Calculating Friction Force and Bore Size for Robotic End of Arm Tooling

[dgansen]

Good morning,

I need some help on a robotics application. We have an electrical component, which fits into a plug to have software loaded; a robot picks up the component, puts it in the plug, and once software is loaded, moves the component to an unload bin.

Space constraints mean we used a smaller pair of grippers to pull the parts out of the plug, and part geometry means we come in with both gripper "fingers" on two flat sides of the part. We need about 12 pounds of lifting force, which we are not getting with our .500" bore air cylinder...

In my mind, since the grip force is normal to the direction of motion (and gravity) it is all about generating enough friction to avoid the part sliding out of the grippers during the lifting operation. How would I calculate the size bore cylinder required to lift the part?

As stated, I need 12 pounds of lift force. Our current cylinder is a .500" cylinder bore, with a max air pressure of 140 PSI. The surfaces contacting each other are 4140 steel to 6061 aluminum on one side, and Acetal to Nylon 66 on the other side. Gripper fingers contact the part about 3" away from the air cylinder.

I can try to post photos later, but need to double check on our disclosure agreement. Any and all help is greatly appreciated!

Thanks,
-Dan

 

[Dr. Courtney]

Pictures would be really helpful.

 

[dgansen]

Here's a photo with a few material callouts... I apologize for the black smudges, had to perform a little CYA...

Edit to fix photo URL

 

[Alex at work]

Investigate the addition of a lip or notch in the part to match a feature created on your gripper.
Sure customers are often dead set in not changing their parts to accommodate automation,
but the best solutions are often accompanied with small compromises. At least this has been my experience.
A small notch or lip could make all the difference for high speed insertion and removal of your part.

:EDIT
Are you inserting with a different gripper and removing with this one pictured?

 

[dgansen]

Unfortunately, the majority of the components have already been made; speed is not the most critical, as they only load 4 parts at a time with a 35 second cycle time... We are investigating the addition of suction cups, but can only fit a 13mm; even with 2 cups, we estimated a requirement for 40mm cups with a 21 Hg vacuum generator; that does account for a safety factor of 4, so we will see; it may come down to a combination of larger gripper cylinder, as well as the two 13mm cups. The question is, how big of a cylinder?

I'm assuming there is a formula that will determine how much friction is required to lift that weight, which can be turned into an amount of force required, which can then be referenced with the manufacturer to determine bore size...

Definitely should have saved my old college physics textbook, tried finding the formulas and it's amazing how much info I've forgotten...

ETA...

Yes, this gripper is only pulling the part out; load bins are sized differently from unload bins, so there is not room for the first set of grippers to unload them.

Where the aluminum backplate protrudes with two mounting holes, the first grippers utilizes those to locate, and the part is held very well. We simply don't have the space in the unload bins for the "side to side" motion of unclamping; we thought of using pins through those holes, but would need an ejector of some sort to push the component forward off those pins. Front to back motion is also severely limited during the unload process.

 

[Alex at work]

you might be overcoming more than just weight and friction in the plug.
the sticking tendency in the plug could have an initial higher (and varying) friction quality we sometimes call by the made up term "sticktion".

 

[dgansen]

Hmm, the vacuum didn't have near enough force, which makes sense; crunching the numbers showed we wouldn't even be close. We may still supplement the grippers with the cups, but we'll still need the larger cylinder; It's a matter of determining the clamping force to grip the component hard enough to overcome "sticktion" (I really like that term, by the way)!

 

[Nidum]

I'm assuming there is a formula that will determine how much friction is required to lift that weight, which can be turned into an amount of force required, which can then be referenced with the manufacturer to determine bore size...

Ratio of friction force to gripping force is just coefficient of friction for materials invloved . Work out the worst case and then apply a sensible reserve factor .

You have other problems though - that gripper looks very flexible .