Forces on Large Piston

robot
2019
#1

Our team is attempting to use large pneumatic pistons in order to lift our robot onto level 3 in the HAB. Our one concern is that all of our weight would be on the pistons, which we hypothesize could bend the stroke/damage it permanently. Does anyone have any prior experience with lifting large weights with large pneumatic pistons. Essentially, imagine our robot on stilts, inching forwards towards level three. Will the pistons be damaged?

#2

You could use 2 pieces of tubing where one fits into another and some plates and bearings to create an extending arm. By doing that any forces would act on the tubing instead of the cylinder.

3 Likes
#3

It depends on how you implement your design. If you are worried about it, I would suggest coming up with a way to help support the piston rod. Something like a telescoping tube, linear guide, or guy wires to either take the forces for the rod or directly support the rod against side loads.

#4

you can have a reversed piston(always extended) and when you want to climb you retract it and that pushes a bar down
you are correct you always want to have piston in tension if you can

#5

Terminology police: A “piston” is part of a “pneumatic cylinder”.

To answer your question, a long fully-extended pneumatic cylinder is VERY easy to damage. It cannot take any side load whatsoever. It should be protecting with some type of guarding.

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#6

How much force are you expecting to get out of this pneumatic cylinder?

That will tell you how large diameter it needs to be.

The large ones are made with large diameter shafts, and can withstand some side load. But they can be damaged by too much side load.

I’d be more concerned about being able to have enough compressed air stored to be able to do what you want. The compressor has a relatively low power motor, so it takes a while to build up enough air pressure in a large enough tank, to lift a robot.

You might want to explore the math on this subject. We can help.

#7

You could but if you run a bar from the axle to the frame you should be fine

#8

1108 has used air cylinders for many things historically and we’re considering a 'gurney-- style" lifting mechanism to get to level three now. a 1.0 inch dia air cylinder can lift 47 pounds at 60 psi. Three of these should be plenty to lift a whole robot.

Last year, we used a 2-inch diam 24" stroke air cylinder in telescoping tubing to both be our cube-elevator and to lift the robot in end game. It worked fine, The only real problem was running out of air. So originally we designed an air-circuit that could keep two unused Clippard tanks at 60 psi and open them as 'reserve." But later discovered that air tanks from the car suspension industry were legal. 2.5 gallon, 3.2 pounds. Never run out of air again. Approved in WI by Big Al himself.
https://www.amazon.com/Air-Lift-12958-Aluminum-Tank/dp/B00GWITQ54/ref=asc_df_B00GWITQ54/?tag=hyprod-20&linkCode=df0&hvadid=316708741155&hvpos=1o1&hvnetw=g&hvrand=12179999147365778427&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9023862&hvtargid=aud-466346483690:pla-589242904769&psc=1

But last evening we tested a prototype of a three-air cylinder system to lift the front and back independently, (drive wheels and motors stay on the floor). Getting pairs of air cylinders to lift together at the same speed seems to be an issue. Robot repeatedly fell over because the cylinders don’t lift together. We put slow-down valve on them, better, but not reliable.
We could put each cylinder on it’s own valve and use ultrasonic sensors to control our way out of this problem, but sounds complicated.

Any ideas other how to solve this problem?

Otherwise, we’re thinking of going to rack-and pinions??

#9

We are exploring a similar idea and would love help with the math. We are hoping to use 4 pistons and use a stroke length of 20 inches. For planning purposes, we are looking at lifting 150 lbs (120 lbs + bumpers, battery, and safety margin). Does anyone have a pneumatics spreadsheet or calculator that we could use to calculate bore size and air volume requirements? Thanks!

#10

doesn’t say ‘air volume’, but that’s just Pi * r^2 * height. And figuring volume under pressure is way more tricky, PV=nRT. But you’ll never know for sure until you’re building it if you have enough air.

#11

Odds are, any cylinder that has enough force to lift your robot, will likely also have a rod diameter adequate to take any side-loads it’s likely to encounter while doing so. While small cylinders tend have small rods that are quite fragile, the larger ones typically have fairly beefy extension rods.

Granted, as others have pointed out, this depends entirely on your particular implementation.

1 Like
#12

What’s a good vendor for rack and pinion? McMaster?

Edit: McMaster definitely has them, but @ around $120 a peice for a 18" rack. Ouch!

#13

CNC Router Parts maybe?

1 Like
#14

prusa for my team (joke)

I’m going to 3d print them in ten-inch sections and put hole down the middle to put them on a aluminum rod.

#15

This.

Always make sure pneumatic cylinders are not subject to large lateral loads. They are designed for motion, not structure. If you are going to have them support a lateral load, at least use one with a larger bore.

Also, look at this video from an ri3d team for an example of getting up to level 2. They used cylinders with a smaller bore (1/4 or 3/8 it appears), which I would not recommend due to the chance of breaking, but the same design with larger bore cylinders and/or a way to support the load like @Robochair suggested would be more durable.

#16

You can also use the pneumatics tab of my calculator. It calculates the force and volume needed, and keeps track of your air storage needs throughout the match.

#17

Definitely minimize the side loading on a cylinder. Keep it to zero if possible. Mounting it such that another mechanism accepts the side loading is one way to do this. Another is to swivel-mount the cylinder at both ends, possibly using a trunion or clevis type swivel, or even better a ball-and-socket joint like air shocks typically use. This won’t work for all situations, but when it does it’s often much lighter.

#18

You need to get speed control valves like these and you can tune the speed of the cylinders.

1 Like
#19

Thanks indysam…but…when I said

“We put slow-down valves on them, better, but not reliable.”

Those are what I meant.

#20

Have to be very careful not to have much side on the rod of the cylinder or that may cause it to bend. We had this last year on our intake as any misalignment between the two sides was taken by the cylinder rod. We bent it quite a few times. I would recommend finding a way to support it if going with this route.