CIM Voltage and Motor Suggestions

[jakep]

Hey Kevin, WOW thanks for that awesome spreadsheet! And it's got those numbers and specifications we all love!

For those Dewalt motors/gearboxes:

What do you mean by "the parts you'll need will run you from $100-$150 per motor. Depending on how much case you want around it." ?

I was looking at the ServiceNet link you gave me, the "motor pack" part number is 389010-00 and the transmission part number is 388974-00. Buying one of each costs around $90. For our project, we have a protective aluminum shield around the underside of our robot. (Where the motors, electronics, batteries, etc. are mounted) So, I don't think we need much case around the motors/transmission. Would we be set if we just bought the motor pack and transmission?

[KenWittlief]

One of the limiting factors of electric motor rpm is the centrifugal force on the wires, armature, commutator...

With no load you can keep increasing the voltage, and the motor will spin faster, until you reach the point where the motor rips itself apart.

I think that is the real issue you will face.

Heat dissapation is the second. You can push the HP rating of the motor as long as you back off on the duty cycle, to give it time to cool between application of full power. This would more of less be a matter of trail and error. The failure mode for a motor that overheats is usually the wires melting/unsoldering themselves from the commutator, which is not easy to do since most motors are soldered with silver solder, and the wire is hooked or twisted to give it extra mechanical strength.

right in the same range of abuse, the varnish on the wires will start to cook off from the heat, making a wonderfull pungent scent that all FIRST teams have experienced at some point. If the varnish burns through and wire shorts to wire, then the motor turns into a space heater, and you get those wonderfull clouds of white smoke spewing like Mt Saint Helens.

[Kevin Sevcik]

Originally Posted by jakep

Hey Kevin, WOW thanks for that awesome spreadsheet! And it's got those numbers and specifications we all love!

For those Dewalt motors/gearboxes:

What do you mean by "the parts you'll need will run you from $100-$150 per motor. Depending on how much case you want around it." ?

I was looking at the ServiceNet link you gave me, the "motor pack" part number is 389010-00 and the transmission part number is 388974-00. Buying one of each costs around $90. For our project, we have a protective aluminum shield around the underside of our robot. (Where the motors, electronics, batteries, etc. are mounted) So, I don't think we need much case around the motors/transmission. Would we be set if we just bought the motor pack and transmission?

Have you looked at the Nothing But Dewalts white paper? You can see a bit of how complicated these Dewalt trannies can be. I suspect you need more than just the motor and tranny to make that work. Specifically, I think you need some of the plastic case parts to screw things into and other parts to hook up a shaft to.

So, I think you also need parts 17, 18, 19, 38, and maybe 24. 18, the gear case, would hold everything together. 38 is a nice round-ish spindle to use for a shaft. 17 and 19 almost certainly engage the spindle with what's probably an odd looking asymmetrical output on the tranny itself. 24 is a spring that probably helps the clutch work, but you can probably simply weld/glue and 19 together instead.

EDIT: oh. and 29. A nice left-hand screw to go into the spindle. And 22,to hold the spindle in. So all that works out to $125. I'd personally want the case as well to give it more support and some of the screws for it. That works out to $154 or so.

[jakep]

Thanks for the quick reply! ,

I looked through that whitepaper briefly, but it looks as if it's meant more for CIMs and FIRST motors hooked up to dewalt gearboxes. If we are using everything dewalt, won't it be easier? Also, would there be much machining to do? We don't exactly have easy access to a machine shop

"I'd personally want the case as well to give it more support and some of the screws for it. That works out to $154 or so." - The case is part number 1 right, but what about the other screws?

EDIT: Nevermind, I think the screws are part 6, correct?

[sanddrag]

Is there a specific reason you just can't use 4 CIMs (at 12V) to power this project?

[jakep]

There is no easy way to mount 4 CIMS with our gearboxes. Doing so would require redoing the gearboxes completely, so I am searching for perhaps something better.

Edit: Kevin, sorry for my lack of knowledge in this subject, but the only machining required would be to attach something to the output of the gearbox, right? I know on regular drills you unscrew the cap, place a drill bit or something inside, then screw it tight. I'm guessing that strategy wouldn't work for us though, right?

[Andy A.]

The CIM motors are sealed units, meaning that the motor can has no venting and the armature (assumingly) has no fan on it like you might find on a Fisher Price motor. This means that all the heat generated by the motor has to migrate out through the motor can, either by radiation or through the bearings.

Neither is efficient or fast, and I suspect that the heat generated is going to fry the armature long before cooling the can with heat sinks and fans will do you any good. The CIM motors get pretty toasty as it is at 12 volts. As was stated, at double the power output, the motor will need to dissipate 1200 watts. To put that in perspective, an electric space heater I own has a maximum of 1500 watts.

You might consider removing the end cap and drilling out some holes in it. Then place a muffin fan behind the motor blowing into the now perforated end cap. The added airflow might help keep the motor cool while running. It will certainly help speed up cooling it in between run times. Just how much of an improvement is might make is difficult to say, but some air flow is better then none.

Modifying motor parts, especially removing metal, can be a bit hazardous to the motor's health, and I wouldn't recommend that you try it unless you are confident you can do so with out risking damage to the motor.

[Kevin Sevcik]

With some further info from jakep, this isn't necessarily a continuous use application. Atleast, it won't be operating at max power for more than a few seconds. It would hit peak power and past for a bit and then level off and operate somewhere below that. So it might not destroy itself too quickly. Perforating both endcaps and using a muffin fan might be enough to handle the extra heat. Certainly it's a cheaper easier option than the re-engineering and building that'd be necessary to use those dewalts. Though the dewalts would work exceptionally at this and I'd be highly amused to see a pair of hammerdrills in this application.

[jakep]

Hmmm... See the problem is that if we upgraded the voltage we were using with the CIMs, we would have to re engineer our gearboxes with a different gear ratio so that extra speed would translate into more torque. Since we need to redo the gearboxes either way, those Dewalt ones are looking awfully nice

The one problem at this point seems to be attaching the output shaft on those to our wheel, which has a key slot-thing already cut inside.

Any ideas? I can get some dimensions for you guys if that would help.

[Al Skierkiewicz]

Very interesting discussion so far...
I would like to add that the output power of a motor is not only a function of input current but also of the available field in the magnet structure. At some point, the magnetic field developed by the electrical structure may exceed the permanent magnets. At this point, no additional speed or power will be available and all excess electrical input is turned to heat. Without some testing at these extremes, we can not determine a point at which this will occur. Please remember that the resistance of the motor windings and brushes are also affected by heat which will change performance. Unfortunately, as nice as this motor is for a two minute match, it is not designed for anything approaching continuos duty, included are heat dissipation, brush life and bearings.

[jakep]

'Unfortunately, as nice as this motor is for a two minute match, it is not designed for anything approaching continuos duty, included are heat dissipation, brush life and bearings."

Which motor are you talking about, the CIM or the recommended Dewalt?

[Al Skierkiewicz]

Originally Posted by jakep

'Unfortunately, as nice as this motor is for a two minute match, it is not designed for anything approaching continuos duty, included are heat dissipation, brush life and bearings."

Which motor are you talking about, the CIM or the recommended Dewalt?

Chalupa, of course.

[jakep]

Hey Kevin, or other mechanically inclined person,

Could we use one of these (referenced fromt he NBD whitepaper) as a motor mount with the 24V Hammerdrill motors?

http://www.chiefdelphi.com/media/img...e988043e_l.jpg

[sanddrag]

Originally Posted by jakep

There is no easy way to mount 4 CIMS with our gearboxes. Doing so would require redoing the gearboxes completely, so I am searching for perhaps something better.

Edit: Kevin, sorry for my lack of knowledge in this subject, but the only machining required would be to attach something to the output of the gearbox, right? I know on regular drills you unscrew the cap, place a drill bit or something inside, then screw it tight. I'm guessing that strategy wouldn't work for us though, right?

What are your gearboxes? I thought you wanted to buy Dewalts but this makes it sound like you have something already. Do you have something already?

Using a stock drill chuck probably wouldn't work because you couldn't get it tight enough. It would slip. To adapt a drill motor to a robot, you could take off the chuck by removing the left hand screw you'll whole chuck will unscrew from the gearbox output shaft. You'll be left with a threaded shaft. On the lower voltage XRPs, it is 1/2-20. I'm not sure if it is the same on the 24V model. So then, you just use a tap (preferably set up in a lathe or a mill) to put internal threads in the bore of your sprocket/gear/flex coupling/etc so you can screw it on the gearbox output shaft. Then you put the left handed screw back in the gearbox output shaft with a nice thick washer and that keeps the sprocket/gear/etc from unscrewing itself. It is all explained in the NBD whitepaper.

[jakep]

sanddrag, I was talking about the 24V hammerdrill motors that Kevin Sevcik was suggesting. We would be buying the Dewalt motor pack, transmission and some other Misc. parts from the DewaltServiceNet. The part we are trying to work out now is a bearing system or something to handle the side loads. Also, we will be doing a shaft drive system, no chains.