how to calculate motor to move an object? - Page 1
Forum section: electronics / beginners

author: diegoterc3
author_rank: Regular Contributor
timestamp: May 15, 2014, 03:49:29 pm
content: Hello I'm trying to build a platform which will have wheels so I can put some weight on it and move it arround different surfaces, for instance: carpet, wood, paviment, and others.The max weight I'm trying to move is about 70Kg. So bassically I need a way to calculate de motor that will move this mass. For calculations I'm using static friction coefficient = 1 from various tables I´ve researched. So bassically I'll have to beat 700N (with g=10m/s2). From there I´m lost. I´m guessing I´ll have to transform that Force into Energy and then get the torque?Thank you in advance for your answers

author: madshaman
author_rank: Frequent Contributor
timestamp: May 15, 2014, 04:04:08 pm
content: http://en.m.wikipedia.org/wiki/Torque

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 15, 2014, 04:32:50 pm
content: You can probably skip the wikcalculus if you reaize you are mostly there with your 700N. Just convert that into the needed N*m based on your drive pully radius and its RPM. Realize that you will lose typically 10% efficiency in each torqe conversion such as a gear ratio. It should get you close enough for final testing.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 15, 2014, 04:37:14 pm
content: Huh? The formula requested is at the top of the page including a cute animation and only involves a division...

author: IanB
author_rank: Super Contributor
timestamp: May 15, 2014, 04:42:03 pm
content: Quote from: madshaman on May 15, 2014, 04:37:14 pmIt looks to me like the equations at the top of the page involve a vector cross product...Also, for the OP, remember that if the friction force is 700 N and the motor applies a motive force of 700 N, then the cart will not move. The two forces will cancel out and you will have zero resultant force. You need extra force to make things move, and it will help to have a good extra margin. If in doubt, use a bigger motor (or lower gearing).

author: madshaman
author_rank: Frequent Contributor
timestamp: May 15, 2014, 04:43:23 pm
content: Yeah, it does, but it can rearranged, in one's head..

author: diegoterc3
author_rank: Regular Contributor
timestamp: May 15, 2014, 05:11:24 pm
content: Ok. I think I´ve got it from the Wiki and the explanation.https://drive.google.com/file/d/0BwBiiNcJ8DICQS13TmFKcFVNaEU/edit?usp=sharingThe radius for each wheel is 4cm which will result in 7Nm of torque for each wheel.Please advice if my calculations are correct.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 15, 2014, 06:52:25 pm
content: Quote from: diegoterc3 on May 15, 2014, 05:11:24 pm(700N x 0.04m)/4 = ?[edit: 7000 -> 700]

author: madshaman
author_rank: Frequent Contributor
timestamp: May 15, 2014, 06:56:13 pm
content: P.S. The way you're designing, the motor choice is constrained, you could also start with the motor you want to use and calculate the wheel size (or gear box etc.) from it's torque at optimal efficiency.

author: diegoterc3
author_rank: Regular Contributor
timestamp: May 15, 2014, 07:31:40 pm
content: Those 700N are distributed into 4 wheels which result in about 175N for each wheel.Also de static friction coefficient is 1 so static friction force is 175x1=175N.Then with the wikipedia Torque=forcexradius = 175Nx0.04m=7Nm for each motor.Just wanted to check if this is ok.P.S. I wont use a gearbox and the wheel size can only be 4cm because of the structure saddly.

author: madshaman
author_rank: Frequent Contributor
timestamp: May 15, 2014, 07:36:41 pm
content: Quote from: diegoterc3 on May 15, 2014, 07:31:40 pmYou calculation is correct (for most formulae involving SI units the units imply the formula itself)Luckily for you, you can get as much torque as you want out of an electric motor if you have sufficient cooling (I'm partly joking, but it's true).

author: Kohanbash
author_rank: Regular Contributor
timestamp: May 16, 2014, 12:05:08 am
content: HiIf you google for finding robot motor torque you should be able to find more information on this.Here are two possible links:http://robotsforroboticists.com/motor-selection/http://www.societyofrobots.com/mechanics_dynamics.shtml