Does your fluid provide enough protection?
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Figure 1. Mining equipment is susceptible to contamination from dirt and dust and from water condensation that results from equipment repeatedly warming up during operation and cooling down when shut off. |
You’ve spent a lot of time and effort designing and building your hydraulic system; now you’re ready for production. After the equipment with its brand new hydraulic system has been shipped, your colleagues in accounting can calculate your profit.
But what will happen when (not if) contaminants enter the hydraulic system? Will contamination degrade fluid performance? Will it cause equipment malfunctions and downtime? If so, you can bet it will eventually jeopardize future orders from customers. Of course, adequate filtration will eliminate or reduce many contamination related failures, but your choice of hydraulic fluid help can help mitigate the problems associated with contamination.
Hydraulic systems have changed dramatically in the last 20 years. Generally, they are smaller, contain less hydraulic oil, and require the same or more output as the systems of yesteryear. The smaller size places a greater burden on the hydraulic oil to help protect the equipment and components.
One thing that hasn’t changed over time, however, is the fact that hydraulic oil can become contaminated in a variety of ways and from a variety of sources. Contaminants in a hydraulic system can be:
- present when the system is new,
- generated by the system during operation, or
- be introduced to the system during operation or even during idle periods.
Examples of contaminants that could be present in a new system include metal chips or dust left from manufacturing processes. Contaminants generated by the system include wear debris and thermal degradation by-products from heat, such as sludge and varnish. Contaminants, including dirt, water, and even other fluids, can enter the system through reservoir breathers, rod and shaft seals, and whenever a hydraulic component is replaced.
You can also classify contaminants by type, such as dirt and wear debris, water, air, by-products of heat, and other fluids such as gear oils, cleaners, and motor oils. Systems are designed to have filters in place to remove many of these contaminants. But what role does the hydraulic fluid itself play with respect to these contaminants?
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Figure 2. Excessive heat can cause oxidation of hydraulic oil, so measures should be taken to keep hydraulic lines and components away from heat sources, such as in steel mills. |
Dirt and wear debris
Eaton Vickers has stated that 90%
of failures due to contamination come
from abrasive wear, which is caused
by metal-to-metal contact between
components. The result is that small
particles of the components break
off and infiltrate the hydraulic fluid.
In turn, they can cause wear of other
components, clog filters, and chemically
react with the fluid. Vane and
gear pumps especially need protection
against abrasive wear. Hydraulic
fluids help prevent abrasive wear
by using an effective anti-wear additive.
Zinc dialkyl dithiophosphate
(ZDDP) is typically the anti-wear additive
used. ZDDP is both relatively
inexpensive and very effective at reducing
mild wear by preventing metal
surfaces from contacting each other.
The result is reduced abrasive wear.
Over time, anti-wear additives can
become less effective, therefore it is
important to use a fluid that exhibits
wear protection over an extended time
period. These fluids are said to have
good durability, meaning that their
performance does not diminish over
extended periods of time. The timing
depends on system operating conditions
such as temperatures, pressures, and the operating environment.
Water as a contaminant
It is not uncommon for water to
enter a hydraulic system through access
plates in reservoirs or simply
condensed from ambient air drawn
into the reservoir. Because water can
cause poor lubrication, hinder filtration,
and potentially form rust, water
content should be monitored closely
by analyzing the oil. Depending on
the application, water content of more
than 1000 ppm (0.1%) generally is
considered excessive.
Hydraulic fluids are designed to separate from water (demulsify) quickly so it can be drained from the bottom of a reservoir. Some hydraulic fluid additives containing some of the less stable ZDDPs also can react with water to form acids that can corrode yellow metals, such as copper and brass.
When a fluid does not react with water, it is hydrolytically stable. It is important to use a fluid with high demulsibility and hydrolytic stability, referred to on fluid suppliers’ technical data sheets as hydrolytic stability — ASTM D 2619 and water separation (or demulsibility) ASTM D 1401.
Rust in a hydraulic system often can be tracked back to water ingression. The interaction of iron, water, and oxygen forms rust. Rust can be prevented by using a fluid containing rust inhibitors built into the additive system.
Water also can cause problems with filtration. Today, many systems are designed to use very fine filters, 3 μm or less, to help remove wear debris and other contaminants. In the development of hydraulic fluids, filtration is measured to make sure that the fluid can be filtered, both in its original condition and as contaminated with water. Generally, fluids will filter much more slowly when contaminated with water. Some OEMs have set specifications to ensure that fluids filter well both in their original condition and when contaminated with water.
Air as a contaminant
Air contains oxygen, and when it is
mixed with oil at high temperatures,
the oxygen converts oil molecules into
acids — a process known as oxidation.
These acids can thicken the oil, which
will decrease pump efficiency. The
thickened oil may also result in cavitation,
which can cause catastrophic
failure. In some cases, acid buildup
forms deposits that can block filters
and strainers. In general, an oil’s oxidationrate doubles with every 10° C
increase in temperature.
To combat oxidation, good quality hydraulic oils are formulated with antioxidants, which interfere with the oxidation process (air reacting with the oil molecules). As a result, the fluid lasts longer and is less likely to increase in viscosity.
Air in hydraulic oil also can lead to foam-related problems. Foam can cause pump cavitation and decrease lubricity, which shortens component life. Foam carried by the fluid will deteriorate system performance and usually can be prevented by eliminating air leaks. Surface foam can be eliminated with proper reservoir design or by using a defoaming additive.
Another general problem related to air contamination is entrained air. Entrained air is made up of bubbles suspended in the fluid. It can be introduced during release of dissolved air within the fluid when pressure is decreased, from leaks on the suction side of the pump, from splashing in the reservoir, or from contamination Entrained air can result in spongy controls, cavitation, noise, and loss of horsepower. The best prevention for entrained air is a fluid formulation designed with proper additive and base oil selection.
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Figure 3. Overhead hydraulic cranes used for ship loading are prone to water contamination that may cause corrosion. |
Varnish
Hydraulic systems often operate at
elevated temperatures. For mineral
oil-based hydraulic
systems, temperatures
from
140° to 180°
F are considered
normal.
If the oil temperature
goes
higher, oxidation
rate increases
and oil
life decreases.
The problem
is exacerbated
by the use of
more powerful
systems with
smaller reservoirs, which have become
a trend in the design of many of
today’s hydraulic systems. Over time,
the additives and oil can degrade to
form sludge and varnish.
Sludge and varnish are by-products of oxidation and thermal degradation of the fluids that are meant to protect expensive hydraulic systems. Varnish is a dark, sticky deposit that adheres to metal parts. It can increase friction of moving parts to cause erratic operation, especially in valves, or prevent valves from shifting altogether. Sludge is a thick deposit that can accumulate through a hydraulic system. It can also cause erratic operation and often clogs filters, necessitating premature element replacement. In extreme cases, sludge and varnish can collect in orifices and narrow passageways and block them.
These problems prove costly not only in the expenses associated with labor and component replacement, but with productivity lost due to down time. Furthermore, the cost of replacing the degraded fluid with fresh fluid, disposal costs, and possibly having to flush the system can also be substantial.
So it is best to keep varnish from forming in the first place. To do that, a combination of carefully selected additives can provide all the traditional benefits expected in a properly formulated hydraulic oil. They help prevent varnish from forming on surfaces, thus keeping the system clean.
Other contaminants
Other contaminants, such as gear
oils, motor oils, and even cleaners,
can contaminate hydraulic oils. This
often happens if poor transfer methods
are used to fill systems or if fluids
or equipment are not labeled correctly.
The result can vary depending on the
contaminant.
Often, viscosity is an issue if a different viscosity grade of another fluid is added. Viscosity increase can result in sluggish pump performance. The viscosity can be too low if a low viscosity contaminant is added by mistake. If contaminated with motor oil, water separation properties can be compromised, leading to potential wear and/or rust issues.
Hydraulic oil contamination is a hot topic with hydraulic systems. The impact on the system varies depending on the type of contaminant. Effective filtration systems help remove particulates in order to minimize performance- related issues. However, hydraulic oil additive chemistry can also be effective in reducing the effect of potential contaminants. Overall, this helps to extend the life of the fluid and boost reliability of the equipment.
For more information, contact the authors at robert.profilet@lubrizol.com or dave.oesterle@lubrizol.com or visit www.lubrizol.com.
