Globe motor modification

[Veselin Kolev]

Is it legal to modify the gearbox of the globe motor?

I needed to speed up the output shaft, so I tapped out the pins holding the planetary gearbox on, removed one of the reduction stages, made a spacer for the shaft, and tapped the pin holes and screwed it back together. I am guessing this is legal because it's legal to remove the gearbox. Any ideas?

[eugenebrooks]

Originally Posted by Veselin Kolev

Is it legal to modify the gearbox of the globe motor?

I needed to speed up the output shaft, so I tapped out the pins holding the planetary gearbox on, removed one of the reduction stages, made a spacer for the shaft, and tapped the pin holes and screwed it back together. I am guessing this is legal because it's legal to remove the gearbox. Any ideas?

You may modify and build your own gear boxes. It is only the "motor" part that you may not modify. Just to ask, how much of a speed increase did you get? Our globe is running a little slow as well...

[Richard Wallace]

Originally Posted by eugenebrooks

You may modify and build your own gear boxes. It is only the "motor" part that you may not modify. Just to ask, how much of a speed increase did you get? Our globe is running a little slow as well...

I'd like to know also.

Using the planetary stage reduction formula (R/S) + 1 for each stage, where R is the number of teeth in the ring gear and S in the number of teeth on the sun gear, and counting the teeth as follows:

First stage S = 21, R = 59, second stage S = 13, R = 59, output stage S = 13, R =59,

I get motor stage reduction = 3.8095:1, second stage reduction = 5.5385:1, output stage reduction = 5.5385:1, so the overall reduction is the product of these, or 116.855:1, which agrees closely with the published figure 117:1. The published free speed is 81 +/- 7 RPM

Veselin, did you remove the first stage and replace the motor pinion with a 13 tooth one made from the first stage planet carrier? In that case you'd get overall reduction 30.675:1 for a new free speed of about 310 RPM.

Or did you remove the second stage, giving a new overall reduction of 21.099:1 for a new free speed of about 450 RPM?

Will someone please check my analysis above?

[ajlapp]

assumption....he removed the second stage.

as far as i remember this removal trick can only be done by removing the second stage.

[MattB703]

On ours we removed one of the 5.5:1 stages. We tached the free speed of the shaft after modification at ~ 480RPM.

Matt B.

[Conor Ryan]

Did Some Rule searching

Originally Posted by [url="http://www.usfirst.org/robotics/2006/frcdocs.htm"

The 2006 FIRST Robotics Competition Game Manual[/url]-Section 5]5.3.4.1 Kit of Parts Rules
<R36> So that every robot’s maximum power level is the same, the motors in the kit may not be modified in any
way, except as follows:
• The mounting brackets and/or output shaft/interface of the motors may be modified to facilitate the
physical connection of the motor to the robot and actuated part.
• The gearboxes for the Fisher-Price, and Globe motors are not considered “integral” and may be separated
from the motors. FIRST will not provide replacements for parts that fail due to modification.
• The electrical input leads on the motors may be trimmed to length as necessary.

[Rick TYler]

Originally Posted by Richard

Or did you remove the second stage, giving a new overall reduction of 21.099:1 for a new free speed of about 450 RPM?

I love you people. We need 450rpm for one application (prototyped with a cordless drill at 450rpm) and I was wondering if I was going to have to resort to a drive belt and pulleys. Stepping up a motor that had already been geared down seemed kind of dumb to me. Thanks!

[Al Skierkiewicz]

Originally Posted by Veselin Kolev

Is it legal to modify the gearbox of the globe motor?

I needed to speed up the output shaft, so I tapped out the pins holding the planetary gearbox on, removed one of the reduction stages, made a spacer for the shaft, and tapped the pin holes and screwed it back together. I am guessing this is legal because it's legal to remove the gearbox. Any ideas?

Veselin,
I have not heard this question asked before and it should be put to the Q&A. The rules state, under R36, "The gearboxes for the Fisher-Price, and Globe motors are not considered “integral” and may be separated
from the motors. FIRST will not provide replacements for parts that fail due to modification." I believe the gearbox is also available separately as a single component. As such it would fit under the acquired and modified language making it a "fabricated item" subject to rules and flow chart for that item. Thinking outside the box again, aren't you?

[MattB703]

We are doing almost the same thing this year, but instead of using a spacer we put the gearbox housing in a lathe and shortened it by the width of one stage. One of our students is panning to write a whitepaper with the details but is may take a little while. if anyone is interested you can drop me a PM or email.

Matt B.

[Peter Matteson]

This is a great trick. We also did this on our 2004 bot because we needed higher RPM and torque wasn't really an issue.

No issues with inspectors once we pointed out the rule. They thought the shortened gear box looked funny.

I'm glad to hearsome one else is doing it. Now if only I could figure out what it's for...

[Tristan Lall]

We were just looking at the same thing last night (shortening it). I don't see how there would be any issue, since plenty of precedent exists for modifying the output shafts of Globe gearboxes, and the F-P gearboxes, which FIRST seems to treat similarly.

[Joe Johnson]

I am confident that it is legal as well. I can think of no rule interpretation that would make this illegal. This is no different than teams modifying the FP transmissions which is very very common.

Joe J.