Why use a synthetic oil for the lubrication of a mechanical device? What advantages are obtained compared to a mineral oil? When is the use of mineral oil not advised? In this article we will analyze the characteristics of a synthetic fluid and a mineral fluid under the following aspects:

• Lubricating properties
• Operating temperature range
• Advantages and disadvantages for the lubricated mechanical device

The last 15 years of mechanical design have produced devices characterized by high reliability and optimization of the quality / cost ratio. The automotive industry and the related OEM components have provided the most decisive impulse: just think of the high standards of reliability and durability that components such as the braking system, the clutch assembly, the chassis, the gearbox, the electromechanical actuators and the internal solutions for the passenger compartment guarantee today compared to the recent past.

Lubrication has thus become extremely important, helping to improve the duration and reliability of the kinematics, through an effective separation of the friction surfaces in all operating conditions: the more persistent the "floating of the surfaces" without contact of the materials in friction, the less the consequent mechanical wear.

The lubricating solutions used in the electromechanical components are still dominated today by formulations created from mineral oils.However, synthetic oil, as the basic constituent of the formulation, has increasingly taken on a primary role especially when considering:

• the extension of the operating temperature range
• the massive use of plastics and elastomers, which requires greater care in the lubricant formulations in terms of compatibility

The first fundamental difference between a mineral fluid and a synthetic fluid concerns the production process to obtain it.

• Mineral oil: obtained through the distillation and refining of petroleum, it is easily available on the market both as regenerated and as newly produced oil.
• Synthetic oil: it identifies a wide family of solutions all obtained through specific synthesis processes starting from elementary chemical bricks. Numerous families exist, each characterized by different chemical-physical and functional properties.

Let's now analyze 5 different aspects by comparing the performance of some synthetic and mineral lubricant solutions: 

(1) Low temperature performance: freezing point and pour point

When temperatures go down, the viscosity of a lubricating fluid tends to increase until it is completely solidified. Smoothness and lubrication are closely related properties. It is essential to know them if you want the lubricating film to function as a separator of the friction surfaces, favoring mutual sliding and not gluing !! Below are the freezing values of a mineral base and 4 synthetic oils, all of which have the same starting viscosity at 40 ° C. The values alone highlight a first important difference to be taken into consideration in the design phase. Which oil is best suited to my project ? 

Freezing temperatures of mineral oil

and synthetic oils compared - Viscosity at 40 ° C - 40cst

Consequences of the wrong choice for the lubricated device: increase in the coefficient of friction, increase in absorbed power, blocking of the device. 

(2)  Viscosity index or viscosity temperature coefficient 

It represents the relationship that links the viscosity variation of a lubricating oil, that is of its internal friction, to the reference temperature. It is expressed with a number without a unit of measurement as it represents the ratio of viscosity at different temperatures.

The higher its value, the lower the change in oil viscosity as the temperature changes.

It is a fundamental parameter, since it has a direct influence on the thickness of the lubricating film at various thermal operating conditions. If we compare the values of the indices with the reference curve in the temperature range between -40 and + 40 ° C (see figure), we can observe how rapidly viscosity for the mineral standard solution increases as temperatures decerase, compared to synthetic "competitor" oils

Viscosity curves - Comparison of lubricating base oil temperatures

Viscosity index and freezing point represent two parameters of considerable interest for the designer. Just think of what may happen if you decided to use a mineral based lubricant (freezing point -12 ° C) to lubricate mechanical actuators operating at - 20 ° C or -30 ° C.

Consequences of the wrong choice for the lubricated device: high mechanical wear, increase in the dynamic friction coefficient, device block. 

(3) Lubricating power at high temperatures: consequences on the lubricant

The increase in temperature at the friction point produces the following effects on the lubricating fluid that separates them

• Thinning of the lubricating film
• Viscosity reduction
• Evaporation
• Primary thermo-oxidation
• Secondary thermo-oxidation

Each of these processes has a different temporal trigger and course. The table below summarizes the specific properties of each family of oils with a synthetic index of quality, together with a graph showing the average minimum and maximum temperatures, sustainable by the fluids, in continuous operation mode: 

Quality index table comparing mineral oil and synthetic oils
Synthetic index rating from 0 to 10

(4) Compatibility of plastics and elastomers

The presence of plastic or rubber details (elastomers) in many electromechanical components means that special attention needs to be paid to the potential interactions existing with the lubricant used and the possible unwanted reactions .We generally speak of incompatibility when the lubricating fluid somehow compromises or reduces the functionality of the polymeric part. The main phenomena observable on the details are:

• embrittlement
• swelling or pinching
• premature ruptures
• variation in hardness and mechanical properties
• weight gain

The image below compares the effects suffered by an o-ring in EPDM material after immersion in a mineral base for 24h at 20 ° C, compared to its original conditions: its dimensions after conditioning are increased by 30%!!

(5) Duration of the lubricating film

The thermal excursions and frictions, which occur during the natural operating cycles of a mechanical device, result in a progressive impoverishment of the properties of the lubricating fluid responsible for the separation of the surfaces in contact, for this reason some kinematic mechanisms may provide for programmed relubrication cycles.

Lubricants formulated starting from synthetic oils have a much higher performance than the corresponding mineral solutions. The figure compares the (average) hours of continuous operation in an interval between 80 and 150 ° C.

Resistance of lubricating film based on the maximum continuous operating temperature

Knowing the duration of the lubricating film in relation to the operating conditions is not easy, only a field test can provide a certain answer. However, it is certainly possible to formulate an appropriate lubricating fluid based on the knowledge of the extent of the following variables:

• sliding speed
• workload
• maximum temperature
• minimum temperature
• presence of humidity
• contact with aggressive chemical fluids

In this article we have compared the performance of mineral oils and synthetic oils in 5 different aspects. We have covered how the advantages achievable with one or the other solution can heavily modify the final performance of a mechanical device.

Below we propose a guide containing a series of indications to be taken into consideration if you want to use a lubricant for low temperatures in the design phase.