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Unread 04-02-2007, 12:30
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Re: Banebot 56mm gearbox - double D - RELOADED

I know this isn't a full solution, but I think there are some things you can do to make the gearboxes live longer, at least through a regional competition, without spending much money or needing exotic machining abilities.

First, make sure the end play is properly adjusted. There is a spacer that goes on the motor to space the gear into the motor the correct amount, this procedure will get that spacer to be the correct thickness. Support the gearbox housing, with the shaft down, and make sure the gearbox is not resting on the shaft or bearing. Use a depth measuring device such as the pin that sticks out the side of a dial caliper, and measure the depth of the first stage planet carrier plate in the gearbox. The place you need to measure is shown in the photo. Now measure the distance D on the drawing from the motor mounting plate, to the end of the gear. The gearbox measurement should be larger than the motor measurement. Subtract the two measurements, this is the end play on your motor/gearbox. I suggest that a desireable end play would be 0.005 - 0.010 inches (0.13 - 0.25 mm).

The fun part is adjusting the end play. If there is not enough, you could turn the spacer in a lathe or file it down to make it shorter, or you could add a shim between the motor mount plate and the gearbox. Paper would probably be sufficient if it only needs a little bit of shimming, normal paper is about 4 thousandths of an inch thick.

If there is too much end play, then you need to make the spacer thicker. You can't use paper for this task, as things are turning, so you need something that won't wear easily, such as nylon, or steel. You might see if you can find a thin flat steel washer, or make your own using "shim stock", which can be purchased at some old-fashioned automotive parts stores, or places such as MSC or McMaster-Carr. You can also buy "round shims" at McMaster-Carr and probably others. If you have access to a lathe you can make some extra spacers, and play around with machining them to the correct thickness. There are probably other relatively easy solutions I have not thought of, hopefully others will let us all know what they are when the figure them out.

There are some other gotchas on the end play issue, such that you should recheck it after you have run in the motor/gearboxes, and again after they have been in use on the robot for a while. There is some end play on the output end of the gearbox, because of how the bearings are mounted. If your drivetrain design pushes in on the output shaft, then you may have problems with binding or excessive wear, so try to make sure it is not doing that.

The next thing to make the gearboxes live is to understand that the problems with the carrier plate being damaged appear to be due to sudden direction change. Talk to your drivers, and tell them that any sudden acceleration of the robot, especially a sudden direction change, is essentially abuse of the robot. The gearboxes will live MUCH longer if when you need to change direction, you make sure to come to a complete stop, then gently accelerate in the other direction. Jamming the joystick from one end quickly to the other end of it's travel is an invitation to disaster.

One possible method of ensuring that the motors are started in a controlled way, would be to have the programming team put a limit on how fast the PWM outputs to the drive motor speed controllers increase. If they allow only a small change per unit time, then the gearboxes cannot be damaged by sudden direction reversals, even if the driver gets a bit exuberant (and I do realize how exciting the matches are!). Look into this, hopefully some of the programmers here can come up with some code to control motor acceleration rate.

I think if you take care of your Banebots gearboxes, you should be able to get them to survive the season. If some of you can try out these suggestions and then take the gearbox apart and see how much wear they have, it might put a lot of teams' minds at ease....
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Last edited by MrForbes : 04-02-2007 at 19:05.
 


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