Technical Topic
Hydraulic System Care & Maintenance Background Hydraulic machines power the moving parts o many kinds industrial machines by applying the orce o a uid under pressure. Some systems are very small, simple and straight-orward to very large, high pressure systems with a complex array o servo valves and pumps. No matter the size or complexity, complexity, proper maintenance o BOTH the system and the hydraulic oil is crucial in maximizing uptime and reducing repair costs.
Hydraulic Fluid Care Hydraulic uids are the lie blood o the hydraulic system. The hydraulic uid transmits pressure and energy, energy, seals close-clearance parts against leakage, minimizes wear and riction, removes heat, ushes away dirt and wear particles, and protects suraces against rusting. Conventional petroleum (mineral) oils are normally used in hydraulic systems, but fre-resistant, synthetic, and biodegradable uids are used in other situations. There Ther e are are our our key key objective objectives s that are essen essential tial to to gaining gaining optimu optimum m service lie o hydraulic uids:
extreme localized heating o the hydraulic uid. This results in
Control the Temperature — Heat develops in the uid as it is orced
nitrogen fxation that, when combined with oil oxidation, can orm
through the pumps, motor tubing, and relie valves. In conventional
deposits which will plug oil flters and cause servo valves to stick.
systems, excessive temperatures will oxidize the oil and can lead to varnish and sludge deposits in the system. Conversely Conversely,, running
Keep Systems Clean — Even new systems may be
the temperature too low will allow condensation in the reservoir and
contaminated and should be cleaned beore use. Prevent
increase the likelihood o pump cavitation.
contaminants such as dirt, water, cutting uids, and metal particles rom entering the system around the reservoir cover, cover,
Typica T ypicall industria industriall hydrauli hydraulic c system system tempera temperature tures s oten oten range range betwee between n
openings or suction and drain lines, through breather fll openings,
110 to 150ºF. Mobil hydraulic system temperatures can operate up to
past piston rod packing, and through leaks in pump suction lines.
250ºF.. Selection o the proper grade o hydraulic oil is critical to ensure 250ºF cold start, high temperature protection and to obtain t he optimum
Keep the Fluid Clean — Keeping hydraulic uids clean
system efciency. efciency. Keep systems which operate on a water based uid
begins with good storage and handling practices. To prevent
below 140ºF to prevent the water rom evaporating.
contamination beore use, store new uid in a protected area and dispense it in clean, DEDICATED containers. Clean the fll cap
The deposits deposits cause caused d by oil oil degrada degradation tion can plug plug valves valves and suction suction
beore removing it to add hydraulic uid. On critical NC systems,
screens and cause high-tolerance servo valves to seize and/or operate
use quick disconnect hoses and flter all oil added to the reservoir
sluggishly.. To allow heat to radiate rom the system, keep the outside sluggishly
through a 5 micron flter.
o the reservoir clean and the surrounding area clear o obstructions. Make sure the oil cooler is unctioning properly and keep air-cooled ed
Full-ow flters designed into the system keep the uid clean while
radiators ree o dirt. Normal temperature drop or most oil coolers
in service. These flters are oten orgotten and go into bypass
is 5 to 10ºF 10ºF.. Reservoirs should be flled to the proper level to allow
mode, thus allowing dirty oil to circulate. Inspect uid flters
enough uid residence time or the heat to dissipate and to shed
requently and change or clean them beore they go into bypass
water and dirt.
mode. Portable flters will supplement permanently installed flters and should be constantly rotated rom system to system
In modern equipment using servo valves, oil degradation can be
regardless i you think the system requires fltering or not. not .
even more damaging. High pressure (up to 4000 psi), high
Systems should be fltered long enough to pass the total volume
temperatures, and small reservoirs stress the uid. With minimal
o oil through the flter at least 10 times. Portable flters should be
residence time and high pressures, entrained air bubbles can cause
used when transerring new oil rom drums or storage t ank to a
gargling marbles.
system — especially or NC machines. Keep an Oil Analysis Program — OEM’s generally speciy that
7. Listen to the pump or the signs o cavitation. Cavitation is
system hydraulic oil be drained annually. However, with an eective
slightly more complicated than aeration, but bares some
oil analysis program, you can saely increase that interval while at
similarities. Cavitation occurs when air is released rom the
the same time provide yoursel with an “early warning” o possible
hydraulic oil during momentary depressurization at the pump
mechanical problems.
suction and then imploded onto metal suraces upon discharge. These implosions are extremely destructive to pump suraces.
At minimum, check your critical and large volume hydraulic systems at
A cavitating pump will emit a high-pitched whine or scream.
least annually by oil analysis. Semi-annual or even quarterly sampling
Causes o cavitation are the same as those o aeration with
intervals may be required or extremely critical machines. Consult your
the exception o suction side air leaks. How do you discern
ExxonMobil lubrication engineer and machine OEM or the best sampling
aeration rom cavitation? One way is to install a vacuum gage
interval and the parameters you should be testing. Also, please reer to
on the suction side and make sure the pressure is equal too
our Technical Inormation Sheet titled “Oil Analysis — The Basics” or
or greater than that prescribed by the pump manuacturer.
a more in-depth discussion on this topic.
Foaming in the reservoir is usually the telltale sign o aeration. 8. Inspect a small sample o uid or color, signs o
Hydraulic System Care
contamination and odor. Keep in mind that visual inspection
Hydraulic system maintenance is just as important, and directly related
is limited in that it will only detect signs o excess contamination.
to, hydraulic oil maintenance. All the fltering and analysis done on a
9. Scan electrically controlled servo valves with an inrared
hydraulic oil would be meaningless and utile i the system itsel is in
thermometer. High valve and solenoid temperatures
a shambles.
(over 150ºF) usually indicate the valve is sticking. 10. Scan the electric drive motor with or housing hot spots and rotor bearing temperatures using an inrared thermometer.
A 10 Point Check — A lubrication technician or operator responsible or hydraulic system maintenance should, at minimum, perorm the ollowing 10 point checklist as part o a routine weekly “quick scan”
Fluid Change-Out Recommendations — These are the proper
o a hydraulic system:
steps to ollow when changing the hydraulic uid in a system.
1. Check uid levels. Add oil (i needed) via portable fltration
1.
contaminants in suspension.
(i available). DO NOT MIX OILS! Use the same oil brand and viscosity grade that is being used in the system.
2.
3. Check flter indicators and/or pressure dierential gages.
3.
Mop, siphon, or pump out oil let in the reservoir.
4.
Wipe reservoir clean with lint ree rags and remove rust and ree paint.
4. Visually inspect all system hoses, pipes, pipe connections or leaks and rays. Hydraulic uid leakage is a common problem or industrial
5. 6.
Refll the system with new uid making sure to vent high points.
reduce the system capacity enough to overheat the system. 5. Check system temperature via built-in thermometers or hand-held
Replace or clean flter elements and strainers and clean flter housings.
systems. Excessive leakage is an environmental and saety hazard, increases waste streams and oil consumption, and, i ignored, can
Empty uid rom cylinders, accumulators and lines that might not drain properly.
2. Inspect breather caps, breather flters and fll screens — DO NOT punch holes in screens in order to expedite adding oil.
**Drain the system while the uid is hot to keep
7.
Restart and check system or proper operation.
inrared detectors. Normal temperature range or most systems is 110-140ºF. I temperatures are high, check cooler operation and
**For systems that exhibit high deposit, sludge and/or varnish
relie valve settings.
formation: a petroleum based cleaner (such as Mobil System
6. Visually inspect the inside o the reservoir or signs o aeration (via
Cleaner) may be required. Follow manufacturers recommendations.
the fll hole using a ashlight). Aeration is a condition in which discrete bubbles o air are carried along in the stream o oil as it
Safety Precautions
enters the pump. Visual signs o aeration in the reservoir are
Hydraulic systems operate under very high pressures. Shut the
generally oaming and/or little whirlpools taking small gulps o air into
system down and relieve system pressure beore opening any
the suction strainer. Causes o aeration include: low uid levels; air
part o the system that is under pressure. Do not allow spray rom
leaks in the suction line; low uid temperature; uid is too viscous to
any high pressure leak to contact any part o the body, as serious
release air or maintain suction at the pump; or aulty shat seals.
injection injuries may result. Pumps, valves and motor may become
When air leaks are suspected on the suction line, smothering these
hot; be cautious o incidental contact between bare skin and hot
points with oil will usually pinpoint the leaks by creating a marked
suraces. Keep hands and clothing away rom moving parts o
change in pump noise. A pump ingesting air sounds as i it were
the system.
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