How to Select the Correct Hydraulic Oil for Your Machine…

Transcription

How to Select
the Correct Hydraulic Oil
for Your Machine…
OR
How to Make Sure
the Hydraulic Oil
You Are Currently Using
Is the Right One!
By Brendan Casey
Copyright © 2010 Brendan Casey – wwwHydraulicSupermarket.com
PLEASE READ THIS FIRST!
Thank you for investing in this report and video.
To get the most from it, I recommend you start by reading the
report from the beginning, then pausing to watch the video where
directed.
The video plays in Windows Media Player 9 or higher. If this
program is not already installed on your computer, the current
version can be downloaded from:
http://www.microsoft.com/windows/windowsmedia/
If you are a Mac user, you can watch the video by downloading the
Flip4Mac WMV player for Quicktime:
http://www.mireth.com/wt/mxmewt545.html
A transcript of the video dialogue is included in the Appendix
accompanying this report. So if for some reason you have trouble
understanding the dialogue on the video, you can refer to this
transcript for clarification.
Prior to watching the video, be sure to read the introduction and
notes on page 17.
Once you’ve studied this report and video a couple of times, you’ll
be equipped to apply this know-how to select the correct hydraulic
oil for your machine… or to check the oil you are currently using is
the right one!
Yours for better hydraulics knowledge,
Brendan Casey
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Table of Contents
Why You Need to Know This ................................................................................................................4
Why Hydraulic Oil is Different From Other Lubes ............................................................................4
The Benefit of Multigrade Oil ...............................................................................................................5
The Problem with Multigrade Engine Oil............................................................................................7
The Controversy About Detergent and Zinc - and How to Deal With It ...........................................8
Why You Should Avoid Automatic Transmission Fluid...................................................................11
The Truth about ‘Biodegradable’ Oils ...............................................................................................13
Hydraulic Oil Type Selection Summary.............................................................................................16
Video: How to Select the Correct Oil Viscosity .................................................................................17
How to Monitor the Condition of the Oil ...........................................................................................18
Appendix ...............................................................................................................................................20
Further Reading ...................................................................................................................................20
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Why You Need to Know This
In a recent issue of Hydraulics and Pneumatics magazine, there was a case-study
about a new hydraulic excavator which had been shipped to site with the wrong type
of hydraulic oil1. The consequence of this was four pump failures - at a cost of
$20,000 each, three swing motor failures and two track drive motor failures – all
within the first 27 months of operation. In fact, the total cost of failures - including
downtime, amounted to $193,872 over 2-1/4 years!
While the hydraulic equipment you’re responsible for may not be on the same scale,
the principle is the same: if the hydraulic system is not filled with the correct oil:
• it won’t perform like it should; and
• it won’t last like it should.
Why Hydraulic Oil is Different From Other Lubes
Hydraulic oil is different from other lubes. Not only is it a lubricant, it’s also the
means by which power is transferred throughout the hydraulic system. So it’s a
lubricant and a power transfer device.
This dual role makes it unique.
To be an effective and reliable lubricant, hydraulic oil must possess properties similar
to most other lubes. These include: foaming resistance and air release; thermal,
oxidation and hydrolytic stability; anti-wear performance; filterability; demulsibility;
rust and corrosion inhibition; and viscosity in respect of its influence on lubricating
film thickness – which is critical for maximum service life of hydraulic components.
To be most efficient in its role as a power transfer device, hydraulic oil needs high
bulk modulus (high resistance to reduction in volume under pressure) and high
viscosity index (low rate of change in viscosity with temperature).
As an analogy, consider the tension on a vee belt. If it is out of adjustment, the belt
will slip. The result is a higher percentage of input power wasted to heat. This means
less power is available at the output to do useful work. In other words, the drive
becomes less efficient.
A similar situation can occur with hydraulic oil. Change in its bulk modulus and/or
viscosity can affect the efficiency with which power is transferred in the hydraulic
system.
The perfect hydraulic fluid for transmission of power would be infinitely stiff
(incompressible) and have a constant viscosity of around 25 centistokes regardless of
temperature – see exhibit 1. Such a fluid does not exist.
1
Hydraulics and Pneumatics, December 2009, pages 35-37.
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Exhibit 1. Temperature/viscosity diagram for the ‘perfect’ hydraulic fluid.
Viscosity flat-lines at 25 centistokes regardless of temperature.
The Benefit of Multigrade Oil
Bulk modulus is an inherent property of the base oil and can’t be improved with
additives. But viscosity index (VI) can be improved by using high VI base stocks such
as synthetics and/or by adding polymers called Viscosity Index Improvers to the
formulation.
Viscosity Index Improvers were first used to make multigrade engine oils in the
1940s. These days, this common and well-tested technology is used to make high VI
(multigrade) oils for other applications, including automotive transmission fluids and
manual transmission gear oils. However, the VI improvers used in oils for the above
applications are not typically shear stable when used in modern hydraulic systems.
But recent advances in VI improver technology means that mineral hydraulic oils with
a shear-stable viscosity index in the 150 to 200 range are now commercially
available.
The most common reason for using a high VI or multigrade hydraulic oil is to cover
wide differences in ambient temperatures between winter and summer, and thus
eliminate the need for seasonal oil changes.
But there is another case for considering the use of a high VI or multigrade hydraulic
oil.
Within the allowable extremes of viscosity required to maintain adequate lubricating
film thickness for hydraulic components, there’s a narrower viscosity range where
power losses are minimized, and therefore, power transfer is maximized.
By maintaining the oil’s viscosity in this optimum range, machine cycle times are
faster (productivity is increased) and power consumption (diesel or electricity) is
reduced.
So using a high VI or ‘multigrade’ oil means the hydraulic system will remain in its
power transmission “sweet spot” across a wider operating temperature range.
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You could think of this as similar to installing an automatic-tensioner on the vee belt
drive we talked about earlier – to maintain optimum power transfer conditions.
But based on simple cost/benefit analysis, if the cost to install the auto-tensioner was
$200, we wouldn’t spend this money unless we were satisfied we can recover this
investment - plus an acceptable return, through savings attributable to more efficient
power transfer and/or reduced maintenance costs.
Shear stable, multigrade hydraulic oil is more expensive than monograde, and so the
same approach should be applied when evaluating the cost and benefits of using a
high VI hydraulic oil. But unlike the relatively simple vee belt drive, savings accruing
from increased hydraulic machine performance can be more difficult to quantify.
But to give you some idea of the economic benefits possible, consider the following
results from a field trial conducted by a manufacturer of shear-stable VI improvers2.
In this trial, the performance of a 40 horsepower compact excavator was evaluated
using an all seasons 142 VI ‘baseline’ oil and compared to the performance of the
same machine using a 200 VI ‘test’ oil.
The test procedure was as follows:
Run baseline data with 142 VI oil
1. Start with new air filter and fuel filter.
2. Top off fuel to fill neck at start of test.
3. Trenching blade width to normal depth.
4. Dig trench for seven hours.
5. After seven hours, record fuel to refill.
6. Measure trench width, depth and length.
7. Repeat steps 2-6 with second operator.
8. After baseline established, change hydraulic oil and filter, run for 2 hours and
repeat oil and filter change with 200 VI oil (due to some dilution of the 200 VI oil
with the 142 VI baseline oil after changeover –the actual VI of the ‘test’ oil was less
than 200).
9. Repeat steps 2 through 7.
The higher VI test oil demonstrated the following advantages over the baseline fluid:
• 15.4% improvement in “Fuel Economy” - cubic yards of dirt moved per gallon
of fuel consumed.
• 14.3% improvement in “Productivity” - cubic yards of dirt moved per hour.
To assign a value to these performance gains, a spread sheet was developed to
calculate an owner’s variable costs over the 1000 hour drain interval recommended by
the excavator OEM. The following assumptions were made:
•
All seasons baseline oil cost $9 per gallon and the 200 VI test oil $18 per
gallon.
2
Gregg, D., Herzog, S.N., “Improving Fuel Economy and Productivity of Mobile Equipment through
Hydraulic Fluid Selection: A Case Study” NCFP Ι08 – 2.4, IFPE March 2008, Las Vegas, NV, USA
6
•
•
Labor and equipment rental cost of $75 per hour.
Diesel cost of $3.15 per gallon.
From extrapolating the results of the trial, it was determined that with the baseline oil,
the excavator could dig approximately 20,000 yards of trench in 1000 hours. And the
same amount of trench could be dug in 874 hours with the 200 VI test oil. No value
was assigned to the additional 126 hours the machine owner would have to undertake
additional work.
Based on the field test results and the assumptions stated above, replacing the 142 VI
all seasons oil with 200 VI oil would save the machine owner $10,000 every 1000
hour drain interval – see exhibit 2.
Exhibit 2. Cost/benefit analysis of changing to 200 VI shear-stable hydraulic oil
As exhibit 2 shows, while the fuel cost savings are meaningful, the greatest potential
benefit from switching to higher VI oil is likely to accrue from machine productivity
improvement.
As the results of this trial show, the potential economic gain from using a high VI or
multigrade oil go beyond the simple elimination of seasonal oil changes.
But the ultimate decision on whether to use a multigrade over a monograde involves
weighing cost against benefit, given the type of hydraulic machine, its duty cycle and
its temperature operating window.
The Problem with Multigrade Engine Oil
Engine oil can work satisfactorily as a hydraulic fluid. BUT, if a multigrade engine oil
is being used in the hydraulics specifically for its high viscosity index (VI) – which,
as explained above, means it has a lower rate of change in viscosity with change in
temperature - then it’s not the correct solution. And it’s all to do with the additives
used to increase Viscosity Index.
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Viscosity index improvers are relatively large molecules which, at low temperatures,
are "curled" into little balls and don't thicken the oil. At higher temperatures, they
"uncurl" into long chain molecules which give the oil greater viscosity.
The earliest VI improvers used to make multigrade engine oils in the 1940s were
basically melted rubber. And this worked well in engine oil.
VI improver technology has advanced a lot since then. But even today, their drawback
is because they are long and complex molecules, they are very susceptible to “shear
down” as the oil circulates. And in terms of shearing forces, a modern hydraulic
system is one of the most challenging for VI improvers.
Because VI improvers are an expensive additive, oil blenders formulate for the
application. So while it would be possible to formulate a multigrade engine oil which
would be shear stable when used in a hydraulic system, it would be overkill for the
engine application and therefore add unnecessary extra cost.
As a result, VI improved engine oils, automotive transmission fluids and manual
transmission gear oils are not typically shear stable when used in modern hydraulic
systems. This means if they are used in a hydraulic system, the VI improvers will
shear down and lose their ability to provide the necessary viscosity improvement,
which defeats the purpose of using them in the first place.
So if the temperature operating window of your hydraulic equipment dictates the use
of a multigrade oil – or you seek the performance advantages described earlier,
though it would be convenient, engine oil is probably not the answer. You should use
a multigrade oil formulated specifically for hydraulic systems.
The Controversy About Detergent and Zinc - and How
to Deal With It
Let’s consider the purpose of zinc and detergent additives - in engine oil firstly. Zinc
(ZDDP) is an anti-wear additive and also an anti-oxidant. Detergent additives disperse
insoluble matter such as soot and keep it in suspension. This prevents these particles
from being deposited on internal surfaces of, in this case, the engine.
The particles in suspension are either captured by the oil filter – if they are large
enough – or drained out when the oil is changed. In diesel engines, suspended
soot/sludge particles are what makes the oil turn black. And if an oil change is
deferred for long enough, they can cause ‘soot-polishing’ wear – to the valve train in
particular. Also, if the oil becomes saturated with soot particles and is unable to
suspend any more, soot and sludge starts getting deposited on the engine’s internal
surfaces.
Detergent oil also tends to emulsify (suspend and retain) small amounts of water,
rather than demulsifying or separating it out. So a large part of the debate about using
detergent or non-detergent oils in hydraulic systems revolves around the question: do
you want the hydraulic oil to demulsify water or emulsify water?
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Certainly, for industrial hydraulic applications, demulsification is the general rule.
And so most hydraulic oils are formulated to demulsify water – so it settles out in the
hydraulic tank.
Caterpillar takes the opposite view. Because there’s a possibility, in mobile hydraulic
equipment, that settled, free water may re-enter the system – due to machine
movement. So Cat reckons it’s better to keep water emulsified in the oil.
Rexroth appears to agree with Caterpillar. On page three of their technical document:
“Hydraulic fluids on a petroleum oil basis for axial piston units” (RE 90220/08.97)
Rexroth states:
“The following fluids are particularly suitable for mobile applications:
…HLP fluids with detergent properties…”
But you’re damned if you do and damned if you don’t. Because if enough water gets
emulsified in the oil, there’s a risk it can get turned into steam in highly loaded parts
of the system. Not good.
Of course the debate around this issue presupposes it’s OK to let water to get into the
hydraulic system and the oil to get wet. Which it isn’t – not if you’re serious about
maximum service life and reliability anyway.
Regardless, Caterpillar recommends their own hydraulic oil (HYDO) as the first
choice, OR diesel engine oil to API specification CD or better. But NOT ‘industrial’
hydraulic oil – because it doesn’t have a high enough zinc concentration, and as
already explained, it demulsifies water – which Cat doesn’t recommend.
Interestingly, Cat’s HYDO hydraulic oil has a zinc concentration of 1200 parts per
million (ppm). Diesel engine oil to API specification CG-4 and CH-4 have about 1400
ppm of zinc. Most ‘industrial’ hydraulic oils have zinc concentrations much lower
than these levels – in the 300 to 600 ppm range.
Now, the oil blenders will tell you when it comes to hydraulic oil, it’s not just the
quantity of zinc that’s important but also the quality. That is, the chemical
composition and therefore the stability of the zinc in the additive package is just as
important as how much of the stuff you plonk in. This being the case, why is
Caterpillar so concerned about high zinc concentration?
Well, at a technical conference I attended a couple of years back, an oil chemist who
works for a company which makes oil additives told me it’s in no small part because
Cat expects cross compartment contamination to occur. Meaning, they expect
hydraulic oil to end up in the engine.
Don’t try this at home, but apparently, if you put a hydraulic oil containing 300 to 600
ppm of zinc into a diesel engine, it’s likely to stop spinning. However, if you put a
hydraulic oil containing 1200 ppm of zinc into a diesel engine, it’ll cope. Cunning
strategy, huh?
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Keep in mind, zinc is an anti-wear additive, so this aspect of the discussion has
nothing to do with detergent additives and water emulsification or demulsification –
or even VI improver shear down, explained earlier.
Komatsu agrees with Caterpillar. While not nearly as forthcoming with its technical
literature, Komatsu recommends 10W engine oil to API specification CD or later for
the hydraulic compartment. In other words, a high-zinc, detergent oil.
It should be noted that SAE 10W falls between ISO viscosity grades 32 and 46 – or
about 37 centistokes at 40°C. This viscosity is too low for warm climates found in
southern North America, most of Australia and many other parts of the world.
Furthermore, Komatsu’s technical literature for their branded engine oil – 15W40 to
API specification CF-4, states that it is suitable for use in the hydraulic compartment.
Once again, this is consistent with Cat’s recommendation.
Of the ‘big three’ mobile hydraulic equipment manufacturer’s, Hitachi is the
dissenting voice. They recommend their own branded zinc-free hydraulic oil – Super
EX 46 HN. This is an ISO VG46 oil containing viscosity index improvers (VI = 125).
But even so, this viscosity is possibly on the low side for hot climates – depending on
the machine’s operating temperature, of course.
The data on Hitachi’s hydraulic oil is silent on whether it emulsifies or demulsifies
water. But perhaps not surprisingly, it gives zinc a bad rap – since its oil is zinc-free.
Zinc anti-wear additives are notoriously unstable in the presence of water and start
decomposing above around 90-100°C. Whereas zinc-free, organophosphate anti-wear
additives, such as Tricresyl phosphate (TCP) remain effective up to about 200°C.
So Hitachi’s hydraulic oil is certainly formulated to contend with hot-running and the
presence of water.
Who’s right and who’s wrong? Well, in considering this question, we first have to
answer a couple of others:
Is zinc (ZDDP) harmful to a hydraulic system? Zinc has proven to be reliable antiwear additive over a long period of time. It is affordable and effective. So it’s likely it
will continue to be a popular choice for the anti-wear package in hydraulic oils for
sometime to come. With the odd exception, for example, the presence of exotic metals
in the hydraulic system or excessive heat and water contamination, there’s not a lot
evidence to suggest zinc is harmful to a hydraulic system.
Are detergent additives harmful to a hydraulic system? This question is not quite so
clear cut. First, there’s the water emulsification / demulsification issue. And as I’ve
mentioned already, if the oil is kept dry – which it would be in a perfect world – this
becomes a non-issue - or a lesser one at least. While not based on hard evidence, my
take on this particular issue is it’s a ‘storm in a tea-cup’.
A properly maintained hydraulic system is not going to live or die based on whether
the oil being used is emulsifying or demulsifying small amounts of water.
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Furthermore, if a detergent oil is being used and the system is properly maintained,
the emulsifying properties of the oil should result in the oil being dried or changed
more often. No big deal.
But there’s another issue to consider in the use of detergent oils in a hydraulic system
- although it only applies to changing from a non-detergent oil to one containing
detergent additives. And it is: can a detergent oil disturb and redistribute existing
sludge deposits?
Switching back to engine oil for a moment, it is generally accepted that detergent oil
will not clean a dirty engine. But this presupposes the dirty engine has been operated
with detergent oil since new and therefore any deposits have occurred in spite of the
presence of detergent additives.
But what if an engine is operated with a non-detergent oil and then switched to a
detergent oil? Well, when detergent engine oil was first introduced in the 1960’s, the
engine repair business increased dramatically. Existing engines which had always
been operated with non-detergent oils were ill advisedly changed over to detergent
oil, which promptly attacked existing sludge deposits. Engine bearing surfaces were
flooded with oil containing high concentrations of newly suspended sludge particles –
to the engine’s detriment of course.
Based on this experience, it may not be wise to switch a hydraulic system that has
been happily running on a non-detergent hydraulic oil, to engine oil. I haven’t come
across a hydraulic system where this has happened, but the possibility that detergent
engine oil may disturb and suspend existing deposits - with potential for collateral
damage - must be considered.
So whose approach is right? Caterpillar / Komatsu or Hitachi? Well, there’s no
overwhelming body of evidence to suggest either camp is wrong. And if in fact
Hitachi’s hydraulic oil does emulsify water, then they’re all pretty much on the same
page - with the exception of their choice of anti-wear chemistry.
Why You Should Avoid Automatic Transmission Fluid
The use of automatic transmission fluid (ATF) as hydraulic oil has a somewhat
chequered history. I remember nearly 30 years ago when my father took delivery of a
brand new combine harvester – the first he’d owned with a hydrostatic transmission
for the ground drive - the label beside the transmission reservoir’s fill cap was clear:
‘Use Dexron II Only!’ (Dexron is GM’s ATF spec). And so we did.
This transmission didn’t give any trouble. But unless you’re a contractor, your
combine only gets used for about one month each year. So this wasn’t a particularly
arduous application. I don’t have much to do with agricultural machinery these days,
but I would be surprised if the hydrostatic transmissions in the current generation of
combine harvesters use ATF.
Similarly, going back perhaps a decade, maybe two, at least one of the major skidsteer loader manufacturers used ATF exclusively in the hydrostatic transmission for
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the ground drive. And unlike a combine harvester, the transmission on a skid steer
loader gets some real hammer. Anyway, they don’t anymore – presumably for
reliability reasons.
But despite the official move away from ATF in this application, there are more than
a few long-time, skid-steer owner/operators who still reckon ATF is the way to go.
Logic would suggest these are the owners who have not experienced any problems
while using it.
Fact is of course, ATF is formulated primarily for hydrokinetic or hydrodynamic use
and not hydrostatic (hydraulic) use – although the actuation of clutches and brakes in
the transmission is hydrostatic. ATFs are complex fluids which contain as many as 15
additives. Here’s an excerpt of what Lubrizol (an additive manufacturer) has to say
about them:
“ATF is the most complex of all lubricating fluids. Not only does it have to reduce
friction to prevent wear like all lubricants, but it also has to allow a certain level of
friction so clutch materials can engage. Since most OEMs use proprietary frictional
materials, virtually every ATF is OEM-specific. In some cases, they're transmissionspecific.”
As this indicates, the main fluid property which distinguishes ATF from a
conventional hydraulic fluid is its friction characteristics. For clutch operation to be
smooth and chatter-free, the value of the fluid’s static and dynamic coefficient of
friction has to be almost the same. This is achieved using so-called friction modifier
additives.
ATFs also have a very high viscosity index (VI). This means they flow well at low
temperatures but still maintain adequate viscosity at high operating temperatures.
Looking at the data sheet for Conoco Phillips Super ATF, it has a viscosity of 35.4
centistokes at 40°C and 7.6 centistokes at 100°C. This equates to viscosity index of
193. Most monograde mineral hydraulic oils have a viscosity index of around 100.
High VI is great to have – for reasons already discussed, but it’s important to
understand that it is achieved with additives. And like most other additives, VI
improvers can be depleted/damaged. Modern high-pressure hydraulic systems are one
of the most severe applications for VI improvers. And while ‘shear-stable’ VI
improved (multigrade) hydraulic fluids are available these days, as already stated, the
VI improvers used in engine oil and ATF are not generally considered ‘shear stable’
when used in hydraulic systems. The reason is cost. The more shear stable the VI
improver is, the more expensive it is.
This means it’s possible for the multigrade properties of ATF to be lost quite quickly
when the fluid is used in a hydraulic system. Obviously this is not good for reliability
if the temperature operating window of the hydraulic system needs a high VI fluid to
maintain adequate lubrication.
The other thing which stands out about the ‘Super ATF’ spec is it only has 30 parts
per million of zinc (ZDDP). This indicates it relies on a zinc-free organophosphate
such as tricresylphosphate (TCP) for its anti-wear performance. As already explained,
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there are hydraulic oils available these days featuring zinc-free, anti-wear additives.
But ZDDP has a very long and distinguished track record as an anti-wear additive in
hydraulic oil. In short, it’s cheap and effective.
With all this said, the Conoco Phillips Super ATF specification states:
“Super ATF may be used in industrial and mobile hydraulic systems operating over a
wide temperature range.”
So its maker appears happy for it to be used as a multigrade hydraulic fluid. But for
reasons already explained, I’d definitely be asking some challenging questions about
the shear stability of its VI improvers before I’d be relying on its multigrade
properties in a hydraulic system of mine.
And what do hydraulic component manufacturers have to say about the use of ATF?
Well on page three of their technical document: “Hydraulic fluids on a petroleum oil
basis for axial piston units” (RE 90220/03.88 – March 1988) Bosch Rexroth states:
The following fluids are particularly suitable for mobile applications:
…ATF fluids.
The latest version of this document I can find is dated August 1997 and the above
section of text remains unchanged. Taken at face value, you could certainly be
forgiven for taking this as an endorsement from Bosch Rexroth to use ATF in a
mobile hydraulics application.
HOWEVER, I think this statement from Lubrizol, mentioned earlier, is telling: “ATF
is the most complex of all lubricating fluids…” Well, I do NOT want to use a
complex fluid in my hydraulics – unless there is a compelling reason for doing so.
If I can, I want to use a simple (and reliable) fluid, not a complex one. This will
usually translate to a monograde, zinc-based, anti-wear hydraulic oil – if the system’s
temperature operating window allows, or a shear-stable, multigrade hydraulic oil if it
doesn’t.
The Truth about ‘Biodegradable’ Oils
This is quite a complex and a somewhat gray area. In no small part because there is no
universally accepted definition of ‘biodegradability’ when applied to oil. We know
what biodegradability is supposed to mean: substances which are digested or
consumed by micro-organisms present in water, air or soil - without harm to
vegetation or organisms, or toxic residue.
However, the US EPA has defined two classifications for biodegradation: readily
biodegradable and inherently biodegradable.
‘Readily biodegradable’ means laboratory testing indicates the substance will undergo
rapid and ultimate biodegradation in aerobic aquatic environments.
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‘Inherently biodegradable’ means the substance can degrade under favourable
conditions – such as in oxygen-rich environments. In other words, the substance has
the potential to biodegrade. This sounds like a reasonable definition until you consider
many synthetic and mineral oils can be classified as ‘inherently biodegradable’. In
fact, most mineral oils are NOT non-biodegradable, but can degrade under favorable
aerobic conditions!
Notwithstanding the somewhat rubbery definition of ‘biodegradable’, there are four
classifications for biodegradable oils according to ISO 15380:
HETG: triglycerides (vegetable oils), water soluble
HEES: synthetic esters, non-water soluble
HEPG: polyglycols, water soluble
HEPR: polyalphaolefins and related hydrocarbons
Let’s take a look at each of these in a little more detail.
HETG. Triglycerides are natural esters extracted from oil seeds such as rape or
sunflower. These vegetable oils are used as the base stock for HETG fluids (and as a
raw material for synthetic esters). A lot of research and development is being done on
these oils, but at the moment, they have poor thermal and oxidative stability compared
with mineral oil. So even though triglycerides have high natural lubricity, which
translates into good anti-wear performance, they are not currently considered suitable
for high-pressure, high-temperature hydraulic systems. HETG fluids also have poor
hydrolytic stability (chemical stability in the presence of water) so they must be kept
dry. Although this latter problem is not unique to HETGs as you’ll see in a moment.
HEES. Synthetic esters are group of substances with a wide variation in chemical
structure. Esters are manufactured by altering alcohols and acids. Depending on their
chemical composition, their performance characteristics equal or exceed mineral oils.
This makes most HEES fluids suitable for use in high-pressure, high-performance
hydraulic systems. Like HETG fluids, HEES fluids must be kept dry because they
have poor hydrolytic stability. But the hydrolytic stability of synthetic ester is
somewhat better than natural ester.
HEPG. Polyglycols can be formulated in a number of chemical variations. And like
HEES fluids, their performance characteristics equal or exceed mineral oils. But
unlike HETG and HEES fluids, HEPGs are not miscible with mineral oil and are not
compatible with many common seal materials. This limits their acceptance, especially
when changing an existing system from standard mineral oil to a biodegradable. On
the other hand, HEPGs have very good hydrolytic stability compared with HETG and
HEES fluids, and have the ability to absorb large amounts of water and still maintain
lubricity. For this reason, HEPG fluids are commonly used in environmentally
sensitive applications where water ingression is unavoidable, such as canal locks and
offshore, subsea applications.
HEPR. These fluids are synthetic or highly refined hydrocarbons - whose degree of
biodegradability varies with chemical composition. Those classified as HEPR are
more rapidly biodegradable than standard mineral oils, but significantly less so than
most synthetic and natural esters. Polyalphaolefins (PAOs), which are included in this
14
classification, are probably the most common type of synthetic base oil used today.
No surprises then that the performance characteristics of these fluids equal or exceed
that of mineral hydraulic oils.
Generally speaking, the more biodegradable the hydraulic fluid, the higher
maintenance it is. And if a high-maintenance fluid isn't looked after, the eventual
result is a maintenance disaster - read: oil failure followed by component or system
failure.
There’s no way to pre-program these biodegradable oils to only degrade AFTER
they've managed to find their way out of the hydraulic system. Given the right
(wrong) conditions, they're just as happy to degrade while they're still in the hydraulic
system.
So unless the equipment user has a proper oil analysis program in place - and by this I
mean she knows what to look out for and so is able to specify an appropriate test slate,
then changing to biodegradable oil is a disaster waiting to happen. You can get away
with a lot of things with a mineral oil that you won't with a biodegradable. They are
NOT "fill and forget".
And how 'green' are these oils anyway? In the Swedish county of Goetheborg, they
have removed regulations against mineral hydraulic fluids. It was found that
breakdowns and leaks were ten fold with biodegradable fluid - resulting in an overall
increase in oil spill!
And biodegradable hydraulic fluid is not harmless to the environment, as this extract
from the U.S. EPA (Clean Water Act) explains:
"Like petroleum-based oils, non-petroleum oils can have both immediate and long-term
adverse effects on the environment and can be dangerous or even deadly to wildlife. For
example, non-petroleum oils can deplete available oxygen needed by aquatic organisms, foul
aquatic biota, and coat the fur and/or feathers of wildlife. For example, when a bird's plumage
is coated with non-petroleum oil, their feathers lose their insulating properties, placing them at
risk of freezing to death.
Birds that are covered with non-petroleum oils can also smother embryos through the transfer
of non-petroleum oil from the parents' plumage to the eggs. Birds and wildlife can ingest oil
directly and may continue to ingest the oil as they eat if the source of their food consists of
fish, shellfish, or vegetation that also are contaminated with non-petroleum oils. Other
adverse effects of spilled non-petroleum oil on bird and wildlife include drowning, mortality by
predation, dehydration, starvation, and/or suffocation."
So despite the spin, biodegradable is not a synonym for environmentally friendly. And
if machine reliability is compromised in any way - and it will be without due care and
attention, then the environment may actually be worse off with non-mineral than with
mineral oil.
15
Hydraulic Oil Type Selection Summary
1. Getting the viscosity right is THE most important aspect of hydraulic oil
selection. This is covered in detail in the next section and in the accompanying
video.
2. As a general rule, use the least complex type of oil suitable for the job. In the
majority of applications, this means a monograde, mineral hydraulic oil (of the
correct viscosity – see video) with a zinc-based, anti-wear additive package.
3. If your application calls for a multigrade oil – due to a wide temperature
operating window, and/or you seek the performance and productivity benefits
a high VI oil can deliver, then be sure to select a multigrade oil which is
‘shear-stable’ when used in a hydraulic system.
4. Be wary of and question any recommendation to use ATF or multigrade
engine oil in a hydraulic system, specifically for their high VI properties.
The VI improvers used in these oils are not generally shear-stable when used
in modern, high pressure hydraulic systems.
5. That said, if you have a hydraulic application which has performed
satisfactorily over a significant period of time using either ATF or multigrade
engine oil, then there is probably no compelling reason to change (if it ain’t
broke, don’t fix it).
6. Detergent oils, such as monograde (or multigrade) engine oil are not harmful
to the hydraulic system. HOWEVER, switching a system which has been
operating with a non-detergent oil, to a detergent oil, may result in surface
deposits being disturbed and suspended, with the possibility of collateral
damage.
7. If your application does mandate the use of a ‘special’ purpose oil, for
example fire-resistant or biodegradable, be aware that these fluids require
‘special’ attention to ensure acceptable service life and performance – so only
use them when necessary.
8. For commercial reasons relating to warranty etc, you should never ignore the
machine manufacturer's oil recommendations. But understand OEM’s are in
the habit of making blanket recommendations, which do not always take into
consideration the wide variations in machine operating environments. So
rather than blindly following (or ignoring) the OEM’s oil recommendations
use the knowledge you’ve gained from this report to question them – with the
objective of getting the correct recommendation. And don’t be afraid to
interrogate the technical representative for the brand of oil you buy, either.
16
Video: How to Select the Correct Oil Viscosity
Choosing the correct oil viscosity is THE most important aspect of hydraulic oil
selection because, as explained at the beginning of this report, it affects both the
performance of the hydraulic machine and the service life of the components in the
system.
In the accompanying video, I use the following application example to show and
explain the process for selecting the right oil viscosity grade:
“I work for a Singapore-based company which manufactures hydraulic winches
and cranes for the marine and offshore industries. On one of our winch
hydraulic systems, we use a Bosch Rexroth A10VO140 pump and Hagglunds
CA-50-50 hydraulic motor. Our continuous system operating temperature is 60
degrees C. How do I work out the correct oil viscosity for this application?”
Important notes about the video:
•
The video plays in Windows Media Player 9 or higher. If this program is not
already installed on your computer, the current version can be downloaded
from: http://www.microsoft.com/windows/windowsmedia/
•
If you are a Mac user, you can watch the video by downloading the Flip4Mac
WMV player for Quicktime: http://www.mireth.com/wt/mxmewt545.html
A transcript of the video dialogue is included in the Appendix accompanying
this report. So if for some reason you have trouble understanding the dialogue
on the video, you can refer to this transcript for clarification.
•
•
I use Shell Tellus oil in the example discussed in the video. This is NOT an
endorsement for Shell oil – it just happened to be the first oil data sheet I laid
my hands on.
•
In the video I use an Excel template to calculate oil viscosity at any
temperature. This Excel template can be downloaded from:
http://www.kittiwake.com/downloads.htm then look for the link towards the
bottom of the page which says: “Download Viscosity Calculation Software”.
Once downloaded and opened in Excel, look for the tab which says:
“Viscosity Any Temperature”.
•
You do NOT have to use the above viscosity calculator when doing this
exercise for yourself. The same result can be achieved by using the viscositytemperature diagram specific to the brand and type (monograde, multigrade,
synthetic, etc.) of oil you are planning to use. The viscosity-temperature
diagram for Shell Tellus oil discussed in the video is included in the Appendix
for your reference.
With the above points in mind, stop reading here and watch the video.
17
How to Monitor the Condition of the Oil
A brief introduction to oil analysis
There are generally only two conditions which necessitate an oil change: oxidative
degradation and additive depletion. And the only way to know if the oil’s life has
expired based on either of these conditions is to do regular oil analysis.
A hydraulic oil’s oxidative degradation is determined by an absolute measure of its
total acid concentration (AN test). When oxygen combines with hydrocarbon
molecules a chain reaction occurs, which results in the formation of organic acids.
These substances darken the oil, increase viscosity, reduce foaming resistance and air
release, and form varnish and sludge. In other words, the oil becomes unserviceable.
The total acid number (AN) test result is expressed by the volume of the alkaline,
potassium hydroxide (KOH) in milligrams (mg), required to neutralize the acidic
components contained in one gram (gm) of used oil.
Due to their additive composition, new, zinc-based, mineral hydraulic oils can have a
rather high initial AN of 1 to 1.5 mg KOH/gm. This number initially decreases as
additives deplete. But as the oil starts to age and oxidize, the formation of acidic byproducts reverses this trend and causes AN to rise.
For mineral hydraulic oils, AN of 2.0 mg KOH/gm is the typical trigger value for an
oil change. But for synthetic esters and some triglycerides (vegetable based oils) AN
can be as high as 5.0 mg KOH/gm before an oil change is required.
If you don’t know the AN value which should trigger an oil change for each of the
hydraulic oils you are using – contact your oil supplier to find out. And if you don’t
know the current AN value of the hydraulic oils you have in service, now is probably
a good time to find out.
Assessing additive depletion, involves comparing an elemental analysis of the used
oil to a baseline of identical new oil. For example, zinc (ZDDP) is both an anti-wear
and anti-oxidant additive, so it gets consumed. The absolute concentration of zinc on
an oil analysis report doesn’t mean much until it is compared to the concentration for
the same oil when new. In other words, it’s the additive concentration relative to new
oil which is important.
But additive concentrations are among the most difficult parameters to measure using
oil analysis. The primary tests used are Elemental Spectroscopy and Fourier
Transform Infra Red Spectroscopy (FTIR).
Elemental spectroscopy is done by exposing the oil sample to an arcing electrode or a
plasma torch. The extreme heat vaporizes the atoms causing them to emit light. And
each atomic element emits light at a known frequency. The instrument quantifies the
amount of light generated in each frequency and converts it into a concentration for
each element – zinc, phosphorus, silicon, etc.
18
Elemental spectroscopy has two main limitations when used to monitor additive
concentrations. Firstly, additives can be spent but their constituent elements remain –
transformed into other molecules, resulting in no change in elemental concentration.
And secondly, many additives are comprised of elements which can also be wear
metals or contaminants. For example, dirt and some anti-foaming additives will show
up as silicon.
FTIR differs from elemental spectroscopy in that it evaluates the presence of
molecules, not atoms. In this test, a known thickness of oil is applied to instrument’s
test cell. Infrared energy is then passed through the oil sample. The various additives
and contaminants in the oil absorb infrared energy at particular frequencies. For
analysis of contaminants, additives and degradation by products, the frequency
spectrum of the used oil is compared to the baseline of identical new oil.
FTIR is limited by various interferences that can occur and its poor ability to quantify
results. For this reason, results from FTIR aren’t always sufficiently conclusive on
their own.
The acid number (AN) test can provide additional insight. Anti-wear and some antirust additives produce an elevated acid number in new oil. As these additives deplete
the acid number usually decreases, followed by an increase once base oil oxidation
commences.
And sometimes with respect to additive depletion, testing of the performance
characteristic the additive provides - rather than the additive itself - may be
necessary. For example, the rotating pressure vessel oxygen test (RPVOT) can
reliably estimate the condition of anti-oxidation additives in the oil.
19
Appendix
The following are included in the file Appendix.pdf attached:
•
•
•
•
Viscosity data for Bosch Rexroth A10VO pumps (referred to in the video).
Viscosity data for Hagglunds Compact motors (referred to in the video).
Temperature-Viscosity diagram for Shell Tellus oils (for determining viscosity
at any temperature – an alternative to using the viscosity calculator used in the
video).
Video Transcript (in case you have trouble understanding the dialogue on the
video)
Further Reading
Hydraulic Breakdown Prevention Blueprint by Brendan Casey
Available: www.HydraulicSupermarket.com/blueprint
20

How to Select the Correct Hydraulic Oil for Your Machine…

Transcription

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