Synthetic high temperature lubricants: 9 reasons to use them

 

The thermo-oxidation process of an oil, grease or lubricating paste takes place through three phases:

The three processes, easily distinguishable in the thermal degradation phase of a lubricant, are sequential but also overlapping, as in the case of the evaporation process, which continues even after the beginning of the primary and secondary oxidation.
We will provide 9 valid reasons to design your electromechanical device bearing in mind a lubricating fluid that has what it takes to withstand prolonged periods of operation at high temperatures.

The lubricated mechanical component, bearing, gear or chain, lives and performs its function in a more complex system -just think of a gearmotor in a beverage dispensing machine, the brake cylinder inside a vehicle, a pneumatic cylinder in a woodworking machine or the spindle bearing of a machine tool. But are we always sure that exceptional conditions cannot somehow exceed the thermal regimes envisaged in the design phase? In reality this happens very often, and once the failure has been highlighted it is not always possible to construct the “thermal” history of the mechanical component.

 

 

 

 

 

The graph highlights the operating temperatures detected during the continuous operation of a worm geared motor, compared with the theoretical ones. The difference settles around an average of 10 ° C constant!

 

 

(2) - Why antioxidant additives can help keep the lubricating film active

A lubricating film in operation can reach very small dimensions (1/20 of the diameter of the human hair), especially when temperatures exceed 100-120 ° C. The presence of specific additives for high temperatures can significantly delay the beginning of secondary oxidation, and keep the lubricating film active much longer.

 

(3) - Why the separation of the friction surfaces is optimal if the film is hydrodynamic and not dry!

There are no optimal operating conditions if two surfaces in friction and in the presence of a load are dry lubricated!! Solid lubricants: graphite, PTFE, boron nitride, molybdenum disulfide, can contribute to lubrication or provide emergency help when the fluid film is absent, but cannot last long in the absence of an active and reactive "fluid" film between the surfaces in friction. As when using a PTFE grease or a Teflon grease, the presence of the solid lubricating additive (PTFE) certainly cannot replace the effectiveness of the base oil. Additives help improve lubrication conditions, but certainly cannot replace them.

 

 

Above: Hydrodynamic lubrication
Below: Lubricant film present and active

 

(4) - Why a layer of charred grease between two frictional surfaces doesn't help!!!

The image below compares 2 lubricating greases subjected to the same test conditions, 24 h at 150 ° C. The image shows:

• (A) a lubricating grease formulated for high temperatures with synthetic oil, and lithium soap.
• (B) a generic lubricating grease formulated with mineral oil and NLGI 2 lithium soap.

Grease B's non-existent lubricating power is very clear: formulated with mineral oil, it is by now deprived of a considerable part of base oil and completely carbonized.

 

Testing in the oven 24h at 150 ° C

 

(5) - To limit mechanical wear from friction

Two metal surfaces, if suitably separated and already subjected to running-in, have no reason to lose material, that is, to undergo mechanical wear from friction. The thinning of the lubricating film at high temperatures reduces the lubrication meatus and increases the possibility of contact between the surfaces. A synthetic base oil has a more stable viscosity as the temperature changes and it is therefore possible to exploit this intrinsic advantage and use more viscous oils, which allow for a more effective separation of the friction surfaces.

 

(6) - Why mineral oils evaporate more easily

A lubricating fluid, grease, oil or paste, is always composed of a base oil, present in the formulation in a minimum weight of 60-65% up to 99% and more. A mineral oil has little chance of surviving when compared with a synthetic oil, just look at the loss by evaporation according to Noack!

 

 

If the tribosystem (lubrication point) does not include any cycles to restore the lubricating film, it's done! Once the most consistent part, responsible for the separation of the friction surfaces has been lost by evaporation, a dangerous vicious circle will begin:

 

• Thinning of the lubricating film
• Mechanical wear
• Abnormal increase in the friction coefficient
• Loss of efficiency
• Increased noise
• Premature blocking of the device

  

 

 

 

The gearmotor of a window regulator in a car, a pneumatic cylinder of a press and the gaskets of the brake cylinders of the car disc brakes, are meant to work for life and obviously cannot be lubricated a second time. Repeated excursions to high temperatures, over 100 ° C, can cause imperceptible but continuous reductions in the anti-friction and anti-wear properties of the lubricating film.

 

 

 

 

  

If the temperature variations are important, and if there is a real risk that the lubricated component works at low temperatures, below -20 ° C, it is important to have a formulation that uses a synthetic base oil (Pao, Pag, Silicone), whose superior viscosity index can guarantee a low coefficient of dynamic friction even at low temperatures.

 

 

 

(9) - Why the cost / benefit ratio is often favorable