One small element of the sorter I did not notice before I bought a PivotPi : how the servo is connected to the lego liftarm. It is connected not by a a lego pin or a gear but by a plastic twist tie.
In other words, and I can confirm this now I have them in hand, the servos do not connect readily and securely to any lego part. So beware - you’ll have to come up with something yourself if you want to incorporate pivot pi servos into an existing lego construction.
There is this:
Can’t quite see what it does yet! Is the short axle on the right in the first picture supposed to spin or just the ring? How useful is the latter?
Another possibility
I’ve found drilling out one end of this part:
and pushing it on to the 9g Tower Pro servo gear and the other end to a lego axle might be secure enough.
I’ve built a robot, using lego ev3 motors, controlling them from a RaspberryPi via a BrickPi.
Basically my problem is I’ve run out of motor ports! There are only 4 on the BrickPi and I need a few more motors for a turntable and a better lift arm. So, I have a PivotPi, and the servos should do the job but they don’t connect to lego axles or gears.
Not surprisingly, the solution I put up does not really work. The rotor will spin inside the drilled out lego part. Perhaps, gluing might work but I’d rather avoid that.
Two much more promising adapters:
(to be printed)
and
The issue now is whether the sg90 micro servo is compatible with either of them! As far as I can tell, both adapters will only fit shafts with 20 teeth. According to Google (and this is weirdly very difficult to find out) the sg90 has 21 teeth. Yet, when I look at the sg90 that came with the PivotPi, it seems to have 25 teeth.
You are now discovering why hardware engineers get grey hair early.
(Apologies for taking so long to respond, but I’m in the middle of doing taxes in several currencies across multiple banks - and that gives people grey hairs too.)
I don’t want to sound trite, but the correct answer is “try it!” and see what happens.
In my case I was working with a pan-and-tilt that had a hub that expected a servo with “X” number of teeth. All the servos I had access to had “Y” number of teeth where X and Y were close, but not the same.
So, what did I do?
I used a heat-gun to soften the plastic hub just enough so that I could press the servo’s gear into it, molding it into a “Y”-toothed hub. It worked, (for a while), but the tough part was making the press-fit exactly 90° - and do it without damaging the servo. Later on I was able to find hubs and servos that matched - and fit the pan-and-tilt.
I keep an assortment of sharp tools, dime-store[1] boxes of emery boards, and various adhesives[2] available for just these kinds of problems. (I also keep a drawer well stocked with band-aids too. )
I guess I’m dating myself with that comment. You can translate that to “dollar-store” if you want.
If you find you need to glue the polycarbonate acrylic sheets that the GoPiGo use as its chassis, (they do break occasionally), I found that Cosmo CA-500.200 (Cosmofen-12) cyanoacrylate, (and its sisters by Cosmo), works very well on polycarbonate plastic sheets if you keep them immaculately clean. In Europe I find it in automotive parts stores and some “big box” hardware stores too. Other kinds of cyanoacrylate glue, or most other adhesives - including epoxies, work poorly on polycarbonate plastics.
One of the best methods for gluing any plastic is by a process called “solvent welding” where you use a liquid that will soften the plastic applied to both sides, press the two pieces together so that they merge, and then allow the solvent to evaporate. Volia’! You have a joint that is as strong as the original plastic. There are solvents for polycarbonate plastics, but I can’t get them where I live, and if you can get them, they’re best used outdoors on a warm sunny day.
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