I have a simple cylinder mesh. It has a 'hole' cut through it using through it (using knife project). The vertices which form the top face of the cylinder are grouped as 'Top'. The vertices which form the hole are grouped as 'Hole'.

I have an armature with bones called 'Top' and 'Hole', so that in b4w I can grab the top and adjust the length of the cylinder, and I can grab the hole and slide it along the cylinder.

But I can't find a way to prevent the hole getting 'outside' - beyond the top of - the cylinder.
All I need to do (afaics) is detect the proximity of 'Top' and 'Hole' bones and react
by preventing any further movement.

In blender I can't set collision bonds on individual bones, only the armature as a whole, so the bones wont collide with each other. I can't use multiple armatures because b4w will only load the first one.
Any ideas please? TIA Phil

Mikhail Luzyanin: Thanks, fixed it now.
I had failed to spot the significance of the bit about non uniform scaling in the docs. For anyone who's interested I got around it by using separate bones which are just moved (location) instead of scaled to get the desired effect. Thanks again. P

The action 'TopAction' runs OK in blender - it just scales the top bone to stretch the vertex group 'Top' (the top face of the cube) upwards.

But when I 'fast preview' or export, the animation doesn't show in B4W. It's there (its name appears and it says it's running, but nothing actually happens.

Its been driving me nuts for days because sometimes I can make a file that works and sometimes not and I can't see what the difference is.
Any help very much appreciated, Phil