Transcript Mechanismsx - jdpillesTECHNOLOGY

Let´s Move It
What are they?
 A mechanism is simply a device which takes
an input of motion or force and outputs a
different motion or force. The point of a
mechanism is to make the job easier. The
mechanisms most commonly used in
mechanical systems are levers, linkages,
cams, gears and pulleys.
Input motion
Input force
Mechanism
Output motion
Output force
 Levers  Mechanical advantage
 Linkages  used to change the direction or type of motion, in
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combination with levers also could change the size or magnitude of a
force.
Cams  change the type of motion from circular to rectilinear motion.
Gear, gear trains and compound gear trains gear ratio and output
speed.
Worm gear, bevel gear, rack and pinion
Pulley systems and hoist  velocity ratio, output speed, output torque
Chain and sprocket
Crank, link and slider.
Levers
1st class
2nd class
3rd class
Can you think of any more
examples of levers?
Mechanical Advantage and
Equilibrium Point
 M.A. =
Load
Effort
d2
d1
Effort
Fulcrum
Load
L·d1 = E·d2
In summary,
 A lever is the simplest kind of mechanism. There are three
different types of lever. Common examples of each type are
the crowbar, the wheelbarrow and the pair of tweezers.
 All levers are one of three types, usually called classes.
The class of a lever depends on the relative position of the
load, effort and fulcrum:
 The load is the object you are trying to move.
 The effort is the force applied to move the load.
 The fulcrum (or pivot) is the point where the load is pivoted.
Linkages
Linkage type
Description
Diagram
Reverse Motion L.
A reverse-motion linkage changes the
direction of motion
reverseLinkage
Parallel Motion L.
A parallel-motion linkage creates an
identical parallel motion.
paralleLinkage
Bell-Crank L.
A bell-crank linkage changes the
direction of movement through 90°
bellCrank
Crank-Slider L.
Converts rotary motion to linear
motion
CrankSlider
Treadle L.
The rotary motion is converted in to an
treadle
oscillating motion
Linkages
 Linkages are mechanisms which allow force or motion to be directed
where it is needed. Linkages can be used to change:
 The direction of motion
 The type of motion
 The size of a force
 A linkage consists of a system of rods or other rigid materials
connected by joints or pivots. The ability of each rod to move will be
limited by moving and fixed pivots. The input at one end of the
mechanical linkages will be different from the output, in place, speed,
direction and other ways.
Cams
 A cam is a shaped piece of metal or plastic fixed to a
rotating shaft. A cam mechanism has three parts:
cam, slide and follower.
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Pear-shaped cam
Eccentric cam
Drop cam
camSystems
Gear
 Gears consist of toothed wheels fixed to shafts. The
teeth interlock with each other, and as the first shaft
(the driver shaft) rotates, the motion is transmitted
to the second or driven shaft.
 A number of gears connected together are called a
gear train.
Gear ratio
 Where there are two gears of different sizes, the
smaller gear will rotate faster than the larger gear. The
difference between these two speeds is called the
velocity ratio, or the gear ratio, and can be calculated
using the number of teeth.
V1·Z1 = V2·Z2
G.R. = Z2 / Z1
V1: input speed
(driven shaft)
V2: output speed
(driver shaft)
Z1: number of teeth
of driven gear
Z2: number of teeth
of driver gear
Compound Gear Train
Where very large speed reductions are required, several pairs of
gears can be used in a compound gear train. A small gear drives
a large gear. The large gear has a smaller gear on the same
shaft. This smaller gear drives a large gear. With each transfer,
the speed is significantly reduced.
gearSystems
Worm gears
 Another method of making large speed
reductions is to use a worm gear.
 Each time the shaft spins one
revolution, the gear turns forward by
only one tooth.
 The worm can drive the worm gear
round, but the worm gear cannot drive
the worm. This means that worm gears
are good to use in hoists, the load will
not fall back when the motor stops.
 Worm gears are a good option when you
wish to alter direction or rotary motion
through 90° and reduce the speed.
WormAndWormwheel
Bevel gears
Bevel gears, like worm
gears, change the axis of
rotation through 90°. The
teeth have been specially
cut so the gears will mesh
at right-angles to each
other.
bevelGears
Rack and pinion
 A pinion is a round cog and the rack is a flat bar with
teeth.
Pulley systems
 Pulleys are used to change the speed, direction of rotation,
or turning force or torque.
 A pulley system consists of two pulley wheels each on a
shaft, connected by a belt. This transmits rotary motion
and force from the input, or driver shaft, to the output, or
driven shaft.
V1·D1 = V2·D2
Velocity Ratio
 If the pulley wheels are different sizes, the smaller one
will spin faster than the larger one. The difference in
speed is called the velocity ratio.
V1·D1 = V2·D2
V.R. = D2 / D1
BeltAndPulley
V1: input speed
(driven shaft)
V2: output speed
(driver shaft)
D1: Diameter of
driven pulley
D2: Diameter of
driver pulley
 Chain and sprocket
 Crank, link and slider
 Crank shaft
 Ratchet and pawl
Torque
 The velocity ratio of a pulley system also determines
the amount of turning force or torque transmitted
from the driver pulley to the driven pulley.