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traight oils can break down over time and fail, just as water-dilutable products do,
but fluid failure can be slowed through best practices in recycling methods. Processes and
process variables—the type of machine operation, type of metal and surface finish and the
cleanliness of parts coming off the machine—determine the choice of a particular straight oil.
Viscosity (i.e., load-carrying capacity) is the major consideration for using straight oils in
metalworking applications. Straight oil fluids are preferable to water-dilutable fluids when a
high degree of lubrication is a higher priority than high cooling capability. Typically straight
oils are used in slower operations and those producing parts with tight tolerances and in severe
operations like broaching, pipe threading and gear cutting or in finishing bearing surfaces.
They also are used in machines, including some older units requiring high viscosity fluids.
WEBINARS
Straight oil metalworking fluids: From use to recycling
Proper maintenance is the difference between replacing every few weeks and indefinite reuse.
KEY CONCEPTS
Processes and process
variables determine the
choice of a straight oil.
Oils can fail, just like
water-dilutable products.
Recycling methods can
slow fluid failure.
S
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Sto
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By Dr. Nancy McGuire
Contributing Editor
4 4 • J U N E 2 0 1 9 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y W W W . S T L E . O R G
Straight oil typesThe general terms straight oil and cutting oil, applied to MWFs,
can refer to true straight oils,
compounded or blended oils
and extreme pressure (EP) oils.
The term neat oil sometimes re-
fers to a straight oil formulation,
although it more often refers to
the base oil before the additives
are put in.
Petroleum oils are the most
common base oils, but esters can
be used when more lubrication is
required. Synthetic oils are not
commonly used, mainly because
of their cost, but they can be use-
ful for specific applications.
True straight oils can be a
blend of several base oils, but
base oils are the only compo-
nents of these products. The
primary difference between
various true straight oils is their
viscosity. These oils are typi-
cally used for light machining
operations like honing or ream-
ing where only small amounts
of metal are removed.
Compounded or blended
oils contain base oils and spe-
cif ic additives, and they are
typically used for medium-du-
ty applications. These oils are
used in a wide variety of appli-
cations, and various products
offer differences in viscosity,
lubrication and wetting.
EP oils mix base oils with
mild or active EP additives, and
they are used in severe applica-
tions that remove large amounts
of metal but still require a good
surface finish. EP additives are
generally sulfur, phosphorus or
chlorine compounds. EP oils
are formulated especially for
heavy-duty applications, so us-
ing them to reduce the amount
of oil needed in lighter applica-
tions is not likely to be effective.
Other additives include lu-
brication enhancers like esters
and fatty materials, over-based
calcium sulfonates and some
polymers. Antioxidant addi-
tives protect against oxidation
from tramp oil and water, and
corrosion inhibitors protect
the finish on the active, freshly
machined surface. Deaerators
(which push air to the fluid sur-
face) are used for applications
that generate turbulence and
entrained air since defoamers
aren’t as effective in straight
oils as they are in coolants.
Anti-mist agents can be used
for machines that aren’t well
sealed from the atmosphere.
Oils that contain additives,
especially EP additives, require
more maintenance in terms of
testing and regeneration than
true straight oils do. Replenish-
ing an oil can be as simple as
topping off the reservoir. How-
ever, before an operator pours
a new oil on top of the old oil,
it’s imperative to check with the
fluid supplier or manufacturer
and find out if the two fluids are
compatible.
Failure processesStraight oil failures fall into two
categories: contamination and
fluid breakdown. Failure mecha-
nisms include contamination by
particles and fines, water con-
tamination, heat, evaporative
losses, extraneous oil contami-
nation and additive depletion.
Oil contamination by parti-
cles and fines represents a non-
permanent failure mechanism:
we can remove the material
almost as quickly as it’s gener-
ated. This is the most common
cause of oil failure, and it can’t
be completely prevented be-
cause one of the functions of a
MWF is to wash particles away
from the work area. Particles,
fines and debris from the ma-
chining process are distributed
throughout the system along
with dirt and debris from the
plant. Overall particle levels
and size distributions are deter-
mined using particle counters
or filter tests. More specific par-
ticle identification is done using
elemental analysis or, less com-
monly, ash testing.
High particle levels can re-
duce the cleanliness of parts
and damage the surface f in-
ish—especially if finished parts
are stacked and rub together.
Particles also can stain parts and
tools. Straight oils are inherently
corrosion inhibiting, but dissimi-
lar metals coming into contact
(like steel chips on an aluminum
part) can set up a galvanic cell
that promotes corrosion.
Smaller particles, such as
those from grinding processes,
are more harmful than larger
ones. They are harder to re-
move, and they can recirculate
over and over again throughout
the system. Abrasive microfine
particles can scar metal pieces
(see Figures 1 and 2 on Page 46). Extraneous oils, also called
tramp oils, are another source
of contamination. These oils
MEET THE PRESENTER
This article is based on a Webinar presented by STLE Education on Dec. 10, 2018. Straight Oils:
From Use to Recycling is available at www.stle.org: $39 to STLE members, $59 for all others.
Alan Cross is a senior project engineer with Houghton International Inc. in Valley Forge, Pa.
He received his bachelor’s of science degree in chemical engineering in 2004 from the University
at Buffalo and has been in the metalworking fluid industry for 14 years. Cross has been involved in
metalworking fluid projects including fluid recycling, industrial wastewater and pure water treatment,
process improvement as well as research and development in the metalworking fluid and fluid power
areas. Cross has presented many papers at STLE and global conferences and holds two patents.
He has co-authored a chapter titled Recycling of Metalworking Fluids with John M. Burke in
Metalworking Fluids, Third Edition. You can reach Cross at [email protected] Cross
Viscosity is the major consideration for using straight oils in metalworking applications.
W W W . S T L E . O R G T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y J U N E 2 0 1 9 • 4 5
can enter a MWF through hy-
draulic leaks, oil dripping from
ways, greases or materials
spilling into an open machine.
Incoming parts, especially
those with rust-preventive
coatings, also can introduce
extraneous oil. The problem
can be worse in a system with
a relatively small sump that
doesn’t allow extraneous oil
to separate out before it is re-
circulated. Extraneous oil con-
tamination can be determined
by measuring general viscosity
trends. FTIR analysis can iden-
tify specific oils and organic ad-
ditives, and elemental analysis
is especially good for identify-
ing EP additives.
High levels of extraneous
oils can significantly raise or
lower a straight oil’s viscosity.
Additives in extraneous oils
can stain parts and otherwise
reduce part quality, and the
problem is especially bad for
yellow metals and aluminum.
Hydraulic fluids with EP ad-
ditives could contaminate a
straight oil without EP addi-
tives. (Some companies switch
to hydraulic fluids that don’t use
phosphorus, sulfur or chlorine
for just that reason.)
Fluid breakdown is a more
diff icult problem to solve. It
can’t be prevented, but it can
be slowed down using best
practices.
Water is a contaminant,
but it also contributes to fluid
breakdown, so water content
should be measured on a reg-
ular basis. This is especially
important in areas with wide
temperature swings where
water condensation could
drip into the oil. Water can
drastically reduce the appar-
ent viscosity of a straight oil,
which increases friction and
tool wear. Certain situations
may require continuous inline
or online monitoring to catch a
water-contamination problem
before it gets out of hand. The
best method for water-content
measurement in straight oils is
Karl Fischer titration, which is
accurate down to the low (~50)
parts-per-million level.
Excess heat can accelerate
fluid breakdown by promot-
ing oil oxidation and additive
depletion. Heat is generated
by the machinery and friction
from the metalworking pro-
cesses and should be removed
through heat exchange equip-
ment. Darkening is one sign of
heat degradation, but a dark-
ened oil isn’t necessarily out of
specif ication—it depends on
what caused the oil to darken
(see Figure 3).Evaporative losses also can
cause an oil to perform poorly.
Base oils with various viscosi-
ties, boiling points and volatili-
ties are blended to fine-tune
the viscosity to the desired
value since it’s impractical to
make one-component oils for
each different viscosity. Low-
er viscosity oils tend to have
lower boiling points and higher
volatilities. As they evaporate,
the overall viscosity of the
straight oil increases. Although
this higher viscosity increases
the load-carrying capacity,
it can present problems with
flow and with the lubrication
characteristics in the interface
between the tool and work-
piece. Kinematic or dynamic
viscosity tests monitor the ef-
fects of heat and evaporative
losses.
Normal operations break
down or neutralize additives,
or they carry additives away
on workpieces moving down
the line. EP additives react
and form new compounds that
are carried out on parts or re-
moved during filtration. Addi-
tive depletion is determined us-
ing X-ray fluorescence,
WEBINARS
Chips Grinding swarf
Figure 1. Smaller particles like grinding swarf are more harmful than large
chips. (Figure courtesy of Houghton International, Inc., and Byers, Jerry P., ed., Metalworking Fluids, Third Edition, 2018, CRC Press/Taylor & Francis. With permission.)
Figure 2. Fine particles are difficult to see when dispersed (left) but are
clearly visible when they settle (right). (Figure courtesy of Houghton Inter-national, Inc.)
Figure 3. Water contamination causes a variety of problems in straight
oils. (Figure courtesy of Houghton International, Inc., and Byers, Jerry P., ed., Metalworking Fluids, Third Edition, 2018, CRC Press/Taylor & Francis. With permission.)
4 6 • J U N E 2 0 1 9 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y W W W . S T L E . O R G
<0.1% water 1.0% water
90
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30
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DEHYLUB®
4030DEHYLUB®
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Biodegradation via OECD 301 [%] Renewable Content [%]
DEHYLUB®
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WEBINARS
atomic emission spectroscopy
or elemental analysis.
Straight oil recyclingRecycling straight oils lowers
costs by reducing the amount
of new oil purchased and by re-
moving sources of fluid failure
to increase service life. Most
recycling processes are simple
and easy to implement. Recy-
cling methods include remov-
ing particulates and water, ad-
justing the viscosity, removing
heat, replenishing additives and
partial fluid dumping.
Filtration and separation
can be used alone or combined
to increase effectiveness. Fil-
ters use a physical barrier (filter
paper, filter media, screens) to
remove particles, and filter aids
like perlite or cellulose can im-
prove effectiveness. Filters are
usually disposable, but many
landfills have stopped accept-
ing these materials because
they contain oils and metal
particles. Metal screen filters
do not present a disposal prob-
lem since they can be used over
and over again. Portable oil fil-
ter units, which can be mounted
in a kidney-loop fashion, are
good for systems with several
individual sumps (see Figure 4). Bulk filter media come in a
variety of porosities for differ-
ent applications. Fine pores will
trap more of the smaller par-
ticles, but pores smaller than
f ive microns can strip some
additives out of a fluid system.
Analyzing the residue on the
filter media will show whether
the filters are contributing to
additive loss.
Magnetic ferrous particles
can be removed using a mag-
netic separator with a rotating
drum. A blade on the back side
scrapes off the magnetic parti-
cles sticking to the drum and
deposits them into a collection
bin so they can be taken to a
recycling facility. This method
removes even particles in the
sub-micron range without filter-
ing the additives out of the oil
(see Figure 5).Liquid-solid centrifuges re-
move large particles like chips
that settle quickly, but they are
less useful for separating out
microf ine particles. Heating
the oil to increase the density
difference between fine partic-
ulates and the oil may not be
especially effective, especially
if this causes the oil to oxidize
and break down sooner. Any-
one considering this practice
should contact the centrifuge
manufacturer for recommenda-
tions and determine whether it
removes enough extra material
to justify the cost of the energy
for heating and cooling the oil
and the effect on the oil’s per-
formance.
Liquid-liquid centrifuges
are only useful in specific ap-
plications where there is a large
density difference between the
straight oil and the extraneous
liquid. Lab analysis can deter-
mine if centrifugal separation is
effective for a particular opera-
tion.
Gravity separation is a less
common option. It requires
time, a large volume of excess
oil and periodic sampling to de-
termine when the oil is ready
for reuse. Some operations al-
low used oil to sit in a settling
tank for as long as two years be-
fore it is put back into service.
Vacuum dehydration is the
most commonly used method
for removing water from a
straight oil. This can be a slow
process depending on how
much water is in the oil. Heat-
ing the oil speeds the water
removal, but the temperature
should not be raised so high
that it degrades the oil. A de-
hydrator should be kept onsite
in areas that are prone to water
ingress or where condensation
is a problem. Portable dehydra-
tors can be moved from sump
to sump using a forklift or a
dolly (see Figure 6 on Page 50). Membrane dehydrators
have a small footprint, and
they can be a permanent ad-
dition to straight oil systems,
especially small systems with
small single sumps. These de-
hydrators force oil through a
membrane-lined tube at high
pressure. The oil component
flows straight through the tube,
but the water is forced through
the pores in the membrane and
is carried away by a vacuum or
a stream of dry air. Membrane
dehydrators can run continu-
ously if necessary (see Figure 7 on Page 50).
Of course, given enough
time, water will separate on its
own, but turning a system off
and letting the water
Figure 4. (a) Metal screen filters can be reused rather than discarded. (b) A
portable filter unit is useful for systems with several small sumps. (Figure 4a courtesy of Rosedale Products, Inc. Figure 4b courtesy of Oil Filtration Systems, Inc. Figure also courtesy of Byers, Jerry P., ed., Metalworking Flu-ids, Third Edition, 2018, CRC Press/Taylor & Francis. With permission.)
Figure 5. A drum magnetic separator captures and collects large and small
ferrous particles. (Figure courtesy of Eriez Manufacturing Co. and Byers, Jerry P., ed., Metalworking Fluids, Third Edition, 2018, CRC Press/Taylor & Francis. With permission.)
(a) (b)
4 8 • J U N E 2 0 1 9 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y W W W . S T L E . O R G
separate out is seldom practi-
cal—and water can do a lot of
damage in a short time. The
time needed to separate the
water depends on how much
water is in the system and
how long it’s been recirculat-
ing and mixing in with the oil.
If a large amount of water has
been dumped into a system, the
best practice is to shut the sys-
tem down and decant as much
water as possible, followed by
an oil analysis to identify any
changes to additives, viscosity
and other properties.
In addition to keeping the
oil dry, it’s essential to continu-
ously remove heat to keep the
oil in specif ication. Thermo-
statted chillers, with or without
refrigeration units, are attached
to machines or sumps to keep
the temperature under control,
using air or liquid as a heat-ex-
change medium. Refrigeration
works better than radiators in
warm climates or shop environ-
ments, but radiators are effec-
tive in cooler surroundings. Us-
ing a sufficient oil volume and
reservoir size also helps keep
temperatures down by giving
the heat a chance to dissipate
from the oil before it is put back
into service. Residue buildup
in a sump can significantly de-
crease the volume of oil it can
hold, so the oil can’t cool down
enough before it’s recirculated.
Keeping oil in a sump just for
the purpose of letting it rest is
generally not necessary. Letting
oils sit idle requires purchasing
larger volumes of oil, and it can
allow the oil to cool down, which
could cause water condensation
problems. When the oil comes
back up to operating tempera-
ture, the added water could re-
duce the oil’s apparent viscosity
and increase oxidation and deg-
radation of the oil. Straight oil
fluids can be run continuously
as long as the operator moni-
tors viscosity, water content, dirt
counts or particle levels and oil
temperature—and as long as
machined parts are checked for
specific pass/fail criteria estab-
lished by the customer.
The chemistryModifying the chemistry of
the oil is sometimes necessary,
but it’s a more diff icult solu-
tion than the fixes previously
described. Verifying the results
requires laboratory analysis
and guidance from the manu-
facturer. Adding base oils can
bring a fluid’s viscosity back
into specification if it has be-
come too high or too low, but
this can throw the concentra-
tions of the other formula com-
ponents out of balance.
Oil additive levels must be
checked periodically, and ad-
ditives must be replenished
as they are consumed and
removed. Again, this requires
laboratory analysis and guid-
ance from the manufacturer,
but ordinary maintenance test-
ing and recommendations can
be a simple, routine process.
Manufacturers often provide
standard packages (e.g., lube
or EP pack) for commonly re-
quired replenishments. These
packages are diluted with base
oil, then dispersed through
the system. Packages that are
supplied as solids must be dis-
solved in base oil (this can re-
quire heating the oil) before it is
circulated through the system.
As a last resort, when a
straight oil has lost its desirable
properties and other recycling
options are not effective, it may
be necessary to dump some
portion of the straight oil and
replace it with fresh product.
The used oil must be disposed
of properly, which generally in-
curs a disposal cost. If this is a
recurring problem, you may be
using an oil that’s not appropri-
ate for your particular process.
When dumping is necessary, the
operator should shut off the sys-
tem and let the water and partic-
ulates settle to the bottom of the
tank where they can be drained
off. This ensures the system is
clean before fresh oil is added.
Just like any other met-
alworking fluids, straight oils
can fail. However, their service
life can be greatly increased
by reducing the mechanisms
that cause failure—this largely
involves keeping them clean,
cool and dry. Proper mainte-
nance can make the difference
between replacing the oil every
few weeks to reusing the same
oil indefinitely with routine re-
plenishment operations.
Nancy McGuire is a free-lance writer based in Silver Spring, Md. You can contact her at [email protected].
Figure 6. Vacuum dehydration is the most common method for removing
water from oil. (Figure courtesy of Oil Filtration Systems, Inc., and Byers, Jerry P., ed., Metalworking Fluids, Third Edition, 2018, CRC Press/Taylor & Francis. With permission.)
Figure 7. A portable membrane dehydrator can be kept onsite. (Figure cour-tesy of Compact Membrane Systems and Byers, Jerry P., ed., Metalworking Fluids, Third Edition, 2018, CRC Press/Taylor & Francis. With permission.)
Verifying results requires laboratory analysis and manufacturer guidance.
WEBINARS
5 0 • J U N E 2 0 1 9 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y W W W . S T L E . O R G