Manipulator joint constraints

edit

Hi, i am trying to implement a manipulator inside gazebo, by writing my own SDF file. I have written the geometry and joint basing on existing examples, but when I spawn the robot, it goes limp. I can't seem to have control using the joints menu part, it seems no matter what input I give to the first joint, it seems to break the whole robot. I haven't calculated the inertia for different links yet, for now I used some placeholder values, might this be the source of my problem? Here is the code for the robot: https://pastebin.com/0vAtGS8P

EDIT: I've changed the inertia values to ones that closely resemble the real parts, but it seems that after inserting the model it just breaks, after getting in contact with the ground. Also, with the previous configuration I noticed that when I changed the position of the base_revolute joint the base part was rotating, not the whole robot.

Here is the pic of the broken model:

Here is the new code:

https://pastebin.com/asvmHPG9

EDIT #2: It seems that the problem is connected with the fourth link, but currently I can't figure out where.