Designing with Pneumatics-Catcher

[PaulEleven]

Hello all,
My team is thinking about using two 3/4 bore pneumatic cylinders to power a linear motion catcher instead of a pivoted one.
The question is: can the two cylinders take the load(~40 lbs) downwards from the stack once it's mounted horizontally?

Also the way I'm thinking about mounting it to be a non-pivotal arm is to foot bracket it fixed to the base and turn the end clevis 90 degrees and run a bolt through to the arm.

Newbies need them feedback! Thanks

[Everett33]

To find the force a pneumatic cylinder produces in the extend direction, you multiply the area of the cylinders bore by the pressure you are applying to it (max of 60 psi).

To calculate the force in the retract direction you take the area of the bore and subtract the area of the cylinder rod, then multiply by air pressure applied.

[MrForbes]

My guess is that they are planning to use the cylinders as a slide mechanism, so that a tote will land on something that is supported by the cylinders, when they are extended.

If I'm right, it's probably not good design practice. If they would post a drawing or picture of what they plan to do, we might be able to answer the question better.

[PaulEleven]

Here's a sketch, would the two cylinders be able to take a stack's weight?

[DaRealSlimShady]

Depending on how your gripping the tote. It looks like your mechanism will easily be able to support six totes, although you'll need to really fine tune it. If the cylinders are off by even a little bit, the whole system is messed up. I speak from experience.

For future reference I find this force calculator very quick and helpful.
http://www.pneumaticsonline.com/calc2.asp

[PaulEleven]

Originally Posted by DaRealSlimShady

Depending on how your gripping the tote. It looks like your mechanism will easily be able to support six totes, although you'll need to really fine tune it. If the cylinders are off by even a little bit, the whole system is messed up. I speak from experience.

For future reference I find this force calculator very quick and helpful.
http://www.pneumaticsonline.com/calc2.asp

Are you talking about misalignment or spacing? What about adding a linear slide in the back of the moving arm?

[Everett33]

If you have some sort of linear slide mechanism to locate and support the tote and gripper weight, the 2x 3/4" cylinders should be able to move it back and fourth. I would not trust supporting the weight of a tote axially on the cylinder rod itself. The 3/4" bore cylinder rods are not very hard to bend (especially with a longer stoke length). After they are bent they probably will not be able to be returned to original working order.

[PaulEleven]

Originally Posted by Everett33

If you have some sort of linear slide mechanism to locate and support the tote and gripper weight, the 2x 3/4" cylinders should be able to move it back and fourth. I would not trust supporting the weight of a tote axially on the cylinder rod itself. The 3/4" bore cylinder rods are not very hard to bend (especially with a longer stoke length). After they are bent they probably will not be able to be returned to original working order.

Won't be able to add a very big one, I'm thinking about the igus 40mm one in the KOP. What about 1" cylinder? Air consumption isn't that large a problem

[Joe G.]

Try to avoid putting significant side loads on any pistons, no matter the size. They aren't really designed to take it, and it takes surprisingly little deformation to hurt a cylinder's performance pretty significantly. Typically, in designs like this, you'll see some other kind of structure which takes the brunt of the downwards load.

I would consider placing your inner jaw on some kind of linear guide (the IGUS components in the kit are a good choice, as is the REV extrusion, the Vexpro elevator bearing gussets, and numerous other off the shelf options), or switch it to a pivoting design. A small four bar linkage may be a good choice here, giving you the strength and ease of fabrication of a pivoting joint, while maintaining your desired linear motion profile.

[PaulEleven]

Originally Posted by Joe G.

Try to avoid putting significant side loads on any pistons, no matter the size. They aren't really designed to take it, and it takes surprisingly little deformation to hurt a cylinder's performance pretty significantly. Typically, in designs like this, you'll see some other kind of structure which takes the brunt of the downwards load.

I would consider placing your inner jaw on some kind of linear guide (the IGUS components in the kit are a good choice, as is the REV extrusion, the Vexpro elevator bearing gussets, and numerous other off the shelf options), or switch it to a pivoting design. A small four bar linkage may be a good choice here, giving you the strength and ease of fabrication of a pivoting joint, while maintaining your desired linear motion profile.

Thanks for putting the four bar idea in my head, I'll never get it out now..... another sleepless night......

[BBray_T1296]

Originally Posted by PaulEleven

Won't be able to add a very big one, I'm thinking about the igus 40mm one in the KOP. What about 1" cylinder? Air consumption isn't that large a problem

It is good practice to design a pneumatic implementation such that the cylinder has NO loading other than in the push/pull vector. The cylinders are only designed with those forces in mind, and while they are materialistically capable of minor cross-forces, over time they could bend and warp, which is bad news.

As a general rule, if you are designing a system that demands forces other than along the the push/pull line of action, other components of the system need to handle those besides the pneumatic pistons themselves.

A go-to mechanism I always prefer is cascading drawer sliders, you can get these in oodles of lengths and loading capacities, with compact, reliable, and robust construction, and they sell them at Home Depot and Lowe's, or lots of places online

[PaulEleven]

Originally Posted by BBray_T1296

It is good practice to design a pneumatic implementation such that the cylinder has NO loading other than in the push/pull vector. The cylinders are only designed with those forces in mind, and while they are materialistically capable of minor cross-forces, over time they could bend and warp, which is bad news.

As a general rule, if you are designing a system that demands forces other than along the the push/pull line of action, other components of the system need to handle those besides the pneumatic pistons themselves.

A go-to mechanism I always prefer is cascading drawer sliders, you can get these in oodles of lengths and loading capacities, with compact, reliable, and robust construction, and they sell them at Home Depot and Lowe's, or lots of places online

Good thing you guys got to us before we built this thing, I was quite afraid of this.

[Joe G.]

Another resource to look at if you haven't seen it already: the Build Blitz claw design guide has some great information.

[PaulEleven]

Originally Posted by Joe G.

Another resource to look at if you haven't seen it already: the Build Blitz claw design guide has some great information.

takes after its shape actually, however we feel pneumatics might be a more versatile option given that we do not have accurate machining capabilities.

[PaulEleven]

Originally Posted by PaulEleven

Won't be able to add a very big one, I'm thinking about the igus 40mm one in the KOP. What about 1" cylinder? Air consumption isn't that large a problem

correction 1 1/16