[size=1][font=Verdana, sans-serif]Hi,[/font][/size][font=Times New Roman,serif][/font]
[size=1][font=Verdana, sans-serif]I would like to make a pivot joint like in the knee or elbow.[/font][/size]
[size=1][font=Verdana, sans-serif]I would like to control the angle by high torque Ac motor.[/font][/size]
[size=1][font=Verdana, sans-serif]How do I construct the joint and the gear? How do I make only one tube to move while the other tube stays in its position?[/font][/size]
[size=1][font=Verdana, sans-serif]I attach a picture that will demonstrate what I thought to be the basis for the joint.[/font][/size]
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[size=1][font=Verdana, sans-serif]Thanks[/font][/size]
Have you already chosen the motor? If so, can you provide a link?
If it is a normal rotational motor, you would fix the frame to the outer metal, and the shaft to the inner metal.
Alternatively, you can fix the inner metal to the shaft, and use a pulley or gears to rotate the shaft.
Thanks for you replay and suggestions.
I looked into this motor servocity.com/html/12_rpm_hd_planetary_gear_motor.html#.VtcE_PkrKM9
it comes with encoder - be glad to hear what you think.
In both options i will need to fix the inner metal to the shaft - how to fix 6 mm shaft to the diagonal inner metal ?
Thank you
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You can mount the face plate of the motor to the outer metal bracket, and use the shaft as half of the joint, connecting it to the inner metal bracket with a hub (6mm bore and perpendicular mounting holes).
robotshop.com/en/6mm-aluminu … wheel.html
The other side of the joint would be a rod with (or without) bearings.
Thanks you making me seeing it 
inner metal bracket - can you give me an example for such bracket ? Let say the Inner tube is 200 mm X 200 mm.
Essentially what you have in the image you attached. It will be up to you to find / select these two parts.
Thank you very very much