Big Disk Energy Hub Motor

After failing to get a 3lb bot in the ring at Mass Destruction in Maker Fair New York, I spent the first day of the competition looking at other bots in the Northeast and thinking about the next design. I decided to abandon the tombstone-esque extended horizontal mid-cutter due to the hub motor not preforming as expected in that configuration.. I needed a better horizontal spinner design, something with a weapon assembly that just won't stop - no matter what it hits or gets hit by.

This rules out any belts in the system because although they have always been successful for me in FRC, I haven't been able to get any system with a belt to reliably run on a combat robot. My first beetle, Amped up, ran a 3mm timing belt which constantly fell off due to inconsistent center-center spacing and an unreliable motor mount. There were many other problems with that robot but even the veteran bots like Debacle experienced these same issues with belt reliability.

Here is one of several intakes I made in FRC that uses 9mm wide 5mm HTD belts. Because the belts were trapped between the intake arms and the roller, the belt could not skip off of the pulley, even when the polycarbonate arms flex. This style of intake could take a beating and easily stay functional.


This is Amped up's second fight. notice how the belt skips off the motor at 1:22.


Amped up also got significant gyro from being a vertical spinner with a relatively large moment. I didn't like how uncontrolled the turning was on this robot, which is another reason I decided to go with a horizontal spinner vs a vertical spinner. I would rather be able to hit a robot once and lose control with a horizontal rather than not being able to hit them in the first place with gyro from a vertical. I am a terrible robot driver.

So I decided that this robot will have to use a hub motor in its weapon.

Remembering the hub motor horizontals from WAR events, Dark Slayer comes to mind. This robot uses a large diameter bearing to go around the motor can and a smaller diameter bearing at the end of the stator to double-support the disk. I liked this concept because it lets you have a large diameter, dead shaft run through the stator that can be used as a 'nub' to run on the ground and is much stronger than other common designs for horizontal hub motors as it does not rely on a smaller diameter shaft internal to the motor.

After the event, I began to design this robot on the way back. I chose to use a 5010 brushless outrunner because it can output a decent of power for a beetle (900w @ 6s, which means I can expect 400W @ 4s), has tons of much-needed torque to spin a large diameter weapon, and should have a large hole through the stator for a bigger dead shaft. The smallest diameter and lightest bearing that could fit around this 60mm OD monster of a motor was a 61813 thin-section bearing.

Here is an isometric and cross section view of the design that I came up with for the hub motor assembly:



The green object is the 5010 stator, the blue is the magnet ring, and the red is the larger bearing. 

Since I put so much time into this hub motor, I wanted it to be re-usable throughout all of the versions of Big Disk Energy so I put a good amount of weight into it. This assembly weighs 0.9lb and is built to never break. As long as there are no direct hits to the stator, this assembly should take any horizontal hits at the same height as the large bearing because the bearing itself should disperse some of the load throughout the assembly.

After I designed the rest of the frame (I will write about the frame next) I began machining and assembling the hubmotor module.



Here is the bearing when I received it in the mail. For scale, it is so big I can fit my entire hand through the inner diameter. This should take impacts from 3lb bots very well.


After machining, the bearing was a slight press fit into its bore, but the center diameter was slightly too small so the stator had to be retained with some green loctite.


This is the fully assembled inner hub. I couldn't get round stock that was big enough easily so it was milled out of a square piece of stock. I later found out that milling this part caused the magnets to slightly rub on the stator because when it was flipped for the second op the zero was off and caused the OD to not be concentric to the ID of this part.


Here is the final hub motor that I put in BDE. In the next blog I will go over how the frame was constructed around this module, then the design for the blade.

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