# Gears with herringbone teeth or double helical teeth

Wheels and pinions with double helical teeth have left and right helical teeth with the same inclination and opposite sense made on the same blank.

The toothing can be “continuously”, i.e. without incision between the left and right teeth; this version leads to the best resistance to tooth breakage. It is advisable for practical and economic reasons, to put a cylindrical incision between the teeth (width from 5 to 6 mm minimum). The herringbone teeth cover less width than a double helical teeth.

When designing a gear with herringbone teeth, one  should take into account certain special features of the process Sykes on which we are also bound by the nature of our machines:

• For teeth according to the “Diametral Pitch”-system (from 10 to 2 DP DP) the arrow angle is always 120° (corresponding to two helix angles of 30°).
• For teeth calculated according to the metric module system, the arrow angle slightly deviates from this value (of module 1.75 to 12). The helix angles for this system are indicated below in a table.
• One calculates always according to transverse module or diametral pitch; also regarding the pressure angle (apparently 20°).
• The tooth depths
 – Addendum = 0,8 x apparent module – In the Anglo-saxon system the addendum is = 0,8 x  (in”) – Dedendum = 1,1 x apparent module – In the Anglo-saxon system the dedendum is = 1,1 x  (in”)
• The transverse pressure angle is always 20° except in a few special cases.

The double helical teeth eliminates axial forces, provided a perfect application and design what is not always the case. This is an advantage opposed to the common helical gear wheels. It is necessary that one of the two elements of the gear pair (preferably the pinion) is positioned axial free, so that for an equal distribution of the load on the teeth this element is axial free to continuously search for the alignment of the arrow points of pinion and gear.

When the incision can be taken sufficiently large (± 10 x module) the open double helical teeth forms two seperate helical gears and the tooth hobbing can be done with the hob. In this case there is free choice for the value of the helix angle.

Herringbone teeth (or when the incision is small) must be manufactured on special machines, such as e.g. the Sykes-machines or via free form milling on 5-axis milling machines. In the case of the Sykes machine the helix angles are defined by the pitch of the internal helix of the machine and by the pitch diameter of the cutters.

The following table indicates the helix angle for the different metric modules; for the teeth in DP the helix angle is always 30°. The arrow angle = 180° – (2 x the helix angle).

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