Selecting Oils with High Pressure-Viscosity Coefficient

Are synthetic oils (PAO - Polyalpha olefin and PAG - Polyalkylene glycol) better gear and bearing lubricants than mineral oils?

At the heart of the issue is a property we refer to as pressure-viscosity coefficient of the various types of lubricant base oils.

The pressure-viscosity coefficient refers to the relationship between the load placed on the oil film (pressure) at the dynamic load zone and the thickness of the oil film (viscosity) at that load, when all other factors (material, temperature, geometry, speed, load) are constant.

The pressure-viscosity coefficient gives us fixed values for lubricant film thickness in a given set of conditions (elastohydrodynamic regime, also known as an EHL or EHD regime) based on a mathematical estimation as noted in the American Gear Manufacturers Association (AGMA) Information Sheet AGMA 925-A03 (see Note).

The actual unit of measure (mm²/N) is less useful than the percentage improvement of the synthetics over the mineral oil at the given temperature points as noted in the information sheet.

The analysis suggests that there are in fact conditions where the mineral oil out-performs the PAO/PAG synthetics. Note in particular the performance level comparison for temperatures below 80°C.

However, as the temperature changes, the relationship changes, leaving us with clear direction for when we could expect to see which synthetics demonstrate superior film-forming capability in a temperature range.

Much is made of the potential benefit of synthetics for energy conserving purposes. Hopefully, this data provides insight into when and why we would rightfully expect to see reductions in frictional conditions leading to improved reliability and energy savings as a consequence of changing to a synthetic-based lubricant.

Clearly, it is not always best to use a synthetic lubricant.

Technical Editor’s Note

A study comparing EHL film thickness vs. temperature for mineral, PAO and PAG lubricants is presented here. The objective is to determine how lubricant ch48ce may influence bearing life calculations.

Measurement Methods for Determining Pressure-Viscosity Coefficient

EHL film thickness was calculated using equations from AGMA 925.¹ The pressure-viscosity coefficient quantifies the EHL film-generating capability of a lubricant.

The pressure-viscosity coefficient is measured either directly by assessing viscosity as a function of pressure using high-pressure apparatus, or indirectly by measuring film thickness in an optical interferometer. In the latter case, the pressure-viscosity coefficient is calculated from measured film thickness using an EHL film thickness equation.

Although direct measurements may be more accurate, they are not readily available. AGMA 925-A03 gives pressure-viscosity coefficients derived from optical interferometry for many lubricants over a wide range of temperatures.

The film thickness equation is the Dowson and Toyoda equation for central film thickness. It applies to components with line contact such as gears and roller bearings.

Lubricants

Table 1 shows values for absolute viscosity and pressure-viscosity coefficient obtained from AGMA 925.¹

Table 1. Absolute Viscosity and Pressure-Viscosity
Coefficient vs. Temperature

Temperature

EHL film thickness is established by the operating temperature of the components. For gears, the temperature of the gear teeth is relevant. For bearings, the temperature of the inner ring and rollers is relevant. A typical operating temperature for the gear flank and bearing ring and roller is 80°C. Environmental factors can influence the actual operating temperature.

EHL Film Thickness

EHL film thickness was calculated using equation 65 from AGMA 925¹:

where

• Hc is the dimensionless central film thickness
• G is the materials parameter
• U is the speed parameter
• W is the load parameter

If geometry, elastic properties, speed and load are fixed, EHL film thickness varies with the pressure-viscosity coefficient () and absolute viscosity () as shown in equation No. 2:

Normalized EHL Film Thickness

EHL film thickness was normalized by dividing equation No. 2 by properties for a mineral oil at 80°C as shown in equation No. 3: A temperature of 80°C is typical of the operating temperature of gear teeth and the inner ring and rollers of rolling-element bearings.

Equation No. 3 normalizes the film thickness to that of a mineral oil at 80°C to allow direct comparison of the film thickness achieved by PAO and PAG synthetic lubricants to the film thickness achieved by a mineral oil.

Table 2 and Figure 3 summarize normalized film thickness calculated using equation No. 3 (see Note).

Discussion

Figure 1 shows absolute viscosity vs. temperature for mineral, PAO and PAG lubricants.

Figure 2 shows the curve for pressure-viscosity coefficient for mineral oil, which is higher and steeper than the curves for PAO and PAG lubricants across the temperature range.

Figure 3 shows that PAO and PAG synthetic lubricants have similar trends for variation of EHL film thickness with temperature change. PAG lubricant gives thicker films than PAO lubricant at all temperatures. Mineral oil has a steeper curve of EHL film thickness vs. temperature than PAO and PAG lubricants.

At T<80°C, mineral oil gives thicker films than PAO lubricant, and at T<57°C mineral oil gives thicker films than PAG lubricant. In the range 70°C<90°C, there is only 5 percent difference between EHL film thickness of mineral and PAO lubricants. In this same temperature range, PAG lubricant gives thicker films ranging from 16 percent to 37 percent thicker than mineral oil.

Table 2. EHL Film Thickness Normalized to
Mineral Oil Film Thickness at 80°C

As an example of the significance of film thickness, consider its effects on rolling-element bearing life. If a bearing inner ring operates at 70°C and is lubricated by an ISO 320 mineral oil, Table 2 or Figure 3 gives 1.35868/1.00000 = 1.36 (36 percent) increase over a mineral oil at 80°C. A PAG synthetic oil has a film thickness 1.57558/1.00000 = 1.58 (58 percent) increase over a mineral oil at 80°C.

The advantage of using a PAG lubricant over a mineral oil is shown by 1.58/1.36 = 1.16 (16 percent) increase in film thickness.

For this example, DIN ISO 281 shows the 16 percent increase in film thickness increases the bearing life more than four times (129,000 hours with PAG compared to 31,500 hours with mineral oil).

For a gearset lubricated with ISO 320 oil and operating with 80°C tooth temperature, the film thickness with a PAG synthetic oil is 27 percent thicker than with a mineral oil. AGMA 925-A03 shows the probability of wear with the PAG oil is less than 5 percent, whereas with a mineral oil the probability of wear is 25 percent.

Practical Guidance for Selecting Lubricant

(as demonstrated by Figure 3):

• PAG oils have significantly thicker films than PAO and mineral oils over the entire range of practical temperature.

• There is little difference between PAO and mineral oils for the temperature range of 70°C to 90°C. In this same temperature range, PAG lubricant gives 16 percent to 37 percent thicker films than mineral oil.

• For temperatures less than 70°C, mineral and PAG oils have significantly greater film thickness than PAO oils.

• For temperatures greater than 90°C, PAO and PAG oils have significantly greater film thickness than mineral oils.