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  • Robotic arm revolute joint design

    Discussion in 'The main mechanical design forum' started by murrdpirate, Apr 4, 2012.

    1. murrdpirate

      murrdpirate Member

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      I'm trying to make an elbow joint for a robotic arm that is actuated with a stepper motor and two gears. Here's a quick sketch of what I'm trying to do: http://imgur.com/mBlg7

      The two bottom plates will be attached to the 'bicep' and the two upper plates will be attached to the 'forearm.' The forearm pivots about the elbow shaft when the stepper motor rotates.

      In order to work, the elbow shaft has to be fixed to the upper two plates but free to rotate within the bottom plates. Allowing it to rotate within the bottom plates is easy enough...could probably just provide large enough holes in the plates and let them act as bushings. What I can't figure out is how to fix the shaft to the upper plates. Anyone have any ideas? Or maybe a better way to go about this entirely?
       
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    3. rahuldhinakaran

      rahuldhinakaran New Member

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      There are a few unknowns here. I dont know what loads would be acting on the arm or the space between the gear and the "inner" AL plate. Did you try using a small flange? The shaft can go through the flange and "inner" AL plates and onto the external AL plates. The flange can be fastened to the shaft using a couple of bolts (i cant specify sizes as you've not mentioned shaft dia). The flange has holes in PCD which can be used to fasten it to the AL plate(again cant specify sizes as you've not mentioned dia,etc) This way when the shaft rotates it automatically pulls the "inner" AL plates along with it.
       
    4. Dana

      Dana Well-Known Member

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      There are dozens of ways to do it, depending on loads, accuracy, etc., required and your budget. The most obvious is a flange as described above, keyed or otherwise locked to the shaft and bolted or welded to the arm. Or you could attach the gear directly to the arm and have no continuous shaft.

      Not what you're asking, but unless the loads and forearm mass is very low, you will probably need more than a single stage gearing to adequately control it.
       
    5. dreamrobotx

      dreamrobotx New Member

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      It may be easier to leave the shaft loose, with bearings in each plate (use can use flanged bearings on the ends to keep the shaft from coming loose or other ways) and then just connect the gear to the plates, and the plates to each other somehow, further away from the shaft. It would probably be easier than trying to secure it at the shaft. Do you have room for something like that?
       
    6. murrdpirate

      murrdpirate Member

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      Well, I'm still in the very early stages of the design, but I think I can expect a load of around 1,000 oz-in (and will need two stage gearing) . I've been looking at metal gears with hubs (pin and clamp types)...any idea of the slipping potential with these things? Do I need to look at welding the gears to the shaft or something?

      I found some shaft collars with tapped holes that could be used to fix the shaft to the plates: http://us.misumi-ec.com/us/ItemDetail/10300019270.html But again, I'm not sure how much potential there is for slipping with these.
       
    7. amrshoof

      amrshoof New Member

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      I personally think that a slotted shaft should work just fine, skip the shaft collars, you might have to design the shaft a bit more to accommodate locking nuts but you'll skip calculating the maximum torque for the collar and such, you'll have to be do some calculations to make sure the materials you're using can take the loads though (shear stress maybe, remember also that failures due to torque are shear failures), there could be some jerky action but that depends a lot on the manufacturing precision.
       
    8. murrdpirate

      murrdpirate Member

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      Slotted as in a 'keyed' shaft? I was actually just looking at those. I think you're completely right - that really looks like the way to go. The set screw stuff might work decently, especially if I flattened the shaft where the screw rests, but keying looks pretty much worry free.
       
    9. Dana

      Dana Well-Known Member

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      Set screws work OK on shafts with low torque, low inertia, relatively high speed, and no or infrequent torque reversals. In an application like yours, I would not recommend it. A keyed shaft would probably be OK, splines better, but if you can attach the gear directly to the arm (so the shaft carries no torque at all) that would be best. Note that a keyed shaft still requires a set screw to preload the key and take the play out of the system.

      You might also look at tapered bushings; they come in various types and sizes and also make for a play-free connection. A Trantorque bushing might be worth a look.
       
    10. murrdpirate

      murrdpirate Member

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      Well, I'll probably need at least two stage gearing (need about 100:1 ratio), so I'll need at least one other shaft. But I suppose I can at least fix the last gear to the arm.

      Anyone have any recommendations of where to buy gears and shafts that are keyed, splined, tapered, etc? I see keyed gears at SDP-SI, but they're pretty expensive and don't have quite many with fewer than 24 teeth.
       
    11. Dana

      Dana Well-Known Member

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      If you need 100:1 gearing, you're better off buying a gearhead that mounts directly to the motor than trying to design and build your own gearing. But if you think SDP stuff is expensive, you're not going to like what a gearhead costs. But look at Alpha or Apex Dynamics; they have nice little gearheads with a small flange output that you could bolt an arm to directly so you might not need any other bearings or support.

      What is the application here? What does the robot arm have to do, and what kind of precision do you require? You mentioned 1000 oz-in, that's 62.5 lb-in, which is above the range of most the kind of little stuff SDP Sells. A bit of your background would help to, to better know what kind of advice to offer.
       

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