How to make automatic swing (Hinge joint 2D?)

I'm trying to make a simple automatic swing that swings to left and right without losing speed. I am able to create object that rotates full 360 degrees using hingejoint2d and using it's motor, but I don't know how I would create a swing (I tried playing with angle limits but with no luck). The closest I was is to use motor and then disable it, which causes the object to swing from one side to other, but it loses speed over time.

 

Reduce / remove drag of rigid body.