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