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Pulleys of the third system, three pulleys (7 : 1)

How does the third system of pulleys work? · Hardware · basis: Xem Studio design following Brown No. 21 (1868)

Also called: third system of pulleys, spanish burton, 7 to 1 pulley, compound pulley, sistema de poleas de tercera clase (7:1), Rollensystem dritter Art (7:1)

Exact dimensions. True size, with no printing tolerance added.

sizes
4 in table

Notes

  • 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.

Pulleys of the third system, three pulleys size chart

parametervalue
fixed blockpitch Ø80
runnerspitch Ø40 and Ø20
rope tensions4P, 2P, P
mechanical advantage7

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

Henry T. Brown (1868, public domain)Five Hundred and Seven Mechanical Movements — scan on archive.orgwebPublisher's page; dimensions are facts
507movements.comBrown's movement No. 21: figure, his text and an animationwebPublisher's page; dimensions are facts

Free files

★ Free files 1 parts. Xem Studio design: free to use, print and sell things from; not to be republished as files. Xem Studio Design License. index.json

third-system-pulleys-7-to-1-mechanism-assemblyAssembled mechanism, all parts placed at one pose (STEP keeps each part separate)STEP · STL
Let your AI pull this. Add https://xemstudio.com/mcp to Claude, ChatGPT, Cursor or any MCP client (free, no account, one minute) and it can look up exact sizes, mating fits and free STEP/STL from every chat.
Set it up →

Plain text: sheet.txt · JSON: /api/geometry?object=third-system-pulleys-7-to-1-mechanism

Pulleys of the third system, three pulleys (7 : 1), isometric render of a generated part
Render of third-system-pulleys-7-to-1-mechanism-assembly from the free STEP; not to scale.

Fits with

Tackle beam 240 × 20 × 12 with Ø3 sockets every 5 mm, isometric render of a generated part
Tackle beam 240 × 20 × 12 with Ø3 sockets every 5 mm240.0×20.0×12.0 mm★ 1 file
Single block for a pitch-Ø80 sheave, stem up (fixed block), isometric render of a generated part
Single block for a pitch-Ø80 sheave, stem up (fixed block)90.4×13.0×108.2 mm★ 1 file
Single runner block for a pitch-Ø40 sheave, rope socket on top, isometric render of a generated part
Single runner block for a pitch-Ø40 sheave, rope socket on top50.4×13.0×55.2 mm★ 1 file
Single runner block for a pitch-Ø20 sheave, rope socket on top, isometric render of a generated part
Single runner block for a pitch-Ø20 sheave, rope socket on top30.4×13.0×35.2 mm★ 1 file
Rope sheave for Ø3 rope, pitch Ø80 (rope centre), Ø84.4 rim, Ø4 bore, isometric render of a generated part
Rope sheave for Ø3 rope, pitch Ø80 (rope centre), Ø84.4 rim, Ø4 bore84.4×8.0×84.4 mm★ 1 file
Rope sheave for Ø3 rope, pitch Ø40 (rope centre), Ø44.4 rim, Ø4 bore, isometric render of a generated part
Rope sheave for Ø3 rope, pitch Ø40 (rope centre), Ø44.4 rim, Ø4 bore44.4×8.0×44.4 mm★ 1 file
Rope sheave for Ø3 rope, pitch Ø20 (rope centre), Ø24.4 rim, Ø4 bore, isometric render of a generated part
Rope sheave for Ø3 rope, pitch Ø20 (rope centre), Ø24.4 rim, Ø4 bore24.4×8.0×24.4 mm★ 1 file
Ø3 ropes for the pulleys of the third system, three pulleys (drawn in place), isometric render of a generated part
Ø3 ropes for the pulleys of the third system, three pulleys (drawn in place)83.0×3.0×281.5 mm★ 3 files
Weight Ø40 × 40 with a Ø3 top socket (394.58 g in steel), isometric render of a generated part
Weight Ø40 × 40 with a Ø3 top socket (394.58 g in steel)40.0×40.0×40.0 mm★ 1 file
Load bar 100 × 10 × 8 with 3 rope sockets and a weight stem, isometric render of a generated part
Load bar 100 × 10 × 8 with 3 rope sockets and a weight stem100.0×10.0×16.0 mm★ 1 file
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325), isometric render of a generated part
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)5 sizes★ 23 files

Questions

How does the third system of pulleys work?

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).