We did consider a clamp arrangement, like brake pads on a car, however I think we'll struggle to resist the forces applied to the arm by the crushing hydraulic cylinder.
We are currently working on the design of the locking mechanism. We have struggled to get a fast acting shot bolt that is strong enough and think we may be better going for a ratchet mechanism.
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We should point out again that this is our first robot. We all have no idea and are working things out as we go along so really appreciate the feedback!
I will go and have a look at buddybox now and save an RC setup