Pulleys

Device efficiency (η) is output tension ÷ input tension for that sheave or device. 95% means about 5% of tension is lost to friction. Published manufacturer and test-lab figures — field rope, dirt, and small sheaves run lower. System AMA ≈ TMA × η^(moving pulleys). AHJ and the manufacturer’s website win.

Devices

• CMC Capto

  Raise 88–91%

  Needle-roller, 44.5 mm sheave. Combined ≈ 86–88%. ~90.8% raising with 11 mm.

• CMC Clutch (11 mm)

  71% in 1:1

  Official CMC test. ≈ 85% when used in a 2:1 or 3:1. Better than traditional cam descenders.

• CMC MPD

  ≈ 88%

  High-efficiency pulley in raise mode.

• CMC ProSwivel (ball bearing)

  85–95%

  Sealed ball bearings. Size-dependent — 2.0″ models up to ~3% more efficient than 1.5″.

• CMC HD / large PMP

  ≈ 90%+

  Large sheave, sealed bearings.

• Petzl Pro Traxion

  95% official

  Sealed ball bearings, 38 mm sheave. Independent tests often mid-80s.

• Petzl Micro Traxion

  91–93% official

  Compact. Independent tests often high-70s to low-80s.

• Petzl Maestro S

  86% official

  Integrated progress-capture pulley.

• Petzl Maestro L

  81% official

  Larger model.

• Petzl Rescue

  95% official

  Independent tests ≈ 86%. Common high-efficiency single.

• Petzl Twin / Minder

  97% official

  Large 51 mm sheaves, sealed ball bearings. Among the highest pure-pulley figures.

• Rock Exotica Omni-Block / PMP

  80–90%

  Same family as ProSwivel. 2.0″ ≈ 89–90%, 1.5″ ≈ 85%, 1.1″ ≈ 80–84%.

• SMC 3″ PMP

  94–96%

  Large sheave, ball-bearing. Frequently cited as one of the most efficient pure pulleys.

• Oilite / bushing pulley

  ≈ 80%

  Plain bearing. CMC teaching examples often use 80–85% for a typical rescue pulley.

• Sealed ball-bearing 2″

  ≈ 95%

  CMC / NFPA development tests: a 2″ sealed ball-bearing pulley is about 95% efficient.

• Round-aspect carabiner

  ≈ 50%

  Not a pulley. CMC and MRA both cite ~50%. Tight bend radius. I-beam designs usually lower.

System AMA

App estimate: TMA × η^(moving pulleys). Matches the Haul systems calculator.

• App AMA ≈ TMA × η^(moving pulleys). Directionals add friction the T-method also counts.
• CMC T-method example: a 3:1 with 85% pulleys is 2.57:1 actual; at 95% it is 2.85:1.
• Carabiner used as a pulley is typically ~50% (CMC, MRA). Do not treat it as a sheave.
• Petzl published: Maestro S 86%, Maestro L 81%, Pro Traxion 95% official (field tests often lower), Twin 97%.
• Independent tests often read 5–15 points below manufacturer claims once the rope is dirty or bent over a small sheave.