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MECHANISMS (Rotary motion mechanisms (3.1. Friction drives (Friction…
MECHANISMS
DEFINITION : Mechanisms are devices that transmit and convert forces and motion from a driving force or input element to an output element. They allow us to carry out certam tasks more easily and more efficiently.
Mechanisms
they have diverse uses:
• transporting people from A to 8
• telling the time
• entertaining children in the park
• lifting and carrying objects
Linear motion mechanisms
2.1. Levers
A lever is a rigid bar that is supported by a fulcrum.
• F is the force or effort appbed at one end of the bar.
• R i s the resistance or load which acts at the other end of the bar.
• d i s the distance from F to the fulcrum
• r is the distance from R to the fulcrum
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Types of levers
There are three types of levers: class l, class 2 and class 3. Theseclassilications are based on the relative position of the lever, the effort
applied and the resistance
Class 1
The fulcrum is between the effort appied and the resistant The effect of the effort applied ran be multiplied or reduced.
Class 2
The resistance is between the fulcrum and the effort applied. The effect of the effort applied is always multiplied ( d > r )
Class 3
The effort apphed is between the fulcrum and the resistence.
The effect of the effort applied is always reduced ( d < r ).
2.2. Fixed pulley
A fixed pulley is a wheel that has a groove around lt into which a rope,
cham or belt 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:
F = R
Uses: wells and gym equipment, for example
2.3. Movable pulley
A moveable pulley ts a set of two pulleys — one ts fixed while the other
can move tn a linear direction.
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2.4. Compound pulley
This is a system of fixed and moveable pulleys, often called block and
tackle
If we use more pulleys, the system becomes more complex, but less
effort is needed to move the load.
Uses: lifts, goods lifts and cranes
Rotary motion mechanisms
These mechanisms transmit motion and effort in a circular way, from the input to the output.
3.1. Friction drives
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The first wheel is called the primary drive wheel . When it moves, it turns or drives the second or output wheel, causing it to move as well.
The ratio between the rotation velocity of the wheels or pulleys depends on the relative size of the wheels.
Uses: often in industry to make metal sheets, rolls of paper or other fine materials. Video players use this system to move video tape
3.2. Pulleys with belt
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Uses: industnal machines, vehicle engines, washing machines and dulls
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3.4. Worm gear
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Uses: lap counters, gear reduction systems, musical tuning keys and
windscreen wipers
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3.6. Gear train
This is a system of more than two gears, connected together as shown in the diagram.
The gear ratio between the drive wheel (I) and the driven wheel (4) depends on the number ol geared teeth m the system.
Uses: machine tools, robotics, vehicle gearboxes and some electrical appliances.
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Variation in velocity
When the wheels and the gears have the same diameter or the same
number of teeth, they rotate at the same velocity.
When they' re different sizes, the smallest rotates more quickly
The ratio of the velocities of the two wheels is given by the gear ratio.
Multiplying system; velocity is increased, Constant system; velocity remains constant, Reducing system; velocity is reduce.
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