Purchasing Pistons

I was wondering how teams go about buying pistons? Do you CAD the robot and then purchase the pistons needed or do you have a set of pistons you use yearly and design your robot with those sizes in mind? If teams do have a stock of extra pistons that they use how did you decide what pistons sizes to buy and where did you get them from?

Thanks

There are usually a list of pneumatic cylinders that bimba will donate to teams each year. We try to design around the free cylinders but also have a stock of cylinders from previous years that we’ll use (mostly from previous year’s bimba donations).

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1293’s in a location where AutomationDirect can get an order by 6:00 PM or so and have it to you the next day for ground shipping money.

1293 orders cylinders when they know what cylinders they need. :slight_smile:

The team I mentored last season had some pneumatic cylinders from Grainger. There is a Grainger location in Lubbock. You can have parts shipped to the store within a few days for free and pick them up some time during the day.

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We figure out roughly what size we need and then go check the box of pneumatic stuff for similar sizes. If we’ve got something that’s close enough to get the job done, then we adjust the CAD a bit and use it. If not, we order a new one.

Protip: McMaster’s pistons are usually Bimbas and they ship either same day or next day. For San Diego, the cutoff for same-day shipping is 10:15AM.

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The teams that I’ve been a part of have always used the free Bimba voucher to stock up on three free cylinders every year, regardless of whether or not the rest of the robot has pneumatics. Here’s some specific sizes to stock up on as a long-term goal (do not buy all of these pre-emptively!):

3/4" Bore
1/2", 1", 2", 3", 4", 5", 6", 8", 10" stroke

1-1/16" Bore:
1", 2", 3", 4", 5", 6", 8", 10" stroke

1-1/2" Bore
1", 2", 3", 4", 5", 6", 8" stroke

Edit: For all of these sizes I’d recommend getting the universal mount (with front threaded mount and rear pivot mount) and ideally with NPT ports rotated 90° (if ordering through Bimba):

image

The key item is to get at least 2x of each bore + stroke combination, as some mechanisms work best with two identical cylinders in parallel. I do not recommend using any cylinders that consume more air volume than about a 1-1/2" bore x 8" stroke cylinder, as you’ll likely run into issues refilling your storage tanks in the six minutes between elimination matches.

Once you begin amassing this library of cylinders, regardless of whether you design in CAD or not, you can either choose to design around cylinders you already have (preferred), or if this is not possible, then design around the cylinder you need (ideally one of these standard sizes), and then buy it as necessary.

Lastly FYI: the term “piston” only refers to the rod that moves in and out. The entire assembled unit is called a pneumatic cylinder.

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Pretty much what @BordomBeThyName said. Calculate the needed volume, check stock, design around that or see what can be ordered for quick delivery and design around that. When using vouchers not for a specific purpose, get something with a volume not already on the shelf.

It also helps to build up an inventory of the mounting brackets needed to mount the cylinders and clevises to connect to the piston rods. Some of them use uncommon thread sizes such as 1/4" - 28 so it would also be good to build up a stock of those nuts.

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We ran a cylinder with a 2-inch bore and 16-inch stroke, with no issues with repressurizing during eliminations at our last 2 offseason events. We had 4 air tanks, and the onboard compressor was constantly running, which proved to work well for us. IIRC, we didn’t even spend time repressurizing off the field, since we were busy taking care of a few issues with our superstructure loosening. (The cylinder was for our hatch mech extension, so we were using it rather frequently)

Quick counterpoint: many teams (ours included, along with 2910) successfully used large cylinders to climb on this year. I’d say the rule is to avoid large pneumatics for anything that needs to work more than once.

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After 2019 we also have an inventory of large bore, long throw cylinders. I ca’t say that we successfully climbed using them, but we did reach level 3 once with them in competition. By champs we had switched to a mechanical climber. I hope we never have to use more than one of those pistons again (zero would be even better).

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We had two on the robot, two on the practice robot, and 2 or 4 spares assuming that we’d damage them in practice or competition and have to replace them. The originals worked all season and never needed replacing, so now we have 4-6 of these gigantic, expensive, cylinders. They’re too valuable to toss but probably not useful anymore.

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Thank you for this in depth answer! Also sorry for my mistake on “piston” vs “Pneumatic Cylinder” i’ll be sure to remember that!

Parker Hannifin is another good brand for pneumatic cylinders. They have fairly wide distribution, so there is a good chance you can pick up a part near a competition site.