Lifting 40kg with small Dc motors - Page 1
Forum section: electronics / projects

author: tlhsglm
author_rank: Contributor
timestamp: December 21, 2019, 03:28:56 pm
content: Hi fellows,For my project i have to lift 3 weights like on the image.The movement's range is about 4-5 cm.I dont want motors back drive when power is off, so i will choose warm geared motors.The calculations is P=F.v , velocity that i want is 0.01meters/seconds. so the power is equals P= 392.4 x 0.01 = 3.92 watts, so i have to buy bigger than 4 watt motor. The motors on the internet sites gives operation voltage, free spinning current and stall current.Is it safe to make motor's power calculation depending on stall current and op. voltage?And do you think that little motor can do the job;Op voltage: 12VVelocity: 100 rpmFree spinning current: 80mAwStall current 1,4AShaft: Vida delikli 6mm D ŞaftAll length: 77mmThe Project;

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 21, 2019, 03:33:18 pm
content: Micah Elizabeth Scott has used similar motors for her project and doesn't recommend them because they break a lot.

author: tlhsglm
author_rank: Contributor
timestamp: December 21, 2019, 03:44:41 pm
content: Thank you for the answer.Are they breaking or not working after been a while?I searched the name but there is nothing in the results about electronics.What do you recommend?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 21, 2019, 04:37:43 pm
content: Motor power rating is electrical input power, not mechanical output power. Efficiency of cheap, poor DC motors can be as low as something around 50%. Efficiency of the cheap gearbox is also likely low, especially for large reduction ratios (using multiple stages of gears). Worm gears aren't very efficient, there's a lot of friction. The combined efficiency of motor+gear is likely around 20-30%.Stall current is defined by the total DC resistance of the motor windings and brushes, and is of little use here. When operated near the stall current rating, the efficiency is likely very poor; you can think about it as the motor iron saturating and being unable to produce more torque, so larger part of that current is converted to heat.Nominal current is something you need to know, this is the on-load current the motor is supposed to work against for the given duty cycle. Then you could apply P = U*I, but this is still electrical power, and you need to estimate the efficiency to know the mechanical output power.So I suggest you take a look at motors rated for at least 10W.This being said, if your duty cycles are short, most motors can produce 2-3 times the nominal torque for a short time (tens of seconds) without overheating.The best way to know is to buy a motor and try it.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 21, 2019, 05:05:00 pm
content: Quote from: tlhsglm on December 21, 2019, 03:44:41 pmThey're designed to be cheap with plastic gears that just don't last.I would suggest modifying a cheap cordless drill - they're pretty robust.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 21, 2019, 07:33:31 pm
content: what you need is the torque figure, and then we can work out the gear reduction from there, rpm (and the reduction caused by gearing) also need to be taken into consideration. whatever the value is, i dont think that small body can withstand 40kg of weight directly at any point. you'll need gear reduction (with support rig bolted to the rigid base to take the 40kg compressive force through secondary/output/last gear shaft) so tangential force on the primary gear (connected directly to the motor worm gear shaft) is down to practical value, maybe 1-5kg. and yes, as others mentioned, you cant take 100% efficiency from input to output, maybe 50% is reasonable for such small (cheap) power 10W is about right, more is better. ymmv.Quote from: NiHaoMike on December 21, 2019, 05:05:00 pmtalking about china cheap plastic gear... i saw plastic gear tooth broken into pieces in nissan car. i have china foot massager's gear broken like that just few minutes on first use. its been like 10 years ago and i still keep the bits in hope i can replicate in metal, i still dont have the technology to do that.. my last attempt using fiber glass material hybrid with some metal on gear tooth, also damaged on first use, there's so much torque/force in there that i think requires carbide type gear... so thats the boring story...