Do you know you use a lot of machines every day without even knowing about it? Remember those scissors that you used to cut paper for your art class? And the stapler that you used to staple extra sheets together? They are all simple machines.
Every day you use machines without even thinking about it. A machine is anything that helps make work easier. Basic tools like staplers, screwdrivers and scissors are simple machines. These machines are all based on simple inventions like levers, planes, pulleys or wheels.
Why do we need machines?
With the help of machines, a small force can be used to overcome a large force. For example, a screw jack is used to lift an object as heavy as a car to change its tyres.
We need machines to:
- Lift heavy loads with a small effort.
- Carry out unsafe and dangerous tasks.
- To increase the speed of a moving object.
- To move, lift or perform an action in the required direction.
- To reduce the risk in performing hazardous tasks.
Simple Machines
Simple Machine:
A machine is a device that helps us to do work with less effort in less time. A simple machine has few or no moving parts.
Simple machines are basically classified into two groups, levers and inclined planes.
- Simple machines like the pulley, screw, wheel and axle and wedge come under these two categories.
- The see-saw in the playground is a simple machine.
You can use these simple machines to build a complex machine. For example, a bicycle is a complex machine that is made using of nearly every kind of simple machine.
Pulley
A pulley is a flat circular disc having a groove in its edge and capable of rotating around a fixed point passing through its central axis, called an axle.
The two commonly used types of pulleys are single fixed pulley system and the single movable pulley system.
Single Fixed Pulley System:
- A single fixed pulley system or a simple pulley consists of a grooved wheel, made of wood or metal, with a rope passing through it.
- The pulley rotates about an axle passing through its centre.
- The axle is fixed to a frame or a block.
- The pulley is normally fixed to a support above the load.
- The load is tied to one end of the rope and the effort is applied at the other end.
- Such a pulley makes our work easier by simply changing the direction of the force, i.e. a load is lifted up using a downward effort. It is easier to lift a load up by pulling it down rather than by pulling it up directly.
- Note that a simple pulley does not reduce the effort required to lift a load.
- A simple pulley is used to hoist a flag, to draw water from a well, etc.
Single Movable Pulley System
- This pulley system has a block of two pulleys.
- In this system, a load of W kgf can be lifted with an effort of W/2 kgf only.
- In a block and tackle arrangement, the pulleys are assembled together to form blocks and then blocks are paired so that one is fixed and the other moves with the load.
- The rope is threaded, or reeved, through the pulleys to provide a mechanical advantage that amplifies the effort applied to the rope.
An example of a practical application of pulleys and levers working together is the crane which is used to lift heavy loads.
The Wheel and Axle
Wheel and axle essentially consist of two cylinders of different radii joined together, such that if one is made to rotate the other also rotates. The cylinder with the larger radius is called the wheel and that with a smaller radius is called the axle.
Examples of wheel and axle system are a doorknob, a knob of water tap, a screwdriver, egg beater, a hand drill, etc.
Inclined Plane
An inclined plane is any sloping flat surface along which a load can be pushed or pulled with less effort. A hospital ramp on which a wheelchair can be pushed easily, a wooden plank used to load heavy boxes into the rear of a truck, winding roads around a hill, winding staircases, etc. are all inclined planes.
Wedge
If two inclined planes are put together to form a sharp edge, it is called a wedge. So a wedge is a double inclined plane. It works on the principle of inclined planes.
A wedge is used to tear apart or cut through objects. Knives, needles, axes, chisel, etc. are examples of a wedge.
Screw
A screw is a rod or nail with grooves on its circular curved surface and is used to hold two objects firmly together.
In simple terms, a rotating (winding) inclined plane is called a screw. The winding edge of a screw is called a thread. The grooved part of the screw is an inclined plane.
The head of the screw has a groove for the tip of a screwdriver. When the screw is held against a wooden block and its head is turned using a screwdriver, the tip of the screw moves into the wood. Because of the grooves, a screw holds the wood more firmly than a nail. Also, less force is required to insert a screw into wood than a nail because of the inclined edge.
Uses of Screws
- Screws are used to join two pieces of wood or metal. As the screw is a winding inclined plane, it cannot be pulled out easily from the attached pieces.
- A nut and bolt arrangement has two winding inclined planes. One inclined plane is on the external side of a metal cylinder and is called the bolt. The other inclined plane is on the inner side of a hollow metal cylinder and is called the nut. When the nut is given a circular motion over the bolt, it moves up or down without slipping and can withstand a lot of load.
- A cork screw is used for pulling out the cork from the bottles of ketchup or wine.
- The screw jack used to lift automobiles for repairs works on the principle of a screw.
Maintenance and Care of Machines
The following points should be remembered about the maintenance of machines.
- Machines should be protected from dust to prevent their wear and tear. So when not in use, machines should be kept covered.
- To avoid rusting due to exposure to moisture, the non-movable iron parts of a machine should be painted.
- To reduce friction, the moving parts of a machine should be regularly lubricated.
Example
Why do we have three types of levers?
A rigid bar resting on a pivot that is used to move a heavy or firmly fixed load with one end when pressure is applied to the other is called a lever. For example, imagine the incline in your parking lot that will take to your parking space in the garage.
Depending on the location of the load, the fulcrum, and the force levers are classified into three classes. They are:
- First class lever: It has a fulcrum between the force and the load, like a crowbar.
- Second class lever: It has a load between the fulcrum and the force, like a wheelbarrow.
- Third class lever: This lever has the force between the fulcrum and the load, like the arm on a human.
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