# Choosing a gripper mechanism

by Femme_physics
Tags: choosing, gripper, mechanism
PF Patron
P: 2,544
So we need to make a gripper in Solidworks only for this object

Our current goal (as per our professor) is to focus on our gripper mechanism.

My classmate suggested the following for the gripper final grip mechanism

 Chebyshev's linkage - alternative form While the picture here looks quite different from Chebyshev's linkage above, these two configurations actually generate the same curve. It is curious that a particular curve may be generated by more than one linkage. For a clue why this might work, have a play with the GeoGebra applet below, in which the two linkages are superimposed.

The idea being that at the end of this straight movement that closes the gripper 2 jaw parallel effectors attached at the tip of this mechanical mechanism grab the object. We thought it's best for both sides of the gripper to close in a straight motion as this would increase accuracy, as opposed to a Two Jaw Cam Actuated Rotary Gripper...which mean they will be closing in an angel and decrease accuracy.

What do you think of the idea so far?
 HW Helper P: 6,165 Well, how does it look... sexy! As for the idea of a linear gripper... do you have a drawing?
 PF Patron P: 2,544 Better yet, I have a video! :) http://www.youtube.com/watch?v=Ttsp7LJMDdo
HW Helper
P: 6,165

## Choosing a gripper mechanism

Nice video. ;)

Or is there more to come?
 PF Patron P: 2,544 No, it's just a suggestion. I wanna see if it can be improved, or if it's a good idea overall. We're still beginners on the issue, so every feedback counts. Do you have a feedback? :)
 HW Helper P: 6,165 Should your gripper operate in 3 dimensions? How would your gripper be fixed in a frame? Did you already google for grippers? Trying to find a picture of a gripper that matches your object? And perhaps one that will look sexy in SolidWorks? Personally, I thinks it's more important that it looks sexy than that it is a functionally correct gripper.
 HW Helper P: 6,165 Btw, is there anything I can get you for your birthday?
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P: 2,544
 Quote by I like Serena Should your gripper operate in 3 dimensions? How would your gripper be fixed in a frame? Did you already google for grippers? Trying to find a picture of a gripper that matches your object? And perhaps one that will look sexy in SolidWorks? Personally, I thinks it's more important that it looks sexy than that it is a functionally correct gripper.
LOL, well the current design is pretty sexy

Yes, it's 3 dimension. But, I guess you can just imagine the 2D construct with volume, or width.

Yes, I googled for gripper, mostly we realize that the 2 jaw parallel gripper is good since it uses less friction than flat friction grip, therefor less energy. My issue is the gripping mechanism, not the actual gripper design. It looks ok, but I'm just a starter so can use the feedback :)
PF Patron
P: 2,544
 Quote by I like Serena Btw, is there anything I can get you for your birthday?
Aww yea what u brought last time just for uppers ;)

HW Helper
P: 6,165
 Quote by Femme_physics Aww yea what u brought last time just for uppers ;) j/k! Your forum help is the best gift I can ask for!
Well, the forum help is for free, so that does not count.

So you'd like something to match cybergrrl?
Well, that alone already gives you +25 intelligence points, let alone the charisma points.
I'm just wondering what more uppers I can get you...?
 P: 343 Just a few comments about the whole thing. 1. The problem statement apparently does not specify the direction that gravity wills act on the combined gripper/object assembly, not does it specify the actions that the assembly must be able to make. This makes it difficult to determine just how the gripper must hold the object and how firmly the grasp must be. For example, can it simply scoop the object up in a shovel and dump it out, or must it pick it up in one orientation, carry it, and reorient it before placing it at the final position. How fast does it have to function; will dynamic loads be a consideration? 2. It appears that you have not made use of any of the features of the object to help secure the hold of the gripper on the object; shaped jaws could certainly help, but the closing/opening motion must not allow for a jam. 3. Your gripper design looks really bulky. Often the gripper needs to insert a part where there is not a lot of room for the gripper. You might want to look at ways to stream line the design. (Make your two operating wheels coaxial, so that they appear one over the other in the side view, for starters. I think you might think you might want to look at other mechanisms as well.) 4. In your second figure, the on labelled Fig 1, what appears drawn as a spring is labelled "cam." I cannot see it being anything other than a spring. 5. Remember that done properly, the grip/release motion is really very small. What you have drawn generates a very large motion which is easy to see, but far too big to be serviceable. Think about the sizes that are really needed, and the motions, and corresponding cycle times that could be achieved with them.
PF Patron
P: 2,544
 1. The problem statement apparently does not specify the direction that gravity wills act on the combined gripper/object assembly, not does it specify the actions that the assembly must be able to make. This makes it difficult to determine just how the gripper must hold the object and how firmly the grasp must be. For example, can it simply scoop the object up in a shovel and dump it out, or must it pick it up in one orientation, carry it, and reorient it before placing it at the final position. How fast does it have to function; will dynamic loads be a consideration?
It has to work in numerous such conditions and be able to adapt.

 2. It appears that you have not made use of any of the features of the object to help secure the hold of the gripper on the object; shaped jaws could certainly help, but the closing/opening motion must not allow for a jam. 3. Your gripper design looks really bulky. Often the gripper needs to insert a part where there is not a lot of room for the gripper. You might want to look at ways to stream line the design. (Make your two operating wheels coaxial, so that they appear one over the other in the side view, for starters. I think you might think you might want to look at other mechanisms as well.)
I agree, it's awfully bulky. Although, in reality, it would only open up 30% larger than the object it needs to catch (from each side), in my solidworks video it opens up really wide. It was an initial sketch idea.

 4. In your second figure, the on labelled Fig 1, what appears drawn as a spring is labelled "cam." I cannot see it being anything other than a spring.
I don't see a spring in our design. You might be confusing the "Cam" from the "CamStudio" software I used to record.

 5. Remember that done properly, the grip/release motion is really very small. What you have drawn generates a very large motion which is easy to see, but far too big to be serviceable. Think about the sizes that are really needed, and the motions, and corresponding cycle times that could be achieved with them.
As I said my video was overdoing the grip. We did think of a less clumsy mechanism, which I'd like to present:

Any thoughts?
 P: 30 Both designs are based on rotary movement converted to linear motion. You may think of using a linear actuator (Pneumatic or Hydraulic) to drive your design in some sort of scissor motion.. Just a thought..
P: 343
 Quote by Femme_physics It has to work in numerous such conditions and be able to adapt.
This is really not a design spec, and there is no way to say whether a particular design meets it or not. This is especially true with respect to the dynamic loading/cycle time considerations.

The use of a rack and pinion, with the rack perpendicular to the main system axis, is going to make it very difficult to ever achieve a compact design. Why not a rack parallel to the axis, driving a wedge to clamp onto the object (much like a Jacobs drill chuck)?
PF Patron
P: 2,544
 Quote by berko1 Both designs are based on rotary movement converted to linear motion. You may think of using a linear actuator (Pneumatic or Hydraulic) to drive your design in some sort of scissor motion.. Just a thought..
The idea is indeed to use a linear actuator in our design to drive both gears :) We discounted scsissor motion because direct linear motion for grabbing seems more accurate, don't you think?

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?

 This is really not a design spec, and there is no way to say whether a particular design meets it or not. This is especially true with respect to the dynamic loading/cycle time considerations.
Well, it has to be designed to grab that SPECIFIC part, in that specific area marked in red. The part weights 7.3 gr and made out of AISI steel 1040. It has to be able to pick it and place it elsewhere.

 1. The problem statement apparently does not specify the direction that gravity wills act on the combined gripper/object assembly
Doesn't gravity always act down?

 not does it specify the actions that the assembly must be able to make. This makes it difficult to determine just how the gripper must hold the object and how firmly the grasp must be. For example, can it simply scoop the object up in a shovel and dump it out, or must it pick it up in one orientation, carry it, and reorient it before placing it at the final position. How fast does it have to function; will dynamic loads be a consideration?
Oh, I see what you mean.Yes, it's for pick and transfer.

 The use of a rack and pinion, with the rack perpendicular to the main system axis, is going to make it very difficult to ever achieve a compact design. Why not a rack parallel to the axis, driving a wedge to clamp onto the object (much like a Jacobs drill chuck)?
How can you grab the object with a wedge exactly?
 HW Helper P: 6,165 How about a sliding gripper? Something like this:
 PF Patron P: 2,544 Well, that's essentially what we've made so far in the last solidworks animation (imagine that at the end there are 2 jaw grippers like in your pic. The difference between my idea and your pic is: A) Mine drives the end-effectors directly. Suppose I could mount on the rails grippers like yours. B) Yours doesn't show internal design
 P: 343 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. 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. 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! Do you know how it is to be oriented in the final position? Which end is finally accessible? This makes a lot of difference.

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