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RE: [beam] Torquosity

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  • Sallis, Jeremy
    Jean Calculating torque is not simply a matter of multiplying the load by the length of lever arm. Probably the easiest way to look at it is to consider torque
    Message 1 of 3 , May 2, 2001
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      Jean

      Calculating torque is not simply a matter of multiplying the load by the
      length of lever arm. Probably the easiest way to look at it is to consider
      torque as force multiplied by the perpendicular distance between the line of
      the force (force vector) and the pivot. (I hope that was clear)

      If you take your case with a 8oz load and a 5.5" lever on a bot leg, when
      the leg is nearly horizontal and the load is acting vertically through the
      foot, the force vector acting vertically will be 5.5" from the pivot so the
      torque will be 44 oz in. When the leg is vertical the force vector acting
      vertically will pass through the pivot so the perpendicular distance is zero
      and the torque is zero. With the leg at 30 degrees from vertical the force
      vector will pass 2.75" from the pivot (5.5 * sin(30)) and the torque will 22
      oz in. So In most positions a simple lever leg on a servo wont experience
      the maximum torque.

      It is also possible to calculate torque by taking the full length of the
      lever and multiplying it by the component of the force vector perpendicular
      to the lever.

      Jeremy

      > -----Original Message-----
      > From: Jean auBois [mailto:aubois@...]
      > Sent: 02 May 2001 02:16
      > To: beam@yahoogroups.com
      > Subject: [beam] Torquosity
      >
      >
      > Servo motors range in torque from perhaps 10oz/in for the
      > tiny ones to say
      > 44 oz/in. for the sort that Solarbotics sells all the way up
      > to nearly 300
      > oz/in. for the kind that people who are building "mega 1/4 scale" R/C
      > airplanes like.
      >
      > Say, for example, that your robot weighs two pounds and is
      > supported by
      > four legs (something faintly akin to Spyder). Each leg, as a
      > result, is
      > going to have to support something (on average) on the order of
      > half-a-pound, that is, 8 oz. This implies that the longest
      > lever arm you'd
      > be able to use is on the order of 5.5" from the center-screw
      > of the servo
      > (barring some clever mechanical scheme).
      >
      > How, then, do you obtain enough more torque just to move the
      > thing? An
      > obvious answer is that the torque you supply isn't
      > necessarily being used
      > to hold the robot up (you can use a suspension system for
      > that) but somehow
      > something seems to be missing here. Also, with
      > "pseudo-static" balance,
      > three legs can hold the robot (obviously with lever arms
      > shorter than 5")
      > whilst the third is being lifted and moved "forward". And,
      > the servo (or
      > other motor/actuator) that then actually thrusts the robot
      > forward isn't
      > necessarily the ones that are supporting it.
      >
      > Another sort of example from the Lynxmotion site:
      >
      > >Lift weight (arm extended) = approx. 3 oz
      > >Weight (without batteries) = 21.3 oz
      >
      > for their 5-degree-of-motion arm. Put batteries in and it will weigh
      > nearly two pounds and the thing can only lift 3 oz when
      > extended and even
      > THEN it isn't autonomously controlled. I don't know -- that
      > just seems
      > rather pathetic.
      >
      >
      > Just two more days until the LANL BEAM Workshop -- the Lab
      > Bulletin was
      > ominously silent about that, but the Santa Fe Art Instituted isn't:
      >
      http://www.sfai.org/2001program.html#beam &
      http://www.sfai.org/directions.html

      and three days short of the anniversary of the Forestry turkeys setting
      the "controlled burn" that became the Cerro Grande Wildfire,

      JaB


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    • Andrew Berry
      All this talk about torque has got me thinking (uh-oh. . .). Does anyone have concrete torque measurements (precise physical dimensions would also be
      Message 2 of 3 , May 2, 2001
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        All this talk about torque has got me thinking (uh-oh. . .). Does
        anyone have concrete torque measurements (precise physical dimensions
        would also be appreciated) on those mac motors we all know and love?
        I may need to rethink my upcoming project (levers, anyone?). . . .

        Thanks in advance.

        -andrew
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