Piston Geometry

[MysterE]

Hey all -

I know that there may already be a great post on this, I’m just not sure what term to search for other than the subject line. We are using pistons to lower and retract our intake mechanism and it’s the one thing that I am having to use the guess and check method on placement. As a result, while it would lower the intake, the pistons could not raise the intake.

What is the appropriate way to account for position in regards to pistons dependent on the location and length of the piston.

Thanks so much,
Daniel

[Cbrosch0]

During the last cadathon we used 973’s video on linkages to determine size and placement. Does your cylinder just not close all the way when retracting the intake?

[paulonis]

There is a fantastic pneumatic linkage calculator in Ari Meles-Braverman’s design spreadsheet. The reverse calculation is especially helpful.

[MysterE]

It would not pull up the intake at all. But if we increase the amount of pressure going to the pistons, it lowers it way too fast.

[MiloB]

My team had this problem as well, we solved it by using a flow restrictor to slow down the speed at which the intake dropped. This allowed us to increase the pressure to the point it could pick itself back up.

[MrForbes]

Cylinders…call them cylinders, not pistons, please?

Start by drawing a free body diagram. Show the pivot point of the moving part, show the center of mass, and the weight of the part. Show the proposed cylinder attaching points. Sum the moments about the pivot point. Use algebra to determine how much force is required to move the intake up, and get cylinders that provide at least twice that much force.

Or just figure out how to put some surgical tubing on it, to help life the intake, and take some of the load off the cylinders

[CIM]

@Cbrosch0

we used 973’s video on linkages to determine size and placement.

This video should be required viewing for any FRC designer. It is too useful.

@MysterE

It would not pull up the intake at all. But if we increase the amount of pressure going to the pistons, it lowers it way too fast.

Design pistons in using vector math to ensure they will have enough force to do what you’re asking of them. Then, to ensure they do not lower “way too fast”, use flow control fittings or in-line adapters in order to slow the flow of pressurized air in the “easy” direction of motion. For linkages that deploy things it is very rare that you will not need to do this.

[GeeTwo]

Adding some spring support will also help equalize the speed of up vs down, and may help your cylinder be enough for the job.

[EricH]

@MrForbes

Start by drawing a free body diagram. Show the pivot point of the moving part, show the center of mass, and the weight of the part. Show the proposed cylinder attaching points. Sum the moments about the pivot point. Use algebra to determine how much force is required to move the intake up, and get cylinders that provide at least twice that much force.

@MysterE, I suspect that when you’re in the down position, you’re trying to pull through a pivot, on top of the moment of the intake itself. When you’re already up, you’re probably near 90 degrees. My team ran into that earlier this week–we raised the back end of the cylinder and the problem went away.