I need help to figure out the torque needed by a motor to lift an arm with fixed weight at the end like in the picture....

Where the green circle is the fixed motor and the arm is welded to the motor shaft and the movement is only 90 degrees goes from vertical to horizontal.

The arm length is 55cm, arm weight is 4kg and the fixed attached weight is 13 kg

I wanna know what is the torque (n-m) required to rotate the arm from a vertical to horizontal position.

enter image description here

  • $\begingroup$ First of all the torque is depending on the gearbox you will decide to use and from the motion law you will implement. Without these two aspects I can't help you. I can for sure help you but at least i need the motion profile that you need. Then i can help you with the design of the system. $\endgroup$
    – it8
    Nov 8, 2017 at 12:40
  • $\begingroup$ hello and thanks for your reply i'm planing to use a worm geared DC motor (12V) there are several types of motors with different rpm and diferent torques i don't want high rpm as my whole motion is only a quarter of round so i guess a 1 rpm motor would be fine and i will use relays or sensors to stop the motor when the arm reaches location so i was asking a question to know how much torque do i need in order to buy a motor .... knowing that these types of motors are available from 10kg-cm to 70kg-cm to 5n of torque so i have no idea which motor will do the job .... $\endgroup$
    – Tak
    Nov 8, 2017 at 18:28
  • $\begingroup$ the motor is some type like this : amazon.com/BEMONOC-Electric-DC-Geared-Torque/dp/B014GLUORC/… $\endgroup$
    – Tak
    Nov 8, 2017 at 18:43

1 Answer 1


As a quick approach you can calculate the torque needed to hold the arm + weight in a horizontal position $$ M=M_{weight}+M_{arm} $$ $$ M=m_{weight} \cdot g \cdot l+\frac{1}{2} \cdot m_{arm} \cdot g \cdot l $$

Your torque (after the gear) must be higher than that to be able to rotate in that position.

$$ M=13\ kg \cdot 9.81\ m/s^2 \cdot 0.55\ m+0.5 \cdot 4\ kg \cdot 9.81\ m/s^2 \cdot 0.55\ m $$

  • $\begingroup$ thanks for your reply , i'm unfamiliar with these stuff i dunno what cap m and small m means ...can you do the calculation for me please :weight of the arm is 4kg ....length of the arm is 55 cm...load at the end of the arm is 13kg ...thanks for your effort $\endgroup$
    – Tak
    Nov 9, 2017 at 15:31
  • $\begingroup$ 'M' is moment. 'm' is meter. 'g' is acceleration due to gravity. It's time for you to do some research and do the maths yourself. Add your calculations into your question (to show your work) and someone will check them for you. Note about units: Your original question mentions 'n-m' which reads as nanometers. You mean 'Nm', newton meters. $\endgroup$
    – Transistor
    Nov 10, 2017 at 12:34

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