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PULLEYS AND LIFTING - 2

V. Ryan 2004 - 2010

 

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With a single pulley, the pulley must be able to move so that mechanical advantage can be increased. Furthermore, the pulley is turned upside-down.

The pulley to the left is suspended and as a consequence the mechanical advantage is increased. This happens because the rope on the left and right of the pulley are both lifting the LOAD, they each lift half its weight. The load is split into 2. The calculation is shown below.

 
 

 

VELOCITY RATIO

   

Velocity Ratio (sometimes called movement ratio)- is defined as the ratio of the distance moved by the effort to the distance moved by the load.

   

Using the example above. If the load is lifted 2 metres, above the pulley wheel - the rope on the right hand side must be shortened by one metre and this also applies to the left hand side. Therefore, the effort can be seen to move twice as far as the load.

   

   

A pulley system with the effort applied from above is very difficult to use. The most comfortable way to use a single moving pulley is combine it with a fixed pulley, seen below. This allows the effort to be applied downwards.
Please note, as one of the pulleys is a fixed pulley the mechanical advantage and velocity ratio is still calculated as if only the moving pulley exists. The velocity ratio and the mechanical advantage remains 2.
When dealing with a fixed pulley and a moving pulley, as shown in the diagram below, always work out which of the efforts move about the movable pulley. When the load is lifted two efforts (e1) and (e2) move. The strain of the load is divided into two whilst one final effort lifts the entire load.

 

   
 
   

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