Choosing a gripper mechanism


by Femme_physics
Tags: choosing, gripper, mechanism
Femme_physics
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#19
Mar16-12, 05:33 PM
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As to the direction of gravity, yes, it is always down. But the part, as shown in the drawing looks like the symmetry axis is horizontal. The point is, will it remain in that orientation, or will it be necessary to rotate it into a new orientation, possibly orienting it into several orientations before finally depositing it in its new location. In that regard, gravity then acts in a variety of directions with respect to the gripper. If all of this must happen quickly, then there are rotational inertial loads that can be significant. It is generally considered bad form to drop the part!
Basically our teacher told us to not limit the consumer, if the consumer want to put the part from a high region to low, or vice versa, it should all be possible. The idea though is basic pick and place movement.

Do you know how it is to be oriented in the final position? Which end is finally accessible? This makes a lot of difference.
No, we don't. Again, it's about not limiting the consumers.

Do you have a drill press, or an electric drill with a keyed chuck? This is called a Jacobs chuck after the original patent holder. If you look inside, you will see that the "teeth" that hold the drill bit are in fact three sliding wedges that come out (toward the user) as they close. When you turn the key to tighten the chuck, you are simply tightening the force driving the wedge into place.
Well, I understand what a drill press, and how it works. But how will it help me grab the part at the section I need, that I can't relate.

I don't think I had previously picked up the part about having to pick up on the red surface if that had been mentioned. That is quite significant.
You're right,I shamefully forgot to mention it. Mind you the first time I made this thread I got an error that deleted my entire post (where I mentioned it), so I'd chalk that to impatience. My bad.
OldEngr63
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#20
Mar16-12, 05:50 PM
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@ Femme_Physics: You said, "Our part weighs 7.3 gr. Not sure about using Pneumatic or Hydraulic. We need to grab the part on the area marked in red, so we figured an electric one would best serve this purpose. No?"

Now as I go back and look at the original spec sheet, I see Mass = 7286.12 grams. Has there been a factor 1000 lost on the mass of the object? This will make significant difference with regard to holding forces required.
OldEngr63
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#21
Mar16-12, 05:57 PM
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Well, I understand what a drill press, and how it works. But how will it help me grab the part at the section I need, that I can't relate.

You have not really looked closely at the drill chuck. What the drill chuck does is precisely what you need to do here - grab hold of a cylindrical surface, move it, and then release it. To understand, you have to look closely at how the inside of the chuck works.
OldEngr63
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#22
Mar16-12, 06:06 PM
P: 343
"Basically our teacher told us to not limit the consumer, ..."

Sounds like your teacher must of been a great sales engineer, the sort that wants to be all things to all men. The truth is, it is not possible to design machines (of any category) with unlimited capabilities. They always turn out to be finite in every aspect. (If you tell him what I said, he will probably hit you right between the eyes and swear at you! Faculty get very sensitive about such things!)

He gave you all of those things, mass, mass moments of inertia in all three axes, cm location, etc. more or less implying that he expects you to use them to calculate reaction forces and moments on your mechanism, but then he gives you nothing at all about the motions that the mechanism will experience, not even max accelerations, max angular accelerations, nothing. Talk about a red herring! Shame on him.
Femme_physics
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#23
Mar16-12, 06:43 PM
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Quote Quote by OldEngr63 View Post
@ Femme_Physics: You said, "Our part weighs 7.3 gr. Not sure about using Pneumatic or Hydraulic. We need to grab the part on the area marked in red, so we figured an electric one would best serve this purpose. No?"

Now as I go back and look at the original spec sheet, I see Mass = 7286.12 grams. Has there been a factor 1000 lost on the mass of the object? This will make significant difference with regard to holding forces required.
You're right, it's 7.3 kg. Sorry, excited. :)

You have not really looked closely at the drill chuck. What the drill chuck does is precisely what you need to do here - grab hold of a cylindrical surface, move it, and then release it. To understand, you have to look closely at how the inside of the chuck works.
You're right, after a closer look...the drill chuck looks much better. Now I just have to understand the mechanism the drives the entire drill chuck to make it work. I'll consult with my project classmate to see what he thinks, also.

Sounds like your teacher must of been a great sales engineer, the sort that wants to be all things to all men. The truth is, it is not possible to design machines (of any category) with unlimited capabilities. They always turn out to be finite in every aspect. (If you tell him what I said, he will probably hit you right between the eyes and swear at you! Faculty get very sensitive about such things!)
Actually my professor is very nice old Ukrainian man, I doubt he's ever been involved to deeply in sales. I'm pretty sure he's aware of the finite stuff--perhaps I do a poor job representing him. If in our design we have no choice but create limitations, then we do it. His main concern is really that we document everything :)

He gave you all of those things, mass, mass moments of inertia in all three axes, cm location, etc. more or less implying that he expects you to use them to calculate reaction forces and moments on your mechanism, but then he gives you nothing at all about the motions that the mechanism will experience, not even max accelerations, max angular accelerations, nothing. Talk about a red herring! Shame on him.
Well, to my understanding yes, we need to calculate a lot of stuff, shows advantage and disadvantage in certain scenarios, basically do a lot of documentations.

As far as max accelerations, max angular accelerations....does he really need to define us that stuff? Perhaps he expects us to choose it ourselves within reasonable margins. It appears to me like quite an open-ended project, indeed not many conditions have been predefined, but we do have to account to what we choose in calculations, diagrams, and etc etc...
OldEngr63
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#24
Mar16-12, 07:28 PM
P: 343
"As far as max accelerations, max angular accelerations....does he really need to define us that stuff?"

That's a pretty good question. I don't see how you are going to determine the required gripper forces without some estimates on these things. In the end, it may be up to you to assign some values for these, but you might want to try to have some basis for them. I see a couple of ways to do that:

1) Look for product data on a gripper that you think is similar to yours. Automation magazines would be a place to search.

2) Define some plausible motions and calculate the loads that result. Then take the worst combination as your limits. For automation work, cycle times can be very, very short, much less than a second to pick and place an object, so things really have to move. Unfortunately, you do not have control over the entire system, particularly the robot arm making the move. If it is jerky, the loads on the end effector, your part, go up seriously. So, some things to think about ...
Femme_physics
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#25
Mar17-12, 01:32 AM
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Thanks a lot OldEngr, these will be important tips I can work with down the road to make a more comprehensive work.

I have a trouble though finding a detailed product description about chuck grippers. I want to see basically the method of operations, mechanics, etc. We're still beginners in the field of mechatronics, with only some solidworks, basic technical mechanics and material strength behind us, this is our first big project. Then again, I don't shy from hard work and a chuck gripper does seem ideal. But, there are so many options in this field, and so little information about it that I can find. I even downloaded 2 PDF's about grippers. None seem to have a special section about the chuck grippers. I'm basically looking to understand the operational mechanism of the chuck gripper itself, and what's more favorable in what conditions for the chuck gripper - electric, hydraulic...etc...

A quick search shows me that it's more popular in the pneumatic field.

I'll refer to my professor and classmate tomorrow with questions about the acceleration, part positioning (even though I suspect his answer is "adapt to conditions"), and what does he think about a chuck gripper.

Thanks :)

Damn, our grabbed part is quite challenging.
OldEngr63
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#26
Mar17-12, 10:01 AM
P: 343
As you said, there are so many options, and that does indeed make it difficult to choose the optimum for any particular situation, particularly when you are supposed to be flexible. You are going into a very interesting field, and one that has a good future, so hang in there and don't be at all discouraged. To be sure it is challenging, but it would not be fun if it were just all cut and dried! Cheers!
I like Serena
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#27
Mar17-12, 06:04 PM
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Oh okay, so I didn't know what to give you, so here goes:



Happy birthday!!!
Femme_physics
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#28
Mar21-12, 03:08 PM
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hehe..thanks ILS.

And thanks, OldEngr. I guess we have time with that, yet, so I'll take my time do more research and see if I can come up with anything interesting. :) Appreciate it.
OldEngr63
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#29
Mar21-12, 07:16 PM
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Please keep us posted on what you decide to pursue for this project. There are so many possibilities, with no one of them being "right" with the others 'wrong." I'm very interested to see what you will come up with on this.

Have you taken a course in the kinematics of machines, something often called Theory of Machines? That would be a big help at this point if you have not. You might look for a ToM textbook for some guidance.


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