Problem with rigid body joint I have two meshes and I need they remain together while they move but being different meshes, so Ctrl+J is not the solution. I've tried to put a rigid body joint but when I activate it the two pieces get apart.

I need the two pieces remain in this position

 

When I press the up key this is what happen



Here is the file: http://www.pasteall.org/blend/29210
Rigid body joints constraints intented use is for situations like:


Animation created from the blend file linked in the docs

Note the collision bounds are set to box and the joint itself doesn't have a collision bound.

Comparing your file you have collision bound set to capsule which should be mesh:



Setting it to triangle mesh would at least avoid that it is blown away when you switch on the physics.



In case you only want to parent both objects you wouldn't need the constraints: 

First select the child object then the parent Ctrl-P will open this popup.

 

To have the influence of parenting animated you could consider to use a ChildOf constrained.