53 replies to this topic

[tazman]

I tried to run my recently broken in Hawk 7 and I was upset to say the least. Broken in at 4 volts for 15 mins. and sounded real good. Clean up and installed the gears and tires - tried it out today and it was a dog!

 

After close examination I noticed that the comm was wrapped with twine like an HO hot stock arm. The spot that was tied was pointed up towards the can hole.

 

What is the purpose for this? I noticed that the worn motors I had did not have this.

 

Has anybody had the same problem? Insights?

 

[Ramcatlarry]

Try an 11t 64p pinion. 

 

The tie is to keep the wires from being thrown off... as most modern motors need to be when they exceed 40K RPM.

[Racer36]

Is that a 6 tooth pinion on a Hawk 7? Are you racing around the inside of a teacup?

[JK Products]

Hi Rick,

 

Larry is correct, that is what is called a "tied comm" - a wrapping around the wires on the commutator so they can better handle the high RPM and not "throw a wire." They are standard on most modern motors. 

 

Secondly, I think you might have the motor very poorly geared and this could very well account for what you are calling "dog" performance. That looks like about a 7 tooth pinion which would not be appropriate for these type of motors. As Larry again correctly indicated, an 11 tooth pinion is much more common, maybe along with a 37 tooth gear or thereabouts. Please ask at your local track what people usually run. 

 

Tim

[havlicek]

  When I first saw this, I thought I just wasn't getting what the problem is... like it was just me.  

[gotboostedvr6]

I used to gear Hawk 7 motors the following:

12:38 / .710" tire
11:36 / .710"
11:35 / .700"
14:43 / .710"
13:40 / .700"
13:44 / .760"
13:42 / .740"

If by chance you actually have a Retro Hawk 7 drop 1 or 2 teeth on the spur.

[havlicek]

Larry is correct, that is what is called a "tied comm" - a wrapping around the wires on the commutator so they can better handle the high rpm's and not "throw a wire". They are standard on most modern motors.

 

In other words, that "twine" is a good thing, rather than a bad thing!

[dalek]

Recently, I bought three Hawk 7s. Two of them behaved as follows:  

 

On my power supply, I started at 2 volts, and increased the voltage.  

 

From about 8 volts to about 9 or 10 volts, the motor RPM didn't increase much, but the amps draw did (up to about 1.3 amps).  

 

As I continued to increase the voltage, suddenly the RPM would jump up and the amps drop off.

 

At 12 volts, the normal current draw for a Hawk 7 seems to be about .75 amps.

 

******************

However, the third motor wouldn't make that leap to the higher RPM. This was the case, both before and after the brushes were fully seated.  

 

By the time I had increased the voltage to 12 volts, the motor was pulling about 1.7 amps and quickly getting hot (holding it between my fingers) but the RPM was only about what a good Hawk 7 would be turning at about 9 volts.

 

As it turned out, that third motor was a dog when installed in a car.

 

******************

 

If you test your bad motor as described above, I'd be interested to know what the results were.

[MSwiss]

Dale King,

 

You shouldn't be running your motors without a load at 12v.

[tonyp]

Absolutely.

[dalek]

The motors are 12 volt rated and I've never seen any info from a manufacturer that says they will be damaged if run at 12 volts with no load.
 
One of those two "good" motors (that I mentioned above) was in my JK Indy car this past Saturday at the Florida Slot Car Series races.  
 
If I hadn't missed 2-1/2 heats reparing a broken guide wire, I would have finished second or first (based on my lap times in the heats that I ran).
 
Apparently, that motor didn't mind having been run at 12 volts with no load.

[gotboostedvr6]

IMO 4 or 5 volts max free spinning no load.

You burn the trailing edge of both the brushes and the comm segments with a low load/high RPM situation.

[DOCinCanton]

FWIW: I never run my slot car motors above 6v without a load. With Group 12 and Hawk 6 motors with the "big dog arm," I never went about 5v. If you ever reach 40,000 RPM, it is time to shut it down.

[havlicek]

The motors are 12 volt rated and I've never seen any info from a manufacturer that says they will be damaged if run at 12 volts with no load.
 
One of those two "good" motors (that I mentioned above) was in my JK Indy car this past Saturday at the Florida Slot Car Series races.  
 
If I hadn't missed 2 1/2 heats reparing a broken guide wire, I would have finished 2nd or 1st (based on my lap times in the heats that I ran).
 
Apparently, that motor didn't mind having been run at 12 volts with no load.

Here's the thing here (and yes, running a motor at more than low volts with no load = no bueno).  

If a motor is rated at 12VDC, that doesn't mean it is rated to run at that voltage with no load. Why?, well for one thing, a motor running at 12VDC will be spinning faster than it would installed in a car. Motor ratings are something of an average guess. Manufacturers look at an average life-span of a motor running at an "average" voltage and then come up with their definition of what is an acceptable life span.  In use, these motors don't run full punch all the time. If they did, they'd fail much faster. 

On whatever else is going on here, all motors are never equal. Inexpensive motors do amazingly well all things considered, but there will always be standouts and others that seem to fall further back in the pack.  It does seem though that some of this is due to the end-user (I'm not saying this is the case here)... bad gear choices, poor set-up, harmful "break-in" procedures all come into play as well as just the "chance" involved in mass-production.

[Racer36]

Any device designed to do work (motor or engine) should never be run without load. Try running your gas engine at redline RPM without load. Stand back a touch so none of the flying innards hit you.

[Gator Bob]

IMO 4 or 5 volts max free spinning no load.

You burn the trailing edge of both the brushes and the comm segments with a low load/high RPM situation.

 

If the brushes are installed 90 deg. off of what used to be 'normal', how do you even tell?

 

"Will the real trailing edge please stand up"

[MSwiss]

John explained it well, but I'll add a few things.

I don't see any spec sheets included with these motors, let alone any that state what they are "12v rated".

Any references to voltage, either state the break-in voltage, which would be no load, and IIRC, it never exceeds 5 or 6v.

Any other references to voltage also refer to a gear ratio.

IOW, these motors are marketed as slot racing motors, not free-spinning noisemakers.

Just because your 12v free spinning didn't damage that particular motor, that doesn't mean it's a good idea.

There is zero point to it, because any amp readings you get are pointless pieces of data, because in real racing, the motors never run at no load, at 12v, of course, unless you spin a pinion, or a crown gear.

[Gator Bob]

Most Mabuchi FK motors are 'rated' at 3 - 6 volts.

That's why they run so good but don't last at 12-14v.
 
They sell 6 volt motors rated at 20 something thousand RPM. If you run it up to 12+ volts and you have a 40 something thousand RPM motor.
 
"Breaking them in" might find more 'speed' but it definitely will reduce lifespan.
 
Seems straightforward to me.

[Fast Freddie]

Just a small point of interest. 

I'd like to know just how much heat it took to attach those wires to the motor. You are aware that excessive heat can weaken the springs in those motors and that could very well be why it ran like a dog.

[tazman]

I appreciate all the insights you guys shared and I forgot to mention two things.

 

One, the gears are 72 pitch 9/43 and second the tires are .700" JK tires. This is the original set-p for this car as I was doing a replacement - it was plenty quick before the original motor ran out of motor brushes. I cut the excessive twine sticking upwards on the comm in the new motor and it sounded better but, it still ran like a dog! I am convinced that this motor was a dog to begin with and nothing would change that!

 

On to a new motor!

[dalek]

I don't see any spec sheets included with these motors, let alone any that state what they are "12v rated".

 

I assume that the motors are rated at 12 volts because on the JK Products web pages, it used to state that the Hawk 7 was a 48,000 RPM motor.

 

From measurements I've made in the past, using my motor test stand with optical tach, I was seeing motor speeds in that range while testing with the smallest prop I have.

[MSwiss]

You went from "The motors are 12 volt rated" to "I assume that the motors are rated at 12 volts."
 
Where does 48,000 RPM imply it's "rated" at 12v?
 
If a slot car motor would be "rated," it just stands to reason to reason it would be for the application the motor is attended to be used, which in this case is with a load on it.
 
Here are the back of the cards of the two different JK motors I have in stock. (I sold the last of my Hawk 7s on Friday.)
 
It's pretty clear that the 12-12.5v and 12-12.8v refers to the track voltage while racing.
 
And also, it only OK, free revving, up to 5v during break-in.
 
There isn't any implication that it's OK to free-rev to 12v, for who knows what non-real slot car world reason.
 
These motors might withstand 12v free-revving, partly because they are fairly mild winds and are amazingly well-made for a cheap motor, but as discussed, it's pointless to do it.
 
And free-revving a powerful wing car, or Eurosport motor, to 12v, is a safety hazard, with a chance of a lamination, or laminations, flying out of the motor, at the point of a big balance hole.
 

[Ramcatlarry]

8-9/43 is gearing we used for built Grp 12 C-cans good for 100K RPM for the ISRA Worlds production cars. Still need a bigger pinion.

[Kevin Donovan]

FWIW, the last two Hawk 7 Motors I bought were “dogs,” too. I’m really tired of these things.

[havlicek]

This all brings to mind something I've wondered about. 

 

Since these motors use a "folded and welded" type com connection (as opposed to a flat tab), is the tying even necessary at all with such a relatively mild wind? I mean, besides (I guess) maybe seeming more secure, there's also a "possible" downside. A simple knotted piece of "string" (and these aren't... can't be... a kind of tying that requires a lot of attention because of cost) around the wires could also cause an itty-bitty "imbalance" as well as some turbulence... I guess.

So I wonder if it's done just to make people feel better. Has anyone taken a bunch of these arms, removed the string from half and then run them all to destruction and tabulated the results? I wouldn't be surprised if the tied ones weren't any less prone to failure than the untied ones.