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Looking for the Smallest physical size Servo

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  • #16
    Originally posted by Subculture View Post
    I said that jack screws are less efficient, not more. If you use a normal v-thread, like all commercial piston tanks and most diy units, they're only about 10% efficient, meaning you lose 90% of your power in friction. Conversely rack and pinion are over 90% efficient, meaning you can use a much smaller motor for equivalent force, but the mechanism can easily overhaul.

    For a mechanism to be self locking it has to be under 50% efficient. All this information is readily available online- school level physics.
    To build on Andy's observation about losses in the jack-screw: The ideal thread cut is called a 'Acme' thread. You'll want that if you're to have any chance of minimizing the load torque to a level a reasonably sized modified servo can handle.

    "... well, that takes care of Jorgenson's theory!"


    • #17
      Jacking gear as applied to model display:

      "... well, that takes care of Jorgenson's theory!"


      • #18
        Acme threads are more efficient than vee threads for sure. Square threads are even more efficient- acme threads are similar but have a lightly trapezoid shape which makes friction a little higher, but they have certain advantages for instance easier to make and allowing easy radial adjustment to minimise backlash/wear in a lead screw.

        The snag with both types is that they're very difficult to find in small diameters, and much more expensive hence why they''re less commonly used.

        Servos can easily shift vee threads, but the actuation will be slow unless a stage or two of the gear train is removed.

        To put some meat on the bones of this- a Towerpro MG996R is a cheap readily available servo, with a full metal gear train and digital amplifier. You can pick them up for a couple of quid on ebay. They're torquey blighters with about ( torque running at about 66RPM. Now if using a rack and pinion system, and a 1cm diameter cog on the servo, you can move a 5cm (2") diameter piston against about 0.8 bar (10psi) of pressure at around 1.4 inches per second- fast and efficient.

        For a vee thread we would need five times the power to move at the same speed, or we need to move five times slower. In reality the inefficiency of threaded actuators are less of an issue than the theory suggests, owing to the simple fact that most model submariners rarely delve more than a few feet below the surface, so the load on the piston never becomes huge enough for friction to seriously ratchet up.

        With tiny stuff the main hurdle these days will be the builders skill level, very little is off limits these days.
        Last edited by Subculture; 08-29-2019, 03:21 AM.
        DIVE IN! Go on, go on, go on, go on, GO ON!


        • #19
          A source of acme thread rod might be as close as your Lazy Boy recliner. The lift motor is smooth and quiet.
          The wife would be mad if I experiment with her new chair. Does sixty dollars seem high for a acme thread?
          The thread is in the aluminum housing and the white collar rides back and forth on it.
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          Click image for larger version

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          Attached Files
          Last edited by Scott T; 08-28-2019, 12:42 PM.


          • #20
            Scott, contact Boston Gear. and Stock Drive.

            "... well, that takes care of Jorgenson's theory!"


            • #21
              8mm is quite a large thread, and enormous for mini/micro subs. They give around a 10% advantage in efficiency, so better than a vee-thread but not sure I'd worry too much about it. The ultimate thread is a ball screw which are around 90% efficient or thereabouts, but like a rack and pinion, they're not self locking and are even more expensive and difficult to source in small diameters.

              Worth delving into Norbert Bruggens tome for more info on all this, as he devotes a whole chapter to piston tank specs, and presents some basic formulae allowing the modellers to calculate motor wattage and gear train requirements for driving pistons of varied diameter and rates
              DIVE IN! Go on, go on, go on, go on, GO ON!


              • #22
                I will need to break out that book and re-read that chapter!
                If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.


                • #23
                  Or just invest in one of these:


                  • #24
                    Nice servos/nice video-would be nicer if somewhere in the video they'd show the units operate ..why bother with a video-nothing moved!


                    • #25
                      Looking at the datasheets for those linear servos, they look to me like they will overhaul, so possibly based on rack and pinion drive or a high lead screw, although couldn't find information to that extent on their site. So perhaps less well suited to piston tank actuation.
                      DIVE IN! Go on, go on, go on, go on, GO ON!