Pulley sets are used in order to make it easier to move a load through the use of mechanical advantage. In simple terms, mechanical advantage is the ratio of the force produced by the system compared to the force applied to the system. For example, a 2:1 mechanical advantage system reduces the force needed to lift a load by half.
4 to 1 Mechanical Advantage System (Set Of Fours)
One of the most common forms of mechanical advantage systems is a 4:1 also known as a set-of-fours. They are usually comprised of two double pulleys that are connected with a rope in order to form a system. These systems usually include a form of progress capture to prevent the load from falling if the free end of the rope is released while in use.
These systems have a a number of advantages since they can be kept assembled ready to use. Because they are usually kept pre-configured in a bag, the training time to use them is lessened. The user does not need to remember the proper way to assemble the individual components. This makes them very convenient to use and lessens the chance of mistakes being made while using the equipment.
Pulley systems designed for personal or system use differ in their size and efficiency.
Type of Use
The two main uses for this type of pulley system is either as a personal haul system or as a vertical lifting system. Personal haul systems, also known as jiggers, are used to move or provide tension to an object. They are most commonly available in preconfigured systems between 1 and 4 meters depending on intended use. These systems can be kept readily available for when more force is needed to move an object than can be applied just by pulling. They are also very useful for positioning equipment while performing rope access or rope rescue activities.
Set of fours designed for vertical lifting systems use heavier components and come with longer ropes. These systems commonly come assembled with up to 60 meters (200 feet) of rope allowing them to lift loads up to 15 meters.
Personal use pulley systems, also known as jiggers, come with different styles and options.
Assembled vs Component Systems
Pre-assembled sets are available from manufacturers that are designed to ensure that the individual components are compatible and work together properly. Since these pulley sets are tested and certified as complete systems they are rated with a minimum breaking strength (MBS) and working load limit (WLL).
Pulley systems can also be created from individual components but it is up to the user to ensure compatibility between the rope, pulley, and progress capture. It is also up to the end user to analyze the system to determine the MBS and WLL.
Pulley System Disadvantages
The main disadvantage of 4 to 1 pulley systems is it requires four times the amount of rope to be pulled through the system as the distance moved. In order to move a load 10 meters, 40 meters of rope must be pulled through the system. This is not a problem with shorter systems but for longer distances care must be taken to ensure the rope does not become tangled.
Longer distances also cause problems as the ropes make contact with each creating friction. This added friction causes the system to loose efficiency making them more difficult to use.
For distances longer than 10 meters winches provide a more efficient option.
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