A pulley is an efficient mechanical device resembling a wheel used for transmitting power. The pulley system consists of the pulley and a rope, cable, or chain to drive it. In everyday life, we find many applications of pulley systems in gyms, construction sites, elevators, etc. The main function of the pulley system is to lift heavy weights with very little force input. The term “Pulley” most probably has derived from the word “pull”.
Examples of Pulley System
Let’s understand the concept of pulley using the following examples:
Assume a barrel is required to be lifted to the ceiling above ten feet from the bottom ground level. This can be easily achieved by using a pulley to the ceiling and simply changing the direction of the applied force as shown in Fig. 1A.
Next, assume a 100 kg block to be lifted above the ground. As you can see in Fig. 1B that the pulley is placed in a specific way to evenly distribute the force between each side of the rope by half of the weight of the pulley. This reduces the input force by half.
Working Principle of a Pulley System
A pulley works on the principle of energy conservation. The number of pulleys used in the system dictates the force required to lift that. As per the conservation of energy rule, the work done on either side of the ring should be equal. Mathematically, it can be written as F1d1 = F2d2. The pulley ring changes the direction of the output force.
Let’s assume, a mass of x kg is to be lifted using a pulley system. The whole area can be divided into 2 parts, part A gives work output and part B shows work input. Assume I is the initial point and O is the final point and F is the magnitude of the input force. It can be seen that if the distance, d is the same and W1 =W2 then the input force, F will be the same as X. The direction of force is also changed by an angle Q in the negative direction.
W1 = W2
X*d = F*d
Hence, X = F
The Basic Construction of a Pulley
A pulley is made up of a circular disc having a given thickness. The outermost part is known as the rim of the pulley (Refer Fig. 3). Other elements of the pulley system, like rope, chain, belt, etc are mounted on the face of the pulley rim. Inside the rim, there are supporting elements known as arms. An arm usually resembles the radius of the circle. In general, there are 3-4 arms in a pulley. Some pulley designs may not have arms, making it a simple solid structure. The inner circular structure of the pulley is called a hub. There is a borehole with a keyway inside the hub. The key acts as a very important element for power transmission.
Types of Pulleys Based on Adjustments
Based on the adjustment, there are 4 basic types of pulleys. They are
- Fixed pulleys
- Movable pulleys
- Compound pulleys, and
- Back and tackle pulleys
As the name suggests, the axle of this type of pulley is fixed at a single position. The main use of a fixed pulley is to change the direction of the force. The simplest form of a pulley, a fixed pulley is used to lift light weights due to the fact that the applied force for lifting the weight is equivalent to the weight of the object. The best example of a fixed pulley is the pulley that is attached to the well for removing water using a bucket from the well.
The movable pulley allows the movement of the axle of the pulley. A single rope around the face of the pulley supports the pulley. The rim of a movable pulley moves with the object that is lifted. Even though in a movable pulley, there is no change in the direction of the force but the magnitude of the applied force is reduced to half. This calls for reduced efforts by the operator.
A compound pulley uses a combination of fixed and movable pulleys for its operation. As the input force required to lift the object is considerably reduced in this type of pulley system, the object feels to be much lighter than its original weight. Also, the force direction is changed. Refer to Fig. 4 below that shows examples of all types of pulleys.
Block and tackle pulley:-
Block and tackle pulleys are an important set of compound pulleys. They are used specifically arranged in such a way as to reduce the input force and change the direction of the force.
Types of Pulleys Based on Their Functions
Based on the functions of the pulleys, there are several types of pulleys available in the market. Some of the most functioning pulleys are provided below.
A split pulley consists of two divided halves clamped with nuts and bolts. Split pulleys find applications to transmit power from a complex shaft, where mounting a simple pulley is difficult. In the following figure (Fig. 5), A and B are two halves of a split pulley. These can be easily installed on the shaft by loosening the nuts and then tightening again for the proper functioning of the pulley.
Step or cone pulley:-
A step or cone pulley is used for speed variations. As the name suggests, a step pulley is made up of various rims with different diameters. These are usually used in lathe machines and bicycles to get varying speeds. In the given figure (Fig. 6) below, let’s consider A is the driving shaft and B is the driven shaft. The diameters of the step pulleys are d1, d2, and d3.
- If the belt is on d1 on the driving pulley and d3 on the driven pulley then the speed will be maximum. This is because d1>d3 hence for one rotation of A there will be more rotations of B.
- If the belt is on d2 on both the pulleys then the speed of A and B will be equal because d2=d2.
- If the belt is on d3 in the driving pulley and d1 in the driven pulley then the speed of B will be less than that of A because d3<d1.
Loose and fast pulley:-
A loose and fast pulley is basically a pair of pulleys where one pulley is attached to the shaft and the other one has a bearing to move freely. This type of pulley is useful in systems where one shaft is required to drive more than one system. In the given diagram A and B are the two pulleys with A1 and B1 being attached to the shaft and A2, and B2 have bearings to move freely. D is the main driver, and x and y are the systems to be driven. Now the number of cases may occur.
- If the user wants only x to function, he will simply put the belt of x on A1 and the belt of y on B2. In this way only x will function and y will stop as B2 won’t rotate due to the bearing present in the pulley.
- If the user wants only y to function, he will put the belt of x on A2 and the belt of y on B1.
- In the same way, if he wants both to function, he will put both the belt of x and y on A1 and B1 respectively.
When two pulleys are working together and one pulley is quite bigger in diameter than the other one (diameter ratio> 6:1). In such a situation, the contact between the rope and the smaller pulley is very less resulting in a slipping condition between the belt and pulley. Hence, another pulley known as a jockey pulley is inserted to increase the area of contact between the belt and pulley. When the jockey pulley C is introduced the contact area of the belt with B increases.
Components of Pulley System for Transmitting Forces
For transmitting force, various attachments are used on a pulley. Some of the basic attachments are as follows.
A rope can be made of simply jute or metal rope for stronger applications. The strength and flexibility of the rope vary depending on the application and function of the pulley.
For machinery applications (Example: Crusher machine), a belt on a pulley is quite common. The Belt is made of rubber or any synthetic material. Depending on the application, the shape and size of the belt vary.
A chain pulley is preferred for high-intensity jobs where when the weight of the object is more. Chains provide better grip with minimum chances of slipping take place. A chain pulley has grooves on the rim of the pulley so that the chain fits the pulley and force transmission takes place.
Parameters Affecting Pulley System Design
Even though the working of a pulley system seems to be so simple, the design of the pulley depends on various parameters to make it efficient and cost-effective. The main parameters that affect the pulley system design are listed below.
Diameter Ratio: The diameter ratio is defined as the ratio of the Largest pulley diameter to the cable diameter. It is a significant parameter in pulley system design and the proper ratio must be decided before buying.
Cable efficiency: Cable efficiency and performance are very important in pulley system design. The cables used are equipped with a jacket and lubrication which helps in extended life and proper functioning.
Grooving: For better design, it is suggested to keep the pulley grooving at 150% tolerance for the cable rope.
There are various materials that are used to manufacture pulleys with different applications. The most common pulley materials are:
Cast iron is the most common pulley material. Cast Iron Pulleys are easy to manufacture and they are cheaper. Due to the fact that a cast-iron pulley is brittle, it cannot take load variations. The durability of cast iron is usually less; the surface is prone to corrosion when exposed to moisture.
A pulley made of carbon steel has better strength and durability as compared to a cast-iron pulley. However, the cost of a steel pulley is more than that of a cast-iron pulley. A steel pulley is usually provided with a bush in the center and there are no keyways provided.
For marine application, a stainless-steel pulley is preferred as it won’t rust easily. The saltwater environment is highly corrosive that can easily corrode the pulleys made up of steel. Stainless steel pulleys prove to be a convenient replacement in this respect. The cost of SS pulleys is higher than that of steel pulleys. A stainless-steel pulley is used with a chain for lifting the anchor of the ships.
Before metal pulleys came into existence, Wood was a widely used material for making pulleys. Many of the Viking ships used wooden pulleys for hoisting the sails using a rope. Wooden pulleys are much lighter in weight than cast iron pulleys. These are often coated with a layer of water-resistant material to keep them away from moisture. Wooden pulleys are made of selected quality wood. It has a hub made of iron with a bush used for clamping the shaft.
Pulleys are also made of thermoplastic materials and have very few industrial applications. A plastic pulley is generally used for lifting very light weights. It can be observed commonly in plastic toys. Manufacturing plastic pulleys are very simple because plastic is easy to mold.
Applications of Pulleys
Pulleys are one of the most widely used mechanical devices having simple to complex uses. Some of the most common applications of pulleys are listed below:
- The oldest application of pulleys is found in wells to hoist the bucket full of water.
- Elevators use multiple pulleys in order to properly function.
- Pulleys are used to pull the curtains apart inside a theatre.
- Cargo lifts use pulley systems to hoist items to higher floors.
- Various types of exercise equipment make use of a pulley system to work.
- Pulleys are extensively used in construction sites to lift heavy objects at higher elevations.
- During flag hoisting, pulleys are used to hoist the flag and bring it down.
- Cranes use a complex pulley system to lift objects easily.
- Several engines use pulleys to appropriately work.
- Extend ladders, Garage doors, Bulldozers, Escalators, etc all use pulley systems for their proper functioning.
- Rock climbers utilize a pulley system for climbing.
Advantages of Pulley
A pulley system offers various benefits to mankind. Some of the major advantages that a pulley system offers are:
- The effort required for lifting heavy objects is reduced to a great extent.
- The Pulley system changes the applied force direction.
- The lifting process becomes smoother and quicker.
- The maintenance of the pulley system is quite easy.
- Assembly and disassembly of pulley system components are convenient.
- Lubrication is usually not required in pulley systems and they are cost-effective.
- They perform the work quietly without making noise.
Disadvantages of Pulley
Some of the disadvantages of pulley systems are
- There is a possibility of slippage as friction is involved.
- When RPM is high, Pulleys are not suitable.
- When the angle of the rope with respect to the pulley is increased, it tends to slip out from the pulley.
- The rope wears out quickly after using for a certain period.