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Pulleys: Classification & Characteristics
Pulleys: Classification & Characteristics
Image from: notchequipment.com
Written in December 2023.
The relevance of information might change over time.
Written in December 2023.
The relevance of information might change over time.
Pulley is a simple device consisting of a grooved wheel that rotates on an axis, and through which a rope or cable is threaded.
Pulleys are used in various fields such as:
And are utilized for:
Pulleys are used in various fields such as:
- Climbing, slacklining, ziplining;
- Construction, work at heights;
- Rescue operations;
- Yachting, and etc.
And are utilized for:
The construction parts common to all types of pulleys include:
- Axis
- Sheave (wheel)
- Side plates
- Attachment point
One of the most common block designs exemplified by the Machined Pulley model from Rock Exotica.
Image from: rockexotica.com
This is what unites all blocks. So, what sets them apart?
Pulley Classification
Number of Sheaves
By the number of sheaves, pulleys can be categorized as:
- Single
- Double
- Triple, and so on.
Single, double, and triple pulley from Climbing Technology.
Image from: climbingtechnology.com
Sheave Arrangement
Based on the sheave arrangement, pulleys can be divided into:
- Standard pulleys, where the sheaves are positioned in parallel, and
- Transport pulleys (tyrolean pulleys, tandem pulleys, trolleys, carriages), where the sheaves are arranged in a single line.
Transport pulleys are specifically designed for movement along horizontal and inclined fixed ropes or cables. They are commonly used in applications such as zip lines, adventure parks, tyrolean traverses, and tensioned highlines. Equipped with two or more sheaves arranged in a single line, transport pulleys provide stability and enable high-speed movement, reaching speeds of up to 30 m/s. Advanced recreational models (trolleys) often feature special attachment systems, allowing quick installation to the rope or cable without the need for carabiners and ensuring reliable protection against accidental opening. In addition, trolleys are frequently equipped with plastic protectors to prevent users from inadvertently placing their fingers in the moving parts.
Some transport pulleys find application beyond adventure parks and extreme recreation. Sophisticated models, such as the Petzl Reeve or DMM Keanu, are widely utilized in rigging and rescue operations. These carriage pulleys stand out with multiple attachment points and enhanced strength, allowing for the construction of complex lifting and moving systems for casualties and heavy loads.
Sheave Design
By the sheave design, pulleys can be classified into:
Bi-directional pulleys are the standard ones, with sheaves freely rotating clockwise and counterclockwise.
The situation is different with uni-directional pulleys. Consider the Spin L1D pulley from Petzl as an example. In this model, the sheave rotates only in one direction – think of it as a 'hauling mode.' However, during lowering, the sheave remains stationary. This, combined with the sheave's faceted design, creates additional friction, allowing the pulley to hold or lower heavy loads with half the effort.
- Bi-directional pulleys (two-directional, two-way pulleys)
- Uni-directional pulleys (one-directional, one-way pulleys)
Bi-directional pulleys are the standard ones, with sheaves freely rotating clockwise and counterclockwise.
The situation is different with uni-directional pulleys. Consider the Spin L1D pulley from Petzl as an example. In this model, the sheave rotates only in one direction – think of it as a 'hauling mode.' However, during lowering, the sheave remains stationary. This, combined with the sheave's faceted design, creates additional friction, allowing the pulley to hold or lower heavy loads with half the effort.
Side Plates Design
According to the side plates design pulleys can be divided into:
- Pulleys with fixed side plates
- Pulleys with swinging (pivoting, moving) side plates
Fixed plate pulleys are generally more robust compared to models with swinging side plates, but they may slightly lag behind the latter in terms of versatility. This is because the attachment eyelets on fixed side plates are spaced wider, limiting the choice of available carabiners and other connectors. For instance, such pulleys may not pair well with D-shaped carabiners, favoring ovals and pear-shaped ones.
- Prusik Minding Pulleys
- Button Locking Pulleys
Main Attachment Point Design
In terms of the main attachment point design, there are:
- Pulleys with an Eyelet Main Attachment Point
CAMP Tethys Pro prusik-minding pulley features the attachment point in the form of a eyelet. Despite its relatively small size, it can easily accommodate a pair of carabiners.
Image from: camp.it
- Pulleys with a Bollard Main Attachment Point
The bollard attachment point can be:
- Fixed
- Openable: screw-style / spring-style.
- Swivel Pulleys
Rock Exotica 'Omni-Block' swivel pulley.
Image from: Gardenforst
The attachment point of the swivel pulleys can take the form of:
- Eyelet: fixed or openable (aka gated)
- Shackle
- Carabiner
Another advantage of pulleys with an integrated swivel is that the attachment eyelet of such models can accommodate multiple carabiners and allows direct connection of ropes and slings.
The attachment eyelet of most swivel pulleys accepts multiple carabiners and allows direct tie-in of textile connectors.
Image from: petzl.com
Pulleys with Multiple Attachment Points
Some models may feature more than one attachment point. The most common example is pulleys with an additional eyelet, also known as a becket, located beneath the pulley wheel. This point is primarily utilized for securing the end of the rope when setting up pulley systems. If the becket has sufficient thickness and curvature, the rope can be tied directly to it; otherwise, a carabiner, quick link, or other metal connector is used. The size of the becket hole does matter; a wider hole accommodates more connector options, while a shorter hole allows the pulleys in a system to be brought closer together, slightly enhancing overall efficiency.
There are also triple attachment pulleys, popular among professionals for their design versatility. For example, arborists use this type of pulleys for ascending and descending in the Moving Rope System (MRS) technique. Another application lies in rescue operations, where the side holes of the triple attachment pulley are utilized to move the device along a tyrolean using the corresponding guiding ropes.
Progress Capture Pulleys
Some pulleys come with an integrated cam brake that allows them to lock the rope in the reverse direction without the need for additional prusik hitches or separate devices. These are called "progress capture pulleys" (PCP).
Examples of PCPs:
- Petzl Pro Traxion
- Edelrid Spoc
- CAMP Turbolock
- Climbing Technology Sneak Peek Cric
- ISC Double Progress Capture Pulley
Examples of PCPs:
- Petzl Pro Traxion
- Edelrid Spoc
- CAMP Turbolock
- Climbing Technology Sneak Peek Cric
- ISC Double Progress Capture Pulley
With the exception of the ultra-compact Petzl Nano Traxion model, progress capture pulleys can be used as regular two-way pulleys by "disabling" the progress capture function.
- Releasable Progress Capture Pulleys
When a release handle is added to a single progress capture pulley, the resulting piece of gear becomes a truly multi-purpose device suitable not only for efficient hauling and controlled lowering but also for smooth descents, ascents, and belaying. However, this pleasure comes with a very high price tag.
Examples:
- Petzl Maestro
- CMC Clutch
- CMC MPD
Examples:
- Petzl Maestro
- CMC Clutch
- CMC MPD
Specialized Pulleys
- Knot-Passing Pulleys
- Pulleys for Cableway Evacuation
- Webbing Pulleys
Integrated Pulleys
- Hand Ascenders with Integrated Pulleys
- Pulley-Carabiners
- Webbing Pulley-Carabiners
More information about these models can be found in my article - "How to tension a slackline."
Integrable Pulleys
- Carabiners with Integrable Pulleys
- Shackles with Integrable Pulleys
- Emergency Pulleys
Pulley Systems
A pulley system, also known as block and tackle, is a mechanism comprising moving and fixed pulleys connected by a flexible link (rope, webbing, or cable). Its purpose is to provide mechanical advantage for lifting or pulling heavy objects.
A pulley system can be constructed from individual components such as pulleys, carabiners, rope, clamps, friction hitches, and more. Alternatively, it can take the form of a ready-made set/kit optimized for a specific task. The following discussion will focus on ready-made models from manufacturers.
A pulley system can be constructed from individual components such as pulleys, carabiners, rope, clamps, friction hitches, and more. Alternatively, it can take the form of a ready-made set/kit optimized for a specific task. The following discussion will focus on ready-made models from manufacturers.
- Rescue Pulley Systems
Examples:
- Petzl Jag System
- Rock Exotica Aztek Kit
- Harken Wingman
- Slackline Pulley Systems
And this is just a glimpse into the world of pulleys! Imagine the myriad of construction pulleys, yachting pulleys, and various other types not mentioned... Omg!
Pulleys Characteristics
In addition to the types and constructions discussed earlier, pulleys are characterized by:
Pulley Efficiency
Pulley efficiency is a measure of how effectively the device mitigates friction. The closer the value is to 100%, the better your efforts are transferred to the load.
Pulley efficiency depends on:
Pulley efficiency depends on:
- The type of device providing sheave rotation – whether it employs bushings or ball bearings—and the quality of these components.
- The sheave diameter and its ratio to the diameter of the rope being used.
- The stiffness of the rope.
Bushing
A bushing is a cylinder made of a specific material, typically bronze, through which the sheave rotates around the pulley's axis. Due to a significant portion of the inner surface of the bushing being in contact with the axis, achieving high efficiency and a low coefficient of friction is challenging. Therefore, pulleys with bushings are generally found in inexpensive models, with efficiency ranging from 70% to 80%.
Ball Bearings
Pulley sheaves equipped with sealed ball bearings exhibit significantly higher efficiency. In simplified terms, the design comprises a cavity filled with metal balls rotating between the inner and outer rings. The contact area of the balls is much smaller than that of the bushing, minimizing friction and enhancing pulley efficiency. The entire structure is sealed, preventing the entry of dirt and dust that could disrupt the operation of the ball bearings. The efficiency of such pulleys typically ranges from 90% to 95%.
Pulley Size and Weight
What effects can the size of a pulley have, aside from obvious factors like weight, compactness, and ease of transportation?
Let's break down the pulley into individual parts and assess the importance of the size of each:
Let's break down the pulley into individual parts and assess the importance of the size of each:
- Sheave Diameter: As a general rule, the larger the sheave diameter, the higher the efficiency. This is attributed to the reduction of bending stress in the rope.
- Sheave Width along with the rope channel clearance affects the diameter of the rope the pulley can work with. Pulleys with larger dimensions in this regard, such as the Kootenay, can accommodate ropes with knots or multiple standard ropes simultaneously.
- Main Attachment Point Size determines compatibility with different types and quantities of connectors.
- Additional Attachment Eyelets Size is crucial in the context of interacting with various connecting elements. The thickness of the material and the curvature of the eyelet profile affect the possibility of tying-in textile connectors directly. In the case of an end rope becket, the shorter it is, the closer the pulleys can be brought together in pulley systems. While it may seem like a small detail, for those dealing with it constantly, it holds significant importance.
Pulley and Rope Compatibility
The diameter of the rope for which the pulley is designed is specified by the manufacturer and should not be exceeded. Even if one manages to fit a thicker rope in the pulley, the friction caused by the rope rubbing on the side plates could damage the rope or, at the very least, negate all the efficiency.
Interestingly, the reverse situation is also possible. On some simple models, there might be a certain gap between the sheave and the inner part of the side plates. If you try to pass a thin cord through such pulleys against the manufacturer's instructions, there's a chance it might slip off the sheave into such a 'groove,' and... who knows what could happen.
Interestingly, the reverse situation is also possible. On some simple models, there might be a certain gap between the sheave and the inner part of the side plates. If you try to pass a thin cord through such pulleys against the manufacturer's instructions, there's a chance it might slip off the sheave into such a 'groove,' and... who knows what could happen.
The Petzl Fixe pulley is a vivid example of a model with wide gaps between the sheave and the side plates. The manufacturer explicitly states that this pulley should only be used with the specified rope diameters, ranging from 7 to 13 mm.
Image from: sestogrado.it
Pulley Strength
This one is simple. All operations must be conducted within the manufacturer's specified working load limit (WLL) or safe working load (SWL).
Relying on minimum breaking strength (MBS) is possible only as a last resort in emergency situations. By default exceeding the WLL is grounds for discarding the pulley (as well as any other device) because the product may have incurred damage not visible to the naked eye but significant for future consequences.
Special attention should be paid to the working loads on pulleys with multiple sheaves since manufacturers often indicate the total load for all sheaves, i.e., those engaged simultaneously. If the sheaves are used individually, the WLL is lower.
Relying on minimum breaking strength (MBS) is possible only as a last resort in emergency situations. By default exceeding the WLL is grounds for discarding the pulley (as well as any other device) because the product may have incurred damage not visible to the naked eye but significant for future consequences.
Special attention should be paid to the working loads on pulleys with multiple sheaves since manufacturers often indicate the total load for all sheaves, i.e., those engaged simultaneously. If the sheaves are used individually, the WLL is lower.
Pulley Material
While the material of the side plates is of secondary importance, the material of the sheave is crucial to consider. For instance, a plastic sheave will predictably be less durable than an aluminum one, and an aluminum sheave will be less wear-resistant than a steel one. Therefore, when using a steel cable, only pulleys with steel sheaves are suitable.
To be continued ;..;
18.12.2023
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For more articles please visit the knowledge section
- Pulley Definition
- Number of Sheaves
- Sheave Arrangement
- Sheave Design
- Side Plates Design
- Main Attachment Point Design
- Pulleys with Multiple Attachment Points
- Progress Capture Pulleys
- Specialized Pulleys
- Integrated Pulleys
- Integrable Pulleys
- Pulley Systems
- Pulley Efficiency
- Pulley Size and Weight
- Pulley Rope Compatibility
- Pulley Strength
- Pulley Material
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