This is by no means a scientific test, but my results may be of use to others considering these servos for their robotic or other projects.
The stall torque is, as I understand it, the maximum torque the servo can withstand or 'fight back against' when applied to the output arm. I should clarify that the servo must be powered up and receiving a good signal e.g. 1.5 ms pulse at 50 Hz.
In my test I had a 20 cm arm attached to the servo horn parallel with the table below. I hung a small bag of coins on the arm at 20 cm from the pivot and ran a small python script on my Raspberry Pi that cycled through several servo positions before returning to the start point.
I had hoped to get around 1.5 to 2 kg.cm of usable torque out of the little SG92R's, but sadly they did not live up to their chinese specs!
The only good thing about the tests is that all the servos I tested started to struggle at roughly the same point, so at least they are consistent.
From my tests I have concluded that the SG92R's:
- Can hold against a torque of approximately 2 kg.cm. i.e. the stall torque is around 2 kg.cm. But they did sag about 15 degrees before holding the weight.
- Have, have a 'usable' torque range of about 0.8 to 1.0 kg.cm. At that load they will move where commanded, although slowly, and will hold their position fairly accurately.
- Will move fairy quickly and accurately with a load of about 0.4 to 0.5 kg.cm.
So, In conclusion, my 9 gram SG92R Servos are only really useful up to about 0.8 kg.cm and that is the absolute limit, I probably wouldn't want to use them above 0.4 kg.cm to be honest.
Unfortunately I can't build my biped light enough to use the SG92R's except in a few lower torque joints, so I ordered some bigger and stronger servos. I'm hoping for slightly better results from those, especially with a quoted stall torque of between 10 kg.cm and 14 kg.cm.
