What are the differences between friction, pressure and shear?

28/07/2015 11:29:51

Shear is a buzz word right now in the world of pressure ulcer prevention. But how is shear different to friction? Or pressure?
Should it affect your decision in selecting a medical support surface?
In this series of blog posts, we explore the differences between friction, pressure and shear, to show what you need to consider in the face of pressure ulcer prevention.
Both shear and pressure can potentially be factors in the cause of pressure ulcers. 
However, shear and friction can often be confused as being the same thing, when in fact they are different forces which act upon the skin in different ways. 
This week we discuss friction - blog posts on pressure and shear to follow in the coming weeks...
Friction can be defined as ‘surface resistance to relative motion(1) ’ – essentially, a force that acts to resist motion.
This means that if a patient on a medical support surface is moving across that surface, there is a danger of friction forces acting upon vulnerable skin to cause damage.
As Newton’s third law of motion tells us, for every action there is an equal and opposite reaction(2) . The reaction to friction forces acting on the skin surface is a shear force in the tissues.
How can friction be prevented?
At Dartex, we test all our polyurethane coated fabrics for coefficient of friction, which is the ratio of the frictional resistance force to the normal force  which presses the surfaces together. 
In this case, the normal force is the weight of the patient on the support surface pressing down – in other words; pressure. Where friction is present, shear forces are present too. 
So keeping friction to a minimum is good to do in the prevention of pressure ulcers.
Our medical fabrics are engineered to keep friction to a minimum, so when the patient moves across the surface, the skin is protected from friction damage. 
By minimising the friction, we also minimise the risk of shear damage too.
Want to know more? 
Speak with one of our technical experts for more information on the impacts of pressure, shear and friction enquiries@dartexcoatings.com 
(1)  http://dictionary.reference.com/browse/shear?s=t 
(2) Newton, I (1686) Principia Mathematica Philosophiae Naturalis. 


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