Pulleys of the third system, three pulleys (7 : 1) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/third-system-pulleys-7-to-1-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 21 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- Every rope ends on the load: rope 1 runs from the load over the fixed pulley to the first runner, rope 2 from the load over that runner to the second, rope 3 from the load over the second runner to the hand. Tensions double up the chain — P, 2P, 4P — and the load gets all three: W = 7P (2ⁿ − 1 with n = 3).
- Pitch-Ø80 fixed block, pitch-Ø40 and pitch-Ø20 runners; rope ends on a load bar at x = −40, 20 and 50, whose weight stem sits on the resultant (x = −10).
- Brown's No. 21 (a combination following the doubling rule; the single Spanish burton, No. 16, is the same system with two pulleys).
- Parts (each has its own page): tackle-beam-240-sockets, tackle-block-40-top-stem, tackle-block-20-top-socket, tackle-block-10-top-socket, rope-sheave-3mm-pitch-80, rope-sheave-3mm-pitch-40, rope-sheave-3mm-pitch-20, ropes-3mm-third-system-pulleys-7-to-1, tackle-weight-d40x40-socket, load-bar-100-sockets-m40-20-50, dowel-pins-metric.
- Brown's movement No. 21 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

parameter  value
fixed block  pitch Ø80
runners  pitch Ø40 and Ø20
rope tensions  4P, 2P, P
mechanical advantage  7

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. 21: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_021.html

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/third-system-pulleys-7-to-1-mechanism/index.json
API: https://xemstudio.com/api/geometry?object=third-system-pulleys-7-to-1-mechanism