My Sky Adventurer was getting harder and harder to turn... it was kind of stuck even if you totally disengage the clutch, it won't turn unless you really force it.
On opening it, the problem was nearly shown, see below:
there was a circular scratch mark on the inner rotating axis! Obviously, it shouldn't be there and it should be caused by some metal dirt or uneven machining? So I cleaned up the surface and found a few small marks, they're so small that even if you touch them, you won't feel there's anything... I bought the finest sandpaper to sand those stuff down, and then put the inner axis in, rotate it a few times, and there are two dirty patch will form, and then I sand them down again and again.
Finally, they were gone.
You couldn't trust your finger, you won't feel it. But if you turn the inner axis with it, dirt patches will form. So after a few runs, they were all gone!
The gear was not machined very nicely, and I sand down those rough edges like the one which is shown below:
Placing some grease back (thank Eric Ng), and now it's very smooth! The machining is not bad afterall, when I put the inner axis back even without grease, it feels like pressing a syringe!
I'll them it along for camping and stargazing trip soon!