Home Rescue Articles The Truth About Pressure – How Hydraulic Systems Work

The Truth About Pressure – How Hydraulic Systems Work

by Marcus

One of the first lessons learned in rescue is ‘always to stabilize to avoid uncontrolled movement’. Why? The trapped victim may suffer from internal bleedings, or even a broken spine. Only paramedics can tell. Rescuers are trained to ensure the trapped victim will not suffer from the object moving while performing the rescue operation. Any movement of the object may result in the victim suffering from this movement resulting in even further and perhaps even more severe injuries. Rescuers use cribbing blocks, and other kinds of stabilization tools to avoid such ‘uncontrolled movement’. Paramedics use spinal boards and neck collars. All different types of equipment with the same sole purpose: ‘to avoid movements that may result in additional injuries to the victim’.


1.Protect the accident scene, to avoid a risk of another collision (e.g. marking out the scene with cones or flares (not advisable if gasolineis leaking), lighting) and to avoid a risk of fire (switching off the ignition, putting vehicle in park, disconnecting the battery, placingabsorbing powder on oil and gasoline pools, fire extinguisher and fire hose ready to use) ;

2. Patient triage and initial medical assessment of the patient by qualified medical rescuers;

3. Securing the vehicle (cribbing), to prevent the unexpected movement (e.g. falling in a ditch) and the movements of the suspension, either of which could cause an unstable trauma wound or cause injury to the rescuers); A vehicle should never be moved, it should always be secured.

4. The opening of the vehicle and the deformation of the structure (such as removing a window) to allow the intervention of a first responder, of a paramedic or of a physician inside the vehicle to better assess the patient and begin care and also to release a possible pressure on the casualty;

5. Removal of a section of the vehicle (usually the roof or door) to allow a safe removal of the victim (extrication), especially respecting the head-neck-back axis (rectitude of the spine);

6. Removal of the person from the vehicle (extrication)

Note: In less complicated cases, it is possible to extricate the casualty without actually cutting the vehicle, such as removing a person from the side door or another part of the vehicle

Basically, rescue is all about saving a human being’s life without causing any further injuries. For the rescuer the assignment is very clear. To secure the vehicle and prevent from uncontrolled movement.

Question is why rescue equipment seems to ignore such important notes. Apparently, heavy and very powerful tools are used for extricating. They create a certain force, and force causes an object to undergo a certain change. Just think of Newton’s cradle. One kinetic ball from one side makes the kinetic ball on the other side move; whereas all kinetic balls in between do not. It proves that force travels and creates movement. With this in mind, can we truly assure ourselves that the rescue equipment used is safe and good enough to actually save a life?

The following chapters will discuss in further detail what force is, how it is generated and its impact in rescue operations and thus the rescue equipment.


In extrication the rescuer has a variety of options in rescue equipment, such as cutters and spreaders. Most of this rescue equipment consists of a hydraulic system; tools that use fluid power to do simple work; cut or spread. However, the work to be done may be simple; the materials in today’s vehicles consist of very strong steel such as HSLA or Boron. Therefore, the choice to design rescue tools using a hydraulic system is logical.

Hydraulic systems can generate very large amounts of power that can be transferred through small tubes and flexible hoses. For that reason hydraulic systems are used in industrial equipment. However, with rescue equipment another important factor should be considered. The reliability and handling of the tool are of utmost importance in all rescue operations. Another major aspect contributing to the functionality of the rescue equipment is the hydraulic pressure which creates the required forces for extrication. The rescuer should always be able to trust the tool that it supports in the rescue operation without compromising on power or creating limitations in positioning.

Rescue tools offer a variety of applications; to spread, to cut and to pull, push and hold. All applications are made possible by the hydraulic system.


Rescue tools are designed with hydraulic systems because these can generate extreme forces. What does force mean?

Force: the strength or energy exerted or brought to bear; cause of motion or change; active power; …

In physics force is defined as ‘any influence that causes an object to undergo a certain change, either concerning its movement, direction or geometrical construction’. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e. to accelerate, or it can cause an object to deform. In hydraulic rescue tools the forces are required to create the cut, spread or pull the high quality materials used in vehicles.


– the piston rod
– the piston
– hydraulic fluid
– cylinder shaft


When pushing a pushpin and a nail into a piece of wood by hand you will notice that it hurts much more to push the nailinto the wood that it does the pushpin, even though the force required to push the thin part into the wood is equal. This is because it requires more pressure on a smaller area to push it in.

The difference is area; the point of the drawing pin has a small area. Force acting over a small area gives a large pressure. The formula of Pressure = F / A (Force / Area) or F = P x A (Force = Pressure x Area).

The unit of force is defined in Newton. Pressure is, according to the official international system of Units (SI), defined in Pascal or Mega Pascal (MPa) which in turn equals to 1MPa = 1 Newton per square Millimeter (MPa = N/mm2). However, non-SI measures, such as bar, are used as well and equals to 100 megaPascal.

In the industry of rescue equipment pressure is defined as bar and the market can be divided in manufacturers offering hydraulic rescue equipment at 700 bar and those at 350 bar. Logically, one would think that the rescue tool operating at 700 bar is much more powerful than that tool operating at 350 bar. However, this is not the case.

It has already been mentioned that materials used in vehicles today exist of strong materials such as HSLA. Rescue tools must be powerful enough to deal with such high quality materials. For this reason the assumption is that rescue tools demand a certain force to be able to do the job it is created for, e.g. the cutting of a car post. This results in ‘force’ being a parameter that cannot be changed. On the other hand, the industry standards of pressure in rescue tools are 350, 620, 630, 640, 700 or 720 bar; another parameter that is not subject to change. This leaves us with only one parameter in hydraulic tools that can be influenced: Area.

In summary, if you need to achieve the same force F (fixed market led parameter, i.e. strength of current car bodies) using a smaller P pressure (say 350bar vs. 700bar) you can do so by having a larger surface area A on which this pressure works.


From the previous section it is clear why manufacturers of rescue equipment choose a hydraulic system; it offers power and is very reliable; which is required to be able to deal with the high quality materials used in vehicles today. Thus besides being powerful enough, are other features or specifications beneficial to ensure safe rescue operations? The answer is quite straightforward.


Handling and performance

Durability and safety

It has been concluded that (hydraulic) pressure is mandatory to create the necessary power to make the tool do its job: to cut, spread or pull. This paper will investigate if there is any relation between pressure and weight and performance. Also the handling as well as the safety and the durability of pressure tools will be examined. We will try to understand if a thing such as the ultimate rescue tool exists; a tool that eliminates the possibility to create any uncontrolled movement.


Rescue operations take time. One should understand the situation and it takes an experienced rescuer to complete the extrication successful. However, research shows that almost 60% of the injuries of rescuers are caused by overexertion and stress. Less weight seems mandatory. What is the effect? Can we decrease weight without any consequences? The answers can be found in the formula:

P = F/A and F = P x A

We know that a certain force is required for a rescue tool to cut or spread or even pull. All calculations are based on the assumption that Force and Pressure remain unchanged and rescue tools operate at either 350 or 700 bar.


We want to generate 1400kN with hydraulic tools operating at 350 and 700 bar. What will be the outcome for A?

1400 = 350 x A | where A = 4
1400 = 700 x A | where A = 2

The formula proves that by changing the area, the same force can be generated. To create the same force with low pressure, the piston needs to be increased and vice versa. Due to identical force exertion the diameter of the piston rod remains the same for both 350 and 700 bar hydraulic tools. On the other hand, the diameter of the piston increases resulting in a larger cylinder shaft, and thus a bigger tool. It is no surprise one would expect a bigger tool to be much heavier, explaining your choice for the smaller tool.

Let’s take a look at the example used before where the drawing pin could create a hole in wood, whereas our finger with increased force could not. This example proves that the size of the area does have a direct relation to pressure. Force acting over a small piston provides high pressure. High pressure automatically results in hydraulic fluids being compressed more and creates large tension on the cylinder wall.

So, the size of the area does have a direct relation to pressure. Force acting over a small area gives a lot of pressure. Large pressure creates tension. Whereas tension demands for strong outer sides which – with high pressure – consist of thicker thus heavier outer sides increasing the total weight of the tool. The outer side of the tool operating at 700 bar is, in fact, 2 times bigger than the tool operating at 350 bar. This to ensure that the outer side does not burst under the high pressure it operates.

Having a quick comparison in the various rescue equipment available one notices that – in a specific product segment -tools operating at 700 bar indeed weigh more than those tools operating at half the pressure; the difference in weight being 2 kilograms and even more.

It appears that you would like to choose a low pressure tool as it implies less weight, beneficial to the rescuer during rescue operations as it would result in less injuries caused by overexertion and stress.


There is no need to explain that easy operation and performance of rescue tools are key indicators for your rescue operations. One does not want rescue equipment that cannot be carried around or operated with ease, or are even restricted to other limitations such as size, preventing the rescuer to operate in more narrow spaces. More important, the rescue tool should never compromise on its performance.


– Force output

– Total weight (as discussed in previous section).

Force Output | F = P x A or P = F / A

This formula proved already that by lowering the pressure the area size must be increased in order to achieve the same force. This force is generated by the piston moving upwards and is transmitted through the rescue tool. In case of a cutter this will result in the forces transmitted through the blades providing the actual cut. In case of spreading the piston transmits forces through the arms moving the material.

In the previous section it was found that with 350 bar pressure tools the diameter of the piston is larger (larger area), whereas the piston rod diameter D1 remains the same for both 350 and 700 bar pressure tools (because equal forces need to be exerted). If the diameter of the piston rod remains the same, but the piston area is bigger because of the 350 bar pressure, the effective area on the rod side of the piston will be larger compared to a 700 bar tool. This is crucial for rescue tools offering both cutting and spreading capabilities, known as combi tools.


When cutting the piston moves; both 350 bar and 700 bar pressure tools generates the same force (F = P x A). However, when spreading, the piston is moving the other way, generating a return force. This return force should generate enough force to enable a tool to spread highly effectively. Testing proves that the decrease in return force is linear with the increase in working pressure (see Appendix B). This would result in a low effectiveness of the return force with 700 bar pressure tools.

In other words, tools operating at 700 bar loose performance when return force is needed for spreading capabilities of a combi tool. Such combi tool (both cutting and spreading capability) will be less powerful and will therefore be less effective and eventually not be able to deal with the high quality materials used in vehicles.

When comparing first stage cutting or spreading capabilities (forces) in tools of high pressure and low pressure systems it is important to understand the previously explained F = P x A formula.

When the first stage pressure setting of a high pressure and a low pressure system are identical, say 120bar it is easy to understand using the formula F = P x A that the low pressure tool with a bigger diameter piston (thus bigger A) will generate more force at that 120bar. This means for a low pressure system that more cutting is done in the first stage. Additional benefit is momentum is maintained because the point of switching to the second stage – with the resulted lessening of flow and thus speed – is not reached as often and as soon as when using a high pressure system.

The available energy (engine power & torque) is simply more evenly distributed over the range of working pressure when using a low pressure system, thereby achieving better tool performance in speed and available forces at the relative pressures.

Because the first stage pressure setting are not likely to be identical it is easy to compare on a percentage level. If you take the first stage pressure setting of a system and make it a percentage of the max pressure of that system you will see that the available energy of a low pressure system is more evenly distributed. The switching point will be more towards the 50% mark, utilizing the available energy in the two independent stages more evenly and effectively.


Integrity, durability and safety of tools are covered by the EN testing etc. It is safe to assume all available rescue tools on the market today that comply with EN and/or NFPA are safe.However it is known that with higher working pressures wear and tear on materials and components is heavier. That said maintenance on these higher working pressure tools, and other system components i.e. pumps, hoses and couplings, is likely to be more frequently needed and expensive.


Stabilization is mandatory to have a successful rescue operation. To achieve this stabilization of the vehicle (or other objects where a victim is trapped) is required to assure the vehicle cannot move. Movements to the vehicle may result in additional injuries to the victim.

In rescue hydraulic equipment is often used because of the power hydraulic systems can generate. The complexity of high quality materials used in vehicles today explains the need for such large powers. To generate such force a certain pressure and area is required (F = P x A) where pressure and force are fixed to be able to let the rescue tool do the job it is designed for (in rescue this may be cutting, spreading or pulling), area is not.

However, further analyzing the engineering of hydraulic systems (cylinders) and further testing indicates that high pressure may not be suitable for rescue tools because:

– Increased weight: high pressure generates large tension on the outer sides of the cylinder which demands for thicker materials to cope with this tension.

– Less performance: high pressure leads to low effectiveness of the return stroke force. This return stroke is mandatory for tools that offerspreading capabilities and must therefore not create any loss in performance. In addition, high pressure provides less power in its 1st stage, whereas in its first stage performance is required to allow for a quick and safe rescue operation.

– Increased maintenance requirements: high pressure results in an increase of temperature because of the energy created. An increase in temperature results in the materials suffering from wear and tear (less performance over time). Extreme heat in the system depletes the lubrication function of the fluid causing wear on seals and bearings and many other parts of the tool.

– Less safe: high pressure tools create more uncontrolled forces that lead to uncontrolled movements when in operation (high pressures resulting in large compression of fluids and energy stored in materials (elastic deformation) which results in more energy release and thus ‘before mentioned uncontrolled movements’).


In rescue it is to be advised to use low pressure (>300bar and <500bar) hydraulic tools. Low pressure is safer, offers more controlled forces and thus controlled operation.

Simultaneously, low pressure does not compromise on its performance even when spreading capability is required (combi tool). Low pressure tools result in less heat production which is beneficial to the lifespan of the materials and demands for less strict maintenance requirements. A combination of more controlled operation, increased safety and less weight are beneficial to the rescuer and will minimize the chance of injuries caused by overexertion and stress.

This article was from Resqtec, for more information please visit https://www.resqtec.com

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