Ventm~ter

u~tng ~ ImpDct uf V~riou~

Luntilbutinq F~ctor~ on ~ Pump~bUit~j

(Using statisticalfactorial experiments to show the impact that grease

thickener content interaction and base oil viscosity have on grease

pumpability)

by: Paul Conley, Dr. Raj Shah, Canlong He, Jesse Kelly

SKF/Lincoln, St. Louis, MO

Abstract:

In this paper the Lincoln ventmeter was used as a tool

to study grease pumpability. The Lincoln ventmeter

has been a useful tool in determining grease suitability

for use in automatic lubrication systems since its

introduction in 1965. The Lincoln ventmeter effectively

determines the grease yield stress/stiffness and shear

thinning index at various temperatures which can

be used to estimate a grease apparent viscosity, and

ultimately, pumpability. The Lincoln ventmeter is a

valuable tool to gauge the impact of various factors

affecting grease pumpability such as thickener content,

base oil viscosity, temperatures, etc. The Lincoln

ventmeter was used to quantify how two levels of NLGI

grades of grease with different levels of base oil viscosity

impact the grease yield stress and pumpability over the

temperature range of 30 °F (-1°C) and 77 °F (25 °C). By

using statistical factorial experiments, the study shows

the impact that thickener content interaction and base oil

viscosity have on grease pumpability~ The data and trends

that are developed from the Lincoln ventmeter testing

are relevant in all types of lubrications system.

Background

Since the introduction of the Lincoln ventmeter to the

industry at the 1965 NLGI conference, the device has

been extensively used for testing of grease. In particular,

the instrument has been used to determine yield stress,

shear thinning index, and low temperature flow limits

of grease. Rotter’ discussed one way to calculate grease

yield stress and shear thinning values which allows for

a method of determining a grease’s apparent viscosity.

A paper by Buehler2 shows how grease stiffness affects

bearing start up torque.

In a follow up paper, He and Conley3, showed how

lubrication system design can be more reliable by using

data provided by the Lincoln ventmeter. Another paper4

introduced an improved method to estimate a grease’s

apparent viscosity. In Conley, Lugt, and He5, the authors

describe a more precise method of calculating a grease

yield stress and shear thinning index for determining

a grease’s apparent viscosity for use in designing

lubrication systems.

– 42 VOLUME 78, NUMBER 6

Lincoln ventmeter testing can be extended to quantify how two levels of NLGI grades of grease with

different levels of base oil viscosity impact the grease yield stress and pumpability over the temperature

range of 30 °F (-1°C) and 77 °F (25 °C). By using statistical factorial experiments, the study shows the

impact that thickener contents and base oil viscosity have on grease pumpability.

It is important for grease manufactures, when developing grease for use in automatic lubrication

systems, to know application requirements and the pumpability limits. Ideal greases are the ones that

can stay put in the bearing and yet have sufficient base oil viscosity to prevent metal to metal contact.

Thus the stiffest grease possible, but still within pumpability limits of an automatic system is best.

This paper serves as a guide to quantify how different types of grease chemistry, two NLGI grades

(NLGI 1 and NLGI 2) and two levels of base oil viscosity (460 cSt at high temperature and 220 cSt at

low temperature), affect grease pumpability. For example, it would be useful to know the extended

temperature range one can expect by using an NLGI 1 with a 220 cSt base oil ((30 °F, -1 °C). Similarly,

how would the temperature range be restricted by using an NLGI 2 grade with a 460 cSt (77°F, 25 °C)

base oil?

As a baseline, it can be considered that a grease yield stress value of 800 Pascal or below with a

maximum apparent viscosity of 50,000 cSt the upper limit for grease pumpability in a normal system

at ambient temperature. Depending on system design, larger diameter pipes or shorter pumping

distances, the maximum yield stress or apparent viscosity can be extended.

Looking at the relationship between grease yield stress, shear stress and shear rates, the graph below

shows that as grease begins to flow, the shear stress increases from the yield stress value (shear rate =0)

but the overall apparent viscosity is reduced as shear rate increases.

4Q~3Q

Figurel

In figure 1, the slope of the tangent line to

any of the curves is the value of the apparent

~ 3M00

2~

viscosity at the respective test temperatures.

As seen in this figure, the lower the grease

thinning index or power law index, the

~ 1,000

U

00

—

500

1000

Shear Rate

(

1500

Power Law of .275 ~~Power Law ~2O0

lower the apparent viscosity and the more

pumpable the product. With any grease, the

value of the yield stress is the starting point

of shearing that shows how stiff the grease

will be and is the basis of the response used

in this paper.

Power Law of .30

________________

Samples Used in Testing

A three-factor, two-level design of

experiments was performed on 4 different

type thickeners. The three factors were

temperature, NLGI consistency grade, and

– 43 NLGI SPOKESMAN, JANUARY/FEBRUARY 2015

base oil viscosity. Base oil viscosity is mineral type. The

two levels for each factor are described below.

a. Temperature High (70°F, 21 °C)

b. Temperature Low (30 °F, -1 °C)

c. NLGI High (NLGI 2)

d. NLGI Low (NLGI 1)

e. Base Oil Viscosity High (460 cSt(70 °F, 21 °C))

f. Base Oil Viscosity Low (220 cSt(30 °F, -1 °C))

–

–

Four types of greases are listed below with 28 total

samples.

1. Lithium Complex 8 samples

2. Lithium 8 samples

3. Calcium Sulfonate 8 samples

4. Aluminum Complex 4 samples

Test Procedure

Lincoln ventmeter as pictured in Figure 1 and

diagramed in Figure 3 was used to perform standard

Ventmeter test 30 °F (-1°C) and 77 °F (25 °C).Grease was

pumped up to 1,800 psi via, a grease gun and then the

vent valve (valve one, figure 3) opened. Pressure reading

was taken and recorded at the gauge, 30 seconds after the

vent valve opened. The test was repeated 4 times for each

sample, and averages were recorded.

(4 replicating tests done on all 28 grease samples)

—

—

—

—

Instrument Used: Lincoln Ventmeter

For each grease category and level, the average

ventmeter value was recorded and plotted. The

corresponding grease yield stress (in Pascal) was tabulated

using equation 1 and 2 and plotted.

Yield stress calculation Y = P*r / 2L

Eq. 1

Y = yield stress [reyns (lb*s/in2)]

(1 reyn=6.89OkPaxs)

P = pressure in psi (6.890 kPa) of gauge reading after 30

seconds.

r= radius of tubing (0.125 inches (0.3 175 cm) for

Ventmeter)

L = length of tubing (300inches (762 cm)) for

Ventmeter)

Having Y = P * .125/2*300 = P*.00021. Using conversion

of 6,890 kPa, the Lincoln ventmeter parameters for radius

and length of tubing, the equation is simplified below.

Yield Point Y (in Pascal)

1.45

Eq.2.

Figure 2

=

ventmeter pressure P (psi) x

25FTø14~N ~76METER(Ø6MM))

COLEEI ~UENG

CHECK

VALYF

PUMP

tLEVER GU~

~

PRESSJ~E

GUAGE

2

Results

70 °F (21 °C) and 30 °F (-1 °C) grease relates to NLGI

Low, Oil-Low level response in yield stress to the rest of

the levels. In all figures 4 through 8, NLGI Low, Oil low is

set a unity of 1.

Figure 3

– 44 VOLUME 78, NUMBER 6

Lithium Complex Group

1200

800

700

800~

~500

~400

609

360

400~

200

~200~

~00

0

0

0

10

20

30

40

50

Thmp(004F)

60

70

130

Lithium Group

1603

1200

14CC

11003

12CC

~800

1003 ~

600

~~NLG~Low0 Low

NLH~h0dH~h

“~“NLG~Low0’ 1~gh

—

—

800~

NLGH1gh0~ Low

~400.

NLGIH0~Koynthetc

200

Factor description

Change Index

(slope)

70”F, 21

“C factor

NLGI-Low, Oil-Low

5.6

1

1

NLGI-Low, Oil-High

3.4

1.4

.95

NLGI-High, Oil-Low

7.73

1.8

1.6

NLGI-High, Oil-High

8.83

2.5

1.94

NLGI-High, Oil-High

~_synthetic

1.88

30 “F,

21 “C

Discussion of results for lithium complex group in

Figure 4.

As seen from the graph, the most distinct jump in

yield stress happens between NLGI 1 to NLGI 2.The oil

viscosity change is less dominant. In the NLGI 2 greases,

the base oil viscosity shows an effect of additional yield

stress. The black dotted line with synthetic base oil

(high VI index) is flat with a small slope of 1.88. This

suggests that mineral base oil has moderate level effects

on pumpability. For the NLGI 2, and high viscosity base

oil grease, the yield stress is greater than 1300 Pascal at

30 °F (-1 °C). A temperature of 65 °F (18 °C) is needed

to achieve a good pumpability (yield stress

70 ~‘F, 21

~C factor

30 ~F,

21 °C

NLGI-Low, Oil-Low

3.6

1

1

NLGI-Low, Oil-High

5.0

1

1.2

NLGI-High, Oil-Low

7

1.6

1.7

NLGI-High, Oil-High

8.25

1.7

1.9

factor

Factor description

—~NLG LowO Wgh

~LG H~h0dL~w

r~one

-fEb

Change Index

(slope>

70 °F, 21

~C factor

NLGI-Low, Oil-Low

5

1

1

NLGI-Low, Oil-High

5.3

1.2

1.1

NLGI-High, Oil-Low

6.1

1.2

1.2

NLGI-High, Oil-High

g.s

1.5

1.6

30 ~F,

21 ~C

factor

Figure 6

Figure 7

Discussion on results of calcium sulfonate group in

Figure 6. As with lithium complex and lithium, the most

dramatic increase in yield stress was between NLGI 1

and NLGI 2. We can start to see a trend develop with

the first few types of grease. The base oil has an effect

but as shown earlier is less dominant. With this set of

data, there is less dramatic stiffening between 70 °F (21

°C) and 30 °F (-1 °C) as evident by the relatively lower

slope index. The change in base oil contributes little to

the yield stress value. When looking at the NLGI 2 high

viscosity base oil grease, the temperature to achieve good

pumpability is 65 °F (18 °C). For NLGI 2 grease with low

viscosity base oil, the temperature is near 55 °F (13°C), a

ten degree difference. With NLGI 1 base oil high and low,

good pumpability is achieved below 30 °F (-1°C).

Discussion on results of aluminum complex group

in Figure 7. With this thickener type, there is less of an

increase in yield stress between NLGI 1 and NLGI2 as

with the previous grease thickener types. The NLGI 2

grease values of yield stress are very close between NLGI

1 with a high viscosity oil to that of a NLGI 2 with a low

viscosity oil. This suggests that the base oil for this type

of thickener plays a more important role in pump ability.

With aluminum complex, there is a more pronounced

change due to base oil viscosity than in the previous

three thickener types. For NLGI 2, base on high, the

temperature to achieve good pumpability is near 70

°F (21°C).For NLGI 1, base oil low, the temperature to

achieve good pumpability is near 60 °F (15°C). Here

a 10 °F (6.56 °C) improvement is recorded. With this

thickener, NLGI 1 with base oil high or low, 55 °F (13

°C) is required for good pumpability. This 55 °F (13°C)

temperature is 20°F (11.11 °C) higher than the other

three grease thickeners, thus the grease is limited to

fewer applications due to its lower pumpability.

– 46 VOLUME 78, NUMBER 6

NLGI

Composite Group

1002

F

000

F’402

120,

shows that NLGI grade is more significant than the base

oil viscosity by a factor of approximately 2.7.

!soo

~

10021

700

~ 600

~500

I

s00~

600~

4,y

~3O0

~

J

200

200

0

0

0

10

20

50

40

50

Temp ( Deg F)

60

70

80

Anova Factor

Source Temp

NLGI Grade

Base 08 Viscosity

dl

-~——NLG~Lo~0 Hgh

L

________

Factor description

Error

Total

NLG~H~gh0JLow

NLGiLowQ Low

Change Index

(slope)

70°F21

~C factor

1

1

1

1

1

1

I

1~5

‘~MS

iF

lEffect I Contrast

735306 73530625 3676.53 -428.8

-3430.C

454950 45495025 227475 33725

2698 C

69432

69432 25 347 16 131.75

1054

-634.C

25122

2512225 125611 -79.25

-678.C

28730

2873025 143.651 -8475

35532

3553225 177661 9425

754 C

1134 C

80372

8037225 401 861 141.75

15 143i046l~

30~F,

21 °C

factor

NLGI Low Oil Low

4.83

1

1

NLGI Low Oil High

5.33

1.1

1.1

NLGI High Oil Low

6.88

1.7

1.5

NLGI High Oil High

10.38

1.9

1.2

Effect ( NLGI Grade + Base Oil Viscosity)=

337÷ 13 1=468

Effect of NLGI Grade 337/468 = 72 %

Effect of Base Oil Viscosity 13 1/468 = 28 %

Table 1

Summary and Conclusion

Figure 8

Testing shows, what would be self-evident with grease

makers and chemists, that lighter grade grease and

lighter grade base oil will result in improved pumpability

of a grease. But the value of this paper and results provide

some level of magnitude on the effect of pumpability

between NLGI Grade 1 and 2 and low and high viscosity

base oil. For an automatic lube system, yield stress of

800 Pascal or below is ideal and can be achieved with

most NLGI 1 grease with either a high or low level base

oil viscosity. The exception in this test is with aluminum

complex grease where NLGI 1 and 460 cSt showed yield

stress values over 800 Pascal near 55 °F (13°C).

Discussions on Composite Group. This graph

represents the combined data from all 4 grease thickener

types. The composite group shows the most distinct

jump in yield stress happens between NLGI 1 to NLGI

2 grade. The oil viscosity change is less dominant. With

the NLGI high Oil high, the greatest stiffening occurs as

evident by the slope of 10.38. The temperate for NLGI 2,

base oil high, to achieve a good pumpability of 800 Pascal

is approximately 62 °F (17 °C). For NLGI 2, base oil low,

the temperature to achieve good pumpability is 50 °F

(10°C). Here a 12 °F (6.67 °C) improvement.

Below in Table 1 is the Anova table for the composite

group. The df column is the degrees of freedom, the MS

column is the mean square of the error, the F column

represents the statistical impact for each factor, and the

Effect column represents the relative impact for each

factor. Table 1 shows that the values for temperature

have the highest impact, but in this paper, the effects of

interest are between base oil viscosity and NLGI grade.

The higher the number in the Effect column, the higher

the impact that factor has on the response. The table

Testing that was performed between 30 °F (-1°C)

and 70 °F (21°C)and it was shown that improvement in

temperature of 10°F can typically be realized between

a NLGI 2, 220 cSt base oil viscosity base oil to that of

a NLGI 2, 460 cSt base oil viscosity. Between a NLGI

2 and NLGI 1, as seen by the graphs an improvement

to achieve good pumpability at 20 to 25 °F (-7 to -4

°C) can be realized. As seen from the lithium complex

group test, when using a synthetic base oil with a high

47

–

NLGI SPOKESMAN, JANUARY/FEBRUARY 2015

NLGI

(VI index >120), a significant improvement can be made

in pumpabiity. It will be the focus of further test to do

more experiments with grease containing synthetic base

oils and compare to the data obtained in this paper with

mineral oil.

Finally from the Anova tablel, the relative magnitudes

of the effects are show as indicated in the effect column.

Ignoring temperature effects as we are mostly concerned

with the NLGI grade thickener level and base oil

viscosity level, the thickener has 72 percent effect on the

pumpability were base oil viscosity has 28 percent effect

on pumpability~

References

1.

2.

3.

4.

5.

F.A. Buehler and H Raich; (1967) “Low Temperature

Flow Limits for Greases”, NLGI Spokesman,pp

49-54.

P Conley and C He; (2007 vol 71 Number

9) “Lincoln Ventmeter Provides Invaluable

Information in Addition to Aiding Lubrication

System Design, NLGI Spokesman ppl7-21.

P Conley and C He, (2009, vol 73, Number 6)

“Lincoln Ventmeter Reading could be used to

Estimate Apparent Viscosity”, NLGI Spokesman pp

18-22.

P Conley, C He, P Lugt, (Nov 2013),”Effective

Viscosity Determination for Lubrication

Systems using the Lincoln Ventmeter” Tribology

Transactions, pp 771-780.

LC Rotter, J Wegmann; (1965)”The Lincoln

Ventmeter and its Possibilities”, NLGI

Spokesman,pp268-273.

PLEASE REME BER to complete your

Grease Production Survey Report questionnaire!

Your participation is vital to its’ accuracy.

Total Lubricating Grease Reported (Pounds)

.

.

.

.

.

.

.

.

19%

North America 19%

Europe 16%

Caribbean, Central & South America 3%

Africa & Middle East 3%

India & Indian Subcontinent 8%

Japan 7%

Pacific & South East Asia 6%

PRC 38%

– 48 VOLUME 78, NUMBER 6

3%

7%

8°o