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