Fixing the knob stress problem

[Images here are linked to larger detail, as usual]
Yoke knob split right in half! I encountered a bit of a hardware problem at Arisia '18 while setting up some Rokboxes for an event.  One of the yoke knobs at the sides suddenly split completely apart, right in my hand as I was snugging it down.  I wasn't turning it particularly hard, and I wasn't trying to twist the yoke around in a way that would have carried the bolt along to excessive tightness; it just went *crunch* and collapsed into pieces under rather modest torque.
  With the design of the knob exposed by this mishap, the cause of the problem becomes evident.  The knob is molded around a knurled part of the bolt, and as the yoke setup comes the knob bears on a plastic washer between it and the outer yoke.  The shaft itself has a small flange more or less flush with the face of the knob that I guess is intended to take some load in a similar fashion to a bolt head, but the problem is that the plastic washer's hole is large enough to let the diameter of that little flange squish right through.  So all the axial bearing stress ends up on the plastic knob face, basically trying to push it outward off the knurled shaft.  Too much force, and something's gotta give.

To its credit, the bond between plastic and metal at the knurled section is deceptively strong, if it could survive even a little bit of this.  In a conversation with Blizzard techs, though, it's clear that the usage as supplied puts it at maximal disadvantage -- they've gotten a number of similar reports of broken knobs.  A related characteristic is that since the plastic washers are a friction element, any clockwise motion of the yoke relative to the fixture will drag the knob along with it and effectively wrench this assembly down really hard.  It's far too easy to do that with the Rokbox, even if you're careful.

How to reinforce the knob assembly What's needed is something to put the stress in the right place, e.g. on that little metal flange on the shaft.  A washer, but just the right sort of washer.  Failing to find anything really suitable in stock, I headed for the hardware store.  The threading is 6mm x 1.0mm pitch, which seems to most closely match 3/16", but flat washers designed for those sizes really have too much play around the shaft.  I wanted something really snug-fitting, even a little too tight if it could be reamed out slightly.

The drawer-cabinet boxes of both metric and english sized washers at the local Ace weren't really turning up anything that wasn't too small or too big -- but then in another box a little farther away, I found these.  A different brand of 3/16" fender washers, but as I tried my bolt through one I had my Goldilocks moment -- these were Just Right.  Plenty wide enough to cover the friction ring, stiff enough to not dish, and even anodized sexy black to match the fixture parts.  All of 32 cents or something each -- sold!

Widening the washer hole a little Some of them did need a little kiss with a drill bit to fit precisely around the bolts.  Chamfering the hole edges just a bit also helped.  By and large, the modified fit went *perfectly* around the outer diameter of the threads and landed solidly on the flange with no slop.

Small molded lip For the most part the shaft flange protrudes just a tiny bit out from the knob face, although that varies just a little between units.  There is also a tiny ridge molded around the perimeter which could unduly bear on the washer, so it seemed prudent to sand that off the knobs and maybe cone off the face a little bit to make sure the shaft would be the only thing taking the load.

Knobs shaved down a tad All the knobs immediately on hand got similar treatment, even if it was a bit variable. 

Tiny gap, force no longer on plastic The point is that once the stack-up is put back together, there should be a tiny gap between the bulk of the knob and the washer.  This configuration tightens much more evenly and nicely, and almost eliminates the "knob drag" problem if the yoke gets moved.  The knobs now work much more like similar hardware on other lighting gear that didn't simply go with an offset "tilt brake" setup like a source4.  Not perfect, but far better than it came.

The knurling undoubtedly still takes a bit of strain from normal tightening/loosening, but it seems aptly suited for that level of use and now if a knob *does* come off, the yoke will still be safely held. 

A note was also taken to obtain and prepare more washers to eventually retrofit the Armory installation as well, as I'm sure their techs would be a little upset by knobs disintegrating when they're working 20 feet in the air.

Batch of new knobs When I reported this to Blizzard, their head tech guy Joel was kind enough to send a handful of replacement knobs gratis.  Two of these would replace the one that had broken on me and its same-fixture companion that was feeling a little loose at the knurling as well, but what was I going to do with all these extra plastic washers??  I'd have trouble believing that they're considered a wearout item.  That aside, to Blizzard's continued customer-service credit, they totally made good on this and also seemed grateful to have the details of my hack-fix to pass back to their suppliers.

_H*   180209