Space Kart - Project Updates

The concern is that we don’t want to put the load of the chain and sprocket on the internal motor bearings as they were not designed for such a load (although we will probably fry something else before the bearings become a concern)

It would also be good to have the option of attaching either a centrifugal clutch or plain sprocket to the motor. Here is a not to scale drawing of what I had in mind, using a clutch might make things interesting as all the examples I have seen put the clutch on the end of the shaft and use a bolt into a tapped hole, so that might require the clutch to be on the end of the intermediate shaft and the two bearings being inboard of that

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Okay, I understand now. For that arrangement, I would suggest a simple lathe turned coupler with set-screw arrangements.

The motor shaft, and the intermediate shaft would need to have flats filed/milled to prevent the burs I mentioned above.

In the future if you need a clutch, the couple could be replaced with a clutch to perform the same function. Only real issue with this approach is alignment, but I would suggest handling that by having some adjustment in the motor mount.

I see much better now. So does this not have the hub pictured?
If the hub is there, easy peasy lemon squeesy flat disk thick enough to clear the motor shaft with holes to mount it to the hub welded to the jack shaft…

I am guessing Walter is right&this motor does not have the hub. In which case your set screw will be your weak point and as Walter said, gotta put a flat side on there. I don’t think the manufacturer built this with the idea that the visible shaft would power anything. It’s too small for the power. …

Our motor has the hub as shown. If you go to the dropbox and look for a file called “sprocket adapter” you’ll find a cylinder with shaft and bolt holes that can be resized to fit the motor. If you look closely t the pict where the motor is mounted on the gokart, you can see the bolt heads holding the belt drive gear to the hub. We could build a shaft adapter to interface the the motor hub to a keyed/splined shaft. Mount everything in a U or rectangular bracket so the motor shaft isn’t leveraged. Set this up so that the keyed/splined shaft floats inside the adapter - no need for set screws. A set of thrust bearings might be needed to keep the gear shaft lined up.

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Sorry to be thick, but I do not think I have any idea what “the Dropbox” is. Is that a DMS resource, or specific to the kart project…?

Aparrently we have many resources.available through DMS I don’t know about. Just last week I learned there are printers to which one can map. I still do not know the password for the member WiFi (not that worried about it). Who knows what other member entitlements I’m missing!?

So anyway… not sure what you mean by “the Dropbox” but that seems promising from your description.

The dropbox he is referring to is a personal file storage shared by Frank with the go kart class.

OK.
So, yeah, having not been able to be a part of the class, I have no knowledge of that, which makes perfect sense.

I am making a 40 amp discharge light rig to make it faster to store the lipos if they haven’t been fully used, say because we blow up the gokart motor for example. (The charger only discharges at .5 amps)

My question is, is it excessive to put 40 amps through a 12-10 awg crimp on ring terminal? I couldn’t find any specs on what is recommended. The setup is 3 12v 20w g4 bulbs in series times 12 in parallel, 36 bulbs total (~13k lumens…)

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We got in our 40" wide 1" rear axle, wheel hubs, sprocket, bearings.

Looking at this much mass that has to start spinning from 0, I will be amazed if we dont have to go with a centrifugal clutch so that the motor can spin up to a thousand rpm before engaging.

Also after measuring the motor hub, turns out the bolts are farther out than the 3/4" shaft, so all we need is a collar like the following to coulpe the intermediate shaft to the motor, and the only modifications to the intermediate shaft would be boring a 10mm hole in the middle so it slides over the motor’s silver shaft.

Would anyone like to 3d print the above collar and see if my measurements line up with the motor hub? I wonder if a really tall 100% density print would work on the actual kart… All of the torque would be going through the contact surface of the 3/16" rectangular key, anyone know what psi abs plastic can withstand without deforming?

$fn = 100;

$boltPatternRadius = 28.575 / 2; // 1.125 inches
$boltRadius = 5 / 2;
$shaftRadius = 19.05 / 2; // .75 inches
$collarRadius = 38.1 / 2; // 1.5 inches
$keyWayWidth = 4.7625; // 3/16 inches
$height = 2;

difference() {
cylinder(r = $collarRadius, h= $height);

// All of the negative space
union() {

  // bolt holes
  for(angle = [0:90:270]) {
  	rotate([0,0,angle])
  	translate([$boltPatternRadius, 0, 0])
  	cylinder(r = $boltRadius, h=$height);
  }
  // key way
  rotate([0,0,45])
  translate([$shaftRadius, 0, $height/2])
  cube(size=[$keyWayWidth, $keyWayWidth, $height], center=true);
  // 3/4" shaft
  cylinder(r = $shaftRadius, h= $height);	

}
}

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Someone let me know if the following analysis sounds plausible at all

From what I could find the compressive strength of ABS plastic is 65 Mpa or ~9,500 psi, lets say 5,000 psi for a printed part.

The radius arm of the key is 0.375 inches, and the surface area is 3/32 wide * 1 inch tall = 0.09375 square inches, so to find the max torque an abs coupler could be expected to survive would be 5000 = x / (0.03125 * 0.09375), so as long as the motor is putting out less than 14.6 foot pounds of torque a plastic coupler might work. Now we just need to rig up our electronic torque gauge and measure the power of the motor from a stop, I’m not sure what to expect

Am I understanding properly this is 2" thick?

Should be easy enough to do in aluminum, too, in the HAAS, except the keyway, if you find the plastic won’t hold up. Heck, this should be fairly easy in steel, even by hand. Again, the keyway seems the hardest…

My physics skills are far too rusty to refute or confirm your calculations, but I do wonder if the parts will get too hot… :smiley:

In regards to your electrical question, I’d point out that the NEC requires 12ga wiring for 20A circuits and IIRC 10ga for 30A. I’d say you’re probably safe, but make sure those crimps are good and tight, and monitor the temperature of things the first time you use it.

Using 12AWG is fine for short distances like in this case. Not sure about the crimp connector though. Be careful when discharging the batteries. Most loads intended for LiPo batteries have an automatic low voltage cutoff. If the bulbs are connected directly to the batteries it would be easy to damage the batteries. Good thinking though, could take all day to discharge at half an amp!

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Got in the remaining components so we can mock up the full drive train this weekend, we are getting closer to finding out 1) Is a centrifugal clutch on an electric motor crazy? 2) What will fail first, the motor, the electronics, or the not at all rated for high speed bearings I got for the intermediate / jack shaft

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We made lots of progress tonight, got the intermediate shaft cut down and bored out on both ends

And the 3d printed coupler looks plausible

But I think due to a combination of my knocking it off the printbed with a heavy object before it fully cooled and overzealous torquing of the bolts I managed to skew the whole coupler, enough that I didn’t even try spinning up the motor. At least I’m hoping that it’s not due to the 3d printer itself, about to try again on lefty.

Edit - Nope, turns out the lefty printer was not printing squarely

Also the light discharge works great, although I would recommend not looking directly at it (welding goggles help) and to have plenty of ventilation to keep the lights cool. It’s important to always have the low voltage alarms plugged into the balance leads, stopping at ~3.65 volts under load seemed to result in ~3.85 volts per cell, and only a quick balance on charger storage mode and they are ready for storage. The wiring seemed to handle the guesstimated 40amps with no problems, didn’t warm up at all.

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You know I just converted inch pounds to a unit I am more familiar with, foot-pounds, and all of a sudden that chart above doesn’t seem sufficient for a go-kart if the range is where our motor falls…

60 inch pounds only equate to 5 foot pounds… A mini cooper has about 115 foot pounds of torque at 4,000 RPM

Yeah, but it weighs 1.3 tons curb…
That’s 22-1/3lbs per lb-ft of torque.

How much is this kart going to weigh?
If it’s less than 113lbs, they’ll have similar acceleration, except the electric hits max torque from a standstill…

edit: Oh yea, except for the driver @ 200lbs avg. :smiley:

Actually it only weighs about 1,400 pounds or about 0.7 tons.

But the rest of your point is valid, I hope. Although in the above chart it only has about 2.5 foot pounds of torque at 2,000 RPM.

10" wheels at 30MPH need to be turning at about 1000 RPM. If we assume a 2:1 gearing ratio. That would mean that we have 5 foot pounds of torque available at the axle at 30mph. And 10 foot pounds of torque at the axle at start. I’d estimate from what I have seen so far that the kart with rider will be about 400 pounds

So, we can estimate the acceleration for the cart with my assumptions as

Tire diamter = 0.833 ft
F = T/R ~ 10 ft-lbs/0.4165 ft ~ 24 lbf
a = F/m ~ 24lbf/400lbm ~ 0.06 ft/s/s

If my math is correct that means the cart will have about two orders of magnitude less acceleration than a typical car (about 6.5 ft/s/s)

Anyone else care to check my math?

I forgot to compensate for the difference between LBF (force) and weight (LBM)

1 LBF = 32 ft-LBM/s/s (or mass in pounds time G)

so the above equation should be 24 lbf * (32ft-LBM/s/s) / 400 LBM ~ ~1.92 ft/s/s

So we are only about 1/3 of a cars acceleration if the rest of my math and assumptions are correct.

Dang, it has been a LONG time since I did these kinds of calculations… @frank_lima would you care to double check?