Since herringbone gears need to have one side floating, I have designed some helical gears to replace the printrbot extruder gears. Helical gears provide smoother power transmission than spur gears. My prints improved immediately after switching from the lasercut spur gears.
Simply bolt them on and enjoy. (Note: this is for the printrbot LC. I am sure it may fit others but I am not sure which ones. The large gear takes a 5/16" bolt. I slightly undersized the hole so I could drill it for the perfect fit. The small gear takes an M3 nut and screw.)