Topics covered:
0:00 Modelling a helical gear using sweep
2:10 Creating a herringbone gear using mirror and combine
3:15 Creating a double helical gear
4:41 How to compute the angle to put into the sweep command
7:28 Positive and negative twist angles, or handedness
9:34 Transverse and normal systems
13:26 Shaft angle is sum of helix angles
15:06 Axial forces in helical gears
17:42 Herringbone gears can't slide into mesh
19:28 Herringbone gears do have a handedness in most cases
20:39 Helical gears have better contact ratio, so they run smoother
22:19 Machining the different kinds of gears
23:57 Comparing the method demonstrated with a helical gear generator
27:34 Herringbone gears and grease/dirt trapping

Full gears playlist:    • All about gears  
Next part (racks):    • How to model Gear Racks, Straight and...  

For the Emmett gear bearing see:
https://www.thingiverse.com/thing:53451

This old Tony did a great video on cutting spur gears:
   • Gears! - But Were Afraid To Ask (Mini...  

These are some resources that go far more indepth on helical gears:
https://www.sdp-si.com/resources/elem...
https://khkgears.net/new/helical_gear...

Section 4.3 on khk's awesome gear guide:
https://khkgears.net/new/gear_knowled...