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Mechanical Design

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Mechanical Design General Concepts AHL Topic 10 Mechanical advantage This is the factor by which a machine multiplies the force put into it. How do we calculate ... – PowerPoint PPT presentation

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Title: Mechanical Design


1
Mechanical Design
  • General Concepts
  • AHL Topic 10

2
Mechanical advantage
  • This is the factor by which a machine multiplies
    the force put into it.

Velocity Ratio
  • A measurement of force amplification.

Efficiency
  • Mechanical efficiency is the effectiveness of a
    simple machine.

3
  • How do we calculate Mechanical Advantage (MA),
    Velocity Ratio (VR) and Efficiency for simple
    mechanical systems?

MA effort load
VR distance moved by effort distance moved
by load
Efficiency MA VR
4
LEVERS
  • There are 3 types of lever
  • 1st class lever (fulcrum is in the middle)
  • 2nd class lever (load is in the middle)
  • 3rd class lever (effort is in the middle)
  • Each class of lever has a
  • Fulcrum
  • Load
  • Effort

5
Examples of 1st class levers
  • On all 1st class lever the fulcrum is in the
    middle.

Seesaw
Crowbar
Scissors
6
Examples of 2nd class levers
  • On all 2nd class lever the load is in the middle.

Wheelbarrow
Bottle opener
Nut cracker
7
Examples of 3rd class levers
  • On all 3rd class lever the effort is in the
    middle.

Tweezers
Broom
Fishing rod
8
  • All of the levers make tasks easier for us.
  • 1st class levers allow us to lift heavy loads,
    and to shear materials accurately
  • 2nd class levers allow us to move and lift
    heavier loads.
  • 3rd class levers allow us to carry out precision
    operations.

9
Equilibrium
  • When a lever is in equilibrium the net moment is
    zero

If this 10N weight was dropped onto the end of
this 1st class lever what would happen?
10
GEARS
  • A gear is a component within a transmission
    device that transmits rotational force to another
    gear or device. A gear is different from a pulley
    in that a gear is a round wheel that has linkages
    ("teeth" or "cogs") that mesh with other gear
    teeth, allowing force to be fully transferred
    without slippage. Depending on their construction
    and arrangement, geared devices can transmit
    forces at different speeds, or in a different
    direction, from the power source.
  • The most common situation is for a gear to mesh
    with another gear, but a gear can mesh with any
    device having compatible teeth, such as linear
    moving racks.
  • The gear's most important feature is that gears
    of unequal sizes (diameters) can be combined to
    produce a mechanical advantage, so that the
    rotational speed and torque of the second gear
    are different from those of the first.

11
Rack and pinion
A rack and pinion is a pair of gears which
convert rotational motion into linear motion.
Rack-and-pinion steering is quickly becoming the
most common type of steering on cars, small
trucks and SUVs. It is actually a pretty simple
mechanism. A rack-and-pinion gearset is enclosed
in a metal tube, with each end of the rack
protruding from the tube. A rod, called a tie
rod, connects to each end of the rack.
12
Bevel gears
  • Bevel gears are useful when the direction of a
    shaft's rotation needs to be changed. They are
    usually mounted on shafts that are 90 degrees
    apart, but can be designed to work at other
    angles as well. The teeth on bevel gears can be
    straight, spiral or hypoid.

This feature is used in many car differentials.
The ring gear of the differential and the input
pinion gear are both hypoid. This allows the
input pinion to be mounted lower than the axis of
the ring gear.
This picture shows the input pinion engaging the
ring gear of the differential.
13
Worm gears
  • Worm gears are used when large gear
    reductions are needed. It is common for worm
    gears to have reductions of 201, and even up to
    3001 or greater.

Many worm gears have an interesting property that
no other gear set has the worm can easily turn
the gear, but the gear cannot turn the worm.
This feature is useful for machines such as
conveyor systems, in which the locking feature
can act as a brake for the conveyor when the
motor is not turning.
14
Gear trains
  • To create large gear ratios, gears are often
    connected together in gear trains

The right-hand (purple) gear in the train is
actually made in two parts, as shown. A small
gear and a larger gear are connected together,
one on top of the other. Gear trains often
consist of multiple gears in the train, as shown
in the following slide
15
In the Gear train to the right, the purple gear
turns at a rate twice that of the blue gear. The
green gear turns at twice the rate as the purple
gear.
The gear train shown below has a higher gear
ratio
  • In this train, the smaller gears are
    one-fifth the size of the larger gears. That
    means that if you connect the purple gear to a
    motor spinning at 100 rpm (revolutions per
    minute), the green gear will turn at a rate of
    500 rpm and the red gear will turn at a rate of
    2,500 rpm.

16
Belts
  • Belt drives are an alternative to chain
    drives. Early motorcycles often used leather
    belts, which could be tensioned to give traction
    using a spring-loaded pulley and hand lever.
    Leather belts often slipped, especially in wet
    weather, so they were abandoned for other
    materials and designs. By the 1980s, advances in
    materials made belt final-drive systems viable
    again. Today's belts are made of cogged rubber
    and operate much the same way as metal chains.
    Unlike metal chains, they don't require
    lubrication or cleaning solvents.

17
Pulleys
  • A pulley (also called a block and tackle) is
    a mechanism composed of a wheel (called a sheave)
    with a groove between two flanges around the
    wheel's circumference. A rope, cable , belt or
    chain usually runs inside the groove. Pulleys are
    used to change the direction of an applied force,
    transmit rotational motion, or realize a
    mechanical advantage in either a linear or
    rotational system of motion.

18
Inclined plane
  • The inclined plane is one of the original six
    simple machines as the name suggests, it is a
    flat surface whose endpoints are at different
    heights. By moving an object up an inclined plane
    rather than completely vertical, the amount of
    force required is reduced, at the expense of
    increasing the distance the object must travel.
    The mechanical advantage of an inclined plane is
    the ratio of the length of the sloped surface to
    the height it spans this may also be expressed
    as the cosecant of the angle between the plane
    and the horizontal.

Screw thread with an inclined plane
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