post-gearbox ~2:1 reduction.
We used the ones from 2015, never had any issues, given it was lightish load. The one thing about them is it cant be forced back, which is good in an application like this.
If you do a motor with a cam you could have it set to stop in the down position where the cam is not quite to the max forced travel and put a ratchet system on the cam, this way the weight of the robot would hold it against the ratchet and the ratchet would allow the motor to not put force into holding the robot in the up position
Why not pneumatics?
Already answered.
I think the simplest solution might be to use a window motor, one on each end of the bot (or maybe one total?). Have that rotate a common shaft that handles both pods on that end. BUT, you’ll need to do the math on that: will any given motor have the required speed and torque, and will the shaft (and its mounts) stay intact under the loads?
Definitely look into the cam option that other people have mentioned or the rod option that allows you to conserve the amount of motors you dedicate to this task. If you need help with the math involved the people here would be glad to help. Using linear actuators for this would significantly increase the price tag on your project, which I’d imagine isn’t desirable.
Maybe a window motor driving an over-center linkage, so that shock loads are not transferred back to the gears? Lock the linkage against a hard stop after it passes over center.
With pneumatics forbidden, I don’t know why you wouldn’t use a ratcheted cam to raise and lower a drop drive.
I wouldn’t be worried too much about shock loads. In 2010 we used a pair of window motors to direct-drive a CAM on our kicker. There was enough force in our kicker that we were pretty close to stall on both motors 
Basically, picture a CAM sandwich with window motors on top and bottom. We never had any issues with those motors.
I guess you left the pins in?
Yup. I don’t even know if my team was aware you could remove the pins back then!