This arm is part of a larger project to create a backhoe loader using Lego Mindstorms and pneumatics. I didn't quite finish, but here is the arm. Other parts that were completed will be added over the next few days.
The shoulder includes two degrees of freedom, one of which is unpowered at this time, the other which has two pneumatic actuators working together. With sufficient pressure built up, it was able to lift fairly heavy objects from this joint (e.g. a billiard ball).
This was in many ways the weakest joint. The poor leverage caused by the need to get a full range of motion at the scoop end combined with the weaker retracting action of the actuator meant that this joint was very poor at lifting.
It took a fair number of tries to get this to work. Notice the elastics which are used as a spring only when the actuator is fully extended and needs a little help coming back. The elastics press against the "bone" of the arm at the point where it joins with the scoop.
The shoulder joint construction is very strong.
This mounting system worked very well since it holds the actuators completely steady. This is necessary because the end of the actuator is a round hole rather than a crossed hole.
This also doubles as part of the shoulder mounting.
It can't be seen in the photo, but the 4 peg diameter round piece is attached to the two yellow 6 peg girders by the use of a 2x2 turntable piece. This is also done at the bottom. The whole mechanism is held together by the blue girders. The full weight of the arm at full extension is easily supported from the grey panel pieces in the foreground of this photo.
