Does pulley size actually matter?

A pulley looks like the simplest thing in the bag: a wheel, two side plates, a hole for a carabiner. It’s also bought on size and color, because the two numbers that decide how it behaves are buried or missing.

Those numbers are efficiency — how much of the force you put in survives the direction change — and the sheave diameter measured against the rope running over it. One is published per product by exactly one of the makers below. The other you work out yourself, because nobody prints it as a ratio.

What does a pulley’s efficiency figure actually mean?

Efficiency is the share of input force that survives the trip around the sheave, and it is always under 100 percent. Petzl is unusually specific about how it gets the number: a 100 kg mass at “a constant pull rate of 1.5 meters/minute, on new devices and ropes,” across ropes from 6 mm to 12.5 mm, with the published figure being “the value obtained with the smallest compatible rope diameter for the device” (Petzl, pulley system efficiency tests).

Read that last clause twice. The percentage on the product page is the friendliest case in the range — thinnest rope, brand new, moving slowly. Petzl says so itself: the results “are given for information only,” because “in the field there are numerous variables to take into account (position of haul system elements, rope diameter and construction, hauling speed, hauling smoothness, the haul load…).” The figures multiply, and that is the whole of the arithmetic: two pulleys at 95 percent return about 90 percent of what you feed in; two at 71 percent, about 50 percent.

Which pulley makers publish an efficiency number?

One of them, per product. Petzl lists an efficiency percentage on every pulley page. The rest publish sheave sizes, rope limits and strengths, and describe efficiency only in prose.

Pulley	Published efficiency	Sheave	Rope window	Bearing	Strength
Petzl MINDER L1	97%	51 mm	6–13 mm	ball bearings	36 kN, 8 kN WLL
Petzl RESCUE M	95%	38 mm	6–13 mm	ball bearings	36 kN, 8 kN WLL
Petzl FIXE	71%	21 mm	6–13 mm	self-lubricating bushings	23 kN, 5 kN WLL
CMC ProTech single	none published	25 mm tread	up to 11 mm	sealed ball bearings	36 kN 3-sigma
CMC Rescue Pulley single	none published	38 mm tread	up to 13 mm	sealed ball bearings	52 kN 3-sigma
DMM Pinto Rig 2	none published	38 mm	up to 16 mm	sealed ball bearings	50 kN, 10 kN WLL
SMC Swiftwater	none published	2 in	up to 13 mm	sealed ball bearings	34 kN
Rock Exotica Omni-Block 2.0 Double	none published	2.0 in	13 mm	not stated	40 kN, 8 kN WLL
Teufelberger tBLOCK	none published	not published	up to 16 mm	Teflon plain bearing	225 kN, 45 kN WLL

An empty cell is a publishing gap, not a zero: every pulley here has an efficiency; most makers won’t say what it is.

CMC is the interesting case. Its product pages carry no percentage, but its explainer on actual mechanical advantage works one example in which “each pulley is 85% efficient (resulting in 15% friction loss)” and another in which “all pulleys have an efficiency of 95%,” and reports that “tests have indicated that the difference in pulley efficiency can be as much as 15%, depending on the stiffness of the rope” without naming the tests (CMC, actual mechanical advantage). The same page puts a carabiner standing in for a pulley at “usually around 50%” — worth knowing before reading much into carabiner strength markings.

Harken Industrial’s catalog covers sheaves of “57 – 150 mm,” fiber rope to 25 mm and rated loads of 1 kN to 31 kN, and describes its bearings without quantifying them (Harken Industrial, pulleys catalog). CMI offers its double-ended pulleys “with bearing or bushing” and publishes no sheave size, rope limit or strength at all (CMI, double-ended pulleys).

How much bigger than the rope does the sheave have to be?

Published minimums exist, they’re higher than the 4:1 figure people quote, and no pulley in the table meets them at its own maximum rope diameter. Inside Towers, summarizing ANSI/ASSP A10.48, gives a minimum 6:1 sheave-to-rope ratio for double-braid and kernmantle and 10:1 for three-strand (Inside Towers, rope and rigging compatibility). Marlow, writing about HMPE, recommends “≥ 8:1 (10:1 preferred)” for general sheaves and warns that “smaller D:d ratios increase point loading, which leads to faster wear, strength loss, and potential failure” (Marlow, HMPE working loads and D:d ratios).

Now divide each maker’s published sheave by its published maximum rope. Petzl’s MINDER L1 is 51 over 13, about 3.9 to 1; the RESCUE M, 2.9; the FIXE, 1.6. DMM’s Pinto Rig 2, 38 over 16, is 2.4. CMC’s ProTech, 25 over 11, is 2.3. Not one clears 4:1, let alone 6:1.

Two caveats. Those ratios are our arithmetic, not anything a maker states. And the inputs may not be the same measurement: CMC publishes a “sheave tread size,” Petzl and DMM a “sheave diameter,” and wire-rope D/d practice is taken at the rope’s centerline, which is larger than the tread. Nobody says which convention applies. The other place bend radius gets decided is where rope crosses structure — see edge protection.

What is a bushing for, if ball bearings are more efficient?

Plain bearings trade running efficiency for static load capacity, weight and cost. Petzl’s FIXE and MOBILE both run “self-lubricating bushings,” both publish 71 percent on a 21 mm sheave, and the MOBILE weighs 68 g (Petzl, MOBILE).

Harken Industrial states the trade directly, describing its ESP blocks as using “sleeve bearings to handle high static loads” while “ball bearings handle sideloads.” Teufelberger’s tBLOCK is the proof: the heaviest-rated block here, 225 kN with a 45 kN working load limit, on a Teflon plain bearing.

One thing Petzl’s numbers can’t tell you: its bushing pulleys also carry its smallest sheaves. Bearing type and sheave diameter move together across the range, so nothing published separates the two effects.

What does the strength number on a pulley cover?

The assembled pulley with rope loaded over the sheave — not the axle by itself. SATRA’s summary of EN 12278:2007 describes two samples “with a loop of rope wrapped round” placed under a 2 kN load, rotated “fully in each direction 10 times,” then pulled to failure against “a minimum requirement of 15kN or the rated load marked on the pulley, if this is larger than the minimum” (SATRA, EN 12278:2007).

CMC’s summary of the NFPA hardware requirements sets a General Use pulley at “3σ MBS of not less than 36 kN (8,093 lbf.)” with a 19.5 kN becket, and a Technical Use pulley at 22 kN with a 12 kN becket (CMC, NFPA 1983 hardware) — the split covered in NFPA G versus T ratings.

The working load limit is a separate and inconsistent number. Petzl writes it as a sum, “4 kN x 2 = 8 kN,” because the sheave carries both rope legs. DMM publishes 35 kN against a 5 kN working load on the Pinto 2, a factor of seven; Rock Exotica’s Omni-Block 2.0 Double, 40 kN and 8 kN, a factor of five; Harken, only that “rated loads are 4:1 coefficient ratio of the type test load.” Three makers, three design factors — and the distinction underneath is what MBS means.

Do prusik-minding side plates change what fits?

Flat-bottomed side plates exist to push a friction hitch off the sheave as the pulley returns to the anchor; rounded plates don’t. Petzl describes the MINDER L1 as having “moving side plates designed for use with a Prusik friction hitch.” CMC lists the plates on its single Rescue Pulley as “Prussik-minding aluminum,” and says machining the ProTech plates means “the thickness of the metal can be optimized to maximize the strength-to-weight ratio, a process that cannot be done with the traditional bent metal side plates.”

None of those pages publishes the cord diameter the plate geometry was designed around — that one gets answered by measuring.

What rope will the pulley actually take?

Most makers publish only a maximum; Petzl publishes both ends. The SPIN L1 lists “Min. rope diameter: 7 mm” and “Max. rope diameter: 13 mm.” SMC’s Swiftwater gives a 1/2 in (13 mm) maximum and no minimum (SMC, Swiftwater Pulley); DMM’s Pinto 2, 14 mm; CMC’s Rescue Pulley, “up to 13 mm.”

A maximum tells you what won’t fit the groove, not the bottom of the range the sheave was cut for — which is the number that matters with thin cord. And the diameter to check is the measured one, not the label (what rope diameter to use).

The short version

Pulley size matters twice over: a bigger sheave and a ball bearing both lift the efficiency figure, and Petzl’s own range runs 71 to 97 percent on that basis alone. The catch is that Petzl is the only maker here publishing a percentage per product, and the only one publishing a method, so there is nothing legitimate to compare it against. The sheave-to-rope ratios standards and rope makers publish — 6:1, 8:1, 10:1 — all sit above what any rescue pulley delivers at its rated maximum rope. Buy the biggest sheave you’ll carry, read the rope window at both ends, and treat an efficiency figure with no stated test as marketing.