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I'm trying to rig the following bridge, but can't wrap my head around this.

enter image description here

When I rotate Bone on its X-axis to hoist the bridge, the bottom part where Bone.002 is attached too comes loose from the part where Bone.001 is attached to.

enter image description here

Note: Bone.001 has a Copy Rotation Constraint and follows the rotation of Bone

enter image description here

What can I do to keep the parts connected?

Here is the .blend file:

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  • $\begingroup$ If you upload a blend I can take a look when I get home. Use this website blend-exchange.giantcowfilms.com $\endgroup$ – SilverRain Feb 12 '18 at 15:44
  • $\begingroup$ I uploaded the blend file, the rig is slightly changed. But the problem is the same. Now only the top part of the bridge comes loose. I use Bone.001 to control (for opening and closing the bridge.) $\endgroup$ – Bram van Vliet Feb 12 '18 at 16:25
  • $\begingroup$ Taking a completely different tack, instead of using bones you could use Rigid Bodies and use a Rigid Body Constraint Hinge for the two platforms. Then use a constraint like copy rotation to keep them in sync. There aren't a lot of youtube videos on how to use the Rigid Body constraints but I found some about using them to animate gears; for example, youtube.com/watch?v=h37WgwkkTFg&t=427s. $\endgroup$ – lumpynose Feb 13 '18 at 21:15
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There are several ways to do this, this is one:

enter image description here

The horizontal bones deform the two platforms, the two vertical bones translate the connecting vertical bars. For the vertical bones, I unchecked inherit rotation in the bone tab. The bottom green bone copies rotations of the top right bone. This is the control bone for the whole setup, you could hide all other bones.

Take a look at the blend:

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  • $\begingroup$ Thank you very much @yann for the very clear answer! This works the way I needed. $\endgroup$ – Bram van Vliet Feb 13 '18 at 10:37
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Like yann said there are many ways to rig this type of mechanical structure.

I used a Track To Bone constraint on the support bone that goes between the top and the road.

The rig set up is rather simple. The road bone (the bone selected in the gif) has a Limit Rotation constraint on it, that is the bone I rotate.

The top bone is simply copying the rotation of the road bone, via a Copy Rotation constraint.

Now the support bone is parented to the road bone, thus the location of the head stays correct. Now in a perfect setup all you would need to do is uncheck Inherit Rotation for that bone, and you'd be done. However it is probably better to use a Track To constraint to make sure the tail of the support bone stays exactly at the top where you want it. I have the Track To to the tail of my top bone.

animated gif of a rigged draw bridge

The blend because I know in rigging it helps to see what is going on.

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  • $\begingroup$ Thank you @David for your very good answer. This is a great example, I stored your .blend file too for future reference and learning purpose. $\endgroup$ – Bram van Vliet Feb 13 '18 at 10:39
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Another way to rig this is with an IK chain. That becomes interesting when the components have different lengths which in turn can mean that the bridge will have a different angle than the beam when raised. The model in the picture is a regular bridge though.

IK rig draw bridge

  • Anim is the bone that gets the keyframes. Rotation only.
  • target is parented to anim with offset
  • IK is parented to bridge and has an IK Constraint pointing to target, length 2

As a result, Blender will rotate IK and bridge so that the tip of IK will rest on the root of target if possible.

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