Improved needle bar driving system for sewing machines, that comprises an armshaft rotating with a predetermined speed; a needle bar driving assembly driven by the armshaft and has a needle bar that performs a reciprocating guided linear movement along a substantially vertical axis; a counterweight driving assembly is driven by the armshaft; and a counterweight piston driven by the counterweight driving assembly, and the counterweight piston also performs a reciprocating linear guided movement along an axis parallel to the axis, wherein the movements of the counterweight driving assembly and of the counterweight piston are always opposite to the movement of the needle bar driving assembly and of the needle bar, wherein the combined torque of the counterweight piston and the counterweight driving assembly acting on the armshaft is substantially equal to the combined torque of the needle bar and needle bar driving assembly acting also on the armshaft, but the two torques have opposite direction, whereby the armshaft gets substantially balanced.