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Choosing drive motors for kids riding toy Answered

I'm building  "robot horse" for my daughter, and have the metal frame mostly complete.  The plan for locomotion is to mount reversible powered 6" wheels to the outside of each hoof, and steer tank-style.

My biggest challenge is finding reasonably priced, reasonably powerful drive motors for each wheel.  My issues are 
1)  I'm inexperienced at specing dc motors.  
    A) I'm not real sure how to translate published specs into real world motion.  I don't know exactly how much power it takes to get this reliably moving over rough ground to a top speed of around 1 mph.  I also don't have a good sense of much voltage you can pump into a DC motors.  (IOW, should I be able to push 2 x the rated voltage for a quarter second to get it moving? Will the motors naturally increase their amperage at a constant voltage (and thus power) as they are resisted by torque?)  How much power will starting from a stop take?  What about a slight uphill slope?
    B)  I'm not sure what form-factor is best:  Gear motor?  Gear motor with final drive belt?  I would like to use worm-gear motors, as I think it would be safest if the wheels didn't move when unpowered.
2)  Hunting around supplier web-sites, I see lots of $20ish gear motors that use 5-20W of power.  Great price, but would 4 of them make enough power?  (See #1A above)  Also, I see lots of fractional HP motors for >$100 that would unquestionably be strong enough, but would cost more than the project justifies.  I found very little in between.  Are there motors and gear boxes that could power this at a reasonable price?  

Relevant clarifications and details:
1)  The horse is about 40" high at it's withers (shoulder)
2)  Frame is made of 1/2" square tube steel.
3)  Normal rider + vehicle will be about 150 lbs.  However, unquestionably, larger kids (or multiple small kids) will at some point hop on it and try to ride it.  At 250 lbs. rider+vehicle weight it doesn't have to go fast, but shouldn't burn up the motors.
4)  It'll be powered by a lipo battery pack that shouldn't have any issues with 20A+ max currents and can be sized as needed for voltage.
5)  I'd like to keep the motors and gears <$200 total, if possible.
6)  "Rough ground" = flat grassy turf or reasonably smooth gravel.
7)  Motors need to be reversible.

Can anyone lend some perspective on what type of motors to buy for this?  How much power/torque should I look for?  Any advantages/disadvantages of any particular gearing style for this application?

I am assuming that motors on each wheel are the way to go, but if there are any other power transmission schemes that would work better, I'm all ears.

Discussions

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SvdSinner

3 years ago

After a LOT of searching, I found these two motors at surplus center. I'll have to gear down the final drive speed, but they are cheap and have so much maximum torque that they'll have plenty of reserves for bumpier ground. As a bonus, their form factor should hide well inside a horse leg.

http://www.surpluscenter.com/Electric-Motors/DC-Gearmotors/DC-Gearmotors/500-RPM-12-VDC-GEARMOTOR-5-1754.axd
(8.5in-lbs@220rpm, @$9.99)

http://www.surpluscenter.com/Electric-Motors/DC-Gearmotors/DC-Gearmotors/500-RPM-12-VOLT-DC-RIGHT-ANGLE-GEARMOTOR-5-1816.axd (16in-lbs@220rpm, @$14.99)

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liquidhandwash

3 years ago

I would try and find a couple of wheel chair motors or electric golf caddie motors, you can pick them up cheap or free

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caitlinsdad

3 years ago

I would try to email this group who is always at Maker Faire showing off their wheeled creations and would probably have some good advice.

http://miters.mit.edu/about.php

Good luck.