Following on from the first part of this feature we will now explore the different types of levers and discuss how they can be used to best effect by rescuers.
Types of levers
In a traffic accident, depending on how we find the injured person and the situation at the scene, sometimes we will have to use a lever. For this reason, it is interesting to know how many types there are and their classification, which is defined by the placement of the three forces that act in this simple machine: power, resistance and the point of support or fulcrum.
First-degree lever: Here the fulcrum is between power and resistance. Depending on the positions in which the forces can act, in this type of lever we have three alternatives:
1. Fulcrum close to resistance: In this case, the power arm is greater than the resistance arm. This means that less effort (power) is needed to overcome the resistance, and at the same time there is a greater displacement of power than of resistance.
This solution should be used whenever circumstances allow, since it gives us mechanical advantage and we can use it when we need to move large loads with small forces.
We must bear in mind that the Bp will suffer a great displacement and, therefore, the tools to be used to generate the movement must be able to make large movements when applying the power.
2. Centred fulcrum: In this situation, due to the balance of forces that exists, both the applied force and the weight of the load are equal, as well as the displacements of power (Dp) and resistance (Dr).
Due to the balance of forces, this manoeuvre does not bring mechanical advantage, but it may offer comfort and convenience. If we want to gain mechanical advantage, we will have to break the balance by increasing the force applied or by eliminating some weight from the resistance.
3. Fulcrum close to power: In this case, the power arm is less than the resistance arm. This means that more effort (power) is needed to compensate for the resistance, and at the same time there is less displacement of the power than of the resistance.
This solution will be used when we have no other options since it does not provide mechanical advantage and is usually used when we want to amplify the movement of the resistance.
The first-degree lever can also be seen in one way or another on our extrication tools, both hydraulic and electrical, as follows:
- Shears: as we can see in the drawing, their blades cut by virtue of the edge they have and the force of their lever when they are working. For this reason, it is important to note that the closer the resistance is to the central support point, the shorter your resistance arm will be and, therefore, the tool will have more cutting power.
- Separators: they also use this type of lever to be able to move the resistance. In this case the power is located very close to the point of support to amplify the movement of resistance, which is our purpose when trying to release a trapped person.
Second-degree lever: In this type of lever we have the resistance between the power and the fulcrum.
In this case, the power arm will always be greater than the resistance arm (Br<Bp) and, consequently, the effort to be made will be less than the load to be moved (P<R).
This arrangement means that the movements of the power and the resistance are always carried out in the same direction, but the load moves less than the power. Therefore, it is a manoeuvre that reduces the performance of the power.
Third-degree lever: In this type of lever, the power lies between the resistance and the fulcrum. The resistance arm (Br) will always be greater than that of the power, which will require greater effort of the force to be applied to raise the load (P>R). These types of levers never have mechanical advantage.
In this arrangement the movements of power and resistance are made in the same direction, but the load always moves more than the power. It is a technique that, while not giving any mechanical advantage, offers increased movement of the load as its main virtue.
When we have to perform a second- or third-degree lever in the rescue of trapped people, we must take into account two important characteristics that the tools we use must meet before starting to perform the manoeuvre.
On the one hand, we must consider their lifting power, since they must have the necessary force to be able to lift the load. And on the other, their ability to extend since they have to achieve the necessary height for the system to release the trapped person. Both conditions are important so as not to have to dismantle the manoeuvre if either of these requirements is not met, which would delay the rescue and cost important time while saving the life of the trapped person.
In rescues of trapped people, sometimes the conditions of the scene can prevent the use of a single lever to allow the release of the injured person. For example, a restricted space, the impossibility of the correct location of the point of application of the power, the difficulty of properly establishing the lever arm, the total weight of the element to be moved, etc., making it necessary to use a combination of simple levers. This set of simple machines is called a composite lever.
The composite lever is governed by the same laws as the lever and the mechanical advantage is achieved through two mounting systems of the elements that generate the power for the different levers that are used.
1. The output of one lever becomes the input for the next lever in the system. That is, the output of each of them is directly connected to the input of the next to achieve the desired effect and, therefore, each of them receives the movement (or force) of the previous one and transmits it to the next. With this we can reach our goal for rescue.
As we can see in the drawing, an overlap of high-pressure cushions would be an example of this technique. With this, we would reach the necessary height to release the vehicle and the injured person.
If the weight of the element to be lifted exceeds the working capacity of our tools, we could use the following type of composite lever.
2. Put the power-generating elements in parallel since they will allow us, with the sum of their partial forces, to generate a combined power that is capable of moving the load in order to release the trapped person.
With this system, we can counter the resistance offered by the load, allowing us to create the necessary space for the release. In addition, it offers greater stability as the load is being moved from several points, as long as it is carried out in an organized and coordinated way.
The composite lever is also used both in shears and in electric or hydraulic separators, giving a greater mechanical advantage through the combination of two levers acting at the same time.
Having seen the different types of levers and how we can work with them, it is also interesting to consider the following when we are about to perform an emergency lift: make an approximate calculation of the weight of the element to be moved. When performing this operation, it is interesting to know that it is preferable to overestimate or even add a few extra kilos to the real weight of the element, since this way we will have a greater margin of safety when choosing the tools and manoeuvres to use.
Take into account the power capacity of the tools to be used, since this capacity is not unlimited. If we foresee that by using only one of them, we cannot execute the manoeuvre with the necessary security and stability, we can use a set of them, combining their efforts, so we will be able to work with guarantees that we can carry out the rescue efficiently and safely.
Any element that we raise by levers, or another system, we must then stabilize for our safety and that of the injured. There are several ways to stabilize, as well as a range of stabilization materials. It is important that we know what tools and materials we have available, along with the optimal way of using them. Thus, we are able to get the most out of our equipment in each rescue situation.
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