A More Precise Determ ination of Gear Oil Viscosity at Low Tem peratures

Kenneth O.Henderson，Joseph T.Mastropierro

（1.McEinri Associates LLC;2.Cannon Instrument Company）

A More Precise Determ ination of Gear Oil Viscosity at Low Tem peratures

Kenneth O.Henderson1，Joseph T.Mastropierro2

（1.McEinri Associates LLC;2.Cannon Instrument Company）

Low temperature fluidity of gear oils isan important fluid property as it directly impacts the useful life ofa gear set.This paper compares low temperature fluidity measurement precision of ASTM D6821 and ASTM D2983.Both tests are identical in the way they thermally condition the sample prior to viscositymeasurement.While ASTM D2983 is cited inmany currentspecifications，ASTM D6821 offers users and formulators amore accurate estimate of gear oil low temperature fluidity.The primary benefit in using ASTM D6821 is better precision.ASTM D6821 accomplishes this by automating the steps from prior to preheat through to viscositymeasurementat end of test.

dynamic viscosity;low temperature viscosity;gear oil viscosity;ASTM D2983;ASTM D6821

0 In troduction

Automotive gear lubricants play a critical role in transferring power between mechanical devices.Today these lubricants seldom draw the interest of today’s modern equipment user.This is in part because most equipment employ closed lubricant systems that a user has little contactwith until there is a problem or failure.Since gear lubricants operate in a closed environment，they are not as exposed to the hostile environment that an engine oil sees.However，there are aspects of operation that stress a gear oil asmuch ormore than an engine oil is stressed.The stress is just applied in a different way.

Gear boxes and transmission cases are designed so that the gears are either partially or fully immersed in the gear oil.In either case，the rotation of the gears isused to circulate the oil.The rotating gear teeth refreshes the oil coating between each tooth－to－tooth engagement.It is this fresh layer which cools the surface of the gears between engagements.This fresh lubricant coating also replenishes the anti－wear additives on the gear tooth’s surface.This fluid coating with fresh antiwear additives protects the gear teeth from metal tometal contact as the gear teeth slide past each other to transfer power from input to output.When twometal surfaces slide over each other athigh loadswith insufficient lubrication，metal tometal contact is likely.Themetal tometal contact will result in not only increased wear butalso galling and pitting.These surface changeswill result in slowly increasing loadsas the contact area decreases.These changes will lead to excessive heat and wear or even breakage of the gear teeth.Thus it is critical for the gear lubricant to have sufficient fluidity so theworking areas are covered at all times by a fresh coating of lubricant.

Low temperature fluidity is a critical aspect of whether a fluid is fit for purpose.Even though low ambient temperature operation is typically a very small portion of a gear boxes service life.Poor fluidity in cold weather operation can be a significant impediment tomaintaining a fluid coating on gear teeth.With an excessively high viscosity，the rotating gears cut a channel through the thick lubricantwithout refreshing the fluid coating on the teeth.When this occurs，the gear teeth quickly lose their lubricant coating and thewear protection it provides.The heat generated by gear teeth sliding past each other indirectly heats the lubricant eventually allowing the gear oil to again coat the gear teeth.While the frictions between the gear teeth are heating the oil by frictional heating，the faces of the teeth are experiencing increased wear or even galling.If the gear lubricant thinsquickly enough then only a small amount of wear will occur.If the lubricant does not quickly recoat the gear teeth significant gallingmay occur.Galling will generate metal particles.The galling also increases the load on the gear teeth due to the reduced load carrying surface.The finemetal particles generated bymetal tometal contact will be circulated by the lubricant.These metal particles will then flow back across the gear teeth and act as an abrasive under load to further increase wear.

Insufficient gear oil fluidity in cold weather operation occurs in a number ofways.One is due to crystallization of the wax in the base stock used to formulate the oil.Another is increased viscosity due to thickening of the gear oil from oxidation which is caused by excessive heat and metal debris.

Oxidation and metal debris isminimized by the choice of rust inhibitors，anti－wear and anti－oxidant components used in the oil formulation.Wax crystallization is controlled through the use of appropriate low temperature flow improvers which are also called pour point depressants.Low temperature flow improvers function by disrupting the growth of wax crystals，thus preventing formation of a strong crystalmatrix.Choosing the appropriate low temperature flow improver and treat rate is critical to keeping wax crystals from forming a strong crystal latticematrix.If a strongmatrix forms，it prevents the gear oil from flowing to the gearsas theymove.This effectively allows the gears to cut channels in the lubricant leaving little or no oil on the gear teeth.A gear oil that doesn’t flow due to wax crystallization can exhibit properties similar to those of a gel while having a viscosity well within specifications.When a gear oil gels，itmay need to reach amuch higher temperature before the gelweakens to allow flow.

1 Specifications＆Methodology

To ensure a gear lubricant will have adequate fluidity at low temperatures，the SAE J306 Gear Oil Specification［1］has low temperature viscometric limits.These are measured by ASTM D2983 Standard Test Method for Low Temperature Viscosity of Lubricants by Brookfield Viscometer［2］which was first published as an ASTM standard in 1971.

Although ASTM D2983 has been in use for close to forty years，it was not the first test tomeasure gear oil fluidity.Prior to ASTM D2983，specifications referenced the‘Channel Test’to assess low temperature fluidity.This test，F(xiàn)TM 791－3426，is still cited in the Mil－PRF－2105E gear oil specification and a few other specifications［3－4］.

The Channel Test is accomplished by placing a 650 mL lubricantsample in a special container.This sample is preheated to 46℃before placing it in a thermostatically controlled bath which is at the final test temperature.It is left in the bath for 16 hours.At the completion of the 16 hour soak，a spatula is drawn through the lubricant cutting a 2 cm wide channel.The time for the lubricant to flow back into the channel is timed.Current specifications call for the channel to fill in less than 10 s.

When gear oil specifications cite ASTM D2983 for low temperature viscosity，they cite themaximum temperature for a viscosity of150，000mPa（s）.These limits are shown in Table 1.ASTM D2983 was accepted as a replacement for the Channel Test in many specifications due in part to it being more precise and a less subjective assessment of low temperature viscosity.

Table 1 SAE J306 viscosity classification for automotive gear oils

Similar to the Channel Test inmany ways，ASTM D2983 is an overnight test.In this test，a tube containing lubricant is heated to 50℃for 30 minutes then allowed to cool to room temperature.After reaching room temperature，the sample tube with the rotor in place is typically placed in a refrigerated air chamber that is at the test temperature.After a 16 hour soak the sample tube is placed in a pre－chilled insulated holder and removed from the air chamber tomeasure the viscosity with a rotational viscometerwhich is typically located on a nearby lab bench.The operator seeks to quicklymeasure the viscosity tominimize the samples’temperature rise from the soak temperature.A typical low temperature thermostatic air chamber is about the size of a chest type home freezer butwith lower temperature capability and forced air circulation.

There are several steps in the ASTM D2983 test procedure where inconsistencies in conducting the test can result in large variations of test results or measurement uncertainty.Some of these issues are:Actual length and temperature of sample pre－h(huán)eat，variation in time between removing the sample from the air chamber to finishing the viscositymeasurement，condition of spindle and disruption of any wax structure in the sample when connecting the viscometer spindle.There are other aspects of the procedure which can contribute to variation in test results.

A few years ago ASTM D6821 was developed as an alternative to ASTM D2983.This test utilizes the same Mini－Rotary Viscometer that is used in ASTM D4684（required for meeting the SAE J300 specification for engine oils）with a few physical and operational changes［5］.The differences in configuration are ASTM D6821 uses a special set of rotorswhich are smaller in diameter and a different weight set as the applied stress is smaller.ASTM D6821 thermal conditioning program mimics the sample heating and cooling defined in ASTM D2983.ASTM D6821 is titled Standard Test Method for Low Temperature Viscosity of Drive Line Lubricants in a Constant Shear Stress Viscometer.

D.L.Alexander led the development of ASTM D6821 with support from Cannon Instrument Company.The development of this test is discussed in SAE paper 1999－01－3672.In summary，for ASTM D6821 ameasured sample is placed in one of the test cells of a Mini－Rotary Viscometer（CMRV）［6］.After the instrument preparation is complete，a temperature program is initiated which warms the sample to 50℃and holds it there for the required time.The sample is then cooled to room temperature.After reaching room temperature it is cooled at the same rate a sample cools in the air bath used in ASTM D2983.After16 hours，the yield stressand viscosity aremeasured with the data automatically recorded by the instrument.

ASTM D6821 provides several advantages over ASTM D2983 which are:A smaller sample size of10mL.Automatic thermalconditioning of the sample.Determines two rheological properties without unnecessarily disturbing the sample.It is believed that these advantages are part of the reason ASTM D6821 is amore precise viscosity measurement technique.A comparison of the twomethods precision is shown in Table 2 at the SAE J306 specification limit.The precision of ASTM D2983－04 is a power function so percent repeatability and reproducibility varies with measured viscosity.The precision of ASTM D6821 is a constant over the viscosity range of the test method and not effected by the magnitude of the viscosity measured.

Tab le 2 Viscosity measurement precision for 150，000 m Pa（s）

The ability to measure yield stress provides another assessment of structure formation that is only visible in the Channel Test.The presence of yield stress is an indication that the sample is not slumping to fill the channel.This is a different phenomenon than seen when a lubricant has an excessively high viscosity.This lack of slumping indicates to a user that the oil has formed a gel like structure.If this structure is strong enough itwill interferewith or prevent lubricant flow around the gear teeth.A high yield stress indicates the tendency of a formulation not to slump.Yield stress cannot bemeasured with ASTM D2983.

The SAE paper by Alexander，et al，demonstrated the value of the ASTM D6821 technique for both gear oils and automatic transmission fluids.Although the precision study was on a limited number of samples，those data showed excellent precision aswell as correlation with ASTM D2983.

Fifteen commercial multi－grade gear oil samples were obtained over a period of time from local resellers.This sample set includes the threewinter grades75W，80W and 85W.In addition to these samples two samples from ASTM Committee D02 ILCP cross－check program for gear oils are included.

Each sample was tested at least twice in two MRV instruments.These were Cannonmodels CMRV－4500 and CMRV－5000.An accredited commercial laboratory provided the ASTM D2983 viscosity data on the commercial oil samples.For the ASTM ILCP samples，the average ASTM D2983 viscosity from the test reportwas used as the reference value.

2 Results and Discussion

Table 3 shows the average of eight ASTM D6821 measurement variations for the 75W multi－grade gear oil.Using ASTM D6821，thesemeasurementshad an average overall variation of5.2%.This corresponds to an ASTM reproducibility of approximately 12%which iswellwithin the precision stated in ASTM D6821 and a third of ASTM D2983 precision.With the exception of sample 957，ASTM D6821 viscosity data is essentially identical to thatobtained with ASTM D2983.Sample 957 had a high yield stress，greater than 34 Pa while the yield stress for the other samples was 11.3 Pa or less.Since there is good agreement between the ASTM D6821 and ASTM D2983 for the other samples，the difference seen with sample 957 is likely due to the presence of yield stress（or a gel－like structure）.

Table 3 75W multi－grade gear oils at－40℃

The variance in ASTM D6821 viscositymeasurement for the80W and 85W multi－grade gear oils is shown in Tables4 and 5.The overall average variation for each grade is 3.8% and 5.3%respectively.This average variance is very similar to that seen in Table 3 for75W multi－grade oils.These variances when converted to ASTM reproducibility are within the precision stated in ASTM D6821 and a third smaller than the precision stated in ASTM D2983.

In Table 4 there are two samples that show significantly different viscosities between ASTM D6821 and ASTM D2983 measurements.Sample 585 exhibits a ASTM D6821 viscosity that is greater than the ASTM D2983measurementbymore than 27%.However ASTM D6821 viscosity for sample 92C is less than ASTM D2983 value by nearly an identical amount （32%）.While in Table 5，the ASTM D2983 viscosity of samples 575 and 104 are both significantly less than the ASTM D6821 viscosity.

Table 4 80W multi－grade gear oils at－26℃

Table 5 85W－xxx gea r oils at－12℃

In each case where there is a large difference between viscositiesmeasured by ASTM D6821 and ASTM D2983 the difference between themethods is very similar－approximately 30%.In three cases ASTM D6821 viscosity is greater than ASTM D2983，while in two cases the reverse is true.Out of the seventeen samples there were five samples with a significant difference in viscosity between the two test procedures.

Without a channel test to evaluate the slump rate，judging the effects of yield stress on the fluids ability to slump is difficult.It is known from the earlier studies on engine lubricants thatyield stressand high viscosity aren’tdependentvariables until you approach the solidification temperature.In this setof data，only sample957 exhibited a yield stresswhich was greater than 34 Pa.For this particular sample，the viscosities measured by these two methods were significantly different.Only one other sample，PB2，consistently exhibited a yield stress greater than 11.3 Pa，which is the lower limit of detection for ASTM D6821.

It is likely that the large viscosity differences seen in five samples are at least partially due to differences in base stock wax type and content.These differences could also be attributed to the low temperature flow improver used，additive component interaction or even additive solubility.

With respect to wax，the current preheat requirement in ASTM D2983 wasmodified in themid 1980’s.Thiswork was initiated by an automotive gear oilmarketer.They discovered that some formulationswithout sufficient preheating had an exceptionally high variance in ASTM D2983 viscosity.After further study，they found that this occurred when the base stock contained significant amounts of micro crystalline wax.By adding a 50℃preheat，they reduced the variability in ASTM D2983 viscosity measurements on gear oil samples containing micro crystalline wax.The reduction in variability was sufficient to bring those results within ASTM D2983 precision.This change did nothave any impact on resultsofgear oilsnot containingmicro crystalline wax.This emphasized the importance of ensuring all wax is in solution prior to cooling the sample in preparation for viscositymeasurement.

The 50℃preheat is effective because it iswarm enough to bring the base stock wax into solution butnothigh enough to cause a change in the gear lubricants additive chemistry.Without the sufficient preheat，there are variable amounts of suspended crystallized base stock wax in the test sample.These wax crystals will act as nucleation sites for the wax in solution as the sample is cooled.These additional nucleation sites reduce the effectiveness of the low temperature flow improver increasing the variability in low temperature viscosity.Using ASTM D6821 provides a consistent sample preheat both in terms of temperature and time，that is difficult to achieve with ASTM D2983.

Poor low temperature flow of automotive gear oils is not likely the sole reason the gears in a transmission case or gear box fail.Poor fluidity that results in poor lubrication will definitely contribute to a shorter useful life，or catastrophic failure in a critical situation.ASTM D6821 can help avoid poor fluidity situations because of its ability to indicatewhether a formulation has a tendency to gel.

3 Conclusion

The data indicate that ASTM D6821 provides amore precise viscosity measurement than ASTM D2983.This significantly better precision is due in large part to consistent sample preparation and subsequent viscosity measurement in ASTM D6821.ASTM D6821 method also eliminates the numerous sample handling steps in ASTM D2983 that can contribute to variable test data.Additionally，the use of ASTM D6821 not only improves the reliability of the test results，it also reduces the amount of technician time required to complete the test.By improving the precision of the test，formulators will gain additional latitude in choosing components that deliver the required performance for their gear oil blends.

The industry and users would benefit by the addition of ASTM D6821 to the Committee D02 ILCP Gear Oil program.Thiswould provide the industrywith a broader sample base for comparing the capabilities of these twomethods.

［1］SAE J306:Automotive Gear Lubricant Viscosity Classification［S］.

［2］ASTM D2983－04 Standard Test Method for Low－Temperature Viscosity of Lubricants Measured by Brookfield Viscometer［S］.

［3］FTM 3426–Channel Point of Lubricating Oils，F(xiàn)ED－STD－791D Federal Standard:Testing Method of Lubricants，Liquid Fuels，and Related Products［S］.2007.

［4］MIL－PRF－2105E Performance Specification:Lubricating Oil，Gear，Multipurpose［S］.1995.

［5］ASTM D6821－02（2007）Standard Test Method for Low Temperature Viscosity of Drive Line Lubricants in a Constant Shear Stress Viscometer［S］.

［6］W A Lloyd，JM Perez，K OHenderson，etal.Viscosity of Drive－Line Lubricants by a Special Mini－Rotary Viscometer Technique［C］∥SAEPaper1999－01－3672.

TE626.3

A

1002-3119（2012）01-0045-05

2011－09－01。

Kenneth O.Henderson is general manager of McEinri Associates LLC，a consultancy in Port Matilda，PA.He hasmore than 30 years of testing and lubricant development experience notably with Ethyl Petroleum Additives，Castrol North America and 11 yearswith Cannon Instrument Company.He is currently Chairman of ASTM Committee D02 and amember of SAE.