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electing Oils with High Pressure-Viscosity Coefficient - Increaseearing Life by More Than Four Times 

rt Errichello, Geartech

In the following article the author explores the ageless question: Are synthetic oils (PAO -

Polyalpha olefin and PAG - Polyalkylene glycol) better gear and bearing lubricants than min

oils?

At the heart of the issue is a property we refer to as pressure-viscosity coefficient of the vatypes of lubricant base oils. The pressure-viscosity coefficient refers to the relationship betw

the load placed on the oil film (pressure) at the dynamic load zone and the thickness of thefilm (viscosity) at that load, when all other factors (material, temperature, geometry, speedload) are constant.

The pressure-viscosity coefficient gives us fixed values for lubricant film thickness in a giveof conditions (elastohydrodynamic regime, also known as an EHL or EHD regime) based on

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 u

than the percentage improvement of the synthetics over the mineral oil at the given tempents as noted in the information sheet.

analysis suggests that there are in fact conditions where the mineral oil out-performs the PAO/PAG synthetic

e in particular the performance level comparison for temperatures below 80°C.

wever, as the temperature changes, the relationship changes, leaving us with clear direction for when we coul

ect to see which synthetics demonstrate superior film-forming capability in a temperature range.

ch is made of the potential benefit of synthetics for energy conserving purposes. Hopefully, this data provides

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ght into when and why we would rightfully expect to see reductions in frictional conditions leading to improve

ability and energy savings as a consequence of changing to a synthetic-based lubricant.

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

chnical Editor’s Note

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

easurement Methods for Determining Pressure-Viscosity Coefficient,L film thickness was calculated using equations from AGMA 925.1 The pressure-viscosity coefficient quantifies L film-generating capability of a lubricant. The pressure-viscosity coefficient is measured either directly by ass

cosity as a function of pressure using high-pressure apparatus, or indirectly by measuring film thickness in ancal interferometer. In the latter case, the pressure-viscosity coefficient is calculated from measured film thick

ng an EHL film thickness equation. Although direct measurements may be more accurate, they are not readilyilable. AGMA 925-A03 gives pressure-viscosity coefficients derived from optical interferometry for many lubric

r a wide range of temperatures.

film thickness equation is the Dowson and Toyoda equation for central film thickness. It applies to componen

h line contact such as gears and roller bearings.

bricantsle 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 

mperature

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

rating temperature.

L Film ThicknessL film thickness was calculated using equation 65 from AGMA 9251:

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ere

• Hc is the dimensionless central film thickness

• G is the materials parameter

• U is the speed parameter

• W is the load parameter

eometry, elastic properties, speed and load are fixed, EHL film thickness varies with the pressure-viscosity

fficient ( ) and absolute viscosity ( ) as shown in equation No. 2:

rmalized EHL Film ThicknessL film thickness was normalized by dividing equation No. 2 by properties for a mineral oil at 80°C as shown in

ation No. 3: A temperature of 80°C is typical of the operating temperature of gear teeth and the inner ring aners of rolling-element bearings. Equation No. 3 normalizes the film thickness to that of a mineral oil at 80°C to

w direct comparison of the film thickness achieved by PAO and PAG synthetic lubricants to the film thicknessieved by a mineral oil.

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

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

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

ure 3 shows that PAO and PAG synthetic lubricants have similar trends for variation of EHL film thickness with

mperature change. PAG lubricant gives thicker films than PAO lubricant at all temperatures. Mineral oil has a st

ve of EHL film thickness vs. temperature than PAO and PAG lubricants.

T<80°C, mineral oil gives thicker films than PAO lubricant, and at T<57°C mineral oil gives thicker films than

ricant. In the range 70°C<90°C, there is only 5 percent difference between EHL film thickness of mineral and ricants. In this same temperature range, PAG lubricant gives thicker films ranging from 16 percent to 37 perc

ker than mineral oil.

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ase reference this article as:bert Errichello, Geartech, "Selecting Oils with High Pressure-Viscosity Coefficient - Increase Bearing Life by More Than Four 

mes". Machinery Lubrication Magazine. March 2004

Issue Number: 200403

Machinery Lubrication

Lubricant Selection