MECHANICAL ADVANTAGE
• Forces we encounter in SAR
• Simple Machines
• Mechanical Advantage of Pulley Systems
MECHANICAL ADVANTAGE
• FORCE
– What is Force
– What types of Force do we encounter
– What are the Units of Force
MECHANICAL ADVANTAGE
• What is Force?
– Force is an external influence that may cause
a body to accelerate. It may be experienced
as a lift, a push, or a pull.
– Force is a vector. All forces will have a
magnitude and direction.
• Forces we encounter in SAR
– Forces due to:
•
•
•
•
Gravity
Friction
Impulse
Applied Forces
• Gravity
• g = 32.2 ft/sec2 = 9.8 m/s2
• F = ma
• F ≈ 0.10197 kg x 9.8 m/s2 = 1Newton (N)
• A newton is the amount of force required to
accelerate a body with a mass of one kilogram at a
rate of one meter per second squared.
• 1 kN = 1,000 N ≈ 224.81 lbf
• Gravity = Weight
– 1 kN = 224.81 lbf
– 80kg (0.8 kN ≈ 1 kN) for a ‘single load’,
– 200kg (440 lbs = 1.95 kN ≈ 2 kN) for a ‘rescue load’
– 280kg (617 lbs = 2.7 kN ≈ 3 kN) for a ‘three-man load’.
• Gravity = Weight
– 1 kN = 224.81 lbf
– 1 Person ≈ 1 kN
– 2 Person ≈ 2 kN
– 3 Person ≈ 3 kN
• Gravity = Weight
– The average rescuer can hold or apply a .2 kN force
with one hand (≈ 45 lbs)
– The average rescuer can hold or apply a .4 kN force
with two hands (≈ 90 lbs)
– Hauling an rope ‘hand-over-hand’ is applying a
force of 45-50 lbs
• Impulse
• Reaction time to a failure or rope movement is 1 sec
• In 1 sec a load will travel 16 feet
MECHANICAL ADVANTAGE
• Simple Machines
MECHANICAL ADVANTAGE
• Machines are affected by factors such as
friction and elasticity
• So the actual mechanical advantage of a
simple machine will usually differ from its
theoretical value.
MECHANICAL ADVANTAGE
Pulley:
• Pulleys change the direction of a tension force on a
flexible material, e.g. a rope or cable. In addition, pulleys
can be "added together" to create mechanical
advantage, by having the flexible material looped over
several pulleys in turn. More loops and pulleys increases
the mechanical advantage.
MECHANICAL ADVANTAGE
Pulley as a Lever:
• The pulley is a variation of the
wheel and axle.
• The size of a pulley does not
influence the MA.
• The size of a pulley does influence
the efficiency of the pulley.
• The larger the pulley, the more
efficient the pulley.
MECHANICAL ADVANTAGE
Pulley Types:
• Fixed pulley
– Provides change of direction
ONLY
MECHANICAL ADVANTAGE
Pulley Types:
• Movable pulley
– Adds Mechanical Advantage
MECHANICAL ADVANTAGE
Pulley:
• Pulleys change the direction of a tension force on a
flexible material, e.g. a rope or cable.
• Pulleys can be "added together" to create mechanical
advantage, by having the flexible material looped over
several pulleys in turn.
• More loops and pulleys can increase the mechanical
advantage.
MECHANICAL ADVANTAGE
Pulley Efficiency:
• Two factors determine a pulley's efficiency:
– Sheave size: the larger the sheave diameter, the
higher the efficiency.
– Bushings and bearings: self-lubricating bushings are
efficient, but they must be regularly maintained.
Ball bearings are very efficient and since they are
sealed, they do not require any maintenance.
MECHANICAL ADVANTAGE
• Effective pulley systems must always have one side
anchored and the other side attached to the moving
load, known as the anchor side and the load side. There
must be something to pull against.
MECHANICAL ADVANTAGE
• The longest distance a pulley system can be stretched,
the distance from the anchored pulley to the moving
pulley, is called the stroke.
• The longer the stroke, the more useful the MA system.
MECHANICAL ADVANTAGE
• Pulling the system down to its smallest stroke is called
compression.
• It is called de-set when the system is compressed so it
will expand again when using the MA system as the
Decent Control Device (DCD) to lower a load rather than
to raise it.
MECHANICAL ADVANTAGE
• Extension means to pull out a pulley system to its
longest stroke.
• “Set” is called when the progress capture device is
engaged, allowing extension to occur.
• “Re-set” is called only when full extension has been
completed during raising operations, and another haul
segment is ready to be made on the main line.
MECHANICAL ADVANTAGE
• All anchored pulleys are Change Of Direction (COD)
only.
MECHANICAL ADVANTAGE
• Pulleys that move with a load (unanchored pulleys) are
simple machines that gain advantage.
MECHANICAL ADVANTAGE
• Pulley systems are either simple, compound or complex.
• Compound pulley systems are made up of at least two
simple pulley systems.
• Complex pulley systems are made up of at least one
simple pulley system and at least one compound pulley
system.
MECHANICAL ADVANTAGE
• If the terminal end of a haul line is attached to the anchor, the simple
pulley system will be EVEN
• 2:1, 4:1, 6:1, 248:1
MECHANICAL ADVANTAGE
• If the terminal end of a haul line is attached to the LOAD, the simple
pulley system will be ODD
• 1:1, 3:1, 5:1, 115:1
MECHANICAL ADVANTAGE
• Simple pulley systems have a greater stroke than
compound pulley systems of the same MA.
MECHANICAL ADVANTAGE
• The Mechanical Advantage of a pulley system can be
expressed as a ratio.
• It is the ratio of the amount of force that must be applied
to a haul line to move a load, divided by the weight of the
object that must be moved.
• It is the ratio of the weight of the object that must be
moved to the amount of force that must be applied to
move it.
• 2:1 = 2 Units of output force will result from 1 Unit of
input force.