My Gripper 3 stopped working

After playing with it a while the gripper just froze. Could not move it at all.

After taking it apart I discovered that the shaft had small burrs on it that must have broken off inside the shaft causing the gear to bind and no longer turn.

Due to the tight tolerances the shaft no longer fits.

Now my fingers are black from playing with the aluminum.

Need to polish the burrs off the shaft so I can put it back together.

Mike


Comments

  • WhitWhit Posts: 4,118
    edited 2020-05-12 - 02:09:58
    When you put it back together, you might want to use some of the grease on every moving surface. Mine had some stiffness to and I disassembled it and greased everything. Works great now.
  • Well now the problem is that because the aluminum is so soft that there are now scratches on the shaft that have to be removed before I can put it back together. The hole also has scratches in it so the two pieces no longer fit and bind.

    I'm going to need to find something to polish the two surfaces to remove the scratches. I don't want to use sand paper as that seems to be too ruff.

    Mike
  • Aluminum-on-aluminum isn't an ideal combination for moving parts, since it tends to be sticky. If the shaft is just a rod, measure it with micrometers, then go to McMaster-Carr to find a steel drill blank the same size. You will probably need to cut it to length. Use a Dremel tool with a cut-off disk; it will ruin a hacksaw. Or, if the shaft is super short, use a steel dowel pin.

    -Phil
  • iseries wrote: »
    Well now the problem is that because the aluminum is so soft that there are now scratches on the shaft that have to be removed before I can put it back together. The hole also has scratches in it so the two pieces no longer fit and bind.

    I'm going to need to find something to polish the two surfaces to remove the scratches. I don't want to use sand paper as that seems to be too ruff.

    Mike

    I use Scotch Brite on steel to remove burrs. Don't know if it too abrasive for aluminum.
  • Could you point to the shaft in question? The only shaft I can find is a plastic horizontal shaft for lifting the gripper up and down. If it is the plastic shaft, I would use 600-1000 Wet/Dry sandpaper with water.
  • PublisonPublison Posts: 11,366
    edited 2020-05-09 - 23:02:16
    Looking at the BOM print, you may be calling the round vertical cylinders on the Arms a shaft?

    Pics are worth, well you know. :) Pics might help Ken get a handle on the problem. Tolerances may be too tight.
  • Looking at the picture there are in essence 12 shafts. 6 rods and 6 holes.

    The problem is that the base shaft started to lose particles of aluminum that ground into the shaft of the arm causing the two to bind together to the point where neither one would move anymore. They became inseparable. After some work I managed to get the two apart so I could clean them.

    After cleaning them the two no longer fit. There where small burrs inside the hole.

    I used a metal piece and tried to smooth the burrs out. After working on it I then applied some grease to the shafts and was able to put them back together. They now work perfectly.

    The instruction don't really say to put grease on the shafts but I see now that is really a good idea if you plan to use them for any amount of time.

    Mike

    842 x 682 - 204K
  • GenetixGenetix Posts: 1,471
    edited 2020-05-11 - 04:47:10
    I am guessing the parts are anodized Type II which is not as durable as Type II or "Hard" anodize.

    When I was working with breast biopsy probes, the glue fixtures would slowly accumulate excess super glue which we would scrape off periodically.
    Eventually the bare aluminum started to show especially along the edges of the inner radiuses.
    I pulled off all those parts and had then stripped and Hard anodized but I forgot to specify the color so the black parts can back a dark gray.
  • The parts are not anodized.

    We should be specifying some grease on that shaft component, but we don't introduce the grease until later than it is needed. We will adjust that in the assembly documentation.

    iseries, please send your shipping info kgracey@parallax.com and I'll get some replacement parts off to you.

    I'll be interested in knowing if others have the same problem, in which case we will adjust the assembly, substitute materials, or add anodizing. For certain, we should specify grease earlier in the process.

    Sorry we caused this trouble for you, too.

    Thanks,

    Ken Gracey
  • @Ken Gracey,

    No need for replacement parts. I was just surprised when the two pieces bonded together.

    Just applied a small amount of grease and it worked like new. I have been playing with it on my desk like a fidget spinner for days now and still works like new.

    Mike
  • OK, run this thing "into the ground" for us and I'm still happy to replace it. Make it clap away all night, for weeks if you wish. This kind of testing we haven't actually done but I'd like to learn about the limits.

    We're good on the offer, even if you take me up on it later.

    Ken Gracey
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 22,627
    edited 2020-05-11 - 22:35:41
    Ken,

    This is an easy issue to fix. Either undersize the shafts or oversize the holes they fit in, and provide users with thin-wall PTFE tube that they can cut to length. The interface between the aluminum and PTFE will remain slick without having to apply grease.

    Here's a link to McMaster-Carr's selection of PTFE tube:

    https://www.mcmaster.com/ptfe-tubing/sleeving-4/

    Nylon or Delrin spacers are another option if you can find one with a thin-enough wall. In any event, it's my opinion that aluminum-on-aluminum will continue to be a problem long-term, regardless of whatever lubrication is applied. A clockmaker once told me that the best and least troublesome interfaces between moving parts are those made of dissimilar materials.

    Another option is to replace the machined shafts with steel shoulder screws; but those can get kind of expensive, since you'd need six of them.

    -Phil

  • These are slow moving parts that don't get a lot of wear and tear. The little amount of grease that I used has worked just fine after several days of playing with it. These pieces fit just fine and are not to snug in my option.

    I would also drop the two screws that fit on the Idler gear as it is held in place by the two end screws.

    I guess the real challenge would be to attach a servo to the part and flex it 1000 times which is far more than any use this item will get on any robot installation.

    Mike
  • Mike, seriously, go ahead and hammer away on the Gripper 3.0. You can be our stress tester!

    See how many cycles you can run it through before the grease wears down. Hook it up to a bench power supply and let it grab itself to death. . .

    We could use the data! If you don't, I shall - maybe tomorrow!

    Ken Gracey
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