Adhesiveness and low friction
The right compromise for optimal lubrication

Mechanical engineers commonly seek out adhesiveness as a characteristic of lubricating grease. For this reason, greases are preferred to oils especially where it is necessary to hold the lubricant in place under high dynamic stress.

Indeed, a very adhesive lubricant makes it possible to limit typical centrifugal losses – as is the case with geared motors, for example. Yet, excessive adhesiveness can be counter-productive and act as a brake on kinematic motion, which is why it is necessary to balance the main constituents (base oil, thickener and additives) in order to prevent:

• Excessive energy consumption
• Increased dynamic and static friction
• Heat build-up and deterioration of the lubricant

It is possible to objectively measure the adhesiveness of a lubricating grease through the identification of:

• • Cohesiveness > the force that a lubricating film opposes to its forced separation
  • Fibrosity > the length of connecting filament in the forcibly-separated lubricating film
  • Viscosity > the tendency to stick to areas affected by friction and loads (stickiness)

  The main formulation parameters that can influence the adhesiveness of a lubricating grease are:

  • Base oil viscosity
  • NLGI grade or consistency 
  • Additivation 

Base oil is the main ingredient of a lubricating grease, present in percentages never lower than 60–70% by weight, with viscosity measured in cSt or cPs identifying its internal friction.  Base oils are conventionally divided into size groups through the ISO VG classification, which assigns each lubricating oil a nominal category whose details are:

• Lower limit > ISO VG 5 equivalent to 5 cSt at 40° C 
• Upper limit > ISO VG 1500 equivalent to 1500 cSt at 40° C

The tolerance range of a viscosity measured at 40° C from the assigned nominal value is +/-10%. Thus, a polyalphaolefin oil of viscosity 35.50 cSt at 40° C will be assigned ISO category VG 32.

The viscosity of a base oil and its chemical composition both condition the adhesive power of the lubricating grease it will form, where the higher the viscosity, the higher its adhesiveness at a given temperature. 

The elevated adhesive power of a lubricating grease formulated with an oil of 1500 cSt at 40° C will significantly reduce the loss of material by centrifugation but will produce an increase in friction and energy absorption, which is even more significant when operating at low temperatures. For this reason, the cohesive, filamentous and sticky action of the lubricating grease in use over a broad temperature range should be evaluated and characterised.

Once the correct viscosity of the base oil has been determined, depending on the thermal exposure of the mechanical component in its application context, two different parameters can be worked on:

• The consistency, which identifies the degree of hardness of a lubricating grease and defined by the NLGI scale according to a progressive numbering from 000 to 9.
  Very fluid greases provide good lubricating power due to their high oil concentration, whilst solid or semi-solid greases are used as sealants or plastic seals.
  
  
• Additives or polymers added in small concentrations can profoundly change the rheology and structure of the lubricating grease, favouring its permanence at the friction point without changing its behaviour at temperature.

Kinematics, linkages, mechanical actuators and uncovered gears often operate in direct contact with water, moisture or process fluids, where proper calibration of the adhesive power can reduce the loss of lubricant from friction zones through runoff.

Using a precision rotational rheometer, the adhesiveness of a lubricating grease can be measured and compared with other formulations.