Minibot motors

[Ether]

Originally Posted by Chris is me

Do keep in mind the motor gearbox is almost laughably inefficient (hovering around 50%)

Yeah, I just discovered that as I was comparing the test data with and without the gearbox. I was wondering if it's even the same motor.

[Al Skierkiewicz]

Joe,
The correct read on R92 is
S. Mechanical fasteners (e.g. screws, bolts, etc),
I would be hard pressed to include COTS shaft couplers in that definition without direct GDC guidance on the subject. Something manufactured from the allowed materials would be OK.

[boomergeek]

baronep1,
Do you have a lathe with a drill attachment and someone that knows how to use it?

Do you have a drill press?

We would have had difficulty with drilling a proper hole or press fitting the aluminum round bar without both machines and some expertise...

If no local expertise, find a local team or machine shop that will lend you a hand to make the parts and assemble them to your specifications.

There are many teams that try to get by with hacksaws, vises and power drills and don't know what a gear-puller or a press fit is.

To me, it's not very sporting to get you to try to imagine what the right machine shop tool is if you have not been trained in a motor machine shop nor have access to one.

Now it might be that you have a motor expert mentor and machine shop and you asked here before asking the local team mentors: but I find that unlikely.

Good luck.

(I would not advise you to try to make the hole in the cylinder or try to attach it to the motor without the proper tools).

[JB987]

Originally Posted by Al Skierkiewicz

Joe,
The correct read on R92 is
S. Mechanical fasteners (e.g. screws, bolts, etc),
I would be hard pressed to include COTS shaft couplers in that definition without direct GDC guidance on the subject. Something manufactured from the allowed materials would be OK.

Right, Update 3 changed hardware to fasteners...But "etc" is so esoteric, Al! I do agree that someone considering purchase of shaft coupler should check with the GDC though.

[boomergeek]

Originally Posted by Chris is me

Do keep in mind the motor gearbox is almost laughably inefficient (hovering around 50%)

Inefficient?- it all depends on perspective...

@$30 a pop and a tendency to quickly burn out when bumping into a stop- the motor with gearbox is very efficient at generating revenue.

The first reason we got rid of the gearboxes is the motor sounded so much better (i.e., not that sound of impending doom) when relieved of their gearboxes. We've had 5 motors: 2 with gearboxes have already burnt out.

[joeweber]

We tried this but the motors would smoke.

[Tom Line]

The motors will smoke because your axle is far far too large: you are not gearing correctly for the torque of the motors. It also looks like you're trying to run square shaft through a round bushing, which isn't good at all friction wise.

Round shafts through those bushings with a much smaller OD.

[joeweber]

Originally Posted by Tom Line

The motors will smoke because your axle is far far too large: you are not gearing correctly for the torque of the motors. It also looks like you're trying to run square shaft through a round bushing, which isn't good at all friction wise.

Round shafts through those bushings with a much smaller OD.

It's not a square shaft, It is the shaft that comes with the tetrix kit with a flat spot. If I lathe down the shaft smaller I would still run into the problem of contact with the pole and the friction required to go up it with out slipping. I place rubber electrical tap over this shaft which is legal but there is nothing else small enough that is legal.

[boomergeek]

Originally Posted by joeweber

It's not a square shaft, It is the shaft that comes with the tetrix kit with a flat spot. If I lathe down the shaft smaller I would still run into the problem of contact with the pole and the friction required to go up it with out slipping. I place rubber electrical tap over this shaft which is legal but there is nothing else small enough that is legal.

We found that precision on centering is critical and we had the accuracy of a lathe.

If the shafts are contacting more than about an inch from to the motor, there is significant wobble.
If the load is more than 1.4 lbs per motor, there isn't typically enough of the shaft in contact with the pole to prevent destruction of the tire.

Having the right amount of normal force on the tires is critical. The optimum is a function of the robot weight.

[Al Skierkiewicz]

Joeweber,
I would agree that a direct drive like that is under the torque needed to climb and so the current is high. You reminded me/us that Tetrix does include shaft collars. Is that what is pictured? I have seen a number of teams using gears scavenged from the Tetrix transmission to reduce the speed and increase torque while mating with the motor pinion perfectly.

[Tom Ore]

There are a couple of easy ways to modify the gearbox. One way results in a ratio of 31.2:1. The other way results in a 10.4:1 gearbox. We ran the 10.4:1 gearbox with roughly a 2" diameter wheel in KC. The minibot climb time is about 2 seconds. This seems a bit slow, but with a reliable deploy it does okay.

[Ether]

For what it's worth, a compilation:

2011-01-28 Tetrix motor+gearbox; spec from Tetrix website:
http://tetrixrobotics.org/Building_S....aspx?moid=721

2011-01-21 Tetrix motor without gearbox; test data from R Wallace:
http://www.chiefdelphi.com/forums/sh...8&postcount=15

2008-05-12 Tetrix motor+gearbox; test data
http://www.chiefdelphi.com/forums/at...0&d=1274226112

[Ether]

Originally Posted by Tom Ore

There are a couple of easy ways to modify the gearbox. One way results in a ratio of 31.2:1. The other way results in a 10.4:1 gearbox. We ran the 10.4:1 gearbox with roughly a 2" diameter wheel in KC. The minibot climb time is about 2 seconds. This seems a bit slow, but with a reliable deploy it does okay.

Tom, you said the 10.4:1 mod was easy. Would you be willing to post a bit more detail how you did this? Thank you!

[Captaindan]

if you have extra motors laying around and money to buy more obviously they are going to burn go ahead and take the gearbox off
but a 3sec robot is possible with proper engineering it is totally up to the builder/team

[Tom Line]

Originally Posted by Ether

Tom, you said the 10.4:1 mod was easy. Would you be willing to post a bit more detail how you did this? Thank you!

Ether, you'll find upon opening the gearbox that the gear that meshes with the motor can be placed on a different shaft. Indeed, it can be put on one of the outside shafts and be the only intermediate gear between the motor and the output shaft gear, if one machines the total height of the gearbox itself down enough. Machining the gearbox down is necessary for the motor to reach the helical gear while the helical gear drives the output shaft. Take one apart and you'll see what I mean - play around with moving the gears around onto different shafts.

This reduces the gearbox inefficiency a great deal, and allows you to use a small wheel directly driven from the gearbox output shaft, increasing efficiency even more.