Hy-Con/T19 part 6: Groove Filler Success, importance of cage stiffness, rotor-centric flywheels

TBNC4 (our 4th major event post-Reboot) was a success in many ways, including the notable lack of T19 jams running the Beta Prime cage. Every ragged-ass old dart I picked up went right through without a single hitch.

Groove fillers work, people. They absolutely, positively make a HUGE difference in reliability. Nuff said.

Because there is no good place to stick these, have a whole bunch of Beta Prime/Gen2 9.5 part images.

However, a discovery came to light with my print of the Beta Prime cage, that being the monumental importance of cage (and overall system) rigidity in these large-format flywheel blasters. I have never seen it discussed much. Anyway, my Beta Prime print used 20% hexagonal infill, 4 bottom and 3 top solid layers and 3 perimeters. This with PETG makes a very rugged part that absolutely isn't going to break, but this cage dropped the T19's velocity by about 15 fps over my previous Beta non-prime, which was printed with a few more solids tops and bottom, with all generations of wheels I tried (which were all of them, including my old PLA prototype set).

Thus, this happened.

It's called Gamma Minor and it has a lot more meat on it. I also cut straight to the chase and printed at 100% infill, resulting in a part that is heavy as fuck and took about 6 hours to run. Velocity went right back up to 176 average with Raytheon waffle blue. I think I overkilled things a tad and can maybe print a bit more sanely in the future while still getting optimal numbers out of this with the inherently stiffer design trend going on (not the Beta's thin motor mount "wings") but at least it proves the point. Cage stiffness, DOES in fact matter way more than anyone has yet given it credit for. Flexibility you can barely feel by hand can affect the dynamics of parts under the shock loads of firing darts. Respect that fact - and stock-cagers should really revisit this with the popularity of the printed cages.

I will probably always print my 'con cages at 100% after this however simply for the NVH reduction. It's a good bit quieter and smoother feeling than my Beta Prime print was.

Now for an installed pic, and...

Wait. What's up with those flywheels??

This is the sort of way development tends to happen with me: in the last few days/hours before a game, with a sudden spark and a mad dash to fab parts and change shit and hope it doesn't explode in my face tomorrow.

So this is the Gen3 Hy-Con flywheel, the "rotor-centric" that I may have referred cryptically to at some point. The profile geometry is the same as the Gen2 with all its refinements in that regard. The big change happens on the other side of the rims. Up to this point I had been designing flywheels with the structural concept of old-school, shaft-mounted wheels from the DC dark ages. I just replaced the shaft bore with a bolt pattern to hook onto the rotor flange of the outrunner, much like an engine flywheel. So did FDL Jesse and most others. I was pondering optimal print parameters and possible design revisions for these Gen1/Gen2 wheels in light of the cage stiffness observation and some concentricity/balance questions with them as well.

Ultrasonic2's Ultracage wheels came to mind, in which there a small step to use the rotor OD as a pilot diameter/locating feature to center the wheel, when it suddenly clicked: Why does the web need to carry a bending load, anyway? The rotor has a large external cylindrical surface, why not just use that to both locate and support side load from the rim?

So there it is!

Consider it a synthesis of multiple older concepts:

* The flange-mounted Hy-Con/FDL or shaft-mounted old style wheels: we still use the web to axially position the wheel and to transmit torque.

* The Ultracage rotor-OD piloted wheel, for being my actual inspiration for using the OD of the rotor on an outrunner in this way.

* Kelly Industries outrunner Stryfe cage, since the press-fit hubless "tire" flywheels transmit their side load in the exact same way. Only mine don't rely on that fit to either transmit torque, or to maintain axial position.

These are dimensioned on spec to get reliable solid fit without movement of the wheel on the rotor. If you early adopters print these, be aware you WILL need to scrape down any layer-change blips first to get a mostly-round bore, and then sand to snug fit on the rotor OD (tight isn't necessary). Careful of the motor bearings when fitting these, avoid excessive force or you could brinell them. Aligning the bolt holes is fiddly; use your allen key for the flywheel bolts while pressing the wheel on. For now the 3mm pilot bore is still there and still needs drilling/reaming to clean up just like before.

The most profound immediate result from this design was better concentricity and greatly reduced NVH. Using the rim ID and rotor OD as a locational fit helps deal with printing tolerances and warping and such in the web and automatically pulls the rims into concentricity with the rotor when bolted down. These things feel almost machined, and my printer is still not as perfectly square as I want it, even. The rigidity also prevents things from wet-noodling at speed under imbalance forces and worsening any imbalance.

Recommended print parameters (with 0.2mm layer height 0.4mm nozzle) for the Gen3 are random start point, 3 perimeters, 4 bottoms and 3 tops (Less mass is priority over pretty top layers) and 20% hexagonal infill. The Gen1/2 wheels benefit from more solids. These do not, because there isn't any bending load on the web. It will just add pointless mass and make you need to crank your delays up and have longer lock time.

At this point I should address printed wheels and inertia briefly. Those rims look awful thick, but keep the above image in mind; there is structured infill in there, thus the rims are mostly air. The majority of rim mass in a printed wheel is in the perimeters. This is why the Gen2 wheel design was admittedly kind of dumb - all I ended up doing was moving the inside perimeter outward, increasing its circumference (hence quantity of plastic) AND its radius, while only saving a tiny tad of infill volume inside. Gen3 is probably not optimized for inertia because the top layers start getting heavier with such a thick rim as well as the infill, but its structural benefits are undeniable and the Gen3 runs the same delay settings as my old Gen1 wheels, which are acceptable. Further mass reduction may indeed be explored within reliability/durability bounds to pep up startups.

I also happened to get this image that ought to embody part of the answer to the question "why brushless?" that I often get. Of course there are about 6 different factors in why brushless (inverter PMSM) drives kick DC's sorry ass all the way to Alpha Centauri and back, but packaging is one of them:

There is no way I could possibly achieve a flat cage package like the Hy-Con with brushed motors. That's, at minimum, a 380, at about 3 times the length.