# How to calculate servo strength && my micro arm

Do what I say, not what I do...

I have told a lot of folks that it is very hard/impossible to build an arm using micro servos. As the arm gets longer and the weight of each servo along the arm is added, the base servos can't keep up. Well, I don't believe myself. Now that I have discovered the wonder of Plastruct, and the lightness thereof, I am going to make a stab at this one.

First off, I wanted to do a little math on the actual strength of a micro servo and thus, how long each segment of the arm can be. I took a little video and whadda ya know, I think I may have explained servo strength along the way.

Watch the video for more info --The servo used is the "standard blue" servo. HXT-900 to be exact and rated at 1.6 Kg/cm --or-- 1600g/cm --In the video, I rounded down to 1.5 or 1500g because A) it is easier math and B) I am running at 5v not 6v.

Enjoy.

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Just wondering.....

The servo rating ( Kg/cm ) is not discussing weight, it's discussing force.

Your initial maths hold is the weight Centre of Mass is acting directly through the middle of the arm. However, yours is hanging down below the arm. In that case, the force the servo is delivering is the moment of the arm + weight -> F + md.

Lets say your weights are 3cm long - difficult to tell from the video - the CoM can be considered to be about 1.5cm below the arm. So you have a force of 0.15Kg/cm at 10 cm. The servo looks like it's managing it well enough, so your easily getting a torque of 1.5kg/cm at the servo.

Looks like the servo is under rated - coping like a trooper.

Thanks. I am glad to see some actual testing. I have four of those 9g servos, (amongst others)