What is a Pulley?
A pulley is a wheel that carries a flexible rope, cord, cable, chain, or belt on its rim. Pulleys are used individually or in combination to transmit power and motion. Pulleys with grooved edges are called sheaves. In belt drives, pulleys are attached to shafts at their axles, and power is transmitted between the shafts by endless belts running over the pulleys.
One or more independently rotating pulleys can be used to gain a mechanical advantage, particularly when lifting weights. Pulleys are assembled to form a block and tackle in order to provide a mechanical advantage to apply large forces. Pulleys are also assembled as part of belt and chain drives in order to transmit power from one rotating shaft to another
The Greek mathematician Archimedes (3rd century BC) is said to have used compound pulleys to pull a ship onto land. In Roman Egypt, Hero of Alexandria (c. 10-70 CE) identified the pulley as one of six simple machines used to lift weights.
Plutarch’s Parallel Lives recounts a scene where Archimedes proved the effectiveness of compound pulleys and the block-and-tackle system by using one to pull a fully laden ship towards him as if it was gliding through water.
What are the uses of pulleys?
The original primary use for pulleys was to make it easier to lift heavy items. A pulley is a simple machine made with a wheel and a rope, cord, or chain.
Pulleys can be found everywhere to help make tasks easier. In addition to lifting things, pulleys can be used singly or with many pulleys working together to transport people or things. They can also be used to provide power from one shaft to another.
Examples of Pulleys in Everyday Life
Examples of pulleys include:
- Elevators: Elevators work via a powerful electronic pulley system. In fact, elevators make use of a system of several pulleys and counterweights to provide the lift system with both power and safety.
- Wells: Old-fashioned “wishing well” style water wells use the simple pulley system to hoist the bucket out of the well.
- Exercise equipment. Most weight-lifting exercise equipments uses pulleys to control the angle at which weights are lifted while keeping the weights themselves in a dedicated location. This makes the process of weight lifting safer and more effective for the user than free weights maybe if used improperly.
- Construction Pulleys. Construction pulleys are some of the most basic and common pulleys, good for viewing the basic functioning of this simple machine. These pulleys consist of a wheel track that can be raised to great heights, fitted with chains or rope attached to hooks.
- Theater Systems. In theaters, curtains and fly systems operate using a system of multiple pulleys. These pulleys are located high above the stage where the audience cannot see them and operated from the side of the stage to raise and lower curtains and pieces of scenery during a theatrical performance.
- Cargo lift system. A cargo lift system that allows for items to be hoisted to higher floors is a pulley system.
- Blinds on windows. Blinds on windows operate using a pulley system to move the blinds up and down. You pull the cord on the blinds and the pulley system causes the blinds to open or close.
- Flagpoles. Flagpoles use pulleys to hoist the flag up or to bring it down. You pull the string on the pulley and the flag runs up or down the pole.
- Crane, Bulldozer, Escalators utilize pulley systems to function.
- Garage doors, Sailboats, Engine, extend ladders, Rock climbers, all Use pulley system to work safer and more effectively.
These are all examples of pulleys. A cable system makes it easier to lift or move objects. Pulleys are required to move and lift very heavy or large objects. The simple principle behind pulleys makes them one of the most effective tools out there.
How does a Pulley Work?
A pulley is a simple machine that consists of a rope and a grooved wheel. The rope fits into the groove in the wheel and pulling the rope turns the wheel. Pulleys are commonly used to lift objects, especially heavy objects. The object lifted by a roller is called a load.
A pulley with a wheel allows you to reverse the direction of your lifting force by pulling down on a rope (looped over the wheel) and lifting your weight. With a two-wheel pulley, you reduce the force you use to lift the same weight. You lift the weight at half strength.
The more wheels you have, and the more times you loop the rope around them, the more you can lift.
A pulley is just a grouping of one or more wheels over which a rope (or a sturdy chain for heavier objects) is looped. Scientists call these “simple machines” because they allow humans to multiply the forces that allow us to lift a heavyweight. Using a roller greatly multiplies the strength of your physical exertion.
A pulley with a wheel allows you to reverse the direction of your lifting force by pulling down on a rope (looped over the wheel) and lifting your weight.
With a two-wheel pulley, you reduce the force you use to lift the same weight. You lift the weight at half strength. This is known as the Mechanical Advantage (ME) and your two-wheeled pulley gives you an ME of two.
The bigger your Mechanical Advantage, the less force you need to lift a weight.
The more wheels your rope can wrap around, the more weight you can lift. This type of pulley system is often referred to as a pulley system and usually consists of a rope wound around two axes several times.
As you pull on the rope, the lower and upper axes slowly come together, increasing your ability to lift the object.
Material use for pulleys
Pulleys can be made from a variety of materials, including cast iron, cast steel or pressed steel, wood, and paper. The cast material should have good friction and wear characteristics. The pulley made from pressed steel is lighter than cast pulleys, but in many cases, they have less friction and can produce excessive wear.
Types of Pulley
There are three main types of pulleys:
- Fixed pulley.
- Movable pulley.
- Compound Pulley.
- Cone Pulley.
- Block and Tackle pulley.
1. Fixed Pulley
A fixed pulley’s wheel and axle stay in one place. A fixed pulley changes the direction of the force on a rope or belt that moves along its circumference. A good example of a fixed pulley is a flag pole: When you pull down on the rope, the direction of force is redirected by the pulley, and you raise the flag.
A fixed pulley is a pulley with the drum attached to a single location. While the force required to lift or move an object is no different than lifting it by hand, the fixed pulley allows you to change the direction of the force required.
For example, if you’re attached to a bucket that pulling water from a well, you can pull sideways with a firm pull to lift the bucket more comfortably than if you were pulling a bucket of water up hand in hand vertically. The water bucket still feels like the same weight, but it’s more convenient to lift.
2. Movable pulley
A movable pulley has an axle in a movable block. A single movable pulley is supported by two parts of the same rope and has a mechanical advantage of two.
A movable pulley is one in which the drum moves as you are moving the load. There is no change in the direction of the force that you need to apply, but the load will “feel” lighter than it is. If you were hauling a heavy hay bale up into the loft of a barn, for example, a movable pulley would make the load feel much lighter, although the direction you are pulling would be the same.
3. Compound Pulley
In a compound pulley system, there is both a moveable pulley as well as a fixed pulley. This means that not only does the load “feel” lighter, but you can also change the direction of the force.
Although this type of configuration makes it possible to move heavy loads very easily, the tradeoff is that considerably more motion is required to do the work.
4. Cone pulley
The cone pulleys is another specialized pulley system that incorporates the basic mechanics of a pulley system while allowing for mechanical adjustments. Cone pulleys are essentially multiple pulleys of decreasing vertical circumference on top of each other, forming a cone shape.
This cone shape allows the pulley operator to transfer the speed of the pulleys’ movements, requiring less work with a smaller circumference, but also producing less work.
Multi-gear bicycles essentially operate on the same system; Cyclists can easily shift between smaller gears that propel the bike at a slower speed, and more gears require more effort but move the bike more distances per revolution.
5. Block and Tackle pulley
A block and tackle are a specific forms of compound pulleys that can dramatically reduce the amount of work required to move a heavy object. A block-and-tackle pulleys system consists of several fixed and movable pulses arranged parallel to each other; a Fixed pulley is aligned with fixed and movable pulleys with movables.
Each compound pair is associated with the next pair, and each set reduces the total work required. This pulley system is attributed to the popular ancient inventor and mathematician, Archimedes.
What is Flat belt pulley?
Flat belt pulleys are used in transmission systems that are driven by flat belts, typically high-speed, low-power applications in textiles, paper making, and office machinery such as computer printers. A flat pulley idler can also be used for the back side of a conventional V-belt.
When the flat belt on such a pulley is off-center and the pulley rotating, the belt quickly moves up to the largest radius at the top of the crown and stays there. The crown is important to keep the belt “tracking” stable, preventing the belt from “walking off” the edge of the pulley.
Flat belts are designed for light-duty power transmission and high-performance conveying. They are best-suited for applications with smaller pulleys and large central distances. Flat belts can connect inside and outside pulleys and can come in both endless and jointed construction.
Flat belts are also used for conveyors. Compared to plied belts of equal horsepower, flat belts are thinner by 25% or more, which allows flat belt pulleys to be smaller than V-belt pulleys. Flat belts are also less expensive than belts used in a serpentine belt pulley.
One safety factor is that in over torque situations, the belt can slip, preventing damage to equipment other than the belt itself. Flat belts require flat pulleys and flat pulley idlers. They do not necessarily require grooved flat belt pulleys. Flat belt drive pulleys apply motive power to the belt.
Types of Flat belt Pulleys
Following are the various types of pulleys for flat belts:
- Cast-iron Pulleys,
- Steel Pulleys,
- Wooden Pulleys,
- Paper Pulleys, and
- Fast and loose Pulleys.
1. Cast Iron Pulleys
Pulleys are usually made of cast iron. The rim is placed on the web from the central boss or by arms or spokes. The arms may be straight or curved and the cross-section is usually elliptical. Cast iron pulleys are usually made with round rims.
When a cast pulley contracts into the mold, the arms are in a state of tension and are very liable to break. Curved arms yield rather than break. Weapons are near the hub. This slight convexity is known as a crown. The crown centers the belt on a pulley rim while in motion. The crown can be 9 mm to 300 mm in the width of the face.
Cast iron pulleys can be of solid or split type. When it is necessary to mount a pulley on a shaft that is already carrying pulleys etc. or its ends are swollen, the split pulleys are easy to use. There is a clearance between the faces and the two parts are easily tightened on the shaft by a bolt. A sunk key is used for heavy drives
2. Steel pulley
Steel pulleys are made from steel sheets and have great strength and durability. These pulleys are lighter in weight (about 40 to 60% less) compared to iron pulleys of the same capacity and are designed to run at higher speeds. They present a coefficient of friction with a leather belt that is at least equal to that obtained by a cast-iron pulley.
Steel pulp is usually made in two parts that are tied together. The clamping action of the hub keeps its shaft pulley; Thus, no key is required except in the most serious service. Steel pulleys are generally equipped with interchangeable bushes with shafts of different sizes to allow their use.
3. Wooden pulley
Wood pulleys are lighter and have a higher coefficient of friction than cast iron or steel pulleys. These pulleys have 2/3 of the weight of a similarly sized cast iron pulley. They are usually made of selected maples that are placed in sections and glued together under heavy pressure. They are kept from absorbing moisture by protective coatings of shells or varnishes so that war does not occur.
These pulleys are either solid or split with an iron hub with screws or have adjustable bushes that prevent the frictional resistance between them and the shaft from being installed. These pulleys are used for motor drives in which the contact arc between the pulley face and the belt is restricted.
4. Paper Pulley
Paper pulleys are made from compressed paper fibers and are formed with metal in the center. These pulleys are commonly used for belt transmission from electric motors when the distance from the center to the center shaft is small.
5. Fast and loose pulley
Fast and loose pulleys, used on the shaft, enable the machine to be turned on or off. A fast pulley is mounted on the machine shaft, while a loose pulley operates independently.
The belt operates on a fast pulley to transmit power by the machine and is transferred to a loose pulley when the machine is not required to transmit power. In this way, stopping one machine does not interfere with other machines that run by the same line shaft.
The loose pulley is provided with a cast iron or gun-metal bushing with a collar at one end to prevent axial movement. The rim of the fast pulley is made larger than the loose pulley so that the belt runs slower on the loose pulley. The loose pulley usually has a longer hub to reduce wear and friction and requires proper lubrication.
Advantages and Disadvantages of Pulley
Advantages of pulley
- The main advantage in the use of pulleys is that the effort becomes less as compared to the normal lifting of the weights. In other words, it reduces the amount of actual force required to lift heavy objects.
- It also changes the direction of the force applied. These two advantages in the use pulleys make them an important tool for heavy lifting. It also provides a mechanical advantage.
- The other advantage in the use of pulleys is the distance between the operator and weight.
- There is a safe distance between them which avoids any disaster.
- Pulleys are easy to assemble and cost-effective.
- The combination of different directional pulleys can change the position of the load with little effort.
- Though there are moving parts in the pulley system they require less or no lubrication after installation.
Disadvantages of pulley
Apart from the above-said advantages while using pulley systems, there are several disadvantages in their use.
- The main disadvantage in the use of the pulley system is that it requires large space to install and operate.
- The mechanical advantage of pulleys can go to higher values but need more space to install them.
- In some cases, the ropes/belts move over the wheel with no grooves, the chances of the slip of ropes/belts from the wheel are inevitable.
- If the system is installed to use for a long time, they require maintenance and regular check-up of ropes/cables as the friction between the wheels and cables/ropes occur causing wear and tear to them.
- Continuous use of the system makes the ropes weak. The rope may break while using the system causing damages to the operator, surrounding place and the load which is being lifted.
A pulley is a wheel that carries a flexible rope, cord, cable, chain, or belt on its rim. Pulleys are used singly or in combination to transmit energy and motion. One or more independently rotating pulleys can be used to gain a mechanical advantage, especially for lifting weights.
There are three main types of pulleys:
1. Fixed pulley.
2. Movable pulley.
3. Compound Pulley.
4. Cone Pulley.
5. Block and Tackle pulley.
Pulleys are commonly used to lift objects, especially heavy objects. The object lifted by a roller is called a load. A pulley with a wheel allows you to reverse the direction of your lifting force by pulling down on a rope (looped over the wheel) and lifting your weight.
Examples of pulleys include:
1. Elevators use multiple pulleys in order to function.
2. A cargo lift system that allows for items to be hoisted to higher floors is a pulley system.
3. Wells use the pulley system to hoist the bucket out of the well.
4. Many types of exercise equipment use pulleys in order to function.
Archimedes. Historians, however, credit Greek mathematician, inventor, astronomer, engineer, and physicist, Archimedes, with the first documented compound pulley system in the third century BCE.