Stethoscope Rig

I modeled a stethoscope and am having a real tough time rigging it. I want to be able to rotate the chestpiece (apparently that’s what the triangular part is called) and then subsequently be able to pull the two ear pieces away from each other. I managed to accomplish these two tasks independently, however, when I combined them things went crazy. Here are them independently:

And here is them together:

The problem is with the bones that have a follow to constraint for the curves. I parented them to the bone of the chestpiece so that they would rotate when the chestpiece rotates. The follow to constraint, though, messes with the positioning. When I rotate the bone of the chestpiece, the rotation of the ear piece bones is not in line.

I appreciate all the help I can get! You guys and girls are a lot smarter than me haha


We need to see how you set up the bones. Here is good picture of stethoscope working parts:

The earpiece tube is rigid tube; a single bone each will do. Now for the spring part, it holds and make it possible to flex / open earpiece tubes apart. There, the rig needs to be done like rigging a bow. Where spring attaches to the earpiece, bone link needs to point and follow earpiece at all times.

Yes, you are right about the spring. I did not take that into account when I rigged the stethoscope. In my rig I have two bones for the “rigid earpiece tubes” and the highlighted vertices of the red rubber portion. I have not gotten into anything fancy here. The verts are all 100% weighted. I planned on changing that. These two bones have IK constaints with targets at the bones they are pointing to. I could have used a track to constraint but that gave me problems (I can’t remember exactly what those were now haha). The targeted bones by the ear piece have a follow path constraint so they would follow the curves seen. I did this so the two earpieces would not intersect when they were pulled apart. The problem is that when I used the follow curve constraint the bones moved and I had to place them manually back at their position at the start of the curve. This works fine if I am just opening the rig, but when I parent these bones to the bone that turns the chest piece, their origin is not at the start of the curve. This creates the distortion seen above.

Study stethoscope action from this image:

Here is the basic rig I had in mind. Main bone is at the base. Next off of its tip is the T bone, spring bone and eartube bone. Spring bone is single bone segmented to look curved.

To open and close I added Copy Rotation constraint on eartube bone. It is targeted on to spring bone. So when spring bone is rotated, eartube bone rotate out with it. This gives the right kind of opening / closing spring movements.

I really like that idea! I will definitely implement that. My only concern is how to avoid having the earpieces intersect or go through each other when it is open. My stethoscope is not symmetrical.