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Thread: Wyvern - Featherweight spinner design and build thread

  1. #1
    Wyvern – New featherweight spinner build thread

    So I thought I would start a thread for this as I think my design is sufficiently progressed. I still have some significant design hurdles to get over, but more of that in a bit.

    I am building a spinner which looks vaguely like this! (some bits are incomplete around the weapon)



    So it is a fairly “classical” bar spinner construction. Spinning bar will be 40cm long hardox, 10mm thick. MOI of 0.017kgm^2. I have put spurs on the end to enable some penetration as opposed to blunt impact. Will be driven by a 100KV brushless outrunner.

    The drivetrain will be two scavenged 12v drill gear motors directly coupled to nylon wheels. I am not prioritising too much speed here, as I want it to be controllable. I have seen enough footage of spinners wrecking themselves on the area wall rather than their opponents! I have demonstrated this drive method on a chopping board testbed so I am happy it works.

    The chassis and armour will probably be HDPE. 15mm thick base (to take the severe shocks the spinner will inevitably generate), 10mm thick lid. The rear panel will be angled HDPE, which I am confident will deflect shots fairly well so I am happy with HDPE. The side panels are naturally more vulnerable, so if I have spare mass I will upgrade these to aluminium or titanium if I am feeling fancy, otherwise 10mm HDPE it is.

    Lid will be 10mm HDPE. Part of it is easily removable for basic maintenance, also with a hole for the removable link. The wider part can still be removed but will obviously involve removing the spinner.

    The details:

    Weapon drivetrain

    So I used the spinner spreadsheet and get the following:


    So it reaches a respectable level of energy in a short time assuming 1:1 gear ratio. The numbers seem reasonable here, but please flag me up if I am missing anything glaring! Checking the FRA rules, the weapon mass just fits within the FRA spec limits, with the length compliant also. RPM is higher than the 500 mentioned in rule 11.2, but one exception is deemed valid.

    This is my intended motor

    I am thinking along the lines of the following for the ESC

    I will likely run at 12s with two of the following

    The burst C-rating is pretty high, so two in series should hopefully be ok. Again am very interested to hear if other people’s experience confirms or defies this!

    The main remaining technical challenges concern the weapon drivetrain.

    I am looking to use a belt drive, with a lawnmower style V belt of 10*6mm section. This belt will drive a very chunky, stainless steel coupling pulley with the weapon blade. The blade sandwiches between two halves of this coupling, held together by two M8 bolts. Flanged bushings run between this coupling and a stationary 15mm SS shaft.


    The first bit I am struggling with is to find a decent source for locking bushings to couple the shaft to the main housing. This is partly a struggle because I have no idea what the consensus name is for one of these!

    What solutions have people used to couple shafts to their housings? Would ideally like to find something off the shelf than build my own, makes repairs far simpler!

    The second challenge I am struggling with at the moment concerns tensioning the belt. I Intend to place an idler pulley somewhere near the front bulkhead (either inside or outside the belt), with some means to adjust it for belt tension. I am struggling to find a suitable tensioner assembly to put in though. Does anyone know of any standard components for tensioning? If I really need to I can design my own, but it would be a hassle. Probably some kind of serpentine moved by a ballscrew.

    I have a decent amount of space near to the weapon motor so I am confident this is a solvable problem!


    More to come on this as I solve these finicky details. Once I have a way forward I will start ordering bits.


  2. #2
    10mm spinning blade is fairly thin for that application. We started the Valkiri series with 395mm diameter 8mm thick, but now we're using smaller disks with 12mm thickness, and those still bend under the forces.

    The 15mm shaft.
    Valkiri 2 used a 20mm silversteel shaft, and that got bend after a few good hits.
    In Nebelwerfer, our 6kg raptor uses a 20mm titanium shaft.

    25mm surface hardened steel will be the shaft for our next feather spinner.
    Last edited by maddox10; 11th March 2018 at 20:31.

  3. #3
    12mm thick hardox bends? Wowzer. What kind of kinetic energy was that running at?

    And I thought 15mm was overkill... Luckily I can thicken that without too much of a mass penalty.

  4. #4
    Valkiri 3 got nearly 8Kj in the disk on full power.

  5. #5

  6. #6
    So I've been doing a bit more design work on this, taking on board Maddox10's advice about the shaft diameter. I have upped it to 20mm now, which hopefully will be ok (given I am running less than 1/4 of the KE of Valkiri 3). I have also been having a think about how to mount it robustly and within my (albeit self imposed) tight space envelope. I am thinking of making the bottom hub and the shaft one monolithic component, welded together from water-cut bits then tidied up on a worktop lathe, which should hopefully achieve ~H9D9 fit tolerances. The exploded view below shows more.

    I will use off the shelf oilite bushings instead of bearings. These will fit inside a rotating drum that also houses the blade. I then have a top running plate for the bushings to run against. The last piece in the puzzle is what to do for the top hub... Still hoping to find something off the shelf here, but I may have to concoct my own if needs be.

    weapon_hub v2.PNG

    Best of luck to anyone competing in Manchester this weekend. I will be an avid viewer on Sunday. Hope to join in next year!

  7. #7
    We broke the 20mm titanium shaft in our raptor, Nebelwerfer.

    I have to admit, it was on the weld, on a very thin centring edge.

  8. #8
    Yes the weld is the bit that has me wondering! I am thinking that some small channels might be a good idea, for the melt to flow and improve contact area with the shaft. Like so:

    shaft base.PNG

    Maybe a chamfer on the outer bit will allow the bead of weld to be deeper too. I will need to trim back the welded joint with a lathe to allow the bushings to run smoothly on it.

    Also I am concerned that the 15mm HDPE base plate will yield first. I intend to spread the mount over as wide an area of the base as possible. 8 M5 bolts currently (need to add countersink chamfers to the CAD drawing). I think maybe more are needed...

    Looks like hardened steel bar is a readilly available and cheap material so this may be the metal of choice for the shaft. Presumably bending yield stress will also depend on the length of the shaft? I am aiming to keep the "unconstrained" length as short as possible.

  9. #9
    Retroman's Avatar

    Paul H
    Pullover V2 (AW), Leopold (BW) & 1/3 of a Feather (Pushover)
    Gtr Manchester
    Maybe an odd suggestion and I've never considered the weight, but have you looked at cam belt tensioner and similar for cars, the more modern designs tend to bolt on then you set the tension, I would imagine the stuff for a/c drives would be smaller too.

  10. #10
    I did have a look at car belt tensioners. They are rather large. Had a look for lawnmower ones too without joy. Will certainly have a dig for air-con tensioners! I do like the look of the "serpentine" type tensioners, they seem least likely to jam under impact shock! Think I have about 500g to spare on the tensioner, will have a look.

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