Direct drive minibot - output diameter?

We’re redesigning our minibot after having seen how effective the direct drive minibots were at WPI, but we have a couple of questions. Have people been leaving the little gear that’s on the output shaft and then press fitting something over it, or have they removed it? Second, what have people found to be effective as an output diameter wheel against the pole? We were looking at around 3/8."

It sounds like you’re on all the right tracks! :slight_smile:

First of all, I’ll preface this by saying that 1519’s minibot has not been optimized… what we’re doing are likely not the best ways of doing things!

Our minibot uses direct-drive and can climb the pole in about 1.3 seconds. To answer your questions, we use a 1/4" shaft with surgical tubing of 1/4" ID and 3/8" OD simply slid on over the shaft. It is “press fit” (snug, but by no means a true press fit!) onto the pinion gear of the Tetrix motor with an 8-32 set screw to help lock it in place on the pinion. That hasn’t failed once in 1 regional plus quite a lot of practice, although making sure your set screws are tight is always important! The other end of the shaft is supported by a bearing.

We are considering increasing the diameter of the motor and trying to remove a couple ounces (we weigh in at 2.3 pounds, currently) to increase our speed slightly. Really though, it’s all about reliability… the primary reason we went with this design was the magnet attachment to the tower - no clamping or grasping involved.

Feel free to ask any questions you like! Best of luck!

3/8 inch is good. Remove the spur gear . The motor shaft is 2.5 mm. A two inch long piece of 6061 drilled with a 3/32 bit can then be carefully pressed on with thread locker.

we removed the gear.

3/8" is in the ball park.

Let’s consider if you are currently using a 4" * PI wheel circumference with a 52:1 gearing

4" * PI divided by the loss of 52:1 efficiency would imply about a 0.075" * PI wheel

Efficiency goes way up getting rid of the gears.

We don’t recommend direct drive on a heavy robot- it did not work for us on a 4 lb’er.

If any of these ideas help… consider paying it forward. If they don’t help, …sorry.

Has anyone drilled the shafts without using a lathe? I would be curious as to how to get it accurately centered enough and straight in without a lathe.

We also removed the brass gear and drilled our shaft with a #40 drill bit. A lathe was used to ensure concentricity of the hole to the shaft. We used the flatted tetrix shaft.

It will be a rough ride to the target if you don’t use a lathe wih a properly aligned tailstock. You could try a drill press, drilling a hole in a block of plastic or aluminum with it securely clamped on the drill press table. This hole would be equal to the shaft diameter. Then change drills to a small center drill, and then step up in crements to your final hole size. Not perfect, but will get you close. Good luck. Isn’t there a local machine shop you could appeal to?:slight_smile: :slight_smile:

Team 241 has a lathe that we spent time to align the tail stock- it started off about 0.07" off center.
My point is there are about 10 threads on minibot direct drive that say “drill a hole”.
(I think some of the audience here on CD sees “drill a hole” and assumes a power drill or a drill press might be sufficient to try;
Or see suggestions like “take off the gear” - but have no idea what a real gear puller is and think of “hammer and pry bar” instead.

I cringe at the potential for wasted time and wasted parts from the possibility of people not having a concept of what the right tools are when someone makes a suggestion to do something.

I think you misread what was posted. I interpreted what they were asking to do as how to drill a concentric hole without a lathe. Making a jig to hold a shaft vertical on a drill press is certainly better than “eyeballing” the center. Sometimes you have to be creative to get the job done.:slight_smile: :slight_smile: The final word was to suggest soliciting a local machine shop to help. The days are long gone when Dad had a workshop in the garage and taught his son or daughter how to use tools, and what they were called. FIRST is helping to fill the gap between engineers and the “hands on” people who actually build the product.

It’s all good. I was the person to bring up the use of the word “lathe” after the word “drilled” was used. In many cases, the person reading “drilled” in this context assumes a lathe… but many reading here on CD may not have enough machine shop experience.

Adding some detail on identifying the right tool or a reasonable equivalent is useful to many folks. I really like your idea of the potential for using a good drill press to drill a concentric hole.

Press-fitting long shafts is also tricky:
We used our heavy duty drill press to hold the shafts and raised the motor on the drill table into the pre-drilled shafts.

If you can’t find a way to make perfectly center drilled shafts, you are not alone.
Here is what we did:
We used 1/4" aluminum tubing with about .172" ID.
Then, put tape over the holes in the motor and connect it to a power source and turn down the brass gear to ~.175" ID.
Wen you go to press on the shaft, you need to be cautious to get it lined up straight. Also, you will need to support the end of the motor AND the gear. We used a pair of steel rulers and a vice for this.
A note on wheel size:
The optimal size is different for each minibot. You will need to know your weight and estimate the amount of drag you have correctly. If you need help with the math, first try your team mentors and physics teachers. If they arent helpful, I’m not sure what they are doing, but nonetheless, then CD can help.

We have a ~1.8 second minibot that is very consistent, and we don’t have access to a lathe (due to some stupid crap… There are three lathes in the shop in which we work, but we aren’t allowed to use any of them due to silly school politics that six years of effort have not circumvented.)

We modified and kept the gearbox, but removed two stages for a total 5:1 reduction. We then added custom wheels that are machined to match the radius of curvature of the pole.

It is very consistent and pretty fast, but not as fast as, say the Thunder Chickens’ press-fit 1/4" (or maybe it’s 3/8") direct-drive minibot.

Obviously after the first 2 weeks of competition the secrets are all out.

If you’re going to use a design that is “heavily inspired” from one you saw on the field, at least take the time to iterate the design and figure out the details yourself.

You can make this into a positive design experience for your team with some methodical experimentation…

An alternative to press-fitting may be to make a shaft coupling from latex tubing. If you get the proper size, it seems it could stretch over the helical motor pinion. Perhaps a zip-tie would help secure it.

Wow. Harsh. We asked one specific question to confirm some of our preliminary calculations. The students never asked for anyone’s complete minibot design, nor do I anticipate that their final design will be “heavily inspired” by any one particular minibot that they saw perform at WPI. They simply felt that, after seeing all of the minibots in action, they wanted to try their hand at a direct-drive system to avoid the mechanical loss that naturally occurs through gearing. The drive train is but one aspect, obviously, of minibot design. As for the output diameter, the students have been experimenting with diameters ranging from 1/4" to 3/8". They simply wanted to hear what others’ experiences had been in order to confirm or dispel their own conclusions. I posted their question for them because I thought that that’s exactly what this forum was for.

For those who offered constructive advice on this thread, thank you!:slight_smile:

Let’s try to assume John was not intentionally implying anything negative in the way he responded. After all, he works for a robotics company that wants everyone to use their products- so why would he ever imply anything negative about any of his customers? I’ll have to assume we must be misreading anything negative.

I offer thoughts and encouragements in order to get teams to try things. Anyone who has tried to build a direct drive minibot knows that significant amounts of trials and experimentation are necessary to be successful.
Does every team need to reinvent the physics description on their own?

I think the minibot challenge this year dramatically increased the amount of time and energy that teams would spend on engineering.
I know that is true for our team. So overall, it is good for the primary goal of FIRST. It causes the most innovative teams to lose some of their advantage over the “incrementing” teams as the season wears on.
But to me, it’s a no-brainer which has better impact on getting more students working on engineering problems for longer periods of time.

For sure, experimenting is good! But I think one example of what John might have had in mind is has anyone taught the students how to read the motor curve for the Tetrix motor and do the calculations? It’s not that hard to do and can be very rewarding and inspiring for a student.


OK, so is ANYone allowed to use the lathe(s)? If yes, have HER (or him) drill the hole for you. end of story.

Not really. Just a reminder that copying is good, but learning and making it even better is, well, better. That’s Engineering.

Could you provide a bit more detail? What exactly didn’t work?


I can’t find it now but I made a post a long time ago on this forum (maybe 2004 or 2005) titled “Gear ratio doesn’t matter.” The point of that post was that gear ratio alone doesn’t matter, you have to take wheel diameter into consideration, too. In addition, weight has nothing to do with the optimal gear ratio either. Higher mass means that the wheel diameter for a given gear ratio (in this case a gear ratio of 1) must be smaller to generate enough force to counteract the weight (mass * g) and other acceleration of the mass. To simply show the relationship let’s use simple F = ma. F in this case is T/Rw (torque divided by wheel radius) and a is g + your desired acceleration at max power. As you decrease your wheel radius you will increase your force which is needed for higher mass.

The bottom line is that the ability to climb the pole at all really only has to do with the motors and mass (in its simplest form). Motors represent the max available power. You just have to find the right combination to lift the mass.

.175 OD, right?