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HS-785HB 3.5 Rotation Servo Metal Gear Upgrade

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  • HS-785HB 3.5 Rotation Servo Metal Gear Upgrade


    For our robot this year we needed some servos that could rotate multiple times and gave position feedback. After searching, we discovered the HS-785hb servo. This is a 3.5 rotation sail winch servo with position feedback and 183 oz/in of tourque. After using them for a while though, we found a large issue. The gears are Karbonite and broke often. We very quickly got tired of needing to swap out a new servo almost every time we ran the robot especially since these servos are not cheap.

    We searched for a set of metal gears that we could use to replace the karbonite ones. We were unable to find any specifically made for the HS-785HB, but we did find some for the HS-755MG which is a similar quarter scale servo. They are $19.95 from servocity.com.

    With a little modification these can be made to work with the HS-785HB servo.


    Tools needed:
    • Hammer
    • Punch and/or nail
    • Dremel or something for cutting the old gears
    • Small phillips screwdriver
    • Sander (dremel sanding drum works well)



    Materials needed:


    Instructions:
    There is a link to an image that goes with each step. There is also a gallery with step by step instructions here. The modifications that are done mostly deal with the last stage gear from the new set. Since it is only for a 180 degree servo instead of a multirotational one there is a pin in the final gear that acts as a stopper. Also, it doesn’t have the part that drives the potentiometer to give position feedback. This piece needs to be removed from the old gear and attached to the new one.

    1. Remove the 4 screws on the back of the servo.
      - image -
    2. Pull the front and back covers off of the servo.
      - image -
    3. Remove the old gears. The first stage gear can stay since it is the same as the one included in the new metal set.
      - image -
    4. Remove the stopper pin from the final stage gear of the new gear set. This can be done with a punch and/or a nail.
      - image -
    5. Cut away the potentiometer drive from the old final stage gear. On the gear pictured, the output shaft that sticks out of the servo had snapped off. If yours is still there, cut it off slightly above the top of the gear.
      - image -
    6. Sand down the potentiometer drive piece so that it fits snugly inside the hole on the new gear.
      - image -
    7. Insert the final stage gear into its place. When you do this, it helps if you tilt the gear so that the potentiometer drive piece that was cut doesn’t hit the potentiometer gears inside the hole. Turn the gear. Make sure you can feel the drive piece meshing with the potentiometer gears inside the servo.
      - image -
    8. Remove the gear and Superglue the potentiometer drive piece into place. As pictured, I inserted the bearing before I did this. The bearing is a pretty snug fit and it is easier to insert before the potentiometer drive piece is in the way. If you do it this way be very careful not to get superglue on or in it. Center the potentiometer drive piece as well as you can and let the glue dry.
      - image -
    9. The output shaft of the final gear seems to be too large for the bearing. Bearings are supposed to be tight, but it seemed to be a little much and I didn’t want to break anything. I made the shaft a little smaller by putting the gear into the chuck of a drill and spinning it against a file. It doesn’t need much. Just enough so that it doesn’t take such an elephantine effort to push it on.
      - image -
      - image -
    10. Assemble the gears. Insert the final stage gear at an angle as before. Apply the lubricant to the gears and attach the back and front plates the same way they were taken apart.
      - image -




    That’s It! We have done this with four of these servos and haven’t had any issues with them yet. They work great now, and we are no longer in dread of putting too much force on them.



    -DakotaSamuel

  • #2
    I'd suggest you ask in the forums, but I don't see how you can claim this is an unmodified servo and legal at that point.

    Comment


    • #3
      Originally posted by korimako View Post
      I'd suggest you ask in the forums, but I don't see how you can claim this is an unmodified servo and legal at that point.
      http://ftcforum.usfirst.org/showthre...ll=1#post20749

      Comment


      • #4
        Admittedly that post seems to reference a servo with some kind of external gearbox. I agree that opening the servo and changing gears is probably not legal.

        Comment


        • #5
          Changing gears is specifically allowed in Game Manual Part I. However, it is assuming you are using direct replacement gears. Step 9 listed above may be an issue. Like above, I would definitely ask this question in the GDC forums.

          Comment


          • #6
            Originally posted by FTC8881 View Post
            Changing gears is specifically allowed in Game Manual Part I. However, it is assuming you are using direct replacement gears. Step 9 listed above may be an issue. Like above, I would definitely ask this question in the GDC forums.
            RE02
            "Approved electrical and electronic devices may be modified to make them more usable; they may not be modified internally or in any way that affects their safety."


            "changing gearboxes is allowed" is what is written as an allowable example. This I imagine is meant to refer to AndyMark, Tetrix motors. I've never heard of anyone refering to this part of a servo as a gearbox. I also think it has been clear that the intent is not to allow teams to change the performance characteristics of motors/servos. Language of previous seasons has been much more explicit here. For this to be legal you would have to consider the gears an external part of the servo, and be convinced that it is not a safety issue now that the servo may draw more current than the original design did.

            Better to ask for clarification, rather than risk a disqualification after competing with an illegal servo/motor; that has happened.

            Comment


            • #7
              From the "Robot Inspection and Build Rules - Mechanical - Answer Thread" post #94:

              Q: In past years, the rules prohibited modification of the motor or servo internal gears. However, rule RE02 seems to allow replacement of the of the gearbox with a gear structure different from the original ratios or using metal servo gears in place of the plastic gears. The example does not specifically state that the replacement gearboxes be identical to those being replaced. Are these changes allowed this year?

              <RE02> Approved electrical and electronic devices may be modified to make them more usable; they may not
              be modified internally or in any way that affects their safety.

              Examples of modifications that are allowed:
              Shortening or extending wires
              Replacing or adding connectors on wires
              Shortening motor shafts
              Replacing gearboxes

              A: Yes, because the gearboxes can be separated from the motors the rules have been expanded to allow teams to modify the gearbox.[/QUOTE]


              Seems pretty clear to me, but I may be biased.


              -DakotaSamuel

              Comment


              • #8
                We won't know if you don't ask. It's a big risk to compete with illegal parts if any one calls you on it.

                Comment


                • #9
                  I posted the question on the game rules forum.
                  http://ftcforum.usfirst.org/showthre...ll=1#post27261

                  Their answer:
                  "Yes, this is allowed."


                  -DakotaSamuel

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