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.