Cranks and pulleys Grade 7 Technology

Показать описание

A crank is simple mechanism which consists of an arm that is joined at one end to a shaft or axle that can turn. The other end of the arm usually has a handle or pedal.
This simple mechanism is an adaption of a second class lever, which is attached at one end to an axle so that it can pivot in a complete circle around the shaft. At the opposite end to the pivot is a pin at 90 degree to the lever, this works as a handle or a pedal to turn the crank. When we are turning the handle, we increase the turning force of the shaft, which then gives the mechanical advantage (MA).
Let’s look at an example a winch, a winch consists of a crank which turns a barrel or drum that has a long length of rope or cable that winds around it, when we are busy turning the crank handle, the drum turns which will then wind up the rope. The cranks help us to easily move loads that are attached to the end of the rope, for example, when we are drawing a bucket of water from a well.
To get mechanical advantage in a crank is almost like other simple mechanics. The longer the crank is, the easier it is to turn the axle although a greater distance is needed to turn it. Because the length of the input arm is long, which is the handle, the crank lever increases the turning force with distance, increasing mechanical advantage.
We have looked at a winch; let’s move onto the pulley, the pulley is a type of wheel and axle or shaft. A cod or rope runs in the groove. When we pull on the rope, the wheel turns, and will lift the object at the other end of the rope. This changes the direction of force, making the object easier to lift.
There are two different kinds of pulleys a single pulley and a compound pulley. A single pulley has one pulley wheel, which is attached to a frame or beam to give it support, and is called fixed pulley. A single fixed pulley makes it much easier to lift a load because you can use the force of gravity and your bodyweight to pull the load upwards.
However, a single pulley does not give any mechanical advantage:
• The amount of effort you need to lift an object is equal to the load.
• To raise a load a certain height, you must pull the rope down an equal distance. For example, to raise a flag which is 10 m, you need to pull the rope a distance of 10 m. So MA=1
Now, moving on to our second type of pulley, the compound pulley, a compound pulley unlike the single pulley is made up of two or more pulley wheels. When we use a compound pulley, less effort is needed to lift a load. In a compound pulley, usually one pulley is fixed and the other pulley or pulleys are moveable. The moveable pulley moves with the load, which means that when you pull on the rope, both the pulley and the load come up.+
In a double pulley system, one fixed pulley is combined with a moveable pulley. This reduces the effort by half. This means that it takes half the effort to lift the load. But because the rope is twice as long, you have to pull it twice as far. MA is equal to the number of pulleys used. So in a double pulley system MA=2.
In a compound pulley system, for each extra pulley used, the effort is decreased, but the distance the rope must be pulled increases. For example, if you use three pulleys, you use one third of the effort, but you have to pull the rope three times as far.