Window motor or FP W/ Gearbox?

Hey everyone,

Our team is trying to decide if we should use a window motor or a fisherprice motor with a gearbox for a spool to lift a 20lb arm.

Any advice on which one would work better and which one has more torque? Thanks.

The window motor has more torque, but that is not the question you meant to ask.

What you want to ask is “Which motor has more power”, which is work done in a unit of time.

And for that, you can use the FIRST Motor Specifications page, which is here: http://www.usfirst.org/sites/default/files/MotorInfo4.1.pdf

If you want more information, just search on “2012 motor” for the many posts here in the last 3 months…

The FP motor WITH the (big plastic) FP gearbox is a very high torque solution. It has 5X the torque of the window motor. By far the strongest combo in the KOP. With a small spool diameter it can lift 20 lbs effortlessly.

How far and how fast do you need it to move?

Also very important, what is the size of your spool?

What is the size of the rope on the spool?

Have you looked at the motor information located at http://www.usfirst.org/sites/default/files/MotorInfo4.2.pdf yet? It doesn’t include the FPs’ stock gearbox, but if you can find the ratio for that, it’s not too hard to calculate it. It will give you power (the real rating you want on a motor–if you don’t like torque or speed, you gear it), stall torque (if it’s moving and you stop it, what torque you are exerting) and free speed (no gearboxes/other loads), as well as current so your electrical crew can plan accordingly.

The FPs have more power, but spin very fast. If you can gear them down enough, they’ll have at least as much torque as the window motors. But, the window motors can’t be backdriven easily due to using a worm gear. They’re also pretty slow–if you’re winching, a window motor’s free speed is about 1.5 revolutions/second, which you can convert to how much line will go onto/off of your winch per second.

Personally, I’d go with the FP–two of them with stock gearboxes and an appropriate cable-lift system can lift a robot very quickly (or, with appropriate gearing, lift a smaller high school student). But, you may need the FP speed elsewhere on the robot…

Just a few of the things to consider besides pure torque in determining which will work better…

We want our arm to move pretty fast, and we have tried the window motor but it was really slow. The spool size for the window motor was a pvc pipe about 1 inch in diameter and 22 inches in length to fit the width of the robot.

A one inch spool will be slow even with an FP and gearbox. The larger the spool the less force you can pull with, but with the benefit of a faster pull.

You can bump it up to a 3 inch spool with a single FP and gearbox and it will move pretty quick. Should cover the 22 inches in little over a second.

I suggest that you consult your local physics teacher or mechanical engineer. He/she can teach you the relationship between power, torque, and angular speed – and how to convert that into a linear time on a 22-inch pull.

With the torque/power specifications of a particular motor/gearbox combination, you will then have all the tools necessary to make the right decision for the application.

22lbs * 1.5 inch lever arm for a 3 inch spool = 33lbsinch=528 ozinch

528ozinch / 121 (reduction in FP gearbox) = 4.33 ozinch

RPM @ 4.33oz is approximately 19000RPM at motor, 157 RPM at output.

24 inches/s assuming 100% efficiency after the motor output.

Using specs for the 801-0673 FP

I was talking to the OP, sorry… I was hoping they’d learn something about torque, power, and speed!

I’ve written a document that attempts to explain how to use the motor spec sheet and do the appropriate calculations: http://www.botsnlinux.net/firstphysics.php. There are also a lot of other great resources here on CD about selecting the right motors and doing force/torque calculations. Search around, especially in the CD-Media white paper section.

Finally, don’t just decide based on your gut or what people here say. Do the math and be sure for yourself!

While the FP and gearbox has much more torque than one window motor, keep in mind the complexity of building a winch for that gearbox. If you use two window motors, one on either side of the winch, then you can build a winch with only 2 parts. We made a winch with a housing of a single piece of 4*2 tubing, and a single winch drum. It “winches” with around 140 lbs of force when both window motors reach stall.

Since it hasn’t been mentioned-
Keep in mind that the window motors have a thermal shutoff that at times can seem to shut off prematurely. We use window motors for bridge lowering and use them twice a match, but teams have run into trouble with constant movement using window motors (ie. swerve steering).

Also keep in mind that window motors and jaguars don’t mix well. (lets not debate it here, do a little searching)

One last consideration- Window motors are anti back drive and FP’s are not.