In many emergency situations firefighters will need to lift large weights to perform the rescue of injured people. The manoeuvres we use must be done safely, quickly and efficiently since the time we have to release the injured is limited.
Renaissance scientists classified six mechanical elements that allowed the direction or magnitude of a force to change. This classification was called ‘Simple Machines’. This set is formed by the pulley, inclined plane, wedge, screw, lathe and, of course, the lever.
Famous is the phrase ‘Give me a point of support and I will move the world’, as pronounced by Archimedes in a mathematical collection of Pappus of Alexandria. But the lever was already used in Mesopotamia and Egypt in the years 5000–4500 BC to move objects.
In this simple machine, whose function is to transmit force and displacement, three forces act:
- Power (P) is applied voluntarily in order to obtain a result. In our case, we usually use pneumatic cushions, hydraulic or pneumatic struts, hydraulic separators, etc.
- Resistance (R) is exerted on the system by the body to be moved. Its calculation can be made taking into account the mass (m) of the body and the value of the acceleration of gravity (g).
R = m x g
Firefighters have to make more or less approximate calculations in situ about the weight of the object to be moved to be able to establish a rescue strategy.
The system that makes up a lever basically consists of a rigid mobile bar that revolves around a fixed point of support or articulation placed in the following way:
- The fulcrum is where the supporting force is transmitted and, therefore, it must have the rigidity and resistance necessary to be able to sustain the whole set.
- The rigid bar is a board, bar or structure that rests on the point of support and in which both the resistance force to be overcome and the power force to raise the load will be applied.
- The load is the element or object that is intended to move or lift. In our case it could be a car, truck, bus, train, etc.
- The force: is the effort or contribution necessary to move the beam and the load.
As first responders in the case of traffic accidents and other emergencies, firefighters must understand the lever principle for the lifting of loads and how to use it correctly and safely.
The lever is a simple machine that allows us to control the intensity of a force, as well as modify its amplitude and movement. For a better understanding of this, we will look at some of the lever’s characteristics.
Force and lever
The ideal lever does not dissipate or store energy, that is, the same amount of energy that is applied is used to move the load. The proportion of the input and output of such energy will depend on the force arms that are created on the beam by placing the fulcrum between the points of application and output of the force. The distances measured from the fulcrum to the points of application of the P and R respectively are called:
- In the case of P, it is the Power Arm (Bp), which is the distance between the point of application of the power force and the point of support or fulcrum.
- In the case of R, it is the Resistance Arm (Br), which is the distance between the point of the resistance and the point of support.
As we can see in the drawing, if a force P is applied at a point of the bar in order to overcome a resistance, then, depending on the length of the bar, where the force P is applied, where the fulcrum is placed and the amount of resistance (R) that we have to overcome, we will need to apply a greater or lesser amount of force to move the load.
To us, when we have to perform a rescue, we need to know how and where those arms of power and resistance are generated and, depending on the situation in which we find the injured person, we can place the lifting tools in the most appropriate way possible to be able to move the load for the rescue. With this we can perform a quick, efficient and safe rescue of the trapped person.
Law of the lever
If friction losses are ignored, the work done by the applied force is equal to the work done by the resulting force. This allows the lever to increase the magnitude of the force applied over a certain distance, by transforming it into the resulting force at the cost of decreasing the distance travelled by the load. The relationship between the applied force and the resulting force is called the mechanical advantage.
Mechanical advantage is a dimensionless magnitude that indicates when the applied force is amplified using a mechanism to counteract a resistance load. We have two types of mechanical advantage:
- Theoretical or ideal mechanical advantage is where the principle of conservation of energy is maintained. The input energy is the same as the output energy. (Friction, wear, deformation, etc. are ignored.)
- Real mechanical advantage is lower than the theoretical because the efficiency of the system is always less than 100%, since there may be friction, wear or deformation of the system.
Mechanical Advantage = Load Resistance Applied Force
When we have to perform a rescue manoeuvre like the one we can see in the drawing, the mechanical advantage we have is the difference in length that exists between Bp and Br.
The greater the Bp the less force will be necessary to overcome the resistance, but there will be a greater displacement of the point of application of the power. Therefore, the tool chosen must have the necessary extension capacity to allow us to fully perform the manoeuvre. Taking into account this concept will allow us to subject our tools to lower stresses and a lower expenditure of energy when performing a manoeuvre.
The mechanical advantage offered by a lever is given when considering the balance of movements or torque on the pivot point or fulcrum. This leads to the next point.
Balancing a pair of forces
It can be understood as equilibrium of pairs or moments of forces to which the force exerted by P with respect to the fulcrum is opposite to the force exerted by R. In physics, the law relating the forces of a lever in equilibrium tells us that the product of power by its arm is equal to the product of resistance by its arm. This tells us that any variation in any of the factors that make up the equation will take the system out of its equilibrium.
P x Bp = R x Br
As firefighters who sometimes have to face a rescue involving heavy vehicles, knowing that we can influence any of the four factors that make up this formula can greatly assist our work.
We can influence the P by choosing lifting systems with the necessary force to lift the load. We can influence the R by removing superfluous weight from the vehicle.
We can influence the force arms by increasing the Bp as much as possible or decreasing the Br as much as possible, which would reduce the power needed to raise the object.
These details can sometimes make significant differences when performing a rescue, since our tools have a limited lifting capacity. If we are aware of these advantages then on many occasions we can release the injured person in safety, both for ourselves and for the trapped.
In the second part of this feature we will explore the different types of levers and discuss how they can be used to best effect by rescuers.
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