Evaluating
A Rotary Screw Lubricant Before You Buy
“ A Product Data Sheet Primer”
Years ago, a New York area clothier ran a TV ad proclaiming;
“An educated consumer is our best customer”.
I loved that ad. From a marketing standpoint it was new,
fresh, and pro-consumer. It basically said, you don’t
have to pay top-dollar for quality merchandise. When you
consider the total operating costs (maintenance, replacement
parts, associated productivity, etc), that are generated
either by purchasing and utilizing the wrong lubricant
and/or inferior product in rotary screw compressors, you
realize that it doesn’t pay to buy cheap or let
some slick salesman sell you on, “ours is the best
lubricant you can use in your compressor”! Or conversely,
“you don’t need that expensive synthetic stuff
just to run your compressor”! But expensive doesn’t
necessarily mean superior either. My grandfather once
told me, “Son, we’re too poor to buy cheap”.
Which basically meant, if you can’t afford a good
quality material that will last a long time don’t
buy anything at all. That is why I’m writing this
paper. A lubricants product data sheet can tell you a
hell of a lot or nothing at all! Be smart, ask questions.
If the data isn’t there or appears to be misleading,
ask questions. Process air is probably the most important
utility in a manufacturing/service facility. It’s
not too hard to achieve long service life with any compressor
if it’s maintained properly. Good compressor maintenance
starts with using a good quality lubricant.
My goal is two-fold; First, I’d like to outline
what tests should be included on a product data sheet
for an air compressor lubricant, secondly, I’d like
to define each of these tests and associate them to what
primary function they effect in regard to performance
in a rotary screw compressor. I’ll also present
to you some additional screening tests that pertain to
this application. The “KISS” method is used
throughout this paper. The “KISS” method,
if you don’t already know, stands for,” keep
it simple-son”. That works for me, hopefully it
will work for you too!!
PRIMARY TESTS-
Viscosity- measured by ASTM D-445. Two (2) viscosity
measurements should be on the product data sheet, a 40deg.C
and 100degC measurement. The forty (40) -degree viscosities
indicate what “ISO” grade the lubricant is
(more on this later). These viscosities will be reported
in centistokes, (cSt). Also known as the kinematic viscosity
further defined as the time required for a fixed amount
of oil to flow through a capillary tube under the force
of gravity. Historically, it was quite common to see additional
viscosity data in “SUS”. “SUS”
defined as Saybolt Universal Seconds (ASTM D 88), was
another means of interpreting viscosity. The Saybolt Company
invented the viscometer, which is the devise that measured
the flow of a lubricant, hence the name.
Compressor company (OEM) engineers specify the “viscosity
grade” or “ISO grade” by calculating
the bearing life under normal operating conditions. The
diameter of the rotor journal, speed of rotors, and operating
temperature all are factored in. Typically, most rotary-screw
applications call for an ISO-46 fluid.
ISO grade- In 1975 the “International Organization
for Standards” (ISO) issued International Standard
ISO 3448 providing a classification system for liquid
lubricants according to viscosity. This standard covers
the entire range of lubricants in normal industrial applications.
As stated earlier, most rotary-screw applications call
for an ISO grade 46 fluid.
Viscosity Index “VI”- Measured
by ASTM D-2270- the viscosity index indicates the rate
of change in viscosity of an oil within a given temperature
range. Simply put, if oil has a high VI (usually >
120) the oil’s viscosity will show less change with
difference in temperature as compared to a low VI (usually
<110) product. Knowing the VI comes in handy if your
application sees wide temperature variations. For instance,
if your compressor runs on an ISO 46 lube, and your ambient
can go down below 0C/32F, you should strongly consider
using a high VI product. A higher “actual viscosity”
will put more load (starting torque) on the electric motor
that drives your compressor. This situation will cause
a higher draw of amps upon start-up, which in turn shortens
the life of the drive motor. A higher VI product would
allow you to draw less amperage at lower ambient temperatures.
In addition, if your VI is too low, less lubricant will
reach vital areas that require lubrication upon start-up
(rotor bearings).
Flash and Fire Points- Measured by
ASTM D-92 (Cleveland Open Cup Test), the flash point of
oil is the lowest temperature at which it gives off sufficient
vapors that will ignite when a small flame is periodically
passed over the surface of oil. The fire point is the
lowest temperature at which oil ignites and continues
to burn for at least 5 seconds. These tests are run to
aid in the generation of MSDS sheets and help plant supervisors’
implement the appropriate safety measures against potential
fire hazards. SAFETY FIRST. Compressors’ generate
heat from the compression cycle. A portion of this heat
is passed on with the discharge air. Flash fires and explosions
have happened due to a combination of hot turbulent air
and oxides of metals (i.e., iron). Though this situation
was more common in reciprocating applications utilizing
mineral oil-based products, it can also happen in rotary
air applications. Synthetic-based products generally have
much higher flash and fire points as compared to mineral
oil-based products and are gaining more and more acceptance
for this very reason. BE CAREFUL!
Auto ignition temperature-Measured by ASTM D-2155 or E659,
Another SAFETY FIRST TEST! This method determines the
temperature at which air oxidation leads to ignition.
Pour Point- Measured by ASTM D-97, the
pour point is the lowest temperature at which oil will
pour or flow under prescribed conditions when it is chilled
without disturbance at a fixed rate. This test along with
the VI, indicates the performance characteristics at lower
operating temperatures. Electric drive motors and overall
lubrication is vital at low ambients. If oil cannot flow,
it will not lubricate properly.
Water Separability- Measured by ASTM
D-1401, This test method measures the ability of the lubricant
to separate from water. Per the ASTM test method, this
test is only to be used as a guide for determining the
water separation characteristics of oils subject to water
contamination and turbulence. It can be used to evaluate
both new and in-service oils. Basically 40milliliters
of water and 40 millilitres of oil are placed in a graduate
cylinder and heated to a determined temperature. Once
the temperature has been achieved a stirrer is introduced
into the mixture and agitated at 1500rpm for five (5)
minutes. The mixture is than inspected every 5 minutes
to evaluate the volume of oil, water, and emulsion. The
amount of time taken to achieve a “split”
is also recorded. The test is usually terminated by 60
minutes if separation has not occurred. The data for this
test should be reported in the following manner; i.e.,
40-40-0 (20). The sequence of numbers represents millilitres
of oil, water, and emulsion respectively. This test basically
indicates how the lubricant will perform in the compressor
oil/water separator. This is very important especially
in high humidity conditions. A word of caution here, oil/water
separators vary in type and design construction not only
from model to model but also from supplier to supplier.
In addition, mechanical parameters (i.e., drain lines,
lubricant velocity) play an important role towards efficiency
of oil/water separation. Good demulsibility is a VERY
important characteristic towards the successful lubrication
of any rotary compressor. So any rating less than 40-40-0
(15) may not be up to snuff.
Specific Gravity-Measured by ASTM D-287
(hydrometer) or 1480 (Pycnometer), this test is defined
as the weight in air of a given volume of material at
a stated temperature to weight in air of an equal volume
of distilled water at the same temperature. The test (D-287)
is run by pouring the test lubricant into a hydrometer
jar, removing all air bubbles, than placing a hydrometer
into the fluid. The gravity is than read off the hydrometers
scale and corrected for temperature. In the ASTM D-1480
test, a lubricant sample is introduced into the pyncnometer,
equilibrated to the desired temperature, and weighed.
The density or specific gravity is then calculated from
this weight and the previously determined calibration
factor, and a correction is applied for the buoyancy of
air. Generally, the gravity of oil was needed for bulk
storage purposes, selling price (sold by volume), and
transportation (weight) issues. However, one could help
determine the species of a lubricant basestock by the
gravity. Petroleum-based products gravity range is usually
from .8200 to .8500. Synthetic base stocks generally are
greater than .8700 with complex esters and glycol-based
products as high as .9950.
Rust Prevention – Measured by
ASTM D-665A & B, this test indicates the ability of
lubricating oil to prevent corrosion during the lubrication
of ferrous parts in the presence of water (A), and synthetic
seawater (B). Normally, part “A” data from
this test is good enough to spec’ out a rotary compressor
fluid. But if you have any long term storage issues or
plan on sending your compressor overseas in the hull of
a cargo ship you should consider part “B”
data also. Remember, compressors make a lot of water,
most lubricants will “saturate” up to approximately
2000ppm water in typical conditions. If your additive
package isn’t up to snuff, you’ll have a lot
of orange color debris (rust) floating around, which can
cause havoc in a rotary compressor, (plugged sep filter,
shorten life of rotor bearings, pit the rotors, etc.).
4-Ball Wear Test- Measured by ASTM D-2266,
this test measures the loss of weight and dimension on
surfaces subjected to friction. In the test, 4 balls are
arranged in the form of an equilateral tetrahedron. The
three lower steel balls are clamped in placed and submerged
in the test fluid. The fourth ball, which is rotating
at 1200 rpm, is placed in the center of the lower 3 balls
with 40kg of applied pressure. The running temperature
is usually set at 75 degC. The test duration is one (1)
hour. During this test circular scars are worn in at the
contact surfaces of the 4-balls. These diameters are than
measured under a microscope and are reported as millimeters
scar. A Four-ball rating of less than 8 is desirable for
most rotary screw applications. Generally, this test indicates
the lubricants performance in regard to friction reduction
at the bearings of the rotor(s).
Foaming Characteristics-Measured by
ASTM D-892, This test covers the determination of the
foaming characteristics of lubricating oils at specified
temperatures, which is accomplished in three (3) sequences,
and a good quality rotary screw lubricant should have
excellent results in all three. In all three sequences,
a specified amount of oil is maintained at the required
temperature and is blown with air at a constant rate for
five (5) minutes, then allowed to settle for ten (10)
minutes. The volume of foam generated is measured at the
end of both periods. Rotary screw mechanical components
have the potential to generate large amounts of foam if
the lubricant used is not appropriately additized or synergistically
balanced to prevent foam generation. The rotors and separator
filter are the main culprits here. Simply, the rotors
and sep. filter act as a mix-master blender and homogenizer
all in one. Add in the velocity, temperature, and pressure
of the lubricant along with any extraneous contaminants
that will chemically cause foam and you have the potential
for one huge foam bath!! Normally, the foam is seen in
the site glass of the sep. filter. Foam causes loss of
lubrication and will eventually result in mechanical failure.
Air-ends are expensive, so any lubricant with foam rating
higher than “nil” should not be used in a
rotary screw application.
Copper Strip Corrosion- Measured by
ASTM D-130, this test measures the lubricants corrosiveness
to copper and copper alloys. In the test, a polished copper
strip is placed in a stainless steel bomb containing the
test fluid, sealed, and submerged into a test bath maintained
at 100degC for three (3) hours. The strip is than wiped
of residual oil and matched against the ASTM tarnish/corrosion
std. A rating of 1a-1b is most desirable, a lubricant
with a rating of 2a or higher in this temperature/time
frame should be avoided. Even though we are in the age
of plastic, there is still plenty of copper and/or copper
alloy materials found throughout a compressor,(i.e., return
lines temperature solenoid valves). Sulfur containing
lubricants, (EP gear variety), should never be introduced
into a rotary screw compressor. Not only do they attack
copper but also build acids in a rotary screw type environment,
which will certainly shorten the life of your lubricant.
Conradson Carbon-Measured by ASTM D-189,
this test measures the amount of carbon residue remaining
in an oil after the oil has been subjected to extreme
heating in the absence of air. In the test, a sample is
weighed into a crucible, covered, then placed into an
oven and burned at an elevated temperature. The remaining
carbon residue is then weighed and expressed in %wt. Well,
I know what your thinking. Your thinking, a rotary screw
compressor introduces air into the lubricant at elevated
temperatures, so what gives here? Allow me to explain-
You usually find this test on the product data sheets
for reciprocating-air lubricants, ISO grades 100 and 150.
It is chiefly used in that area to help predict the cleanliness
of the exhaust valves while in service. In general, once
oil oxidizes, it’s nice to know approximately how
much carbon will be produced from the fluid. So if you
narrowed your lubricant search down to two (2) different
lubes from two different suppliers and the make or break
deciding issue comes down to the Conradson carbon values,
pick the lube with the lower Conradson carbon value.
Alchor High Temperature Deposition Test-
This test was originally designed to evaluate turbine
oil’s thermo/oxidative performance characteristics
at elevated heat regimes. Alchor is the name of the company
who invented the test rig. Well, rotary screw compressors
circulate lubricant also at elevated temperatures. This
test has tremendous value for screening lubricants that
are being considered for use in rotary-screw applications.
In the test, oil is circulated through a sump pan (cabinet)
at a constant temperature (usually 200deg. F). It is then
pumped and passed over a metal tube, which is kept at
a higher temperature (usually 450deg. F). Air is passed
through this cabinet and over the rod at a rate of 1000cc/min.
The test duration is 48 hours. The oils viscosity and
total acid number changes are then calculated. In addition,
the tube that the lubricant flowed over is evaluated for
deposits. You’ll probably never find any Alchor
deposition test data on any rotary screw lubricant product
data sheet. But, if you’re a stickler for detailed
data, call up the lubricant company’s technical
service department and ask them if they have this data
available, if they don’t you may want to consider
looking elsewhere for a rotary screw lubricant.
Rotary bomb oxidation- Measured by ASTM
D-2272, Usually listed as “RBOT”, this test
utilizes an oxygen-pressured bomb to evaluate the oxidation
stability of new oils. The measured amount of oil, water,
and copper catalyst coil are placed in a glass bomb container
equipped with pressure gage. The bomb is charged with
oxygen then placed in a constant-temperature oil bath
and rotated axially at 100rpm. The number of minutes required
to reach a specific drop in gage pressure is the oxidation
stability of the test sample and is reported in minutes.
A word of caution here. Some synthetic basestocks (PAO’S)
perform better than others, (diesters/polyolesters), regarding
this test and should not be solely used as a critical
factor in deciding on whether to use an ester or PAO-based
product. Some compressor OEM’s utilize a combination
of PAO/ester technology to best capture the positive characteristics
of both.
Hydrolytic stability-Measured by ASTM
D-2619, Other names for this test are, “Coke bottle
test” or “beverage bottle test” hydrolytic
stability test for good reason. A weighed sample of fluid,
specific amount of water, and a copper test specimen are
sealed in a pressure-type (glass) beverage bottle-hence
the name. The bottle is rotated, end over end, for 48
hours in an oven at 93deg C (200deg F). The weight change
of copper is measured along with the viscosity and acid
number changes of the test fluid. In addition, the acidity
of the water layer is determined. Though this test is
a regular for hydraulic oils, it could also be used for
screening any circulating fluid.
For more information regarding compressor lubricants,
please contact;
Darren J. Lesinski
Technical Service Manager/Industrial Fluids
Royal Lubricants and The ANDEROL Co.
(973) 887-7410, X221
e-mail: dlesinsk@royallube.com
