Minibot motors
 

Does anyone know how to attach an 3in. long axle directly to the tetrix motor, no gearbox at all?

Re: Minibot motors
 

Is it legal to remove the gearbox from the P/N 739023 Tetrix motor, and use the motor (without the gearbox) on the MINIBOT ?

Re: Minibot motors
 

Yes, that's pretty much the only way to get up in less than 3 seconds.


I saw a team put shaft collars onto the remaining gear and then tinker around with that, but it kills the gear.

PVC and (lots of) PVC cement?

Re: Minibot motors
 

It doesn't say you can't. I saw team 16, 148, and 118 at the Alamo regional with these modified motors and the officials didn't say anything.

Re: Minibot motors
 

Ether,
It is legal to remove the gearbox and use the Tetrix motor alone.
From the Q&A...

Tetrix Motor/Gearbox
Posted by 2011FRC2775 at 01/13/2011 05:10:19 pm
In regards to to R47 and R93 Is the Gearbox on the Tetrix motor considered to be integral to the motor? Can the gearbox be removed and/or modified as long as the motor itself is not changed.

Posted by GDC at 01/16/2011 11:47:05 am
No, the gearbox is not considered integral to the motor. Yes, the gearbox may be removed and modified.

Re: Minibot motors
 

Modify Gearbox: http://forums.usfirst.org/showthread.php?t=16779
Remove Gearbx: http://forums.usfirst.org/showthread.php?t=16187

Re: Minibot motors
 

Off the shelf collars would not be legal. We made them by machining a bolt.

Re: Minibot motors
 

Does anybody have a link to the motor curves of the motor without the gearbox?

Re: Minibot motors
 

Re: Minibot motors
 

Back to the original request now:

Yes, we do. I suppose you want us to tell you?:p

Here is an example, you can experiment from here:
Take a Tetrix metal shaft. Carefully drill a hole with a #40 drill bit into the very center of one end. Press this onto the motor shaft carefully, to avoid damage to the motor (meaning: Hammering is bad).

Some tips:
Hot metal expands, which may make an interference fit easier to assemble.
Long shafts under radial load must be supported at the far end.
Latex tubing is not very abrasion-resistant.
Rubbing alcohol makes a fine lubricant for assembling rubber onto metal.

Good luck.

Re: Minibot motors
 

Thanks. I just found this, which has speed and current in addition to power and efficiency.

The gear ratio is 52:1, so the motor free speed is something >8000 rpm.

Re: Minibot motors
 

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

Re: Minibot motors
 

R92, S. Indicates "mechanical fasteners" are allowed. A shaft coupler/collar mechanically fastens two shafts together...there were several examples of minibots that passed inspection with such couplers/collars this weekend.

Re: Minibot motors
 

Pretty sure they changed that to "mechanical hardware" in an early update, might wanna check the current version manual.

Re: Minibot motors
 

Nope, they changed it from "mechanical hardware" because all sorts of people were getting confused. It reads "mechanical fasteners".

Re: Minibot motors
 

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.

Re: Minibot motors
 

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.

Re: Minibot motors
 

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).

Re: Minibot motors
 

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.:)

Re: Minibot motors
 

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.

Re: Minibot motors
 

We tried this but the motors would smoke.

Re: Minibot motors
 

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.

Re: Minibot motors
 

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.

Re: Minibot motors
 

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.

Re: Minibot motors
 

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.

Re: Minibot motors
 

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.

Re: Minibot motors
 

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

Re: Minibot motors
 

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!

Re: Minibot motors
 

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

Re: Minibot motors
 

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.

Re: Minibot motors
 

Or you can do away with the transmission altogether.

Other than being ridiculously long and complex I don't see what's wrong with the above solution that 1322 posted.

Re: Minibot motors
 

We were attempting to design a minibot with surgical tubing rollers, but couldn't find the sweet spot for the rollers to actually work.

Re: Minibot motors
 

Here is how we make 10.4 to 1 gear boxes.

We make a spacer out of legal material that fits between the gear on the output shaft and the output end bushing that makes that gear mesh with the input gear.

Then you must machine the input end of the output shaft to fit in the bushing and provide the proper thrust clearance.

The input gear stays on its' original idler shaft with the original spacers and there is no need to modify the case. This keeps the machined lip that keeps the end plate square.

The down side to this method is that the output shaft is a precision polished piece and you can't replicate that surface finish. That means accelerated wear of the input end bushing.

In our prototype boxes there is noticeable wear, however that includes a number of minutes of run-in time and dozens of runs between the different wheels and frames we tried. That set is getting retired since we have more transmissions than motors, having let the magic smoke out of a couple.

We also tried the rod wheels w/o the trans and we are not going that route. Yes the direct drive gave us our record time but was no where near as consistent.

Re: Minibot motors
 

If you look at the four shafts in the gearbox, there is a 10 mm center distance between the first three with a total of 40 teeth in the mesh. The first pair of shafts have a 10T / 30T combination, the second pair has a 15T / 25T combination. All three of these shafts have a 12.5 mm center distance to the output shaft. The final reduction is a 10T / 40T combination or a total of 50 teeth in the mesh. Since all three shafts are 12.5 mm from the output, the final reduction gear can be placed on any of the first three shafts and still mate properly with the output gear. To make the 10.4:1 ratio, attach the output gear directly to the input gear. This leaves a stack of 4 gears with only the first and last actually being used. You have the 26T helical input gear, the 10T gear, a 25T gear, and the 10T output gear. The transmission ratio is then 26/10*40/10 = 10.4:1. The advantage of this arrangement is that the only modification required is to attach the input gear and output gears together. The gear box and spacers do not need any modification - the spacers just need to be moved around a bit. The output gear has a very thin spacer under it - we moved that from the third shaft from to the first.

Re: Minibot motors
 

Does anybody know what size the screws that connect the tetrix motor to the transmission plate are? The really small, short flat head phillips screw. We have lost some and it is an expensive venture to buy more tetrix motors/gearboxes just to get the screws. We are definitely more careful when pulling the motors now.

Re: Minibot motors
 

Last night I was able to grind down the shafts and use them in direct contact with the metal pipe. It did work but it still was under powered and just the slightest drag would smoke the motors. The couplings are the aluminum slugges that go inside the tubes, we drilled the center to fit the motor gear and the shaft than drilled setscrews into it. I think we will stay with our original design (design #7) even though it is not the fastest, it is fast and solid.

Re: Minibot motors
 

those are M2.5 on my motors

Re: Minibot motors
 

*cough* latex tubing *cough*
THREE seconds? Goodness, we're down to one point three seconds now.
I agree with above, I get 2.5 mm x 0.45 pitch (which is standard). McMaster has them for about $4 a box of 100, depending on length.

Re: Minibot motors
 

So these two gears are on the same shaft (1st) and the other gear is gone? Connection via roll pin? welding?