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Mechanisms ̿ ̿ ̿'̿'\̵͇̿̿\з=(•_•)=ε/̵͇̿̿/'̿'̿ ̿ (Mechanisms…
Mechanisms ̿ ̿ ̿'̿'\̵͇̿̿\з=(•_•)=ε/̵͇̿̿/'̿'̿ ̿
Around us we can see many moving obiects and mechanisms that produce movement that we can also call motion.
The chain gear system of a bicycle, the gears of a clock or watch, the
lever in a seesaw and the pulley in a well are some of the simpler
mechanisms that can be fuund in many objects.
They have diverese uses:
transporting people from A to 8
lifting and carrying objects.
Telling the time.
entertaining children in the park.
All of these things can make our lives easier and more pleasant.
The most important element in all of them is the driving force that
initiates the movement.
The dnving force can be a spnng, an electric motor or our own muscles.
The motion produced by a motor is transformed and converted by
those mechanisms to receiving elements such as wheels, clock hands, and so on, so that they can do the lob for which they were constructed.
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 certain tasks more easily and more efficiently.
Mechanisms that transmit motion and force in a straight line from one point to another.
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
The following equation indicates a balanced lever.
F x D=R x r
There are three types of levers: class 1, class 2 and class 3. These classilications are based on the relative position of the lever, the effort
applied and the resistance.
Fixed pulley
A fixed pulley is a wheel that has a groove around lt into which a rope, chain 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
Pulleys are used to raise and lower loads easily because we use our own
weight to counteract them.
Movable pulley:
A moveable pulley ts a set of two pulleys — one is fixed while the other
can move in a linear direction.
A moveable pulley is balanced when it satisfies this equation:
F=R/2
The effort required to move a load with a moveable pulley is half the
effort needed to move the same load with a fixed pulley.
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.
Linear motion mechanisms
Rotary motion mechanisms
These mechanisms transmit motion and effort in a circular way, from the input to the output.
They include friction drives, pulley systems, gears and worm gears.
Friction devices:
Friction drives are made up of two or more wheels that are in contact.
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 output wheel rotates in the opposite direction of the pnmary wheel
lf we use more than two wheels, each one rotates in the opposite direction to the one next to it.
I he ratio between the rotation veloaty of the wheels or pulleys depends on pulley 2
the relative size of the wheels.
It is expressed by the followmg equation:
N
D=N2
D2 ---->D/D=N/N
N1 and N2 are the velocities of the primary dmve wheel and the output wheel These velomties are expressed tn revolutions per minute (rpm)
D1 and D2 are the corresponding diameters. They are expressed in units of length: usually millimetres
The ratio D1/D2 is called the gear radio.
Pulleys with belt
They consist of two pulleys or wheels that are a certain distance apart. Their axles are parallel and they rotate simultaneously due to the effect
of the belt. The rotation of one axle is transmitted to the other through the
connected pulleys. Uses: industnal machines, vehicle engines, washing machines and dulls.
Gear mechanisms and cogwheels
:
Cogwheels are sets of wheels that have teeth called cogs. The cogs fit into the spaces between the cogs of another wheel, so that one wheel moves the other. They transmit a rotary motion between the two connected axles, which can be parallel, perpendicular or oblique. Gears can be cyhndncal or conical.
All the teeth must be the same shape and size
The two wheels and the two axles rotate in opposite directions. The ratio between the rotation veloaues of the wheels depends on the
number of teeth on each wheel. It's expressed by this equation: N
x Z=N
Z
Z/Z=N/N
N, and N, are the velocities of the corresponding wheels
• Z , and Z, are the numbers of teeth.
The ratio Z,/Zi is called the gear ratio.
Uses: in industrial machinery, vehicles, domestic appliances such as drills and food processor.
Worm Gear
:
This is a screw that moves a helical cogwheel that is set perpendicular
to the screw Each time the screw rotates, the gear moves forward as many teeth as
there are grooves in the screw, usually a small number 1, 2 or 3. This is used to reduce velocity as well as functioning as a brake system.
A worm screw fulfils this equation: Nwheel=Nscrew x Ngrooves
/Zwheel.