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  1. #1
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    Default Need help with a rubber gripper...

    Folks,

    We're using some rubber gripper pads to lift some parts with out robot. Part has a yoke with a "U" shaped opening. Gripper goes inside of this "U" shaped opening, then 'opens' to grab & lift the part. This is a friction grip.

    Gripper in question is a polyurethane rubber gripper pad, McMaster-Carr part number 9306k71.

    McMaster-Carr



    We're having trouble with some of our painted parts slipping nearly, or sometimes completely out of the grippers.

    Tonight, in McGyver fashion, I used some gorilla tape to fasten some emery cloth to the outside of these rubber grippers, and saw a humungous improvement.


    Is anyone aware of any commercially available gripper pads, which are rubber, or compressible, which also have an abrasive grit for a little extra 'bite' to them?


    Thank you!


    FWIW, serrated metal grippers don't really work. The robot's gripper doesn't have enough force to indent the metal grippers into the part material (steel, approximately 35hrc, with a slick coat of hardened paint), and thus, they just slip right out...

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    Could you take your existing pads and use a urethane adhesive to attach segments of garnet cloth?

    I seem to remember some woodworkers conformal rubber pads with embedded abrasives, but can't find them in a quick search.

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    80A is a pretty hard rubber. It'd be easy to make your own out of lower durometer polyurethane sheet, perhaps 30-40A. The hard part would be making the counterbore, so if possible remake the jaws so that the screw heads are in a spot that won't interfere with your part. I do this a lot to make custom vacuum grippers for robots. Something like this: McMaster-Carr

    Another option would be to mold your own jaws in any shape you want. I've made simple molds, either machined from aluminum or 3D printed, and made custom polyurethane parts. I've used urethanes from other sources, but McMaster sells them too: McMaster-Carr. Be aware that, unlike silicones, you'll want to use a mold release with urethane. Lots of youtube videos on how to do it and it's quite simple. Kinda fun actually.

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    I sometimes mold wheels with garnet embedded into them to provide extra grip. PMC-790 or 780-DRY (Shore 90A and 80A respectively) from Smooth-On is my go to. Works excellent. Like Pumpkinhead mentioned, you should be using a mold release (the universal variant from Smooth-On works well).

    If you want to try it on the cheap you can 3D print the mold and go from there. I've tested a number of urethane/silicon molds that way before heading to the CNC machine.

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    Bury a magnet in the rubber?

    Or, get some of those pads they sell to keep your phone on the dash of a moving car... They seem extra-grippy, especially on nice smooth surfaces.

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    Just for further clarity on the situation. (Last night/OP was a pretty late night...) The part pictured is not the problem child, as more parts come in with a shot-blasted finish which greatly improves grip with our current setup. The painted parts however still cause us grief. Current mix is probably 60% shot-blasted, 40% painted. Last night's problem child painted part weighs in the neighborhood of 55-60lbs.

    grippers.jpg

    Picture of current gripper setup, showing the McMaster-Carr rubber gripper pads in combination with serrated hardened steel grippers, with rubber washers mounted underneath. The idea is that the serrated metal grippers give some kind of positive/rigid gripping authority, while the rubber is allowed to conform best-fit. We have tried about every combination of serrated metal gripper & current rubber grippers, and this seems to be the best arrangement SO FAR, but obviously not good enough.

    What you cannot see, is that the rubber grippers are notched in the center, so as to allow clearance for the parting line on the part.



    I have ordered some of these pre-made strips from Fixture Works, the idea being that they will be oriented parrallel to the parting line (90* from the current red rubber gripper pads,) to give us a little more contact against the part. I've ordered both smooth & fine-hatch and will try both.

    Strips - Fixtureworks



    Last night sort of confirmed a suspicion I've had for a while though - that an abrasive grit on the gripper pads would help with these painted parts. Unfortunately, unless I can find a commercially available rubber stock with abrasive grit, it looks like I'm going to have to go the Rube Goldberg route, and go "homemade." I'd like to avoid that if at all possible.

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    It looks like the gripper geometry could be extended with a section that overlaps the round boss that's currently the contact area, and so give a geometric holding feature. I'm sure you've considered it and have a reason for not doing so, but could you explain why?

    If the throw of the grippers is great enough, the pad could entirely surround the boss, yes?

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    I agree with Milland - if at all possible you want a positive grip based on geometry. Depending on friction is a last resort. This assumes your workpiece location and size is repeatable enough that you can make a gripper the wraps around some piece of geometry, like those round bosses.

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    Cratex ?..

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    Quote Originally Posted by Milland View Post
    It looks like the gripper geometry could be extended with a section that overlaps the round boss that's currently the contact area, and so give a geometric holding feature. I'm sure you've considered it and have a reason for not doing so, but could you explain why?

    If the throw of the grippers is great enough, the pad could entirely surround the boss, yes?
    We run different (yet similar overall) parts through the cell, so unfortunately not all of them have the rounded boss' for a geometric fit.



    Quote Originally Posted by CarbideBob View Post
    Cratex ?..
    I stumbled onto them, but my gut tells me they wouldn't be course enough?

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    Garryflex blocks?
    Sort of a sanding sponge and down to 36 grit but likely to soft.
    Bob

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    So friction has little to do with surface area, its Pressure X Friction Coefficient. So increasing the area doesn't actually increase the grip. So test the rubber blocks by dragging the same weight across the painted surface and compare to other grades, types of rubber to get the best Coefficient.

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    Perhaps you'll need to go to a grip actuator with a greater outward force capability. That, and some carbide pyramidal contact pads will get you the reliable and positive face contact you need.

    Solid Carbide - Fairlane Products

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    Another possible do-it-yourself hack would be to cut strips of old-school mousepad and fix them to the metal jaws. If you don't have any around, McMaster sells Neoprene sheets, you want the ones "with criss-cross texture" and I would probably go with 60 durometer. Maybe try 50 if you want soft and really grippy, but the physical properties (i.e. durability) will be lower.

    Thing about Polyurethane is that there are two major types. One gets all weird when it gets wet, and the other gets all weird when it gets oily. Neoprene is a solid all-around rubber compatible with most common stuff. And with the polyurethane it looks like you have a harder material with a lot of thickness, I might try a softer one with a metal jaw behind it.

    I attached a sketch of how I would make the jaws so that they firmly hold the sheets but allow the rubber to be replaced easily. Might need to make holes in the sheet if you can't make the jaws longer than the part. The step should be about 75±5% of the rubber sheet's thickness for the best balance of holding force and longevity of the material. Always run force flow through a metal-metal hard stop, not through the rubber. Radius about 3x the sheet thickness or more.

    rubber-gripper.jpg

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    If you're able to share the drawings of the part and the gripper I'd be happy to spend some time to come up with some specific solutions. With the group here you would get some stellar solutions rather quickly.

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    I looked into custom molding something from rubber, perhaps impregnated with loose abrasives on the mixed in, or on the surface.

    I looked up a video on YouTube, and am slightly surprised at how easy it looks. This video caters more to the home-DIY crowd, but certainly piqued my interest in molding custom grippers - which I was previously pretty strongly opposed to, mainly out of lack of knowledge on the subject.
    YouTube



    Now, this video speaks to molding silicone. Most of what I can find regarding industrial applications suggests using urethane rubbers. Personally, I know nothing of the difference. McMaster-Carr seems to sell urethan molding kits though.
    McMaster-Carr

    I would appreciate any insight others would have on urethane vs. silicone rubbers for such an application.





    Another idea I'm toying with, would be a metal/rubber sandwich with non-slip adhesive tape for the gripping surface. Such as...

    Non-slip tape
    Thin metal strip
    Medium-soft rubber
    Metal backer

    Non-slip, adhesive backed tape...
    McMaster-Carr

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    The most important thing to consider when molding castable stuff yourself is that silicone is self-releasing, and urethane is not. That means if you make either the mold from silicone or the parts themselves silicone, there is no need to worry about releasing it from the mold. Urethane is not, so it will require some kind of mold release.

    I think the diamond pattern neoprene sheets will be a big upgrade in terms of grippiness over plain 80A durometer urethane, some of those non-slip tapes are basically sandpaper and I'd worry about marring a finished painted surface.

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    If you are going to do a mold with a top and bottom then here are a few tips:
    1. Use mold release on both parts of the mold (this would apply if doing an open faced mold as well)
    2. Mix more than you need.
    3. Add cutouts on the sides of the mold halves to allow you to separate the mold with a prying tool once the mix has cured. Jack screws work too.
    4. Add vent holes every few inches (1/8"-3/16" works fine). The little nubs will need to be trimmed but that's better then getting air pockets.
    5. Add a hole large enough to inject the rubber/silicon with a syringe. If adding garnet, then size the nozzle accordingly.
    6. Degass the rubber mix if possible. This will eliminate bubbles. If you don't have access to a vacuum chamber (or a vacuum pump, a can with a lid) then try not to introduce bubbles when mixing. If I do it this way I'll stir vigorously to get everything mixed well and then I'll start to slow down and tilt the container to draw the mix up the side in an attempt to eliminate the large bubbles.

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    For your application I'd use urethane because it has better abrasion resistance than silicone. I've only used Smooth-On brand urethanes and silicones and they're very easy to use. I built a little vacuum chamber using one of my wife's pyrex bowls and a pump I had laying around. You can also use a venturi style vacuum generator - that's what I started with because I had a few on the shelf from robot vacuum grippers. The 120V pump is just a little faster and less noisy. I screwed an air fitting into the bottom plate to pull the vacuum and lined the plate with a sheet of 35 durometer neoprene so the bowl would seal. A paint pressure pot would also make a good vacuum chamber if you have one. Pulling a vacuum not only gets rid of entrapped bubbles but also helps the material flow into tight spaces and undercuts. For release I use Smooth-On Universal Mold Release which is available from Amazon and some hobby shops.

    In the pictures I'm molding a custom vacuum cup using a 3D printed mold. I'm molding urethane around an aluminum detail that attaches to a robot gripper. The aluminum part has undercuts to hold the vacuum cup on - the urethane cup snaps into slots in the aluminum piece. I molded a bunch of them as spares, peeling them off the aluminum and putting them on the shelf for later. You could do the same with your gripper.

    img_20200202_082029-m.jpgimg_20200202_080205-m.jpgimg_20200202_080551-m.jpg

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    You might cut this material to your needs

    https://www.amazon.com/Adhesive-Weat...xpY2s9dHJ1ZQ==


    Another engineering trick/method is to have a belt going around a part, and an idler finger/roller or sealed bearing that snugs the belt on the going-to side, the side before the part.

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