Through the centuries the block and tackle system has been optimized to meet the evolving needs of the professionals who use it. Today, block and tackle systems power innumerable daily tasks from recreational yachting to helicopter rescues and more. Here’s a closer look at how the system works and the industries to which it is indispensable.
Put simply, a block and tackle is a system designed for lifting heavy loads involving two or more pulleys with a rope or cable threaded between them. The pulleys are assembled together to form blocks, and those blocks are paired with one another so that one is fixed and the other moves with the load. The rotational effect of the pulley allows for the user to change the direction of the force being applied, effectively trading distance for effort by amplifying the tension force in the rope.
For example, consider you’ve got a 100-pound load to lift via rope. You’ll need to apply 100 pounds of upward force to the rope to suspend that load in the air. If you were to attach a single pulley to the ceiling and thread the rope through it, this will change the direction of the force you’ll need to apply, as the load will then require downward force to lift, but it will not change the amount of force needed. Adding a second pulley, however—this one attached to the load itself—will decrease the burden of force you’ll need to apply by half.
This process is essentially a trade of force for distance. If you need to lift the load 100 feet into the air, you will need 200 feet of rope to get it done, but only 50 pounds of force. This burden can decrease by another half—and require twice as much rope—if you were to add a third pulley, and the burden would continue to decrease if you were to add a fourth rope, and so on.
This burden reduction is referred to as the purchase of a block and tackle system. A 4:1 reduces the pull load by 4 and increases the rope requirements by 4. It will also double the load on the pulley attachment point: 100 pounds for the load and 100 pounds for the lifting force.
Block and tackle is a compact, easy-to-install system with countless functions across a variety of industries. The principles at work in the system have revolutionized the way we lift or move heavy objects. The earliest accounts of block and tackle use date back around two millennia, to Greek mathematician and engineer Archimedes, but the system or something similar was likely used in the construction of much older structures like England’s Stonehenge or the pyramids in ancient Egypt.
For example, Hero of Alexandria’s first century Book on Raising Heavy Weights depicts the use of early block and tackle systems. The system and the principle powering it led to the birth of the crane, which revolutionized the way loads are lifted and remain ubiquitous today.
Blocks come in all shapes and sizes and are designed to fit a multitude of applications. What separates a block from a standard pulley is that it contains one or more sheaves (pronounced “shiv”) between each side plate that are capable of handling shock loads. Sheaves are constructed in a range of materials, each designed for a different application. As a general rule of thumb, metal sheaves are used for higher breaking load pulleys while plastics are used with lower loads. The profile of the sheave (V or U shape) determines if it is intended to be used with wire rope or fiber.
Tackle is an assembly of blocks with a rope threaded through the pulleys. Polyester or hybrid martial ropes are the most common type used in a block and tackle system, especially for outdoor or marine use, as they function well in adverse weather conditions. However, other types of rope, including cables, are also used for different jobs.
Mechanical advantage, the basic principle at work in a block and tackle system, can be defined as the ratio of force produced by a device that acts on a load to the applied effort needed to lift or move it.
Basically, the more parts of a load that acts on the rope, the easier it is to lift. For example, if two parts of the rope act on the load—meaning two pulleys are being used—then the system has a mechanical advantage of 2. If the load being lifted weighs 100 pounds, it will require 50 pounds of force to move. This is called a two-to-one block and tackle.
A block and tackle system can be “rigged” one of two ways to increase its mechanical advantage. If the fixed and moving blocks are arranged so the rope is attached to the moving block and is pulled in the direction of the lifted load, then the system is considered “rove to advantage,” as the hauling part is pulled from the moving block.
Alternatively, if the pull on the rope is in the opposite direction to that in which the load is being moved, this system is considered “rove to disadvantage,” as the hauling part is being pulled from the fixed block.
The decision to reeve to advantage or to disadvantage depends on the ergonomics of the job at hand. Reeving to advantage is considered to be the best use of equipment and resources, as it allows for obstacles to be managed more easily when the load is to be hauled parallel to the ground. Reeving to disadvantage adds another sheave and changes the direction of the pulling line, which is advantageous when a fixed load needs to be lifted from a fixed point overhead. In this circumstance, the weight of the lifter offsets the weight of the load.
Friction, or the pulley’s resistance to turning, is the primary force working against a block and tackle system. An increase in friction requires an increase in force to overcome it, which can generate destructive heat and cause thermal expansion in any pulley systems, increasing friction. Each sheave in the system represents an increase in friction, which will eventually lead to a decrease in productivity. This is why finding the right rope and the right sheave for the job is key to the system’s functionality.
Innovative solutions to fight friction have evolved over the years. Where the ancient Egyptians once used water as a lubricant to reduce friction between sandstones, today ball bearings are used to reduce friction in a block and tackle system. Ronstan engineers have developed a dual bearing system to accommodate marine industry conditions which require pulleys to work at high speeds with little to no load and then handle high loads for extended periods of time. The dual bearing system contains an outer ball bearing surface for the former circumstance and an inner bushing on a polished shaft system for the latter—for when the ball bearings have deformed—leading to bushing contact.
Another method of combating friction is to include plastic polymer material like Torelon or Acetal in the system. These materials have a low expansion rate through a wide temperature range, so as the bearing heats up, the surfaces hold their shape and size for longer, keeping friction levels constant.
There are a number of block and tackles systems to choose from, so to help you become acquainted with all of the options out there, and the ways they can each be used, take a look at the list below.
Gun tackle represents the principle of block and tackle in its simplest form. This system uses one fixed and one moveable pulley with a single rope threaded through them, creating a mechanical advantage of 2.
A double luff tackle uses pulleys with two grooved wheels instead of one. This system creates a mechanical advantage of 4 by threading the rope four times through the pulleys.
Gyn tackle uses a three-wheel pulley for its fixed point and a two-wheel pulley for its moveable point, creating a mechanical advantage of 5.
A threefold purchase block and tackle systems provide a mechanical advantage of 6. These are commonly found hoisting anchors on fishing boats.
Block and tackle systems are used to lift or move all sorts of heavy loads across a litany of industries, from lifting construction gear and powering suspension bridges to moving mountain chairlifts and erecting circus tents. Block and tackle is an essential piece of equipment in the agriculture, commercial, construction, manufacturing, logging, oil drilling, and marine industries.
Block and tackle systems are most commonly found on boats and sailing ships, where much of the work is done manually. Sailors have been using the system for centuries to set their ship’s sails to their best advantage at sea. Sailing ships are filled with different types of blocks set for different purposes.
Block and tackle systems are also used in the medical industry and are often found in powering essential apparatus like bed-hoists. These are generally portable devices that can lift disabled or infirm patients. It can be a struggle to move a patient or loved one from a bed to a chair or any other surface, and as a result, healthcare workers experience high rates of occupational injury due to falls and strenuous patient moves. However, block and tackle can remove a considerable amount of burden from the caretakers and create seamless transitions for themselves and those in their care.
Mountain and water rescue equipment is also powered by block and tackle systems. Helicopter rappelling and winching, for example—the process by which a person or an object is let down or picked up by means of a hoist or a winch which is fitted to the helicopter—relies on block and tackle to function. These techniques are integral to outdoor rescue missions.
Working at extreme heights is the main cause of fatalities in the construction industry. For the most part, this is the result of inadequate equipment and improper training in the use of that equipment. However, block and tackle systems have played an integral role in keeping workers safe in the sky for decades.
Recent years have a seen a rise in outdoor treetop rope courses, in which adrenaline-seekers are suspended high in the treeline by block and tackle. Pulley systems have long been used in abseiling, whether for leisure or for occupational use like window cleaning services, but the treetop adventure industry provides a new and exciting future for the system.
The invention of block and tackle has undoubtedly altered the course of human history, as it has rendered movable what once was considered immovable. Today, the system can be found lifting heavy loads and keeping people safe across most, if not all industries, and all the products that power it will continue to evolve and expand well into the future as new uses for this ancient system are still being uncovered today.
Archimedes reportedly moved a warship full of men single-handedly by way of a compound pulley. It is said that afterward, he declared he could move the entire earth if given the right place to stand. One need only to look to the countless achievements the system has enabled to see that this anecdote, and the hyperbolic exclamation that succeeded it, captures well the scope of possibility unlocked by combining pulleys to lessen loads.
Ronstan has been manufacturing and distributing world-leading hardware and rigging products since 1953. In the Ronstan store, you’ll find everything from pulleys of all sizes and materials to rope manufactured using high-tech fibers. Whether you’re a single sailor or the head of an enterprise-level company, if you need to lift or move a heavy load, Ronstan has all the pulleys, sheaves, and ropes you’ll need to get the job done.