Various parts for "Flexbell" adjustable dumbbells

I bought a pair of (broken) adjustable dumbells off Craigslist.  These are biggies that go up to 32 kg (70 lbs) each.  One was in very bad shape, so I had to fix a bunch of stuff.  Including straightening out a bent core rod.

They appear to be gray-market copies of “Flexbell” adjustable dumbells, which I think were predecessors of the current “Nuobell” dumbells from Nuo Athletics.  Also sold/rebranded by SMRTFT in the USA.

As well as making these pieces, I also had to order some springs to replace missing ones.

If anyone is interested, let me know, and I'll include more information about how to dismantle/reassemble, and any other details.  Also happy to upload FreeCAD models to make it easier to remix, or as starting-point for fixing other versions of the dumbbells.

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Various notes:

• Consensus on best material for gears is nylon (PA).  This is strong, tough, and has good sliding friction properties.  Any nylon variant would probably be fine.  This is not a highly-stressed or high-wear part.

  • I printed mine in Polymaker “Polymide CoPA”, which was surprisingly easy to work with.  Biggest issue is keeping filament dry.  I didn't need a heated enclosure.

  • Use lots of perimeter layers, and high infill density.

  • I found that I needed to do some fine-tuning of the center hole size (+/- 0.1 mm) in order to be able to push the steel shaft through with the right amount of friction.

• I don't think material choice matters too much for other (non-gear) parts.

  • The bridge support that spans between the two base rods is pretty stout and can be printed with lots of perimeters and high infill density.  Ends up nearly bulletproof, even in ‘weaker’ materials like PLA.

  • I used a ‘PLA+’ filament that I believe is a blend of mostly PLA and a small amount of TPU to make the PLA less brittle.  I was able to stand on it without it breaking.

• Disassembly is slightly tricky.

  • Firstly, watch out for ball-bearings and springs flying out when you unscrew the end caps and release the sleeve-locking mechanism.

  • It's hard to extract the central shaft that goes through the two pinion (small) gears.  I drilled a hole through the number sleeve on the opposite side (through ‘16’ digits on my 32 kg version) then used a blunt nail to push the shaft out.

• Reassembly is a pain.

  • You need to hold the gears in place while you push shaft through.  (The central gear is particularly difficult – I toyed with the idea of designing and printing a extra piece to hold it in position.)

  • And you also need to get both of the split rods positioned exactly right so that their ends are flush with surface when dumbbells are set to minimum weight.   Not too bad, but fiddly and time-consuming.

  • The (laughably wimpy) spring-loaded locking mechanism has a tendency to explosively disassemble.  I guarantee that you'll spend some time on your hands and knees trying to find the two miniscule springs and plastic cap that will launch across the room and vanish into a dark recess.  In fact, the reason that I made models of these pieces was because I lost one of mine.

  • The limit mechanism that prevents the dial rotating past the max (32 kg) and min (2kg) point is actually the far end of the small shaft that threads through the two pinion gears.  Consequently, I found it useful to thread the shaft only ~95% of the way through, then align the two split rods, then finally tap the shaft the final couple of mm.

    • I only realized this after my first attempt at reassembly, when I noticed that I was able to rotate the dial without hitting any limits. 

• It is worth persevering, though.  My previously broken dumbbell now works much nicer than the untouched ‘good’ one.  Operation is noticeably smoother and more consistent.