I think the IK constraint prevent rotation but I don’t know how to fix it.
When rotating the crane by arm2 then the
arm boom should follow.
Ok, if you select the “control boom” go to the ‘Custom Properties’ panel, I put a switch so you can turn On/Off the IK. 0 = off, 1 = on. hope that helps, I should have put that on the main bone
Sounds like a workaround.
No, not really. As you have already discovered bones that are a part of an IK chain can’t be transformed while being part of that chain, so, the only way to transform them is to turn the IK ‘influence’ down or off. Many rigs use this technique, I always add it to mechanical rigs that use an IK set-up.
Now I think it was not a good idea for me to mechanical rig a foldable knuckleboom crane.Will the animator understand what the rigging person meant by switching IK off and on?
Here you see an example made by someone with solidworks. Its just for showing.
Well I can not comment on that because I really don’t know you or your experience with 3D rigging, but you are doing fine so far as I can tell. I and the others here can provide you with info, knowledge and help to get you to where you need to be.
If they are experienced in animating then they should know what an ‘IK/FK switch’ is, which is what that is, If you do not wish to use an IK set-up, you can [A] set it to ‘0’, [B] delete the IK constraint and the IK target bone, [C] umm I’ll think of another.
So, I would like to be as much help to you as possible, after all your not here to hear me ramble on - true? If you are stuck, then I can offer you any help I can - including rigging the actual file, I’m on my winter ‘vacation’ right now.Just tell me how I can be of more assistance to you.
Thanks I will continue rigging. I find it important that animators can use its true.
Happy Winter holiday 
