Sunday, January 28, 2018

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.


  1. What slicer are you using? After trying out all the infill types in Cura 2.7 and I've settled on cubic infill for all my prints. It is a 3d infill that prints closed 3d cubes, which might help improve axial stiffness on the wheels.

    1. It's Slic3r 1.2.9. A similar infill has been implemented in later Slic3r versions, at least PE.

      Outward deflection of the wheel rim edges under the axial component of the normal force captured within each flywheel is an interesting thought now that web stiffness is no longer a greater concern with Gen3. Then again, motor shaft stiffness is also a factor as well. At a certain point, the system is rigid enough.

      The main trouble source I had was the overhung motor mount "wing" structure being poorly designed in the first place; that is resolved. As to infill for this part, I like the noise reduction benefits of solid anyway, though perhaps a superior infill geometry could save some material with acceptable rigidity.

  2. I feel like you’re a little harsh on DC. It’s simple - cheap - approachable. Sure in a perfect world everyone would run brushless but for those not on the bleeding edge brushed is acceptable for superstock level blasters.

    I recognize you never made any argument, only that brushless is objectively superior but it could be read as downplaying all of the progress that pushed you and others to this level.

    1. I'm definitely more harsh than not on DC.

      I freely admit to downplaying the progress that led to this point.

      As far as nerf and blaster design is concerned, I live in the moment, especially these days. I have been in this game since 2010 and to me it is an endless journey onward and upward.

      What I would add, however, is that I do not like that your comment equates velocity with blaster (system) performance, which is a false equivalence indeed (velocity is one of indefinitely many parameters that constitute the performance of a blaster); also that it positions motor technologies as matters of being suited for certain fixed cases or with "high performance/low performance" cases. My point is more that the inverter PM synchronous technology has general merit and continuous improvement (including cost and accessibility), and there is virtually no case in this hobby for which it ought not be considered. If not now, in the future.

      It is also my position as far as DC is concerned... I assure you, sir, this ship CAN sink. Take a look around the motor technology space right now. Inverter/AC machine pairs are how things are increasingly done as a general rule. We have some forklifts at work that are much like my blasters, in that in a total and radical reversal of how things were done years ago... they do not contain a single DC motor. The traction and hydraulic pumps are big PMSMs, coffee can sized inrunners, with sophisticated, probably FOC drives associated with them but every little tiny thing... even the cooling fans, are brushless. I am fairly open that I want to see the inverter revolution come around and hit nerf, just like it did to RC hobbies, and full-scale electric vehicles, and lift trucks, and consumer appliances, and is now in process with cordless power tools, sooner rather than later.