PULLEY BLOCK AND TACKLE Compound Drive Pulley Trains

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PULLEYBLOCK AND TACKLECompound Drive Pulley
Trains
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Pulleys
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Building Drive Robot Trains
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Building Robot Drive Trains

1. Building Robot Drive Trains is the first volume
   in the new "Robot DNA" series by McGraw-Hill/TAB
   Electronics.
2. It is just what robotics hobbyists need to build
   an effective drive train using inexpensive,
   off-the-shelf parts.
3. Leaving heavy-duty tech speak behind, the
   authors focus on the actual concepts and
   applications necessary to build and understand
   -- these critical force-conveying systems.
4. If you want a clear, straightforward guide to the
   nuts-and-bolts of drive trains, this is the way
   to go.

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Building Robot Drive Trains

• Chapters include
• The Basics of Robot Locomotion
• Motor Types An Overview
• Using DC Motors
• Using RC Servo Motors
• Using Stepper Motors
• Motor Mounting
• Basic Motor Control
• Advanced Motor Control
• Electronics Interfacing
• Wheels and Treads
• Locomotion for Multipods
• Glossary of Terms, Tables, Formulas 75
  illustrations

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Pulleys

• Pulley are wheels and axles with a groove around
  the outside
• A pulley needs a rope, chain or belt around the
  groove to make it do work

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Lifting Pulleys

• Lifting Pulleys in transmission systems
• Single pulley
• Moveable pulley
• Block and Tackle

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Pulley Examples

• Reverses the direction of force
• Two or more pulleys connected together permit a
  heavy load to be lifted with less force

http//www.sciencemuseum.org.uk/on-line/leonardo/I
nventorsToolbox.asp
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Fixed (single) pulley

• Fixed Pulley
• Wheel attached to surface
• Changes the direction of the applied force
• NO mechanical advantage same amount of force is
  required

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Single Pulley

• The resistance arm and the effort arm are both
  the radius of the pulley.
• Since they are equal, there is no mechanical
  advantage.

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Single Pulley

• Provides change of motion, but no mechanical
  advantage

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Movable pulley

• Movable Pulley
• Pulley moves along the rope
• Wheel supports the load
• Effort is in the same direction as movement
• Reduces the forces needed to move an object

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MA in a Moveable pulley

• A moveable pulley is a second-class pulley.
• The effort arm is the diameter and the resistance
  arm is the radius.
• MA dE dR
• MA 2 1 2

Mechanical Advantage 2
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Another Moveable pulley

• Provides change of direction and a mechanical
  advantage of 12 (at the expense of having to
  increase the amount of line to pull)

These kinds of diagrams are easier to analyze
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In the second picture, the weight is held by two
ropes instead of one. That means the weight is
split equally between the two ropes, so each one
holds only half the weight.
http//www.swe.org/iac/LP/pulley_03.html
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COMBINED PULLEY

• Combined (double) Pulley
• Has at least two wheels
• The more complex the pulley, the more the effort
  needed to move the object decreases

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COMBINED PULLEY

• The effort needed to lift the load is less than
  half the weight of the load.
• The main disadvantage is it travels a very long
  distance. 

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Mechanical Advantage

• Mechanical Advantage is the same as the number of
  ropes supporting the object
• MA2 MA2
  MA4

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How about these?
Calculate MA for these and similar
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Illustration of Efforts and Loads on pulleys
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Calculate MA
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Pulley Calculations
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Pulleys as used in robots

• A pulley is basically a wheel with a groove in
  the circumference of the wheel, with a belt or
  cable being fed around the groove.
• Pulleys create a mechanical advantage by
  connecting a small input drive pulley to a larger
  follower with a belt.

Driver (Effort)
Follower (Resistance)
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Mechanical Advantage of a Pulley Drive Train

• MA FOLLOWER DRIVER
• The Pulley Drive Train above produces an MA of 4.
  This means 10 lbs of input torque will create 40
  lbs output, but the driver must rotate 4 times to
  rotate output 1 time. Speed is reduced by 4.

Driver 1mm
Follower 4mm
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Mechanical Advantage

• MA FOLLOWER DRIVER

Driver 3mm
Follower 9mm
Driver 2mm
Follower 14mm
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Increasing Speed

• Increase the speed of the output by using a
  larger driver gear and a smaller follower gear.
  Speed increases and power decreases.

Driver 4mm
Follower 1mm
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Compound Drive Pulley Trains

• A Compound Drive Pulley Train has pairs of
  pulleys working in combination, with the follower
  of one pulley sharing the same axle as the driver
  of the next sequential pulley train.

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Example 1 Compound Drive Pulley Trains

• MA (FOLLOWER 1 X FOLLOWER 2) (DRIVER 1 x
  DRIVER 2)
• Driver 1 2mm Follower 1 8mm
• Driver 2 3mm Follower 2 9mm

Calculate MA
(89)/(23) 72/6 12)
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Example 2 Compound Drive Pulley Trains

• MA (FOLLOWER 1 X FOLLOWER 2) (DRIVER 1 x
  DRIVER 2)
• Driver 1 3mm Follower 1 9mm
• Driver 2 2mm Follower 2 14mm

Calculate MA
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Problem 2

• Sketch the following two pulley trains
• A Simple pulley train with belt wrapped so both
  pulleys rotate in same direction.
• B. Simple pulley train with belt wrapped so both
  pulleys rotate in opposite direction.

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Solution to Problem 2

• A Simple pulley train with belt wrapped so both
  pulleys rotate in same direction.

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Solution to Problem 2

• B. Simple pulley train with belt wrapped so both
  pulleys rotate in opposite direction.

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Problem 3

• Calculate the mechanical advantage of the
  compound pulley drive train model.
• Show your work.
• Small Pulley Diameter 25mm
• Large Pulley Diameter 100mm

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Last slide on Tuesday Jan7
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Block and Tackle
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Block and Tackle

• Multiple pulleys providing a greater mechanical
  advantage.
• MA is determined by the number of pulleys.

100/4 25
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Block and Tackle
Gun Tackle
Luff Tackle
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Block and Tackle
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Block and Tackle Luff-upon-Luff Tacke
1200/100 12
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Pulleys are used to gain mechanical advantage
trading the amount of rope you have to pull to
lift an object for how heavy the object can be.
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The more lines of support (ropes) a pulley has,
the more mechanical advantage it has!
http//www.swe.org/iac/LP/pulley_03.html
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Review of Pulleys

1. Give real-world example of each of the following
   pulleys.
2. Force Multiplier
3. Speed Multiplier
4. Direction Changer
5. Give one example of a simple machine or
   transmission that changes output motion. Identify
   both input and output motions.
6. Give examples of using Pulleys in Robotics,
   especially humanoid robot design.

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Pulley Engineering
Elaboration

• Design, Create, and Verify a pulley system with a
  mechanical advantage of
• 4
• 5
• 6
• Be sure to include drawings of your systems and
  how you verified the MA of the systems
• For block and tackle systems, what effect most
  greatly reduces the MA?

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Pulley Puzzlers
Evaluation

• The girl in the illustration is trying to lift
  herself and the swing using the rope.
• What would you predict would happen if she pulled
  down on the rope?
• If she weighs 90 pounds and the swing weighs ten
  pounds, how much force does she need to apply?

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Pulley Puzzlers
Evaluation

1. We should assume that it is an extensionless,
   massless rope, a massless pulley that does not
   hinder its rotation, and a rigid support.
2. We should imagine a box around the girl and the
   swing with only the rope extending out from the
   box to isolate the forces acting on the objects
   inside the box from the external forces.
3. The rope supports the box once, goes through the
   pulley and supports the box again.
4. The tension of the rope going upwards is T on
   each side, so the total tension is 2T.
5. The combined force from the tension in the rope
   must be greater than the combined weight of the
   girl and the chair for the chair to be lifted.
6. For a 90 pound girl and 10 pound chair, she must
   use a force of 50 pounds of more to be able to
   lift herself off the ground.

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Pulley Puzzlers
Evaluation

• The man in the illustration below is trying to
  lift himself and the block on which he is
  standing off the ground.
• Is this possible?

1. This movable pulley allows the man to pull
   upwards and the block on which he stands will
   rise with him on top.
2. He must apply a force equal to his weight and the
   weight of the block because there is only 1
   supporting rope outside the system.
3. There is a mechanical advantage of 1.
4. Yes, it can be done.

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Pulley Puzzlers
Evaluation

• A long rope passes over a single pulley and has
  bananas on one end and a monkey on the other.
• If the monkey and the bananas have the same mass,
  can the monkey ever reach the bananas if he
  climbs the rope?
• Assume that the rope is massless, there is no
  friction, and the rope is limitless.

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Pulley Puzzlers
Evaluation

• The opposite external torques produced by the
  bananas and the monkey will cancel each other
  out.
• Any upward movement of the monkey will be equaled
  by an upward movement of the bananas.
• If the monkey started below the bananas, the
  distance between the two will always stay the
  same.
• Because we assume that the rope is limitless and
  the bananas will never wedge into the pulley, the
  monkey will never be able to reach the bananas.

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How to use pulleys in your robot?

• Look to the pulleys used in other robots.
• Look to gears, chains and other similar
  constructions in our robots.
• How to use old Goodwill/Home Depot components?

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Pulleys in history
Engagement
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Ancient Pulleys
Engagement
Archimedes, who was kinsman and a friend of King
Hieron of Syracuse, wrote to him that with any
given force it was possible to move any
given weight, and emboldened, as we were told, by
the strength of his demonstration, he declared
that, if there were another world, and he could
go to it, he could move this. Hieron was
astonished, and begged him to put his proposition
into execution, and show him some great weight
moved by a slight force. Archimedes therefore
fixed upon a three-master merchantman of the
royal fleet, which had been dragged ashore by the
great labors of many men, and after putting on
board many passengers and the customary freight,
he seated himself at a distance from her, and
without any great effort, but quietly setting in
motion with his hand a system of compound
pulleys, drew her towards him smoothly and
evenly, as though she were gliding through the
water. - Plutarch
Instructor Read this in your best theatrical
voice ? What else did Archimedes study?
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Archimedes Advantage
Engagement

• Have two volunteers attempt to hold two metal
  rods apart while the instructor attempts to force
  them together with a pulley system.
• Discuss the Mechanical Advantage offered by
  pulleys

Teacher 2
Teacher 1
Instructor
Ask volunteers to keep the bars parallel and to
hold the ends not the middle
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Sources

• R. Lindeke,
• Gerald Rothenhofer

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