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Servo weight limit — Parallax Forums

Servo weight limit

AndroidAndroid Posts: 82
edited 2013-03-18 13:16 in General Discussion
Hi guys i want to make a robotic arm out of wood using two servos for 2 degrees of freedom. I would like to know how much weight the servos can hold up horizontally. Anything helps!
-Android

Comments

  • LeonLeon Posts: 7,620
    edited 2013-03-17 10:47
    It depends on the servo.
  • AndroidAndroid Posts: 82
    edited 2013-03-17 10:59
    I am using a continuous rotation servo from parallaxes boe bot.
  • Mike GMike G Posts: 2,702
    edited 2013-03-17 11:06
    Servos have a torque rating usually in oz-in or kg-cm measured at stall. An HS-322 has a torque rating of 51 oz-in at 6V. I think that is similar to the Parallax servo. The HS-322 can produce 51oz-in of twisting force at the drive shaft. A 2 inch level attached to the drive shaft will feel 51oz-in/2in = 25.5 oz of perpendicular force at the end of the lever given a lever with no mass. Of course the torque rating is at stall so you would not operate the servo at max torque.

    I would not recommend a continuous servo for a robotics arm.
  • AndroidAndroid Posts: 82
    edited 2013-03-17 11:09
    Ok thanks for your help. Do you have any ideas on motors that are good for a robotic arm.
  • Mike GMike G Posts: 2,702
    edited 2013-03-17 11:12
    Standard hobby servos, digital servos, or the newer multi-drop serial servos.

    You'll have to look at the servo specs and find a servo that meets your arm requirements. Asking the question Do you have any ideas on motors that are good for a robotic arm. has no context with out specific details.

    What kind of ice cream should I buy?
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-03-17 11:13
    You can calculate this from the torque.

    The Parallax CR servos says it has 38oz*in torque. The amount of weight it can hold horizontally will depend on how long the arm is. A one inch arm can hold 38 ounces. A 12" arm can hold 38/12 or 3.17 oz.

    The 38 oz*in figure assume a 6V supply. The torque will be less at 5V. You should also not drive the servo at its max torque for long so you should make sure you have a good margin of more torque than you need.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-03-17 11:15
    Android wrote: »
    Ok thanks for your help. Do you have any ideas on motors that are good for a robotic arm.

    I have links to some of the servos I like in post #10 of my index (near the top of the post).
  • ZootZoot Posts: 2,227
    edited 2013-03-17 11:42
    Why do you want to use continuous rotation servos for an arm? It seems like it would be better to use regular servos for an arm, so they can be positioned.
  • ercoerco Posts: 20,256
    edited 2013-03-17 14:02
    Mike G wrote: »
    What kind of ice cream should I buy?

    Vanilla, duh!
  • W9GFOW9GFO Posts: 4,010
    edited 2013-03-17 14:40
    You can also pre-load the arm so that it is balanced - that will reduce the load on the servo.
  • AndroidAndroid Posts: 82
    edited 2013-03-18 11:52
    Thank you for all the help. I am not to experienced with this kind of thing so i did not know what kind of motor to use and i happened to have continuous rotation so I just went with that. All of you have been very helpful thank you so much!
    -Android
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-03-18 12:49
    Android wrote: »
    Thank you for all the help. I am not to experienced with this kind of thing so i did not know what kind of motor to use and i happened to have continuous rotation so I just went with that. All of you have been very helpful thank you so much!
    -Android

    IMO, you're better off doing it the wrong way than not trying at all.

    I have lots of failed robots that were successful experiments.

    Here are two of my favorites.

    Hexapod with ridiculously long legs. Destined to fail, but I learned a lot.
    Expanded rims on the Rover 5. I tried the exact opposite of what I would eventually work. It was only because I posted my failure to the forum that I learned the right way to do it. BTW, After erco's help, I can't get a tread to come off even when I try.

    So go ahead and make mistakes. I hope you share the mistakes you make here. I also hope you share your inevitable successes.
  • ercoerco Posts: 20,256
    edited 2013-03-18 13:16
    IMO, servo precision & durability are as important as "max load". If you overload a servo, the gears wear out. Even if they don't break, they will get so sloppy with backlash that the overall arm accuracy & repeatability suffer. I have built three servo based arms and their payload capacity is minimal. I pick lightweight objects to move in order to make the arm last a long time. All my arms are still swinging.

    Rich suggested preloading to "balance" the load, which certainly helps. Counterspringing is usually better than counterweighting if done properly.

    Per Duane, try something and learn. And share your findings here!
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