Subject:
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Re: torque requirements of 200lb robot
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 6 Jul 1996 18:06:13 GMT
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Original-From:
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Chuck McManis <[cmcmanis@netcom.]AvoidSpam[com]>
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Reply-To:
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cmcmanis@netcom/StopSpammers/.com
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Viewed:
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1566 times
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Mike McGargle wrote:
> I was also thinking of doing this with a couple of drills. How much
> power do they have? I want to move a 200lb platform on 8" wheels.
> Do you think they could do it?
In my freshman physics class (back in the stone age :-) we went over
the notion of torque and how gears "amplify" torque. And what we
showed was that you could move an arbitrarily heavy object with any
motor your wanted if you had enough frictionless, massless gears ;-0
A couple of comments on the above...
A 200 lb robot should have a rolling friction that is much much
less than 200 pounds. Consider a 500 lb box sitting on a bunch of
ball bearings spread out on the floor. I can push it with a single
finger. HOWEVER, the trick comes when the robot is not on a level
surface and has to go "up" hill. Then the force to move it is its
rolling friction plus a percentage of its weight determined by
the formula :
added weight = Weight * sin(angle of incline) 1.
For a 200 lb robot going up a 7 degree incline that is an additional
24.4 lbs of force needed to counteract the force of gravity on your
'bot.
Another thing to worry about are load bearings. Those electric drills
were never designed to carry much side force (after all the designer
expected all the pressure to come from the back, not the side.) So if
you do the "easy" thing and just put an axle into the drill's chuck
and then strap the drill to your robot you are looking for trouble.
The bearings in the drill will fail when you exceed their limits.
A much better solution is to put your wheels on their own axle with
a real bearing (some wheels have the bearing built into the wheel
rather than expect it to be on the axle.) Then attach a "timing pully",
which is a pully with teeth in it, to the drill, and another timing
pully to the wheel. Then connect the two with a timing belt. You can
get these at good hardware stores or small engine shops (they are used
in riding lawnmowers and such.) If a timing pully isn't available
then bike gears and bike chains can be substituted.
So once the mechanical linkage is done, and you have calculated the
rolling friction of your robot (One way to do this to a first
approximation is to pull it with a fish scale and to read the
fish scale. This only works on wheeled vehicles though.) Then
decide what the steepest grade you will consider. (let's say its
7 degrees which is I believe the max wheel chair grade but if it
isn't, the wheel chair grade is a good one to use since the AWD
act requires all building to have such access) You need 24.4 lbs
plus your rolling friction (say 10 lbs) and you want your robot
to accelerate reasonably quickly to its cruise speed. You can do
the math (remember that accelleration = force/mass) or you can
just wing it and say you want twice the needed force (or 34.4 * 2,
or 70 lbs.) Since you have 8" wheels (which have a radius of 4"
or .33') the torque required at the rim of the wheel is 70 lbs
and the torque required at the drive axle is 70 / .3 or 233 ft-lbs
(which on reading it, sounds a bit high to me.) Anyway you can
get away with less, but it has to be at least 35 / .3 or 117 ft-lbs
so that you can move at all up the 7% grade.
Also remember that adding drill motors help so haveing two is better
than one, each needing only to product half the torque.
For further reading I reccomend the Mobile Robots book,
(ISBN 1-56881-011-3) and Android Design (ISBN 0-8104-5192-1)
which is out of print but a book search service might find it
for you.
--
--Chuck McManis http://www.professionals.com/~cmcmanis/index.html
All opinions in this message are those of the author. No warranty as to
the suitability or accuracy is stated or implied. Use at your own risk.
cmcmanis@netcom.com +1.408.524.4805
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Message is in Reply To:
 | | Re:
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| System User wrote: snip (...) have? I want to move a 200lb platform on 8" wheels. Do you think they could do it? My partner (who also reads this list) thinks I'm nuts to suggest buying two new drills and cutting them into pieces. My resoning is that (...) (28 years ago, 5-Jul-96, to lugnet.robotics.handyboard)
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