Double-helical (herringbone) gears, 20 : 30, helix 20° — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/double-helical-gears-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 41 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- Each gear has two rows of oblique teeth of opposite hand meeting at the middle of the face: like single helical gears the teeth engage gradually and several are always in contact, but the two axial thrusts cancel, so no thrust bearing is needed.
- Here normal module 1.5, 20 and 30 teeth, helix 20° each way, 16 mm face; centre distance (r₁ + r₂) = 39.91 mm with the transverse module 1.6. Every transverse section is an exact involute spur mesh.
- Brown's No. 41 shows a second pair of oblique-toothed wheels; this page draws the double-helical form, the answer to the end thrust of No. 40.
- Parts (each has its own page): double-helical-gear-mn1-5-20t-beta20-bore6, double-helical-gear-mn1-5-30t-beta20-bore8, linear-shafts-h6.
- Brown's movement No. 41 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

parameter  value
normal module  1.5
teeth  20 / 30
helix  20°, both hands on each gear
face  16 mm
centre distance  39.91 mm

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Henry T. Brown (1868, public domain): Five Hundred and Seven Mechanical Movements — scan on archive.org — web — Publisher's page; dimensions are facts — https://archive.org/details/fivehundredseven00browiala
- 507movements.com: Brown's movement No. 41: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_041.html

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/double-helical-gears-mechanism/index.json
API: https://xemstudio.com/api/geometry?object=double-helical-gears-mechanism