Tuesday, September 8, 2015

Crescent Rose Fix #1: Motor Replacement

(From the Otakon 2015 Full Day Test Run Results post)
Future possible improvements:
-Change material of the claw
-Replace the 3-wire Vex motor with a 2-wire Vex motor.
- Fix the leaks
-Find a better air source.

 One improvement that Crescent Rose needed was a replacement of the 3-wire Vex motor to the better 2-wire Vex motor. First we have to make sure that the geared 2 wire motor can handle the weight of the new motor.

Vex 2-wire motor specs(from:Vex)


2 Wire Motor 393
  • Motor 0.192 lbs (87.1 grams)

  • 6-32x1/4"
    0.0014 lbs (0.617 grams)

  • 6-32x1/2"
    0.00209 lbs (0.948 grams)

  • Motor
    0.004 lbs (1.814 grams)

  • Motor
    0.002 lbs (0.907 grams)
 Total weight: 91.83 grams


2 Wire Motor 393
  • Free Speed:100 rpm (As Shipped)/160 rpm (High Speed Option)
  • Stall Torque:1.67 N-m (14.76 in-lbs) (As Shipped)/1.04 N-m (9.2 in-lbs) (High Speed Option)
  • Stall Current:4.8A
  • Free Current:0.37A

  • Note: All motor specifications are at 7.2 volts. 

What we are looking for is the Stall Torque and Weight. Stall torque means at what force does the motor fail. In our numbers this means that we can not exceed 1.67N-m. Torque is Force*Distance so we need to calculate force, distance, and then multiply them together to get our final number.

 F=m*a=.09183 kg*9.81=.90085N

With this weight being held out at 16.5"=0.4191 meters


1.67N-m  >  .0378N-m
Rough approximation says this was possible.

And so I went about buying the 2 wire Vex motor to replace the 3-wire Vex motor.

Pictured here is the 2-wire Vex motor to replace the 3-wire.

So the first step is to take the claw apart.

Crescent Rose claw in pieces. When you design for movement, you also design for attachment and reattachment.

The only difficult part about this was undoing a hot glue fix I had to apply during Otakon 2015. I had permanently attached the foam board to the moving piece for strength. What ended up happening was that the foam board piece couldn't handle the load and started wiggling from side to side. Not good! So I decided to back the foam board piece with metal.

The main backing piece is a Vex steel bar1x25 holes attached to another steel bar 1x8 (cut from 1x25)

Next was to attach the 2 wire motor. The way that the 2 wire motors are oriented means I had to attach the motor on the outside. I could have geared the setup in order to more properly hide the motor but that meant more weight on the motor so I didn't bother.

Metal is attached to the claw by two bolts in addition to the green gear.

Now to properly attach everything back together.

Other side of the claw

And back on the main frame it goes.

Future possible improvements:
-Change material of the claw
-Replace the 3-wire Vex motor with a 2-wire Vex motor.
- Fix the leaks
-Find a better air source.

See next update here.

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