What is a Pulley ? | Part 1 | PhysIcs

| What is a Pulley? Single Fixed Pulley |

A Pulley is a disk like object which has grooved rim. A string or rope is sent around the groove around its rim. The metallic disc rotates about an axle which passes through its center. The axle is fixed strongly to a frame by means of nails. It is mainly used to bring a change in direction of the effort. A typical pulley looks like this:

A single pulley can be used, probably, in two ways:

(1)   As a fixed pulley by keeping its axle stationary or, by keeping the frame of the pulley fixed, or in other words stationary.

(2)   As a movable pulley by leaving the axle free, i.e. by keeping the frame of the pulley movable.

The first sight of pulley came when it was seen to be used in Mesopotamia in 2^nd millennium and in the Ancient Egypt during the reign of Twelfth Dynasty. In the Roman Egypt, Hero of Alexandria ensured the pulley as on the six simple machines used to lift weights.

Its uses

(1) A normal example of pulley can be found in the wells where it is used to lift bucket of water from deep down the well.

(2) They are also used in construction equipment as seen in cranes.

(3) They can also be seen in mountain climbing equipment as well.

SINGLE FIXED PULLEY

A single fixed pulley, as its name suggests, is a type of pulley which has its axis of rotation fixed to a rigid support.

This figure shows a single fixed pulley in which the axle is fixed to the rigid support and an in extensible fine thread of negligible mass passes around the grooved rim of the fixed pulley. One end of the string is connected to the load, which is on the left side of the diagram, and the other end is left free where the effort has to be applied. Both the Load(L) and Effort (E) acts in the downward direction. It comes to functioning when the string on the Effort side is pulled downwards and the load moves in the upward direction. Also, a tension force acts on the string a reaction force to the load and the effort in the opposite direction to Effort and Load. Hence, for a fixed pulley the tension force will act towards the upward direction. This type of pulley is mainly found in wells for lifting a bucket of water.

Mechanical Advantage, Velocity Ratio and Efficiency of a Fixed Pulley:

On If we assume (i) the mass of the string and, (ii) friction between rim and string, and the bearings of the pulley to be 0, then in the balanced position of the load, we will have:

L = T and E=T [when pulley is not rotating]

From the above statement, we get that since L=T and E=T, hence L=E.

Therefore, in case of a single fixed pulley, when efficiency is 1, the effort needed will be equal to the load itself.

Thus, Mechanical Advantage = L/E = T/T = 1.

Hence, there is no gain in mechanical advantage. So, there is no role of force multiplying.

If the point of application of Effort moves for a distance of d downwards, then the Load also moves for a distance of d in the upwards direction, that is distance moved by effort=distance moved by load

Thus, Velocity Ratio = distance moved by effort / distance moved by load = d / d = 1.

Hence, it can be proved that for a single fixed pulley, mechanical advantage and velocity ratio are equal.

Thus, Efficiency= Mechanical Advantage / Velocity ratio = 1 or 100%.

But in actual practice, there is always a little friction at the axle or the bearings, so the effort needed is always little more than the load which has to be lifted. Thus, Mechanical Advantage is always less than 1, but velocity ratio remains unchanged. Hence, the Efficiency is also less than 100%

Then you might have a question what’s the importance of using a pulley if there is neither gain in mechanical advantage nor in speed, then why should we use it?  A fixed pulley is generally used to change the direction of effort to be applied. With the use of a fixed pulley, the effort can be applied in a more convenient direction. It is normally difficult to apply effort upwards to lift a load up directly, but it becomes much easier with a fixed pulley because the effort can be now applied in the downward direction to lift the load. Further to make the work easier one can apply his own weight as effort for easier and faster result.

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