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.

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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 across@houghtonintl.com.Alan 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

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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 nmcguire@wordchemist.com.

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