sheaves are more efficient than ones with small...

Some of the factors affecting pulley efficiency in theoretical and real‐world rescue systems.

2011International Technical Rescue SymposiumPresented by John McKently

It is generally accepted that pulleys with large diameter

sheaves are more efficient than ones with small diameter

sheaves. We wondered, “Well, how much more efficient?”

for pulleys typically used in rope rescue. We also

wondered “What other factors should be considered in

deciding on the right pulley for a given application?” In

other words, given all the other pros and cons, is a pulley

with a bigger diameter sheave always a better choice?

No pulley is 100% efficient, as there are always losses due

to friction. While testing pulleys in a lab can provide a

reasonable estimate of their efficiency, variables such as

line stretch, bending and unbending, and line/sheave

friction will affect the results. Even the use of steel cable

cannot completely eliminate these variables.

In this exploration we were more interested in the

relative efficiencies of the various pulleys tested, not

their absolute efficiency.

While we acknowledge that the results obtained in this

testing will differ somewhat from results using different

pulleys and/or ropes, we believe they are representative

of the results that would be obtained using pulleys, ropes

and system configurations that are typical to rope rescue

operations. As such, we find them valuable as a lead-in

point for discussions on equipment selection.

Single sheave pulleys used (all with sealed ball bearings):CMC Rescue 1.5” tread diameterCMC ProSeries 2.25” tread diameterCMC ProSeries 3.75” tread diameter

Test masses used: 300 and 600 poundsRopes used:

1/4” steel cable3/8” Static-Pro (100% polyester kernmantle)7/16” River Rescue Lifeline (nylon/polyolefin, braid-on-braid)1/2” Static-Pro (100% polyester kernmantle)1/2” CMC Lifeline (100% nylon kernmantle)

Other pulleys tested in a 1:1 system:

Steel carabiner with 0.5” diameter frame

DMM Pinto - 0.75” sheave with Oilite bushing

Petzl Rescue - 1.5” sheave with sealed ball bearing

CMI RP121 - 1.625” sheave with bronze bushing

CMI RP121A - 1.625” sheave with sealed needle

bearing

For the 1:1 system tests, one end of the haul line was connected directly to the mass. The line was run up through the pulley to be tested, then down to a COD pulley anchored to the floor, and up to an electric winch used to lift the load.

A load cell was placed in-line between the pulley being evaluated and the COD pulley to record the force on the line at a rate of 200 times per second. The interior angle of the line through the pulley being tested was small, approximately 5-7° depending on the sheave size.

For the M/A tests, the

COD pulleys were

anchored above and the

moving pulleys were

connected to the mass.

The load cell was

placed between the end

of the haul line and the

winch.

The mass was lifted 24” at a rate of 2” per second.

As can be seen in the graph from the sample raw data, the

force on the haul line leveled off and remained essentially

constant after the mass was in motion and the stretch had

been taken out of the line.

For each set-up, two evolutions were recorded. The force

on the line was averaged over a 4 second span after it had

leveled off, and then averaged for the two evolutions. The

following graphs are based on these averages.

559.68

443.96

378.44358.91 358.45 350.91 345.14

330.80

200

300

400

500

600

700

0.5" Steel Carabiner

DMM 0.75" CMI 1.625" bushing

Petzl 1.5" CMC 1.5" CMI 1.625" bearing

CMC 2.25" CMC 3.75"

Poun

ds Force Req

uired

Pulley Type

1:1 system with 300 pound mass on 1/2" Static‐Pro

53.6

67.57

79.27

83.59 83.6985.49 86.92

90.69

40

50

60

70

80

90

100

0.5" Steel Carabiner

DMM 0.75" CMI 1.625" bushing

Petzl 1.5" CMC 1.5" CMI 1.625" bearing

CMC 2.25" CMC 3.75"

Effic

ienc

y (%

)

Pulley Type

1:1 system with 300 pound mass on 1/2" Static‐Pro

80

85

90

95

100

1.5" 2.25" 3.75"

Effic

ienc

y (%

)

Sheave Tread Diameter

600#

300#

300

400

500

600

700

800

1.5" 2.25" 3.75"

Poun

ds Force Req

uired


600#

300# 1:1 system with 1/2” Static-Pro

Observation: measured efficiency is slightly higher with a larger mass

251.86

228.57

220230240250260270280

1.5" 3.75"

Lbf R

equired


2.38

2.63

2.0

2.2

2.4

2.6

2.8

3.0

1.5" 3.75"

Actua

l M/A


79

88

50

60

70

80

90

100

1.5" 3.75"

Effic

ienc

y (%

)


21

12

0

10

20

30

40

50

1.5" 3.75"

% M

/A Los

t


System configuration: one COD pulley anchored above, one moving pulley attached to massRope: 1/2” Static-Pro

181.79

151.88

140150160170180190200

1.5" 3.75"

Lbf R

equired


3.3

3.95

3.03.23.43.63.84.04.2

1.5" 3.75"

Actua

l M/A


34

21

0

10

20

30

40

50

1.5" 3.75"

% M

/A Los

t


System configuration: two COD pulleys anchored above, two moving pulleys attached to massRope: 1/2” Static-Pro

66

79

50

60

70

80

90

100

1.5" 3.75"

Effic

ienc

y (%

)


107.13

84.21

708090100110120130

1.5" 3.75"

Lbf R

equired


5.6

7.13

5.05.56.06.57.07.58.0

1.5" 3.75"

Actua

l M/A


38

21

0

10

20

30

40

50

1.5" 3.75"

% M

/A Los

t


62.2

79.2

50

60

70

80

90

100

1.5" 3.75"

Effic

ienc

y (%

)


System configuration: two COD pulleys anchored above, one moving pulley attached to mass, second moving pulley attached via 8mm Prusik to input side of first moving pulleyRope: 1/2” Static-Pro

600 pound mass on 1/2” Static-Pro

251.86228.57

181.79151.88

107.1384.21

050

100150200250300

1.5" 3.75"

Lbf R

equired


3:1

5:1

9:1

M/A

79

88

66

79

62

79

50

60

70

80

90

100

1.5" 3.75"

Effic

ienc

y (%

)


3:1

5:1

9:1

M/A

5.6

7.13

3.33.95

2.38 2.63

0

2

4

6

8

10

1.5" 3.75"

Actua

l M/A


9:1

5:1

3:1

M/A

38

21

34

2121

12

0

10

20

30

40

50

1.5" 3.75"

% M

/A Los

t


9:1

5:1

3:1

M/A

A larger diameter sheave is more efficient, by as much as

7% in the tread diameter range of 1.5” to 3.75” when

hauling in a 1:1 system.

The advantage in efficiency gained by the larger sheave is

magnified somewhat in M/A systems. It was as high as 17%

in the 9:1 M/A.

The consistency of the results indicates that the test, as

conducted, is repeatable.

290300310320330340350360370

1.5" 2.25" 3.75"

Poun

ds Force Req

uired


1:1 system with 300 Pound Mass – Lifting Force Required

1/2" Static‐Pro

1/2" CMC LifeLine

3/8" Static‐Pro

7/16" River Rescue Lifeline

1/4" Steel Cable

Rope Type

828486889092949698100

1.5" 2.25" 3.75"

Effic

ienc

y (% )


1:1 system with 300 Pound Mass – Efficiency

1/4" Steel Cable

7/16" River Rescue Lifeline

3/8" Static‐Pro

1/2" CMC LifeLine

1/2" Static‐Pro

Rope Type

With steel cable, the largest diameter sheave was only 1.4%

more efficient than the smallest.

For a given sheave size, a smaller diameter rope of the

same type (Static-Pro) was as much as 3% more efficient.

1/2” CMC Lifeline, a softer rope with more elongation,

was approximately 1.5% more efficient than 1/2” Static-Pro.

River Rescue Lifeline, the softest rope with the most

elongation tested, was the most efficient, by as much as 4%.

130.87125.95

123.15

113.64

110115120125130135140

#1 #2 #3 #4

Lbf R

equired

System configuration

2.292.38 2.44

2.64

2.0

2.2

2.4

2.6

2.8

3.0

#1 #2 #3 #4

Actua

l M/A


24 21 1912

0

10

20

30

40

50

#1 #2 #3 #4

% M

/A Los

t


3:1 M/A System with 300 Pound Mass System configurations:#1 – 1.5” COD, 1.5” moving #2 – 3.75” COD, 1.5” moving#3 – 1.5” COD, 3.75” moving #4 – 3.75” COD, 3.75” movingAll with 1/2” Static-Pro

76.4 79.4 81.288

50

60

70

80

90

100

#1 #2 #3 #4

Effic

ienc

y (%

)


If building M/A systems with a mix of sheave sizes, placing

the largest sheave at the moving pulley position and the

smallest at the COD position provides more efficiency than

the other way around.

The difference in efficiency between an M/A with all small

pulleys and a mixed system was not as significant as that

between a mixed system and one with all large pulleys.

251.86228.57

181.79151.88

107.1384.21

050

100150200250300

1.5" 3.75"

Lbf R

equired


3:1

5:1

9:1

M/AWhen building an M/A System, the

total weight of the pulleys may be

a consideration. A general market

survey of rescue pulleys found an

average of 8.8 oz. for 2” sheaves,

17.6 oz. for 3” sheaves and 26.2

oz. for 4” sheaves. In our test,

the larger pulleys added more

than 6 pounds to the 5:1 and 9:1

systems.

Extra weight of carrying 3.75” sheaves vs. 1.5” sheaves

3:1 5:1 9:1

4.675 lbs 6.225 lbs 6.225 lbs

Extra force required to lift a 300 pound mass with 1.5” sheave vs. 3.75” sheave

3:1 5:1 9:1

23.29 lbs 29.92 lbs 22.92 lbs

Let’s go back to our original question of “Is a pulley with a larger sheave the better choice?” and carry it a step further: “And if so, how much better?”

True, there is a difference in pulley efficiency, and with all other things being equal the larger size is more efficient. However, that increase in efficiency needs to be tempered by the additional cost, weight, and bulk of the larger size. Those factors would be very important to a mountain rescue team that might carry or fly their equipment deep into the back country. Industrial or urban fire responders might downplay size and weight as significant factors, yet hauling an extra 6 pounds of pulleys up a tower crane will slow you down. Rope seems to be another factor. Should you use softer rope for haul systems? Is the stretch a concern for hauling as much as it might be for a belay?

Some of the results of these tests confirmed generally accepted theories, but they varied slightly from those previously stated. The relatively small difference between the 1.5” and 3.75” diameter sheaves when tested with cable also follows the results we obtained when testing anchors (ITRS-2007: McKently, Parker, Smith) which tend to disprove the idea that anything less than 4X the rope diameter adversely impacts rope strength and system efficiency, at least in a 1:1 system. Finally, while our results were measured with very precise electronic instruments, can the individual or team doing the hauling really tell the difference in the field?

Sheave tread width

Sheave material

Interior angle

Haul team performance (grip strength, footing, fatigue, etc.)

Rope construction (Kernmantle, braid-on-braid,

three strand, etc.)

Rope condition (icy, muddy, wet, old/fuzzy)

System setup (pulling up vs. down, etc.)

For more information on this presentation, please contact John [email protected]

2011International Technical Rescue SymposiumPresented by John McKently

mailto:[email protected]

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