Servo idler bushings are extremely stiff

[Zoid]

I just built my second Bioloid bot (hexapod). I'm noticing something I have not seen mentioned in the KB or forum. The bushings (part "BU") that form the servo idler joint are so tight within the mating frame holes (part "F2" for example) that they squeak terribly, and require significant torque to pivot. What usually happens is that the securing screw backs out after a few minutes of play. I guess I'm glad it's not a sloppy fit, but clearly it was meant to be a bearing/bushing like glide. Have others seen this? How do I combat the problem? Reaming the hole by a few mils would be extremely difficult, and lubricants are messy and tough on plastic.

Thanks for any help you can provide.

(edited to fix typos)

[limor]

this sounds unusual. the free rotating horn will probably loosen up eventually from the regular friction.

[StuartL]

You're probably over tightening the screw holding the bushing in place, distorting the bushing and making it too tight. Put some mild thread lock on the screw, back it out half a turn and check that the bushing sides are parallel again. You may need to replace the bushing if it's distorted enough that the elasticity of the plastic doesn't restore its original shape.

[Zoid]

Thanks for the reply. Unfortunately these bushings were very tight before the screw even came into the picture. It took a fair amount of hand pressure to press them into place. If I use threadlock at least the screws won't fall out, but I have no doubt that the bushing will spin on the screw (just as it does now) instead of letting the frame spin on the bushing.

I may try some wax on the bushing, and if that doesn't work I'll see about chucking it in the drill-press as a makeshift lathe, and sand off a couple mils.

[Zoid]

Stumbled onto a solution.

It turns out that one of my Dremel mandrels fits snugly inside of the plastic bushing. It fit so well I didn't even need a securing screw. I then spun the bushing up and held a flat screwdriver against the mating side of it. I had done that originally to scrape off some wax I put on there, but then discovered that the screwdriver was scraping a minute amount of plastic off. It was perfect. I'm now working my way around the bot, removing, machining, and replacing the bushings. The hardest part is getting the bushing out of the frames without disassembling the bot. Everything else is a snap.

Hopefully nobody has to endure this, but if you do, GO SLOWLY. You cannot undo a mistake. Depending on how crisp your screwdriver edges are, it takes only a brief, light touch. After I found a good fit (no slop), I judged my progress on the others by the amount of white plastic dust that fell on my black pants.