machines and mechanisms -...
TRANSCRIPT
MACHINES AND
MECHANISMS
Dept. Tecnologia IES Cap de Llevant
MACHINES
They are used to avoid or reduce the human efforts in order
to do a work.
SIMPLE MACHINES
WHEEL INCLINED
PLANE LEVER
SPIRAL PULLEY
ELEMENTS in a MACHINE
MECHANISM
It is a device which takes an
input motion and force, and
outputs a different motion
(output) and force. The
point of a mechanism is to
make the job easier to do.
STRUCTURE
It is an element or a group
of elements that bear forces
in a machine (like a bicycle)
or in a static system (a
bridge or a building).
INPUT
(your effort)
MECHANISM OUTPUT
(load)
REVERSIBLE / NON REVERSIBLE MECHANISMS
REVERSIBLE MECHANISMS are those mechanisms where you
can use the output as an input and the input as an output. For
instance: a seesaw.
INPUT
(your effort)
MECHANISM OUTPUT
(load)
NON REVERSIBLE MECHANISMS are the mechanisms with only
one input and one output. For instance: a car jack.
THE MECHANISMS
How can a bicycle move?
How can the wheels of this machine turn?
How can we get the water from this water well?
Pulley
String
Turn of the pulley and the shaft
Resistance
TYPES OF MOTION IN MECHANISMS
Some mechanisms have the same kind of motion for input and output:
Linear Motion Linear Motion
Rotary Motion Rotary Motion
Other mechanisms take one type of input motion, and output it as a different
type of motion.
Linear Motion Rotary Motion
Rotary Motion Linear Motion
TYPES OF MOTION IN MECHANISMS
There are two basic types of motion in mechanical systems:
LINEAR MOTION is motion in a straight line.
ROTARY MOTION is a circular motion.
LEVERS
LEVERS
A lever is the simplest kind of mechanism.
It is composed by:
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.
When we apply a force (F1) in a point of the lever, we are creating a linear
motion, which transforms in another linear motion and a Resistance Force (F2)
appears.
F1: Applied force or Effort
F2: Load or Resistance
L1: Effort arm (distance)
L2: Resistance arm (distance)
F2 Load or
Resistance
Force or Effort F1
Resistance arm
L2
Effort arm
L1
Fulcrum
LAW OF THE LEVER
The formula is:
F1 x L1 = F2 x L2
Units:
F = Newtons (N) = m (kg) x g (10 m/s2)
L = metres (m)
The longer is L1, the less force you must do (and the other way around).
F1 F2
L2 L1
Example (calculation about levers):
Joan and Anna are seated in a swing. The distance between Anna and the fulcrum is 5m and her weight is 300 N. Joan is at a distance of 3m from the fulcrum.
How many Newtons must weight Joan to raise her?
NL 5003
53002
F1 x L1 = F2 x L2
300 x 5 = F2 x 3
Joan
F2 Anna
300 N
L2 = 3m L1 = 5m
TYPES OF LEVERS
Class 1 levers: has the fulcrum between the effort and the load.
Ex: scale, pliers, crossing gate barrier,...
Load Effort
Fulcrum
Class 2 levers: have the load between the effort and the fulcrum.
Ex: wheelbarrow, nutcracker,...
TYPES OF LEVERS
Load Effort
Fulcrum
Class 1 and class 2 levers both provide mechanical advantage.
This means that they allow you to move a large output load with a
small effort.
TYPES OF LEVERS
Load
Effort
The formula to calculate it is:
Mechanical Advantage = Load / Effort
Class 3 levers: have the effort between the fulcrum and the load.
Ex: hammer, tweezers,...
TYPES OF LEVERS
A class 3 lever does not have
the mechanical advantage.
More force is put in the effort
than is applied to the load.
Load Effort
Fulcrum
LINKAGES
LINKAGES are mechanisms which allow force or motion to be
directed where it is needed.
LINKAGES AND TYPES
Reverse-motion linkage Parallel-motion linkage Bell-crank linkage
It changes the direction
of the motion.
It creates an identical
parallel motion. It changes the direction of
movement through 90°
GEARS
GEARS
GEARS are rotating machines with teeth, which mesh with another
toothed part . When the first shaft moves (it is called driver shaft), the
other one (called driven shaft) turns or moves in the opposite direction.
They can have different functions:
· Increase or decrease forces
· Change the direction of the force
· Increase or decrease the speed of rotation.
TYPES OF GEARS
Depending on the teeth shape and the axis position there are
different kind of gears:
Bevel gears
Simple spur gear train Compound spur gear train
Worm gear
We can find them in watches, cassettes, bombs,....
Rack and pinion
GEARS - FORMULA
The formula used to calculate de velocity is:
V1·D1 = V2·D2
V1 = velocity of the driver shaft (rpm: revolutions per minute)
D1 = number of teeth of the driver shaft
V2 = velocity of the driven shaft (rpm: revolutions per minute)
D2 = number of teeth of the driven shaft
CHAIN AND SPROCKET
This mechanism uses a chain and a spur gear (with teeth) called
sprocket
CHAIN AND SPROCKET
The formula used to calculate de
velocity is the same as in gears:
V1·D1 = V2·D2
PULLEY SYSTEMS
PULLEY SYSTEMS
This mechanism is like the Chain and Sprocket, but in this case the
pulley wheels don’t have teeth and they are connected by a belt.
How can they modify the velocity?
MULTIPLY the velocity
Using the bigger pulley shaft like
the driver shaft in order to move
the smaller pulley, which will turn
quicker than the bigger one.
REDUCE the velocity
The smaller pulley drives the
bigger one and this one has a
slower velocity.
REVERSE the direction
It can be done by twisting the belt.
CALCULATE THE VELOCITY OF THE PULLEY
The formula is: V1 · 1 = V2· 2
1 = diameter of the driver pulley (in metres)
V1 = velocity of the driver shaft (rpm: revolutions per minute)
2 = diameter of the driven pulley (in metres)
V2 = velocity of the driven shaft (rpm: revolutions per minute)
PULLEYS
A PULLEY is a wheel with a groove along its edge, for holding a
rope or cable.
Pulleys are usually used to reduce the amount of force needed to
lift a load.
Fix pulley
TYPES OF PULLEYS
1.- FIX PULLEY (SIMPLE PULLEY):
It doesn’t reduce the effort we have to do to.
It is like a class 1 lever, but it allows to apply the
effort in the desired direction.
2.- Tackle:
It is formed by a fix pulley and one or more moving
pulleys connected by a rope. One extreme of the
rope is fixed to the pulley shaft and on the other
extreme the force is applied.
TACKLES CALCULATIONS
• When there is one moving
pulley then F=W /2
(Effort = load/2)
• When there are two
moving pulleys then F=W/4
(Effort=load/4)
If there are one or more
moving pulleys, the effort
is reduced.
It can be calculated by the
formula:
F = load/ 2n
n = number of moving pulleys
RATCHET
RATCHET
It is a mechanical device that allows continuous linear or rotary
motion in only one direction while preventing motion in the opposite
direction.
Ratchet mechanisms are used in, for example, load-lifting machines,
since ratchet mechanisms prevent the movement backward under the
weight of the load.
CAMS
It is a shaped piece of metal or plastic fixed to a rotating shaft.
CAM
A cam mechanism has three parts: cam, slide and follower.
The cam shaft rotates continually,
turning the cam. The follower is a rod
that rests on the edge of the turning
cam. The follower is free to move up
and down, but it cannot move from side
to side because of the slide or guide.
TYPES OF CAM
CIRCULAR CAM or
CRANK, LINK AND SLIDER ECCENTRIC CAM
PEAR-SHAPED
CAM
DROP CAM
SCREW THREAD
SCREW THREAD
It is a helical structure used to convert between rotational and
linear movement or force.
A screw thread is the essential feature of the screw as a simple
machine and also as a fastener.