PULLEY
BLOCK AND TACKLE
Compound Drive Pulley Trains
Pulleys
Building
Drive
Robot
Trains
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 nutsand-bolts of drive trains, this is the way to go.
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
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
Lifting Pulleys
• Lifting Pulleys in transmission
systems
–Single pulley
–Moveable pulley
–Block and Tackle
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/InventorsToolbox.asp
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
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.
Single Pulley
• Provides change of
motion, but no
mechanical
advantage
100 lbs
100 lbs
100 ft
100 lbs
100 lbs
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
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
Another Moveable pulley
• Provides change of direction and a mechanical
advantage of 1:2 (at the expense of having to
increase the amount of line to pull)
These
kinds of
diagrams
are easier
to analyze
50 lbs
50 lbs
50 lbs
100 lbs 100 lbs
50 lbs
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
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
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.
Mechanical Advantage
• Mechanical Advantage is the same as the
number of ropes supporting the object
MA=2
MA=2
MA=4
How
about
these?
Calculate MA for
these and
similar
Illustration of Efforts and Loads on
pulleys
Calculate
MA
Pulley
Calculations
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)
Mechanical Advantage of
a Pulley Drive Train
MA = FOLLOWER ÷ DRIVER
Driver
1mm
Follower
4mm
• 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.
Mechanical Advantage
MA = FOLLOWER ÷ DRIVER
Driver
3mm
Follower
9mm
Driver
2mm
Follower
14mm
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
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.
Example 1: Compound Drive
Pulley Trains
MA = (FOLLOWER 1 X FOLLOWER 2) ÷ (DRIVER 1
x DRIVER 2)
– Driver 1 = 2mm
– Driver 2 = 3mm
Calculate MA
Follower 1 = 8mm
Follower 2 = 9mm
(8*9)/(2*3) = 72/6 = 12)
Example 2: Compound Drive Pulley
Trains
MA = (FOLLOWER 1 X FOLLOWER 2) ÷ (DRIVER 1
x DRIVER 2)
– Driver 1 = 3mm
– Driver 2 = 2mm
Follower 1 = 9mm
Follower 2 = 14mm
Calculate MA
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.
Solution to Problem 2
• A: Simple pulley train with belt wrapped so
both pulleys rotate in same direction.
Solution to Problem 2
• B. Simple pulley train with belt wrapped so
both pulleys rotate in opposite direction.
Problem 3
• Calculate the mechanical advantage of
the compound pulley drive train model.
– Show your work.
– Small Pulley Diameter = 25mm
– Large Pulley Diameter = 100mm
Last slide on Tuesday Jan7
Block and
Tackle
Block and Tackle
• Multiple pulleys providing a greater mechanical
advantage.
• MA is determined by the number of pulleys.
25 lbs
25 lbs
25 lbs
25 lbs
100/4 = 25
100 lbs 100 lbs
25 lbs
Block and Tackle
Gun Tackle
Luff Tackle
Block and Tackle
Block and Tackle: Luff-upon-Luff
Tacke
1200/100 = 12
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.
The more lines of support (ropes) a pulley has,
the more mechanical advantage it has!
http://www.swe.org/iac/LP/pulley_03.html
Review of Pulleys
1. Give real-world example of each of the following
pulleys.
1. Force Multiplier
2. Speed Multiplier
3. Direction Changer
2. Give one example of a simple machine or
transmission that changes output motion. Identify
both input and output motions.
3. Give examples of using Pulleys in Robotics,
especially humanoid robot design.
Elaboration
Pulley Engineering
• 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?
Evaluation
Pulley Puzzlers
• 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?
Evaluation
Pulley Puzzlers
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.
Evaluation
Pulley Puzzlers
• 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.
Evaluation
Pulley Puzzlers
• 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.
Evaluation
Pulley Puzzlers
• 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.
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?
Engagement
Pulleys in
history
Engagement
Ancient Pulleys
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?
Engagement
Archimedes Advantage
• 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 1
Teacher 2
Instructor
Ask volunteers to keep the bars parallel and to hold the
ends not the middle
Sources
R. Lindeke,
Gerald Rothenhofer