Does anyone have any tips on balancing load when using pneumatics? Some of our projects use standoffs to add spacers from where the piston mounts to the shaft, but the shafts of the pistons always are either bent or violently screech since they are not perfectly straight when they extend. As context, we are using pneumatic cylinders from McMaster-Carr to create an extension for our Super-Pit build, and we keep getting nails- on- chalkboard feelings from when the pistons fire.
Just for clarification for the super pit solution, are you saying you are trying to open the lids with pneumatics? I think you’re looking for gas shocks, not pneumatic cylinders, which is common with a lot of the teams(Citrus used this to provide some help when opening). The idea of trying to pneumatically open the lids to the pit boxes gives me pause, and then heartburn.
I mean to raise the upper part of the platform. This allows the gantry to be lowered for storage, and raised in the pit so you don’t hit your head.
@ARDEngineering Are you mimicking this same setup from 973’s Super Pit?
We made our Super Pit following 5460’s version of the 973 Super Pit. Instead of a piston, 5460 used linear actuators off Amazon. We’ve been using the same setup and have been very happy with the results.
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Thanks! Question, how long did it take to build the Super Pit?
Do you have any plastic slides inside the tubes to reduce friction, or are the tubes sliding against each other? Have you tried to physically lift the moving upper assembly by hand with the pneumatics disconnected, to make sure the slides are not binding?
Here is the thing. As in the diagram we showed earlier in this thread, we are using the same design as Greybots. Two unpowered pistons to guide the one driving piston. The problem we have is spacing the piston off of the metal bar. Because the piston is attached to the outside of the metal slide, the top of the shaft, or the piston head, is spaced further from the metal tube than the base. Because of this, we have about an inch of space, causing the piston head to bend when it fires, and the connection point to the top of the gantry is also now bent due to the weight of the top of the pit setup.
The usual solution is to use trunnions or ball joint at both ends of a cylinder so that the rod does not have to bend if things are not perfectly aligned.
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Cool. I will try it out.
Be aware that offboard compressors aren’t allowed anymore, I know this applies to charging robot storage tanks, but I thought this was just a blanket event/pit rule. Hopefully someone else can chime in with the rule/place in the manuals this is covered as my quick search didn’t turn anything up.
These aren’t related. The “unpowered pistions” are actually gas springs. No external compressed air is necessary.
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Yes.
So I believe the original 973 super pit photo that was referenced was 2 gas shocks and 1 pneumatic to lift it. I just wanted to make sure that anyone reading was aware that I don’t think that’s legal any more.
Also he mentioned “driving piston”
As this is part of the pit and not the robot, this isn’t against any of the robot rules. Nor is it against other FIRST rules as far as I’m aware, though some venues will not allow shop compressors, mostly because there isn’t enough electricity available.
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To me seems like a great excuse to use a piece of threaded rod, some brass nuts, a thrust washer /bearing or two, and a pair of drills (or actuators like @BrendanB mentioned). Sure you need to be careful to sync it between the two people, but at least the screw actuators are (hopefully) self locking by default. Having said all than it is important to consider how much weight you are lifting in the first place, counterbalanced or not.
You do need to be careful with small compressors at events. you may be able to get away with some that are 12V tankless (like you would buy from an auto parts store)
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We have a few compressors that work with FRC robots, the small 12v one that’s just a motor with piston. Since we are using Bimba pneumatics, we can just use a threaded pneumatic coupling and some pneumatic air hoses for the job. That’s not a problem. The pistons not being balanced is more of a problem. I think we need to either have a second linear slide attached next to the piston or behind the workbench to balance it, since whenever the piston fires, the shaft bends and screeches since the top of the gantry is heavier on the opposite side of the workbench. Here:
(This is from Greybots, I don’t have access to mine right now since I’m not at the workshop)
aw. So there is a moment (torque) on the upright of where the tubes come together. Got it.
First thing, I hope compressed air isn’t the only thing holding this up and there is some sort of failsafe. There is a reason we us jackstands or blocks when working under vehicles.
Now to the question.
You are getting binding because of this offset of center of mass vs applied force. To address this you will likely want to reduce what can bind from the clearances. The gas springs and pneumatic cylinder are 2-force members, which will not be taking up any torque load the tube is experiencing. You can tackle this a few ways.
- Use something like PTFE (Teflon) Tape. Everything must be very clean and deburred. Apply this on:
- the front most upright inner tube (whole length that slides)
- the front inside top ~3" of the outer tube
- the inside back of the outer tube (whole length that slides)
- the back bottom ~3" of the inner tube
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Apply a bearing or bushing situation - a bit more involved, but you are basically making a tube-in-a-tube slider. You will need bearing down inside the outer tube attached to the inner tube likely as well.
-
Use a much more powerful gas spring on the front, as this is closer to the center of mass. This spring force is calculable relatively easily using a free body diagram if you know where you center of mass is on the upper portion.
Keep in mind that when selecting gas springs and cylinders these are for a relatively narrow range of mass up top and it may be sensitive to changes (including tipping, moving, or other dynamic scenarios)
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Yes, thank you for the summary of my problem. Someone finally gets it. Finally. Yes, we are using pins and holes to hold it up. Yes, we have PTFE blocks that allow the tubes to slide in each other. Yes, I have tried all of these things, and they are helpful, but I think the solution is to use a thinner-wall aluminium tube for the overhang. I also think having bars that can be detached from the top to support it (kind of like a car hood or a piano) will also help.
Thanks for understanding! I appreciate it.
The way we solved this is to use linear actuators with feedback, and a dual-axis controller that automatically keeps both sides lifting and lowering in sync.
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Interesting. I will check it out, it seems more reliable than pistons.