HOT drill motors

FIRST General Forum
1

Posted by nuts4first.

Engineer on team #340, GRR - Greater Rochester Robotics, from Churchville-Chili and Nortel Networks.

Posted on 2/14/2000 3:54 PM MST

While testing our mobile platform, we were able to run it about 20 minutes before the drill motors reached a temperature at which they where hot to the touch. Now that we have added our subsystem (effectively adding more weight) to it, we now have lessened that time to about 5 minutes.

Our drive system uses a manufactured drill output shaft, which we are supporting with one pillow block at the far end of it… We are looking into adding an additional pilow block closer to the gearbox.

Should this be a concern? Hot much abuse can the motors take?

2

Posted by Michael Betts.

Engineer on team #177, Bobcat Robotics, from South Windsor High School and International Fuel Cells.

Posted on 2/14/2000 4:52 PM MST

In Reply to: HOT drill motors posted by nuts4first on 2/14/2000 3:54 PM MST:

I don’t have an absolute answer. My experience tells me that you may be in trouble. I have a lot of respect for what these motors can do and we have all pushed the envelope from time to time.

But… If you are hot to the touch with just 130 lbs in 5 mins, you will be really cooking after 2 minutes of hard accelerations, pushing and shoving.

  1. Removing the side loads is a must. Your second pillow block is a heavy but good idea.
  2. Take a hard look at sources of friction in your system.
  3. Consider backing off on your gear ratio. Since it sounds like you are a direct drive, smaller diameter wheels.
  4. There is a 12V muffin fan in the kit. You can make a wind tunnel.

Remember that you may only have a minute between rounds… Not much time to cool off.

Last thought. If your motors are getting hot, you are probably border line for tripping your circuit breakers. Once they start tripping, your problems grow by an order of magnitude. The CBs are thermal devices. They reset but don’t really cool off for a while. Once tripped, it does not take much effort to trip them again and again.

Good Luck!

Mike

3

Posted by Daniel.

Coach on team #483, BORG, from Berkeley High School and NASA Ames & UC Berkeley.

Posted on 2/14/2000 5:17 PM MST

In Reply to: Re: HOT drill motors posted by Michael Betts on 2/14/2000 4:52 PM MST:

I’m not exactly an engineer, but my gut is telling me that a second pillow block isn’t the answer. I suppose this would depend mostly on where the motor is being held. If you’re holding the motor in the front, then there’s essentially a pillow block built in. Why would a second one help? Seems like all you’d be doing would be increasing friction (though minimally of course). Although, maybe transfering the side load from a bushing to a bearing would dramatically decrease the friction? If that’s the case I worry about my team. We are supporting the end of the drill motor shaft with a ring of self lubricating plastic.

I think the biggest factor is the gear ratio and that’s most likely the cause of the problem. What is the gear ratio on your bot?

4

Posted by nuts4first.

Engineer on team #340, GRR - Greater Rochester Robotics, from Churchville-Chili and Nortel Networks.

Posted on 2/15/2000 7:04 AM MST

In Reply to: I’m no expert, but… posted by Daniel on 2/14/2000 5:17 PM MST:

Our drive system is like this…

4 wheel drive, using 3/8’ chain to power a forward and aft 6’ wheel. The drill is in the center powering a 15 tooth sprocket with two loops of chain going to each of the wheels. The sprockets on the wheels are 18 teeth. We have set the drill gear box to drive in low gear all the time.

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Posted by Dodd Stacy.

Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.

Posted on 2/15/2000 7:51 AM MST

In Reply to: Clearifications on our 'bot posted by nuts4first on 2/15/2000 7:04 AM MST:

: Our drive system is like this…

: 4 wheel drive, using 3/8’ chain to power a forward and aft 6’ wheel. The drill is in the center powering a 15 tooth sprocket with two loops of chain going to each of the wheels. The sprockets on the wheels are 18 teeth. We have set the drill gear box to drive in low gear all the time.

Nuts,

IMHO, you are NOT over-geared. I make your top speed around 6 ft/sec or slightly less, and that gearing should be ok, in terms of the current you draw in accelerating.

In terms of friction, binding, etc in your drive line, exercise extreme care when adding constraints like extra pillow blocks to align them so they don’t fight each other. It’s easy to cause more problems than you solve. You might try blocking your bot off the floor and running the drive lines as you loosen the various mounts for the shaft bearings and the motors. (Careful with fingers!) Listen for changes in speed as you relax the positional constraints. More importantly, follow this procedure as you snug, then tighten the mounting constraints back up. Maybe run your chains a tiny bit looser than you otherwise think you should.

Then go back to your original diagnosis. How hot is hot? What is very hot to the touch may be quite comfortable for the motor. Yes, the efficiency and power output suffer as the temp rises, but I think you’re worried about damage rather than performance. I gather your motors are exposed, so you can touch the housings? If so, you might periodically cool them during practise. Joe suggests the canned freon stuff. A very slightly damp (with water) sponge or rag on the housing is also good, as long as the water evaporates rather than runs in and shorts out your motor.

We don’t have much direct experience with how hot these motors run during ‘normal’ use in drilling/screwing. This is a great area for a FIRST ‘White Paper’ science project. Why don’t we DO this some time? Good luck, nuts. Let’s hear how you make out.

Dodd

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Posted by Greg Mills.

Engineer on team #16, Baxter Bomb Squad, from Mountain Home and Baxter Healthcare.

Posted on 2/15/2000 9:09 AM MST

In Reply to: Clearifications on our 'bot posted by nuts4first on 2/15/2000 7:04 AM MST:

:
The problem may be in the design - the four wheels that far out cause alot of sliding on the carpet to make a turn. It takes alot to slide the wheels sideways on the carpet. Every turn requires some degree of ‘sliding’ while a 180 or 360 really works the motors. I dare say that if you had room to run straight for five minutes that the motors would not heat up.

We used a similar drive design in maybe '96 or '97 and had the same experience. I would just be very aware and try to be prepared with a quick change motor arrangement and/or cooling between rounds. We didn’t have any failures but it was a point of concern. In the finals cooling time will become a problem, just be ready.

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Posted by Thomas A. Frank.

Engineer on team #121, The Islanders/Rhode Warrior, from Middletown (RI) High School and Naval Undersea Warfare Center.

Posted on 2/15/2000 1:14 PM MST

In Reply to: Clearifications on our 'bot posted by nuts4first on 2/15/2000 7:04 AM MST:

Hello;

It depends on what your definition of ‘hot’ is. If you reach in and touch the motors and say ‘gee, that’s hot, hey Joe, come here and feel this’, I would not be overly worried. If you reach in and touch the motor, scream and run to put your fingers into a glass of ice cold soda, you have a problem.

More specifically, if the motors are getting over about 130 deg F then I would be nervous; below that, I would’t be too concerned.

From your desciption of the drive (it sounds reasonable), I would be looking for excess friction. Is everything lined up properly? We’ve had trouble in the past with wood chassis warping and changing driveline geometry.

Also, keep in mind all the nice little fans FIRST provided. One blowing on each motor would be an ideal cooling approach. And if you make the air intakes accessible, you could have an externally powered fan that could provide cooling air to the motors between matches (I disagree with JJ here about the cool in a can approach…thermal shocks worry me a bit…but lots of forced air is a fine way to go).

Also, remember the format of the competition - you’ll have 2 minutes of run time, with at least a minute of down time (probably more like 2 or 3). So maybe you should consider running the machine in that manner to allow a little cooling time. Only practice is more demanding.

Tom Frank

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Posted by Ken Patton.   [PICTURE: SAME | NEW | HELP]

Engineer on team #65, The Huskie Brigade, from Pontiac Northern High School and GM Powertrain.

Posted on 2/16/2000 1:10 PM MST

In Reply to: How hot is hot? posted by Thomas A. Frank on 2/15/2000 1:14 PM MST:

I think you should listen to Tom here. I would have said exactly the same things. We are currently running a lower numerical gear ratio than you are, and are not experiencing ‘burn your fingers’ temperatures during longer drives. The motors do get hot, but not blazing hot. We know we have very low powertrain friction (imagine that from Powertrain engineers :)), so I would look for friction in your system.

I’d stay away from the thermal shock in a can… just my opinion, no data to back it up.

Ken

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