low gear vs high gear

FIRST General Forum
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Posted by Anton Abaya.

Engineer on team #419, Rambots, from BC High / UMass Boston and NASA, Mathsoft, Solidworks.

Posted on 2/9/2000 6:46 PM MST

using the recommended design (a little modified of course), we are using the drill motors for our drive train, attached to sprockets and chains. tank driven.

initial problem was the motors would buckle up on HIGH gear and refuse to turn. while, LOW GEAR did fine but still showing significant effort.

as this is a recommended design, I was curious what other teams have done to combat this problem. is it even possible to stay on high gear?

need some useful tricks of the trade here. :slight_smile:

thanks.

Anton

2

Posted by Joe Johnson.   [PICTURE: SAME | NEW | HELP]

Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

Posted on 2/9/2000 7:45 PM MST

In Reply to: low gear vs high gear posted by Anton Abaya on 2/9/2000 6:46 PM MST:

Anton,

I think you have either:

TOO high of a ratio (by which I mean not enough ratio actually).

TOO much friction in the system.

How big are your wheels?

Assuming the drive wheels are 8 inches in diameter, I really recommend against any ratio lower than about 80 to 1 between the armature of the motor and the wheels (especially if you are rookie team and desparately need to have a reliable drive train in order to have a good year). In fact I really don’t think you should be considering a ratio much lower than 100 to 1, but perhaps I am in the minority on this issue – see other messages on this very forum.

The gearbox for the drills can give you either 64 or 20 to 1. So, if you are in High, you should have at LEAST another 4 to 1 from the gearbox shaft to the wheels. If you are in Low, you should have at LEAST another 1.5 to one from the gearbox shaft to the wheels.

The ratios scale by the size of the wheels, by the way, so if you have 16 inch wheels you need twice the ratio I recommended above.

As to friction, you need to watch this carefully. What are your bearings for the drill shaft? What are your bearings for the wheels? Is your chain routing causing you to have too much friction? Is your drill mounting causing too much friction?

As a test, you should be able to backdrive your motors (assuming you followed DrJoesDrillAdvice and removed those nasty no-backdrive pins ;-). If you cannot easily rotate your wheels with your battery unplugged, you probably have a fiction problem.

Good luck.

Let us know how things are going.

Joe J.

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