Technics Mount for RC type Servo Motors

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I have tried a number of methods of mounting Radio Control type Servo Motors to Lego Technics models over the years. Non were overly successful, mostly due to alignment issues. Then in the wee hours this morning, whilst trying to sleep, the answer hits me. Out of bed I jumped (tip-toed), because finally there was a solution, thanks to Lego’s new new Part #64179: Gray Technic, Lift-arm with 5 x 7 Open Centre. 55gram RC Servo Motors fit tightly inside these new frames, accepting the Servo output shaft doesn’t align with the mounting holes in the 5 x 7Liftarm Frame. But this is a relatively easy fix that takes about 10 minutes, plus a day for the glue to reach full strength.


Radio Control type Servo Motor mount to Technics 5×7 Lift-arm Frame

To start the process of mounting the Radio Control Servo into the Technics Frame with correct alignment of it’s drive shaft, you need to get out your Minicraft, Dremel, or Proxxon (Die Grinder) out of it’s box to grind away some of the Technic frame at one end as shown below.



Readying the Technics 5×7 Lift-arm Frame for the RC Servo

Next you need to ready the RC Servo Motor for mounting on the Technics Lift-arm type Frame. You will need to remove 3-4mm from the Servo’s mounting lugs which are furthest from it’s shaft so the legs don’t protrude past the end of the Technics Lift-arm type Frame.You also need to make sure the top surface of the Servo’s mounting lugs on both ends are nice and smooth before mounting onto the Technics Lift-arm type Frame.


The Technics 5×7 Lift-arm type Frame & RC Servo Motor readied for Gluing<

Now you should have both components machined ready to glue together. Double check they fit well together and the RC Servo Motor’s Drive-shaft aligns on the X and Y axis with the Technics Lift-arm type Frame. The best method of gluing these two parts is by using Plumbers P.V.C. Pipe Glue which actually melts the two plastics together so they form a very strong weld. As can been by the blue material on the RC Servo is left over from a failed mounting attempt I made awhile ago. The weld was excellent but the alignment was way-out. I had to grind the old Technics Lift-arm off the Servo as I was unable to remove it any other way without braking the Servo’s mounting lugs.


The finished Technics Mounted RC Servo Motor readied for use

The most difficult part I have found with the whole exercise was finding a suitable way of adapting the Nylon Servo Horns to Technics components like Pulleys, Gears and Beams. Shown above is a large Technic Gear mounted onto a large round Servo Horn by drilling 3/16″ holes in the Horn and using Lego Technic Pins to to join the two parts together. If I’m not attaching to the Servo via a Lego Technics shaft, I used a long screw into the Servos drive-shaft to hold the Gear and horn Together. But as my Servo Drive-shafts are brass, I find that in 90% of applications the screw isn’t necessary.

NXTServoI’ve found that the MG995 Hi-Speed & Torque, Metal Geared RC Servo available for around $10 each to be very suited to this application. These Servos also work well with the Mindsensor 8 Channel Servo Controller for NXT (NXTServo-v2). These servos have very powerful Coreless Motors producing 13kg at 1cm on 4.8v batteries or 15kg on a 6v battery. In fact, there’s enough torque available to brake Lego parts.

MG995 Servo Specifications
  • Weight: 55g
  • Dimension: 40.6 x 19.8 x 37.8 mm
  • Stall torque (4.8V): 13kg/cm
  • Stall torque (6.0V): 15kg/cm
  • Operating speed (4.8V no load) 0.17sec/ 60°
  • Operating speed (6.0V no load) 0.13sec/ 60°
  • Operating voltage: 4.8 – 7.2 Volts
  • Temperature range: 0 to +55°C
  • Dead band width 10us
 

The finished Technics Mounted RC Servo Motor readied for use

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