Restrainting pin question

[Jeffy]

Our team this year wants to use a pneumatic clyinder as a restraining pin for our spring loaded kicker. My question is whether or not we can directly use the piston rod as the pin. I know it follows the rules, but would using the piston as the pin damage it?

[Tom Line]

Originally Posted by Jeffy

Our team this year wants to use a pneumatic clyinder as a restraining pin for our spring loaded kicker. My question is whether or not we can directly use the piston rod as the pin. I know it follows the rules, but would using the piston as the pin damage it?

Absolutely it will damage the shaft. Though most people don't realize it, metal isn't "hard". It's actually quite soft. Any time you see metal on metal interaction, for instance in the form of a pin, or a long shaft, etc, it's probably been hardened.

Machine a sacrificial pin that threads onto your shaft, and make sure that pin is supported against side loads by traveling through a hole in a plate. That way when you destroy that pin (and you will!), you can replace it and keep on rockin'.

[Gary Dillard]

Originally Posted by Tom Line

Absolutely it will damage the shaft. Though most people don't realize it, metal isn't "hard". It's actually quite soft. Any time you see metal on metal interaction, for instance in the form of a pin, or a long shaft, etc, it's probably been hardened.

Machine a sacrificial pin that threads onto your shaft, and make sure that pin is supported against side loads by traveling through a hole in a plate. That way when you destroy that pin (and you will!), you can replace it and keep on rockin'.

Hmmmmmmmmmmm

Say your load is 100 pounds, you are using a .75 cylinder (.25 dia steel shaft) for the pin, and you use a double shear connection to support the shaft (a plate on both sides to ground, a plate in the middle connected to the kicker with close fit).
Cross sectional area of the shaft is just shy of 0.05 in^2. The shear force (draw a free body diagram) is 100/2 on each side or 50 pounds. That gives you a shear stress of about 1 ksi. Shear yield strength of mild steel / annealed stainless steel is a little over 20 ksi. That means you would get, oh, about a gazillion cycles before you have to worry about damaging the shaft by my calculations.

Actually there's a more complex beam in a socket calculation because there is actually minimal bending in the shaft (ideal shear doesn't occur in the real world), so you might only get half a gazillion cycles out of it.

[Teched3]

It is not advisable to use the piston rod as a direct mechanism. It should operate a mechanism that will take the load instead.

[Homsar66]

That would depend on the size of the piston, how long it was extended, how many bands, mechanical advantage, ect. But generally, if you don't extend the cylider all the way out and use a thick rod, you should be fine for a while.
Better to have it operate a latch or something though,it might wear down and then start to leak/bend

[Matt H.]

I agree with the above posters that the cylinder piston won't work as a pin--however using Mcmastercarr part #
97175A329
or any similar threaded pull out stainless steel down pin will work wonderfully.
They cost about $1 each; the stainless steel is extremely hard; and they will thread right onto the cylinder.

[Scott358]

Beyond potential damage to the rod as noted above, cylinders are not designed to take side loads. Therefore I would suggest some sort of bearing set-up to support the pin.

[Freeztech]

Originally Posted by Jeffy

Our team this year wants to use a pneumatic clyinder as a restraining pin for our spring loaded kicker. My question is whether or not we can directly use the piston rod as the pin. I know it follows the rules, but would using the piston as the pin damage it?

Our team used the Pneumatic clyinder for the restraining pin in one of the prototype kickers that we build. We did no damnage to the piston because we used a lexan slot to stick it in. We also grinded the treads off the piston and had it resting on there. The lexan was only 1/8th inch bent around a steel pipe and bolted on. So you have an idea, we were dealing with two 300 lb springs fully compressed. So about 600lbs of force on the system.

so a pneumatic piston will work, long as its not metal on metal.

[Bruceb]

I believe your modification of the rod by grinding of the threads is illegal by R73.
<R73> Items specifically PROHIBITED from use on the ROBOT include:
A. Any pneumatic part or component rated for less than 125psi.
B. Any pneumatic part or component that has been altered, modified, machined, coated, or changed from its original “out of the box” condition, except as required for normal assembly with other components. The only acceptable modifications are:
• Tubing may be cut.
• Wiring for pneumatic devices may be modified to interface with the control system.
• Assembling and connecting pneumatic components using the pre-existing threads, mounting brackets, quick-connect fittings, etc.
• Removing the mounting pin from a pneumatic cylinder, provided the cylinder itself is not modified.
• Labeling applied to indicate device purpose, connectivity, functional performance, etc.

[Leav]

Just wanted to chime in that in 2005 team 1574 used a piston to grab the tetras (the tetras would rest on the piston's extended shaft).

After about 3 or 4 games the piston became damaged and started spitting out air at 60psi making a high pitched noise and spewing decompression vapors...

quite spectacular really..

[Freeztech]

ahh. i didn't see that. But it was for a prototype. we've already scraped that, but what we did to provent the piston from bending out of line was we had them slide through two blocks that the lock on the moving arm sit inbetween. so it had those to hold it in place and provent shearing pressure