window motor backlash

[nic_radford]

Has anyone encountered the problem with the amazing amount of backlash that develops over use with the window motor? This seems to get worse with use. I'm looking for away of removing the backlash. Is there away to tighten up the worm gear shaft or something?

[Tristan Lall]

We encountered this last year, but never took apart a failed motor to determine the cause. Try popping the ring clip off, and digging around under the gear. If it is the worm itself, there probably isn't too much you can do for it. They're designed to be replaced, rather than easily serviced.

[Cory]

Last year we popped the cover off the face of the motor and jammed a piece of keystock in between the gears. This worked quite well, although it isnt the best solution.

Cory

[Matt Adams]

popped the cover off the face of the motor and jammed a piece of keystock in between the gears. This worked quite well, although it isnt the best solution.

However, I'd have to say since modifying the motors is explictly against the rules, this is obviously something that GP FIRSTers wouldn't ever consider doing in any FIRST sanctioned (and just about all non-sanctioned) competitions. Fortunately, the problem that you're having should be the same problem that everyone else has, assuming they follow the rules, so it's fair for all.

To get around the problem, there's no obvious quick-fix, so your best bet is to try to plan ahead by not using these for any position intricate applications.

Good luck!

Matt

[Paul Copioli]

Guys,
It's a WINDOW motor. The backlash is there because it can be. If it gets worse over time, I've never noticed it; because it is horrible to begin with. I bet it is at least 5 degrees, probably 10 degrees right out of the box. This motor, when used for its designed puprose, is always gravity loaded! The backlash doesn't even come into play because gravity keeps the teeth loaded.

So, use the motor in a situation that best fits its original design. That is part of the design challenge.

-Paul

[sanddrag]

We are at 45 degrees slop after last season. Would anyone like to trade us them for something else? We have a lot of stuff to trade.

[nic_radford]

gravity loaded while rolling the window down? Gonna have to think about that one. My experience with windows (at least car windows) is they don't really 'fall' down without any linkage attached to them. There quite a bit of friction there with seals and such. However, not to argue that point.

yes, I'm familiar with design challenges. That motor fit all of our requirements minus the backlash and I was looking for a workaround.

So if anyone has a workaround, those type of posts will be appreciated.

[Paul Copioli]

Nic,

Judging from your response, I think I may have come across rude. That was not my intention. When I see people posting things advocating breaking the rules (not your post, but one of the responses) I tend to get a little hot under the collar. Not directed at your post. To specifically answer your question, we have found the best workaround is to use that motor in applications only where the backlash will not hurt you. We tend to use it for lifts, or extensions, or winches. The anti-backdriving capability of those motors is awesome, so we try to use it to our advantage. Modifying the motor is just not allowed within the rules.

I know you don't want to argue the point, but I just can't help myself. The seal drag and other resistive forces are nothing compared to the weight of the window. Even though the window is going down, gravity is still acting on the motor (just helping). The motor's backdriving resisistance (or the linkages' backdriving ability) allows the window to stay up when the motor is off. The second the motor starts to move, the backdriving ability of the worm is greatly reduced (sliding friction vs. static friction) and gravity actually pulls the window down and the motor just controls the fall. The motor doesn't actually pull the window down. Now I know all windows don't operate this way (actually my truck has a very small motor with no backdriving resistance and a huge spring and linkage mechanism), but this is what I meant by always gravity loaded. Gravity is always on.

[nic_radford]

Reason for answering your question the way I did, is I have taken motors out
of car doors and had to push the window through the door seal it was so tight. (no, it wasn't bound)

Anyway - Our FIRST team always abides by the rules and breaking them never once crossed our mind. As soon as the gear head modification post
was posted, a fellow engineer and I double checked the rule and found that yes it is illegal to mod the gear head and quickly disregarded it.

[Cory]

<R62> The motors in the kit may not be modified except as follows:
• It is acceptable to modify the mounting brackets and/or other structural parts of the motors (output
shaft, housing, etc.) as long as the electrical system is not modified and the integral mechanical
system of the moving parts (bearings, bushings, worm gear output stages, etc.) is not changed or
removed.

The way this rule is written, we followed it. We did not remove or change the worm gear output stage, or anything else. One could consider what we did a modification to the output shaft. I do not believe this is or was illegal, we don't plan to do it again, but Im going to submit this to Q&A to make sure.

Cory

[Matt Adams]

Originally Posted by Cory

It is acceptable to modify the mounting brackets and/or other structural parts of the motors (output
shaft, housing, etc.) as long as the electrical system is not modified and the integral mechanical
system of the moving parts (bearings, bushings, worm gear output stages, etc.) is not changed or
removed.

Cory-

First off, I definitely wasn't out to make your team sound like un-GP cheaters. However, I think it'd be in everyone's best interest to ask FIRST about that modification. To me, the rule intent sounds like FIRST gives the thumbs up to modify the exterior of the motor and the piece that spins. To shove an extra piece of metal within the gear train is something that could be considered a more grey area.

It might be better to "play it safe" and realize that opening the cover of any motor (minus removing the pins in the drill) to modify the internals could be entering some sort of a grey area. When I think "output shaft", I think of the metal cylinder protruding from the housing, and I consider things within the motor housing itself to be the "holy ground" that dare not be touched.

However, it'd be best to get an answer straight from the source- the FIRST Q & A.

Good luck!

Matt

[jskene]

In response to a question I posed a few days ago on the subject on the USFORST Q&A site, FIRST has just indicated that adding an internal spacer to the window motor to reduce backlash is LEGAL.

see Q&A Section: 5.3.1.4 Date Answered: 2/10/2004

Now, I just have to figure out how to do this .

[jskene]

We just modified our window motor output shaft coupling by placing a small length ~(1/4") of 3/16 key stock between the gear plate and the output shaft. Play is now reduced to a few degrees of rotation.

You can remove the gear plate by first removing the retaining ring and then pulling on the gear plate.

[blindguyinanorg]

the rule states that you cannot modify the motor. now questions have come up many times about modifying the gear boxes attached to the motors. this is of course ok becuase it is just like removing the gearbox completly(is legal) and making your own. the violation would occure if you opend the motor and added your own busihings or something of that sort.

as for the back lash we experienced that last year quite abit, so much that they stripped themselves silly. part of the problem is the high amount of give in the worm gear.

[jskene]

The give appears to be in the coupling of the output of the worm gear to the gear plate. The spacer compensates for this. The worm itself seems to have little backlash.

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