Jockey (tightening) pulley engaging a slack belt — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/jockey-pulley-belt-tightener-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 5 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- The belt is made a little long, so it slips idle on the pulleys; a pulley on a lever (the jockey or tightener) pressed on the back of the slack strand takes up the slack and the drive starts. Lift the lever and it stops — a clutch without moving the belt.
- Here Ø40 and Ø80 pulleys at 140 mm, a 16 × 1.5 belt of 472.33 mm (0.98 mm longer than an open belt) and a Ø30 jockey pressing the upper strand in 8 mm. The jockey also adds wrap on the small pulley: 169.91° instead of 196.43°.
- Brown's No. 5. Put the jockey on the slack side (here the upper strand when the small pulley drives clockwise).
- Parts (each has its own page): flanged-flat-pulley-40, flanged-flat-pulley-80, flanged-flat-pulley-30-bore6, flat-belt-16x1-5-jockey-drive-40-80-c140, jockey-lever-bore6, dowel-pins-metric.
- Brown's movement No. 5 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

parameter  value
pulleys  Ø40 and Ø80 at 140 mm
jockey  Ø30, 8 mm deflection
belt  16 × 1.5, 472.33 mm
wrap on Ø40  169.91°
lever  51.48 mm + 40 mm handle

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. 5: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_005.html

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