Motor that can be dropped off a building.
 

Has anyone ever run across a motor that is about the same size as the banebots 28mm 20:1 gearmotor that can basically be dropped off of a cliff and still work? I have been looking around at Maxon, MicroMo, and a couple others but I havn't found anything that seems extraordinarily satifying.
I need a gearmotor that can fit in a 1.25"(32mm) space and can't be longer than 2.9" (72mm) not including the shaft. Price isn't really an issue for this project either. Im going to be running this rig on 16.5 volts (the new A123 Lithium cells are amazing by the way) so I'll probably be overvolting a 12 volt motor.
I just really don't trust those banebot transmissions anymore.
Thanks

Re: Motor that can be dropped off a building.
 

How about this. Since price isn't an issue you could buy a couple of the banebot units and try them out and see if in fact they will work.

I doubt anyone wants to guarantee that their units are cliff-proof but it would be fun science.

Re: Motor that can be dropped off a building.
 

The whole reason I ask is because I am already using those banebots transmission and they just cant take the stress i'm putting them under.
I guess I forgot to mention that part.

I'm not actually dropping them off cliffs either, Just the back of our 7 story tall football stadium. But only once. It is all for my 15lb battlebot. I figure, if I can drop it 7 stories onto concrete and nothing breaks (except some concrete) it should be able to handle anything battlebots can throw at it.

Re: Motor that can be dropped off a building.
 

next question:

I don't know how to calculate it, but if I did, I'd do an analysis to see if the stress anticipated can even be met with the designs and materials in the unit.

If it doesn't pass that then that would be a real strong indicator.

I tell my students that one of the things that differentiates engineers from gadget builders is the ability to predict what is gonna happen before you even try.

But I'm a EE so that makes me a gadget builder on this topic......

Re: Motor that can be dropped off a building.
 

I've actually modeled up the whole motor gearbox and did some FEA analysis of it in solidworks cosmos. I had to gues what alloy of aluminium the ring gear was but other than that it is dead on accurate. I came to the conclusion that it wasn't strong enough because there is a gear carrying plate that binds and rubs with the other stage of the gearbox ( i think this is why they stopped selling the 3 stage versions of this gearbox) that eventually tears itself apart. The FEA analysis indicated this problem and so I opened up one of the motors that had some running time under its belt and low and behold, stress fractures and deformation already starting at one end of the plate. I slow the process down with a pillow block supporting the opposite side of the shaft but it is still like a ticking time bomb. To tell you the truth, I wouldn't expect anything better at the price they are sold at.

Unfortunatley, Knowing what the problem is only half the battle. I don't have the capabilities to machine a gearbox inhouse or the funds to outsource it and so I am stuck finding an off the shelf part that fits my needs.

Re: Motor that can be dropped off a building.
 

In the combat robot community people have found *some* rather decent drills with gearboxes that are pretty robust; finding one that fits in 1.25" may be an issue, but you have to loosen up your criteria.

I wonder if 7 stories are really necessary, my now dissembled 12 could take a 3 story drop and hits from dead blow hammer to all but the top and bottom.... but I'm not sure it could survive a 7 story drop. If you run the numbers on it, a 7 story drop is a lot of energy, significantly moreso than a 2-3 story, and what kind of forces you would see I imagine (Although, you should probably compare the amount of Kinetic Energy some of the more powerful spinners can unleash to this).

I also imagine that in a drop test such as that, if the gearbox shafts are well supported, it is unlikely the drop that is killing the gearboxes, it is probably something else. Also, the banebots gearboxes really do require shaft support on the other side, but on top of that the shaft support must both be rigid and aligned. If it isn't aligned nicely, it cause premature failure, and if it isn't rigid enough it could allow forces to misalign the shaft and also cause premature failure. What may be potentially good news is banebots is redesigning the gearboxes to be stronger and more robust; Not sure if they'll be available within your time limit.

Re: Motor that can be dropped off a building.
 

Matt,
I would think the seven story drop would far exceed anything you could encounter in the ring. Globe Motor has all kinds of devices, one of which might fit your needs since price is no object.

Re: Motor that can be dropped off a building.
 

OK, since there aren't any numbers to quantify this level of abuse I'll chip them in.

Assuming a very conservative 3 meters/story, 7 stories is a 21 meter drop.

Looking up the relevant information on-line I find the following equation useful, X - Xo = Vot + .5at^2
Assuming that the item is dropped an not thrown we can set Vo=0
If we use the drop height as a zero reference we can also set Xo=0
This simplifies the eqn to X=.5at^2

Assuming that we are coducting this test on earth, not mars, we can use an acceleration, a, due to gravity of 9.81 m/s. X=21 meters from the drop above.

Solving the eqn for 't' yields t=sqrt(X/(.5a)) I'm neglecting the time travel required to make the negative root useful. A quick plug and chug gives ~2.07 seconds.

So how fast is that at impact?
Going back to the web I can pull V = Vo + at from the same page.
Again Vo=0. Plug and chug... V=at=20.3 m/s, or 66.6 fps

So what is the force at impact?
F=ma
m= 15 lbm as stated previously in the thread, or 6.8 kg for my metric calculations.

What about 'a'? Being optimistic, let's assume that the concrete is able to decelerate the item along a constant acceleration profile; it keeps the math simple. Realize that this is a best case scenario.

Pulling another eqn from that page (note: bookmark these kinematic equations, you'll use them over, and over, and over.)
V^2 = Vo^2 + 2a(X - Xo) Simplifing based upon the stuff we set to zero... V^2 = 2aX
Solving for a... a=V^2/(2x)

Now I don't know how much the concrete will compress, x, but since it is not known as a squishy material I'm going to guess, 'not much'. I'll try three values to get a trend. 3mm, 2mm, 1mm i'd guess it is much less than any of those.

So at 3mm of compression we have an acceleration of 68.7 m/s and a force of 467 N or 105 lbf
at 2mm -> 103 m/s -> 700 N or 157 lbf
1mm -> 206 m/s -> 1402 N or 315 lbf

Keep in mind, that load is going into the point on your bot that first contacts the concrete.

Another way to put this in perspective is to look at the energy involved.
The kinetic energy of the bot at impact, KE = .5mV^2
Based upon our previous calculations the energy at impact would then be 1402 Joules. If the impact takes a full second it is like getting wacked by a 1400 Watt device... Power=Energy/Time If it is faster, say 0.1 seconds it is like getting wacked by a 14000 Watt device (18.7 HP).

Now I'm not saying what is appropriate for your testing, but understanding what you are asking is probably useful in interpreting your results. I'd hate to destroy my bot during excessive testing and never get it onto the playing field. I'd also hate to not test it hard enough and get destroyed in it's first match.

I think I've left enough of a trail to create a spreadsheet out of this so that you can modify the assumptions to fit your needs. I can always be PM'd for assistance.

Re: Motor that can be dropped off a building.
 

Numbers? Quantify?
Ok Mark... you get a cookie too.

Re: Motor that can be dropped off a building.
 

Before dropping your bot, try dropping something you're sure won't break - I dunno, like a 2" x 3 foot steel pipe, or a pillow block. See what happens to it (you may be surprised) and then decide if your specs are a bit too tight.

Don

Re: Motor that can be dropped off a building.
 

I make what is coined a full body spinner. For anyone not familiar with the world of battlebots, a full body spinner resembles a hockeypuck in apearance. A full body spinner has a spinning outside shell with sharp blades on it with a stationary inner chassis that has the drivetrain and spin motors as well as all the other robot essentials on it.

The reason I picked a seven story drop is because I make two assumtions. The first is that the most energy my robot will ever have to contend with is a little more than twice the amount I can dish out. The main component I am really examining with the seven story drop is the center shaft that the shell rotates on and the plates that this are bolted to. The worst thing that could happen to me in battle is my center shaft bending or the plates that support the center shaft being tweaked so the center shaft iss no longer on the vertical axis.

The reason I want to replace the banebots motors is not so much related to the drop test because they are mounted securley enough in the chassis to survive that. My previous drop tests have proven that. It is related to the FEA analysis and the real world results I was getting with them last year. They would start binding in as little as one fight. Also, just a note, I do support the other side of the shaft very well.

Re: Motor that can be dropped off a building.
 

just to be really clear.......

I'd bet any old transmission can survive the drop....

It is the 'undropping' process that is the real problem. :)

Re: Motor that can be dropped off a building.
 

Seven stories is a bit extreme, and way more than anything you'd ever see in combat robotics. Negating air resistance, any object through off a seven story building will reach a speed of about 20 meters per second (or about 45 miles per hour) when it hits the ground.

If you want to do some simple physics equations, you can calculate the exact final speed at impact from a seven story height, as well as how much kinetic energy the robot would have when it hits the ground. Compare this to the kinetic energy that the spinner would have at full speed, and you may find that a one or two story drop would probably* be a lot more realistic (and easier to design for).

* Just a swag. Do the math for an exact answer. There is great truth and wisdom to be held in the numbers.

Re: Motor that can be dropped off a building.
 

Have you looked at cordless drills? I have no personal experience, but a higher quality cordless drill is designed to take a beating. (Consider their normal operating environment on a construction site.)

Re: Motor that can be dropped off a building.
 

A 7-story drop is rather excessive. It doesn't properly simulate the battle field in my opinion. Have you tried reinforcing the motors/gearboxes? I would think that some soft plastic around them would absorb some shock forces.

-Vivek

Re: Motor that can be dropped off a building.
 

I'm assuming this means that you mount the spinning outer shell of the robot directly on to the output shaft of the gearbox? (A photo or CAD image would be helpful) If so, I can see why the gearbox would be prone to failure.

Might I suggest building a custom shaft to support the spinning part of the robot ( you could make it out of 1" diameter solid titanium if you wanted) and weld that in to your frame, then use a pulley from the gearbox output shaft to spin the outer shell? That way (espeically if you build a bit of slip at high impact in to the belt) your gearbox output shaft will not be supporting the load... or impact... faced by the spinning outer shell.

Jason

Re: Motor that can be dropped off a building.
 

yes the drop aint the problem it's that whole slowing down thing

Re: Motor that can be dropped off a building.
 

I don't have the luxury of being able to design for anything less. My robots's spinning weapon strikes the opponent at 258mph maximum. that is more than my robots terminal velocity. This means I could drop the thing from 12 strories and it still wouldn't make a bit of difference.


I Have looked quite extensivley into them and they just wont fit.

I don't support my spinning shaft off of the gearbox. I already do something similar to what you describe. However, the motors for the spinning part are not the ones being questioned.

As far as a CAD image is concerned. here yah go. What that is am image of is the bottom of the robot with no bottom plate or spinning weapon visible.

Yes indead, so are you suggesting installing an airbrake on the robot? ;)

Re: Motor that can be dropped off a building.
 

have you actually tested the motors themselves in actual fights yet?

I think your problem may just be the gearbox being too weak. I'm not sure though.

You could always make your own gearbox :P.

EDIT: This whole concern is about your gearbox shaft getting damaged right? Have you considered using a racheting mechanism so that when it rapidly decelerates, it won't harm the gearbox shaft?

Re: Motor that can be dropped off a building.
 

yeah. I used the motors all last year in fights and even after one good fight you can already see the signs of them breaking.