# Use of the fisher price motor ALONE

ok, i have a demand for an output rpm of 4800, and I was wondering if the fisher price motor would be good enough with gear reduction down to this speed. Is torque higher if i use the fisher price motor alone, or is it with the gearbox. As you can see, the rest of my team members (3 of 'em) and I are rookies. I don’t even understand what mNm means. If higher mNm means more torque, than the motor alone has higher torque? Also, how do the units progress for torque (what comes after/before mNm) and i would also appreciate if someone could link me to a site with basic electricity facts, about watts, amps, atc., since i don’t know what those terms mean in real life. I’m really sorry if i’m asking a lot, but any and all help is greatly appreciated. Thank you, on behalf of team 1514.

mNm is milli-Newton-meters, that is thousandths of a Newton-meter, 1000 mNm = 1 Nm. A Newton-meter is about 0.7365 foot-lbs.

Force and radius are inversely related at a given level of torque. One foot-lb of torque with give you a one pound force at the rim of a wheel with a one foot radius, or two pounds at the rim of a wheel with a six inch radius.

Torque and speed (RPM) are inversely related. Gear boxes generally gear things down to a lower speed and higher torque. So, the globe motors with the gear boxes have a slower speed (RPM) and higher torque than the motor without the gearbox.

There are also inefficiencies, frictional losses, in gear boxes. So if you cut the speed in half, you’ll get a little less than twice the torque. The losses depend on the type of gearing, but generaly get worse with each stage of gearing.

Electricity is a pretty broad subject. Is there a physics teacher you can ask? If you have a specific question, post it here and someone will help.

Today I came across this article in the on line Machine Design magazine. it gives a very basic explanation of torque, power, inertia, etc.

http://www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=57854&strSite=MDSite&Screen=CURRENTISSUE&CatID=3

Also thoroughly read the “Guidelines, Tips, and Good Practices” document in Section 5 of this year’s First documentation.

As the folks who have already posted have said, you get more torque with the gearbox, but more speed without. As far as getting a speed of 4800rpm, you might try opening up the FP gearbox, taking out the first gear, and just using it with the motor. We were doing some calculations earlier this week, and the reduction between the motor and the first gear should get you somewhere in the neighborhood of 4500-4800rpm. (I forget the exact number that we figured.) Not only will that gear mesh with the gear on the FP motor shaft much better than something you can get off the shelf since they’re meant to go together, but the gear weighs almost nothing because it’s made of plastic, which is always nice. The inside of the FP gearbox is a little disgusting, but that’s probably the best way to get what you’re looking for.

Thank you indefinitely for your help. As I mentioned, there are only about 4 people in this year’s team, and most of the teachers are also shying away from it. Hopefully when they realize what robotics really is, they might join next year. Thankfully, though, our supervising teacher is a physics teacher, and she has been really busy with organization for the whole event. I’ll try to get a hold of her. Once again, thank you.

Guy,
If you have specific questions, please PM me and I will answer whatever you need. If you need help just ask.

Guy,

Look in the White Papers section of this website. There are several very good presentations of how to apply motors. Specifically, please download and read WRRF motor selection workshop lecture notes… posted by Ken Leung where he expands upon work by Joe Johnson and Mike Ciavaglia in 2001.

The motors are a bit different from those we have in the kit now but the physics, engineering and design methodology is timeless.

Regards,