What exactly is friction force? How is it measured? In what forms does it arise? What is the difference between the static coefficient of friction and the dynamic coefficient of friction? How can a lubricating grease reduce these?
Friction! Thanks for existing! Life wouldn't be that simple without you. Walking, climbing stairs, grabbing objects ... an absence of mechanical friction would prevent the simplest daily activities from being performed.
Any kinematics to perform its function correctly requires two surfaces in contact and in contrast. Simply think of the transmissions - a variation in torque and rotation speed would not be possible without contact friction.
In this article, we will introduce and analyse the concept of friction with particular reference to the lubrication aspect, mechanical wear and the possible ways of reducing it thanks to the use of synthetic lubricating greases and oils.
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Complete guide of key characteristics
A complete identikit of a lubricating fluid with high load capacity and low friction coefficient
The term "friction" means any dissipative phenomenon that is generated between two solid or fluid bodies in contact and in the presence of movement.
To better explain the concepts underlying the phenomenon, it is necessary to analyse in more detail the main protagonists: the contact surfaces.
If we interpose a lubricating fluid between the two surfaces, we can distinguish between two types of friction:
TRIBOSYSTEM
Between the BODY and COUNTER-BODY
You can see the external friction on the left and the internal friction on the right
Depending on the morphology of the surfaces, friction can be further distinguished into:
Mechanical wear
The reduction of friction between two bodies in contact and in reciprocal sliding can be obtained by interposing a substance capable of reducing the shear stress necessary to allow sliding. This substance, which can be solid, liquid or gaseous, is called LUBRICANT.
The lubricant can intervene by producing a reduction in the friction coefficient, defined as the ratio between the force Fr opposing the movement and the driving force Fa.
Depending on the magnitude of the two contrasting forces, we can distinguish two important values:
Coefficient of friction and speed: the Stribeck curve
In each tribosystem, it is possible to identify variable interface conditions according to the sliding speeds of the parts in contact. A method for identifying the extent of the variation in relation to the reciprocal sliding speeds is the use of the Stribeck curve, in which the coefficient of friction is related to the reciprocal sliding speeds of the surfaces in contact.
From the image, it is possible to identify three different operating zones with the relative course of the coefficient:
A good lubricant formulation has as its objective the reduction in the coefficient of friction and the reduction of the relative mechanical wear that is generated. 5 fundamental requirements shall be analysed in the guide below for such purpose:
Adequate chemical composition of the base oil
Use of solid lubricants in the formulation
Use of anti-wear (AW) and extreme pressure (EP) additives
High viscosity of the base oil
High thermo-oxidative capacity