How to Calculate weight motor can handle

Forum 2005-2010 (read only) General Frequently-Asked Questions
#1

Hi, I'm thinking of starting a new project and am new to using motors. The basic idea is create a sort of robotic ottoman you could sit on. Picture a large rectangle with 4 wheels.Not sure what my wheel size is yet or if I will have 4 or 2 motors. My question is , how do you calculate how much weight a motor can move? I'm looking to move human weight , I guess roughly 200 lbs. If we look at this motor from Robot Shop , what do I need to know to calclate the weight it can move? Also, if I use 2 motors can I double how much weight it can handle? http://www.robotshop.com/PDF/hs645mg.pdf

Speed (sec/60o): 0.20 Test voltage - 4.8v ? Torque (Kg-cm/Oz-in): 9.6/133 Stall Torque - 7.7kg.cm(106.93oz.in) operating Speed 0.24sec/60 AT NO LOAD ? Size (mm): 41 x 20 x 38 ? Weight (g/oz): 55.2 /1.94 ? Metal gears (Alumite) ? Dual ball-bearings If anyone can point me in the right direction it would be much appreciated.

#2

In the example you have quoted the output torque of the motor defines the amount of “pull” the motor can exert on a radius arm. The stall torque (the motor’s absolute limit) is stated as 7.7kgcm with a 4.8volt supply. In other words it can lift or pull a force of 7.7kg on an arm of 1cm length, or say 1kg on an arm of 7.7cm length.

From my memory of mechanics, motor power is defined as the product of force and distance per unit of time. For example a 1hp is defined as 33,000ftlbs/min ie a 1hp motor can lift a 33,000lb weight 1 foot in 1 minute. A 200lb body will be lifted 165feet in 1 minute or 16.5 feet in 6 seconds (1/10 minute)

The above is irrelevant to moving a body across a flat surface since, once the body is moving the only effort required is that necessary to overcome losses (friction, surface roughness, etc). One of Mr Newton’s laws states that any body in motion will remain so unless acted upon by other forces. Motor size (power) will determine how fast your body accelerates.

Wheelchair motors, of which two are normally used are usually rated at around 1/4 hp each. The go-fast pavement junkie mobility scooters might well use larger motors since they are capable of around 14mph and have to carry some grossly over-weight people.

I’d therefore suggest that for your application you should be looking at a motor of at least 1/2hp or 400watts. Two motors of the same rating will do twice the work of one, so two smaller motors could be used and may well be easier to build around. They also give the advantage of steering ability since running them at a slightly different speed will make the “trolley” turn.

jack

#3

The only possible answer is that it depends.

The Seattle Space needle has a revolving restaurant that weighs 125 tons. The whole thing is rotated by a 1.5 horsepower motor.

#4

I suspect that an motor(s) you first calculate and select will either be too large or too small. ;)

Lefty

#5

If you go with the "wheelchair" or "mobility chair" motor route (and as already suggested, for this application, I would) - you're going to find a couple of things:

1) The motors are expensive 2) The motors are expensive to interface with

First off is finding the motors. Your best bet is Ebay or Craigslist. On Ebay, for the motors only, look up keywords like "mobility", "chair", "wheelchair", "motor(s)", "scooter" and of course combinations thereof. Another good word to use is "transaxle", but be careful that you are getting a dual-motor differential drive transaxle that has two motors in it, and not a single motor transaxle with a differential (the first is used for chairs like the Hoveround, the second for chairs/scooters like the 3-wheel Rascal). These things weigh a lot, around 30-40 pounds for the motors and gearboxes, more with wheels; if you can get the motor controller with the motors, and it is in good shape, then get it.

On Craigslist, you are likely to only be able to find the complete chair; generally, the way it goes is that people get a chair thru Medicare, then pass away, and their family has to donate it or sell it - sometimes you can find them cheap, sometimes not, but generally they are waaay cheaper used than if you bought "retail" from a medical supply store (without Medicare). You can also find complete chairs on Ebay, too. I have also seen complete chairs at Goodwill (rare, though).

As noted, if you can get the motor controller (and it is in working shape), you want to get it. Otherwise, you are going to have to shop around for a suitably sized controller for your motors (after figuring out what the motor specs are - which may involve some phone calls, either to the manufacturer of the motor/transaxle, chair, or a repair shop who may know). Controllers are not cheap - but if you have to buy one, look into controllers for "battle robotics" platforms. There are also custom chair controllers out there designed for the mobility chair "modding" community (yes, such a group of people exist).

Finally - an option to consider (dubious, though, I'll admit) is to get the gearboxes, wheels (or at least the hubs), and motors from a dual-gearboxed 12 volt PowerWheels ride-on toy. Sometimes you can find these cheap on Craigslist, Goodwill, yard sales - sometimes people are looking just to throw them away. If you can rebuild the drive system for your custom use, they'll easily haul 200 lbs (though the motors are noisy like you wouldn't believe - unlike powerchair motors, which are designed for smooth and quiet operation).

:)

#6

I agree kids electric riding toys might be the easiest to start with. Generally built to withstand some abuse. Most of the larger ones are 12v powered.

#7

I agree kids electric riding toys might be the easiest to start with. Generally built to withstand some abuse. Most of the larger ones are 12v powered.

I think it is worth trying, if you can find such a toy cheap enough (or free). I have a PowerWheels H2 for my robot project that has no problem carrying my fat butt (240 lbs) on level concrete, but I don't know how well it would work on say, carpet.

Also, it might take some work modding the drive system to get it to fit the requirements of the project, while maintaining the structural stability. You might try removing the plastic wheels used by the toy, and attaching smaller rubber-tired wheels (like for lawnmowers or dollies); note that this will require some custom work to match the wheels up with the output hubs on the gearboxes (PowerWheels gearboxes are weird in this regard).

If you plan to pursue using a toy ride-on like a PowerWheels vehicle, check out the following resources:

http://powerwheelscd.com/ http://www.modifiedpowerwheels.com/

:)

#8

Wow. Thanks for all of the suggestions. I especially the idea for trying to get a kids riding toy to take apart.

#9