The current mechanical coupling between the prono-supination motor and the corresponding joint does not handle strong torques (see also bug tracker). In particular, the application of strong torques at the wrist may produced unwanted backlash at different levels: (1) at the level of the two M2 screws blocking the motor on the chassis; (2) at two different levels of the assembly that couples the motor with the joint: (2a) at the level of the C-shaped piece (2b) at the level of the (custom made) screw
In order to prevent high torques at the joint, the current solution will be to reduce the maximum current given to the motor. According to the mechanical experts, the final solution will be redesign completely the motor-joint coupling.
The joint/motor coupling
The joint/motor coupling relies on three main parts: (1) the motor shaft, (2) the D-shaped coupler and (3) the joint shaft (which is directly connected to the wrist). The D-shape coupler is composed of two main subparts represented in pink and yellow in the pictures. The yellow part is responsible for coupling the motor and the pink part: in order to couple the two parts correctly, the yellow part has a flat surface which should be coupled with the flat part of the motor shaft (see the picture taken from the motor data sheet). Once a correct alignment has been obtained, the assembly should be tightened using the two Allen screws on the yellow part. In order to do so, the only way to access the two Allen screws is to use the two small windows on the upper part of forearm (i.e. close to the elbow). Obtaining a correct alignment is a complex operation and even a little misalignment results in unwanted backlash. Therefore, this operation has to be performed with great care.
The motor shaft can then be screwed on the pink part (remember to insert the proper bearings which are not represented in the side pictures). Once it has been screwed, the coupling can be tightened with the Allen screw on the pink part.
In iCub v1.2 or later the pronosupination issue has been partially improved with a new D-shaped electro-eroded coupler .
How to check the pronosupination joint
We just added a new section in the iCubHardware repository called "How To", you can find it here.