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  • Servo City Linear actuator help

    Hi, my team has pre ordered 2 Linear actuator kits from servo city for R&D purposes and also for competition robot use. Servo city recommends the use of the goBILDA 5202 series Yellow Jacket 1150 rpm motor, this motor would allow us to lift 50 lbs in 6 seconds. I would like to know if I my team can use one of the motors that we have access to and still get the same performance, the motors that we have access to are Neverest 20, 40, 60 and Neverest orbital 20 as well as a Tetrix Tourquenado. I would also like to learn how to compare motor specs as well, so don't just give me the fish, teach me how to fish as well, thanks.

  • #2
    You can find the specs of various Neverest motors at the Andymark's website. The 8 mm lead screw provided in the kit, makes the mating nut travel a distance of 8mm in one turn. The suggested motor run at the 1150 RPM which translates to about 19 rotations per second will make the mating nut travel 19*8mm = 152mm = 6 inches in one second. The Andymark orbital 20 has 300 RPM which is 5 rotations per second and will make the lead screw move 5*8 mm = 40 mm = 1.5 inches per second. A six inch travel will be accomplished in 4 secs vs 1 sec with goBILDA motor. Hope this helps. Cheers !

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    • #3
      The following from Rev has some good information on evaluating motors:

      http://www.revrobotics.com/content/docs/Motor-Guide.pdf

      The following from Wikipedia has information on this type of mechanism:

      https://en.wikipedia.org/wiki/Screw_(simple_machine)
      (in particular, see the section on actual mechanical advantage and efficiency)

      For the suggested motor, 1150 rpm is the maximum rpm with no load. When you add a load (including torque from the weight of the robot plus friction), the rpm will decrease, ultimately going to zero when the load reaches the stall torque of 109 oz-inches. Even if friction is zero (a bad assumption), a 30 pound robot will result in torque of about 24 oz-inches. The rpm would then be about 897.

      In practice, the performance will be highly dependent on friction. With non-zero friction (i.e. efficiency of less than 1), the torque required by the load would be determined by (see Wikipedia article):

      torque (oz-inches) = (weight (oz) * lead (in))/(2 * pi * efficiency)

      Examples:

      For robot weight of 30 lb (480 oz), lead of 8mm (0.315 inches), and efficiency of 0.2 (which, according to the article is not a bad guess), torque is 120 oz-inches. This is above the stall torque of the suggested motor, so the system would be incapable of lifting the robot.

      If this mechanism had an efficiency of 0.33, the resulting torque would be 72 oz-inches, resulting in rpm of about 390. The system could then lift the robot 6 inches in 3 seconds.

      If anyone has tried this specific system (ServoCity linear actuator kit with goBilda 5202 motor), it would be interesting to hear how it worked under a realistic load.

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      • #4
        Just FYI - The servocity linear actuator is not legal - https://ftcforum.usfirst.org/forum/t...-rules-answers .

        Not being mechanically inclined our team is still very confused. We have two questions :-
        1) The lead screw by itself has been called legal in the Game Manual 1. How is the lead screw supposed to be driven if adding a motor creates two degrees of freedom ?
        2) Servocity is now using a language stating that "that this kit is not legal for the 2018/2019 season. All of the components in this kit however, when purchased individually, are completely legal. " What is it supposed to mean ? If you buy all the components individually is the assembly still legal or not ? If its not legal then how is any lead screw based mechanism legal ?

        Any insights or clarification is welcome. Meanwhile we will start exploring non lead screw based lift mechanisms.

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        • #5
          Originally posted by voltemort View Post
          ...

          2) Servocity is now using a language stating that "that this kit is not legal for the 2018/2019 season. All of the components in this kit however, when purchased individually, are completely legal. " What is it supposed to mean ?
          ...
          It means if you buy all the parts in the ServoCity Linear Actuator Kit separately and follow the posted assembly instructions to assemble the actuator, the actuator will be legal. Just don't buy the parts all at once as a kit. Also, make sure you use a legal motor. Although the actuator will be legal, you may not do as well in the judging competition if you assemble key components of your bot from a kit.

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          • #6
            This is truly confusing. One of the rulings cites <RM02>:
            Purchased mechanism kits (for example, grippers) that violate the single degree of freedom rule, either assembled or requiring assembly, are not allowed

            How does it matter whether you purchase one line item or you expand the bill of materials and buy 10 line items? They are the same components. And where is the line? Are tilerunners illegal based on this definition? Does the vendor matter?
            I agree that judges will generally prefer a home grown design - but even then, many designs converge.

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            • #7
              The TileRunner, if bought as a kit, is legal, but the ServoCity linear actuator, if bought as a kit, is not legal. So yes, confusing and contradictory. Simple fix is to make all kits legal provided each part in the kit is legal. Teams that don't rely as much on kits will do better in the judging.
              Last edited by Alec; 10-04-2018, 11:46 PM.

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