Mutilated spur gear moving a rack, a spring bringing it back — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/mutilated-gear-spring-rack-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 81 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- A spur gear with teeth on half its rim turns continuously above a rack: while its teeth are in mesh it drives the rack one way and compresses a spring; when the toothless half comes round the spring pushes the rack back.
- Catalog parts: the module 1, 20-tooth mutilated pinion (10 teeth) and the 120 mm full-depth rack; stroke = 10 × π = 31.42 mm. The rack slides on a base against a side rail; an Ø8 spring between the rack's end and the base's end wall returns it. Drawn in mesh.
- Brown's No. 81 (mutilated spur gear giving alternate rectilinear motion to a rack rod, a spring returning it).
- Parts (each has its own page): mutilated-pinion-m1-20t-half, gear-rack-m1-120-full-depth, compression-spring-od8-wire1-l30, rack-guide-base-170-spring-seat, dowel-pins-metric.
- Brown's movement No. 81 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

parameter  value
pinion  module 1, 10 of 20 teeth
stroke  31.42 mm
spring  OD8 × Ø1 wire

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. 81: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_081.html

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