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Cutting Performance – Theory versus Reality

by Marcus

Cutting performance – Theory versus Reality

When it comes to assessing performance, firefighters are generally predisposed to being impressed by big numbers. Whether this is the megapixels on their new camera, the top speed of their latest car or the screen size on the new TV; big numbers attract and hold their attention. You must, however, look beyond the headline digits, especially where it is only a theoretical figure. In this instance, it is referring to the maximum forces attributed to hydraulic rescue tools.

This article will focus on cutters. The maximum cutting force is without doubt the first consideration for most people when it comes to choosing cutters and is generally at the heart of a purchasing decision. Of course, there are other considerations such as build quality, weight, ergonomics and other unique features but once the conversation regarding those has ceased, it always comes back to cutting force. The question here is: what is cutting force?

Maximum theoretical cutting force

The maximum cutting force is 100% theoretical. This is because it is the result of a calculation relating to the cylinder (or piston) cross sectional area, multiplied by the pressure at which the tool works e.g. 720bar (10400psi). It is also important to understand that the maximum (theoretical) cutting force has nothing to do with the blade design, shape or geometry. If two identical cutters were placed side by side with one having standard blades and on having blades made out of wood, they would both have exactly the same maximum theoretical cutting force. There is far more to the performance of hydraulic cutters than a theoretical calculation which results in a big number.

Cutting performance

The real test of a cutter (or any hydraulic tool for that matter) is the actual performance. This can only ever be assessed with the tool in hand and on a late model vehicle. Although there are still many older vehicles on the road, the true test of modern hydraulic rescue equipment, is their performance on new cars. It is only when a cutter is used on such vehicles that an assessment of its performance can be made. In use, its weight, ergonomics and blade geometry all become apparent. This is because we use the tool at waist height in a neutral standing position; you mostly use them above or below waist height. Blade geometry has more of an impact than you think. The blades are (of course) the point of contact with the vehicle; they will determine how effective the tool is at penetrating and surrounding cuts and they will influence any tool movement during the cutting process. Remember too that how the blades are mounted to the tool, i.e. inclined (see pictures below), may provide increased ergonomics and safety.

Residual capacity

It is also useful to actually assess how ‘hard’ the tool is working. This can be achieved by placing a pressure gauge between the pump and tool. This is important as it will give a visual indication of residual capacity. That is to say that if a cutter performs a cut on a modern-day vehicle using only 50-60% capacity, it has lots of reserve capacity in the event of increased vehicle strength in the coming years.

EN and NFPA norms

The other thing to keep in mind when looking at hydraulic rescue equipment is the norms which they conform to (EN and NFPA norms). These are industry tests which allow comparison to be made between manufacturers. These norms test hydraulic cutters on steel profiles such as round bar. It is important to ask yourself how relevant these tests are when considering the actual ‘real world’ use during vehicle extrication. Vehicle profiles are very different from round bar.
It is very easy to be drawn to and impressed by big numbers and in many things in life it is very often what people look for when making a purchasing decision. However, having an understanding of the numbers sometimes attributed to hydraulic rescue tools, how they are calculated and their operational relevance (or lack of) will allow you to make a more informed decision, by focusing on ‘real world’ performance rather than any theoretical figure. Appreciating that the cutting tests performed for norms only rely on steel profiles rather than vehicle construction means that a real assessment of a cutter’s performance can only be made by using it for its true purpose.

Put simply, whilst numbers are interesting, compared to actually using the tool, they do not really tell you a great deal at all.

The Cutting performance – Theory versus Reality article has been written by Ian Dunbar. For more information, please visit his blog.

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