What Is Pulley?
Pulleys are simply a collection of one or more wheels over which you loop a rope to make it easier to lift things. Pulleys are examples of what scientists call a simple machine. This does not mean that they are loaded with engines and gears; It simply means that they help us to multiply the forces.
If you really want to lift heavy weights, your muscles can only provide so much force, even if you are the strongest person in the world. But use a simple machine such as a pulley, and you can effectively multiply the force produced by your body. Pulleys are a wheel that carries a flexible rope, rope, cable, chain, or belt on its rim.
Pulley is used singles or in combination to transmit energy & motion. Pulleys with grooved rims are called sheaves. Nowadays, a plastic pulley is also available in the markets for carrying small loads. It can be rotated freely about an axis passing through its center.
It can change the direction of the force, making it easier for people to lift something. In Roman Egypt, the Hero of Alexandria identified the winch as one of six simple machines used to lift weights.
The pulleys are assembled to form a block and handle it in order to provide a mechanical advantage for applying large forces. Pulleys are also assembled as part of belt and chain drives to transmit power from one rotating shaft to another.
The Life of Plutarch describes a scene where Archimedes proved the effectiveness of a mixed winch and block-and-tackle system by using one to pull a fully loaded ship as if it were gliding through water.
How Does a Pulley Work?
A winch does easy work by redirecting and amplifying the force. It helps to reverse the direction of a person’s lifting force. As the user pulls the rope down, the winch lifts the weight. When two ropes and wheels are used, a pulley reduces the effort required to haul the weight in half.
Pulleys are simple machines consisting of a rope and an oval wheel. The ropes fit into the groove in the wheel, & the wheel spins by pulling the rope. Pulleys are commonly used to lift objects, especially heavy objects. The object carried by the roller is called weight.
Pulleys with a wheel allow you to reverse the direction of your liftings forces by pulling the rope, the loop on the wheel & lifting your weight. With two-wheel pulleys, you reduce the forces used to lift the same load. You lift the weight at half your strength.
The more wheels you have, & the more times you loop the rope around them, the more you can lift. With a two-wheel pulley, you reduce the force used to lift the same load.
You lift the weight at half your strength. This is known as Mechanical Advantage ME, and your two-wheel pulley gives you an ME of two. The higher your mechanical advantage, the less force you need to lift the weight.
The more wheels your rope can wrap, 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 ropes, the lower & upper axes gradually come closer together, increasing your ability to lift the object.
Functions of Pulley:
The function of a winch is to lift heavy objects by changing the direction of the force on a flexible cable. It also consists of a wheel whose outer edge has a groove and an axle. With the help of ropes, chains, or cords, a winch makes moving objects upwards easier.
The pulley reduces the force given to do work. When using a pulley system, the direction of force required to lift an object is changed from pushing down to pulling down. Pulley manufacturers aim to make heavy work more manageable and are classified as movable, stationary, or combined.
Multiple pulleys can also reduce the force required to lift or pull a load. The pulley also primarily serves to distribute the weight more evenly, thus making the lift lighter. Pulleys are used in many different industries to lift loads, apply force, or transmit power.
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Formulas of Pulley:
As in the figure below, we have to move an object with a single pulley, and we will calculate the force without friction between the wheel and the rope.
- M: Mass (kg)
- g: acceleration of gravity (m/s²), g = 9.81 m/s²
- F: Force to pulls the object. (N)
- α: angle of the ropes.
- We can calculate the forces as formula: F = Mg/cos(α) = 9.81 M/ cos(α)
- For the examples, M = 1o kg, α = 60 degree -> F = 10*9.81/cos(60) = 98.1* 2 = 196.2 N
For Movable Pulley & compound pulley.
- M: Mass (kg)
- g: acceleration of gravity (m/s²), g = 9.81 m/s²
- F: Force to pulls the object. (N)
- n: number of the wheel
- We can calculates forces as formula: F = Mg/n = 9.81M/n
- For the example, M = 1o kg, n=2 -> F = 10*9.81/2 = 98.1/ 2 = 49.05 N
Types of Pulley:
Pulleys are a simple machine composed of a wheel on an axle that supports the change of speed and direction of a taut cable or belt or the transfer of power between the axle and the cable. Pulleys may have a groove or grooves between the flanges around their circumference for running along the cable or belt. The drive element of a pulley system may be a cable, rope, belt, or chain.
There are three main types of pulley:
#1. Fixed Pulley
A fixed pulley system consists of a wheel that is attached to a surface such as a wall or a floor and a rope that hangs free. The pulley remains stationary. A fixed pulley minimizes the amount of force needed to move a load but allows a person to redirect the necessary force.
So instead of lifting a heavy object directly with upward force, a person can use a pulley to lift the object by pulling down on the rope. They are used in flagpoles, curtain binders, and wells. Pulley can be made of wood, plastic, or metals, depending on the application.
#2. Moving Pulley
A moving pulley, also known as a movable pulley, consists of wheels that are not attached to a particular surface, but instead, a rope or cable is attached to a stationary surface. This provides a mechanical advantage, meaning that the pulley reduces the amount of force that a person must apply to move the load.
The load attaches to the wheels instead of the ropes; the act of pulling the rope slides the wheel over the rope, bringing the load with it. For this action, less force is required than to lift the load directly. They are used in cranes, utility lifts, and lifting machines.
#3. Compound Pulley
The compound pulley system consists of both a movable pulley and a fixed pulley. This combination of pulleys means that not only is less force required to lift a load, but a person can also change the direction of the force.
While this type of configuration makes it possible to move heavy loads very easily, the tradeoff is that it requires significantly more speed to do the job.
#4. Block and Tackle
A block and tackle pulley is a variant of a composite pulley that can dramatically reduce the force required to move a heavy object. A block-and-tack pulley system is comprised of a number of fixed and movable pulleys that are arranged parallel to each other.
The fixed pulley is aligned with other fixed pulleys and is movable with the movable. Each of these compound pairs is connected to the next pair, and each set reduces the total force required. They are used in sailboats, rock climbing, shops, and garage shutters, and theater curtains.
#5. Cone Pulleys
Cone pulleys consist of several pulley wheels that decrease in circumference as they are stacked on top of each other to form a cone shape. This cone shape allows the winch operator to transfer the speed of the pulley, with a smaller circumference requiring less force but also generating less force. Multi-gear bicycles essentially work on this system.
The cyclist can easily shift between smaller gears that rotate the bike less & higher gears that require more effort but move the bike a greater distances per revolution. They are also used in drilling & roving machines.
#6. Conveyor Pulleys
Conveyor pulleys in a conveyor system change the direction of the conveyor belt, drive the belt, and provide tension to the belt. They are made of rolled shells with flexible end discs and locking assemblies. They are usually located at one end of the conveyor or near the motor.
What Is Flat Belt Pulley?
A flat belt pulley is used in transmission systems that are driven by flat belts, typically in high speed, low power applications in textiles, paper making, and office machinery such as computer printers. Flat belts are also used for conveyors.
Flat belts are also less expensive than the belts used in snake belt pulleys. One safety factor is that in high torque situations, the belt may slip, causing damage to equipment other than the belt. Flat belts require flat pulleys and flat pulley idlers.
They don’t necessarily require a flattened flat belt pulley. A flat pulley idler can also be used behind a traditional V-belt. Flat belt drives apply a driving force to the police belt.
Types of Flat Belt Pulleys:
#1. Wooden Pulley
The wood pulley is lighter and has a higher coefficient of friction than cast iron or steel pulleys. These pulleys are 2/3 the weight of cast-iron pulleys of similar size. They are protected from absorbing moisture by protective coatings of shells or varnish to prevent corrosion.
This pulley is either solid or splits with iron hubs with screws or adjustable bushings that prevent frictional resistance from being installed between them and the shaft. This pulley is used for motor drives in which the contact arc between the face of the pulleys & the belt is restricted.
#2. Paper Pulley
A paper pulley is made from compressed paper fibers and with metal in the center. When the center to center shaft distance is small, these pulleys are commonly used for belt transmission from electric motors.
#3. Fast and Loose Pulley
The sharp and loose pulleys used on the shaft enable the machine to be turned on or off. Fast pulleys are mounted on the machine shaft, while loose pulleys are operated independently. The belt is driven by the machine onto a fast pulley to transmit power and is transferred to a loose pulley when the machine no longer needs to transmit power.
The rim of the fast pulley is made larger than the loose pulley so that the belt continues to move at a slower speed on the loose pulley. Loose pulleys usually have a longer hub to reduce wear and friction and require proper lubrication.
#4. Cast Iron Pulley
Pulleys are usually made of cast iron. The rim is held on the web from the central boss or by arms or spokes. The arms may be straight or curved, & the cross-section is usually elliptical. Casts iron pulley is usually made with round rims.
When a cast pulley shrinks in the mold, the arms are in a state of stress and very liable to break. Curved arms yield rather than break. The arms are near the hub. This slight convexity is known as the crown. Cast iron pulley can be of solid or split type. When it is necessary to mount pulleys on a shaft that already has pulleys etc., or has swollen ends, split-pulleys are easier to use.
There is a clearance between the faces, & the two parts are easily tightened onto the shaft by bolts. Eccentric keys are used for heavy drives.
#5. Steel Pulley
Steel pulleys are made of steel sheets & have great strength and durability. This pulley is lighter in weight, about 40 to 60% less than iron pulleys of similar capacity, and is designed to run at high speeds. They present coefficients of friction with a leather belt at least equal to that achieved by a cast-iron pulley. Steel pulps are usually made in two parts that are bonded together.
The clamping action of the hub holds its shaft pulley; Thus, no key is needed except for the most serious service. Steel pulleys are generally equipped with interchangeable bushings with a shaft of different sizes to allow their use.
Advantages of pulleys:
- When the pulley is mounted on the engine, it may also be called a belt. It can be easily transported and exposed to oil or metals other than oil.
- It provides a mechanical advantage when lifting heavy loads. The profitability of the types of equipment is known as the effort-to-load ratio.
- A moving pulley is a load pulley. Mutes the distance range. Long-distance running ropes act as props.
- It is easier and fewer expensive to make than ordinary gear.
- Lubrication is not required.
- It helps pulley systems in any direction to apply force and rely on the relationship between load and effort.
Disadvantages of pulleys:
- It uses friction when transferring motion, so there is a possibility of slipping.
- If the rope usually slips when it is being pulled to the sides. This causes the ropes to fall off the track on the wheel, which can cause injury. It is not a suitable solution to move at high rpm for very high power.
- Comparison with gears is not reliable.
- A combined pulleys systems use multiple wheels and additional ropes to support the weight of a heavy object.