MECHANISMS Part II

1.1    Linear motion mechanisms

Linear motion mechanisms transmit motion and force in a straight line from one point to another.

·      LEVER: is a rigid bar that is supported by a fulcrum. Fulcrum is the point of a bar around which it pivots, moves or is supported. The following equation indicates a balanced lever:

F . d = R . r

It´s called the law of the lever. There are three types of levers:

Class 1	Class 2	Class 3
The fulcrum is between the effort applied and the resistance. The effect of the effort can be multiplied or reduced. Pliers and pins are class 1 levers.	The resistance is between the fulcrum and the effort applied. The effect of the effort applied is alwais multiplied (d>r). Wheelbarrow, nutcrackers and paper cutter are class 2 levers.	The effort applied is between the fulcrum and the resistance. The effect of the effort applied is always reduced (d<r). Rod, broom and tweezers are class 3 levers.

·      FIXED PULLEY: is a wheel that has a groove around it into which a rope, chain or belts fits.

It rotates around an axle that is fixed to an immobile surface. A fixed pulley is balanced when the effort F is equal to the resistance of the load, R. It´s used to raise and lower loads easily.

·      MOVABLE PULLEY: is a set of two pulleys. One is fixed while the other can move in a linear direction.

A movable pulley is balanced when F = R / 2

·      COMPOUND PULLEY: is a system of fiwed and movable pulleys, often called a block and tackle.

If we use more pulleys, the system becomes more complex, but less effort is needed to move the load. There are always an even number of pulleys. Half is fixed and half movable.

A block and tackle (compound pulley) is balanced when F = R /2n where n is the number of movable pulleys.