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    Maintenance for Hydraulic Systems

    Routine maintenance on your hydraulic system can alleviate performance problems and downtime, prolong hydraulic component life and reduce operating costs.

    We’re confident that we’ll be able to help if things do go wrong, but we’d rather help you get things right in the first place; following the six simple steps and routines below is time and money well spent.

    1.  Maintain the temperature and viscosity of hydraulic fluids within their optimum limits.

    You need to know the appropriate (minimum and maximum) fluid operating temperature and viscosity ranges for the ambient environment in which your hydraulic system is operating and then select a hydraulic fluid with a suitable viscosity grade and additives.

    2.  Keep your hydraulic fluid clean.

    Contamination is one of the most frequent causes of hydraulic equipment failure or malfunction. Selecting the right hydraulic filter is absolutely vital to ensure that particle contaminants are removed from the hydraulic fluid before components are jammed or damaged through abrasive wear. Remember that air is a contaminant too!

    3.  Maintain hydraulic system settings…

    … according to manufacturers’ specifications and build this into your own maintenance schedule as a minimum.

    4.  Follow the correct commissioning procedures.

    Don’t try and cut corners or you could inadvertently damage hydraulic components during their initial start-up, even though this damage may not always be apparent at the time.

    5.  Replace hydraulic components.

    Replace hydraulic components before they fail and cause further problems elsewhere in your hydraulic system.

    6.  If things do go wrong…

    Conduct a full failure analysis to ensure that you learn from your findings and change your procedures accordingly. Don’t make the same mistake twice, or again after that.

    Here are a few other topics which we hope will be useful…

    Follow this list of hints and tips to avoid unnecessary problems and systems downtime when assembling your hydraulic equipment.

    • Most important – maintain cleanliness; contamination is the reason for many hydraulic problems;
    • All openings in the reservoir should be sealed after cleaning;
    • No grinding or welding should be done in the area where hydraulic components and systems are being installed; all cylinder, valve, pump and hose connections should be sealed and/or capped until just prior to use;
    • Mineral spirits should be kept in safety containers;
    • Air hoses can be used to clean hydraulic fittings and other components, but the air supply must be filtered and dry to prevent contamination of the parts;
    • Examine pipe fittings and hose assemblies prior to use to ensure that burrs, dirt and/or scale are not present; all pipe and tubing ends should be reamed to prevent restriction and turbulent flow;
    • Never use Teflon tape on straight thread connections;
    • When installing hydraulic pumps or motors, always align coupling halves as closely as possibly, within 0.007 inch. When using flexible couplings, follow the manufacturer’s recommendations or allow 1/32 to 1/16 inch clearance between the coupling halves;
    • Do not drive couplings on pump or motor shafts; they should be a slip fit or shrunk on using hot hydraulic oil;
    • Always use a dry spray-on lubricant on splines when installing; this prevents wear and adds to the useful life of the splines. When using double universal joint couplings, the shafts must be parallel and the yokes must be in line;
    • When installing V-belt pulleys on pumps or motors, line up both pulleys as closely as possible. Always install the pulleys with a minimum amount of overhang as close to the pump or motor face as possible – this increases bearing surface life.

    Before you set-off troubleshooting, make sure that your own senses are firing on all cylinders.

    STAY SAFE and before you approach the system make sure all loads are lowered or mechanically secure.

    DO NOT rely on a hydraulic control valve to hold a load; exhaust the pressure locked in the system and isolate power supply systems and electrical controls.

    TOUCH – Heat is a real give-away – the strongest indication of a problem in the system. Feel different areas; if there’s discomfort when you touch a component then there’s a problem – the hottest component is faulty. In addition to damaging seals and reducing the service life of the hydraulic fluid, high fluid temperature can damage system components through inadequate lubrication because of  excessive thinning of the oil film (low viscosity). A fluid temperature alarm should be installed in the system and all high temperature indications investigated and rectified immediately.

    SMELL – An unpleasant, harsh or bitter smelling oil is a sign of excessive heat and indicates that the additive packages in the oil have broken down. Remove and replace the oil as soon as possible to avoid major damage to hydraulic components.

    LOOK – Reduced performance, e.g. longer cycle times or slow operation, is often an early indication of problems within the hydraulic system. In a hydraulic system, flow determines actuator speed and response – a loss of speed therefore indicates a loss of flow. Inconsistent, erratically moving actuators are a strong sign of entrapped air – see our article on article on Air Contamination in Hydraulic Systems.

    LISTEN – Abnormal noise, banging or knocking, in a hydraulic system is often caused by aeration or cavitation – again, see our article on Air Contamination in Hydraulic Systems. Cavitation causes metal erosion, which damages hydraulic components and contaminates the fluid. While cavitation can occur just about anywhere within a hydraulic circuit, air usually enters through the pump’s inlet. It is therefore vital to make sure that the pump intake lines are in good condition and all clamps and fittings are tight.

    Taking the time to proactively monitor noise levels, fluid temperature and cycle times can pay dividends – allowing you to detect changes and conditions that can cause costly component failures and unscheduled downtime of hydraulic equipment.

    A contaminant is defined as anything that impairs the performance of the hydraulic system and its component parts.

    Air in a hydraulic system is a common contaminant and can be found in hydraulic fluid in four different forms:

    • Dissolved air – hydraulic fluid typically contains 6-12% of dissolved air by volume;
    • Free air – for example a pocket of air trapped anywhere within the system. Pre-filling components and proper bleeding of the hydraulic system during start-up will usually eliminate free air;
    • Foam – larger bodies of air, typically bigger than 1mm in diameter, found congregating at the top of the fluid; small amounts of foam are usually cosmetic and generally do not cause problems;
    • Entrained air – very small bubbles of air, less than 1mm in diameter, dispersed throughout the hydraulic fluid.

    The last of these, entrained air, causes the most problems and can lead to:

    • Unacceptable noise levels;
    • Reduced compressibility of the hydraulic fluid creating poor, spongy component response;
    • Reduced fluid viscosity, leaving surfaces vulnerable to wear;
    • Increased heat-load;
    • Reduced thermal conductivity;
    • Severe fluid degradation, leading to component damage as a result of reduced lubricaton, overheating and burning of seals;
    • Cavitation damage;
    • General decreased systems efficiency.

    Cavitation HydraulicsCavitation is the quick formation and collapse of vapour cavities, “bubbles”, in liquids as a result of forces acting upon the liquid.  It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low.
    When subjected to higher pressure, the voids implode and can generate an intense shockwave. Cavitation causes extreme wear in engineering contexts – to propeller blades and hydraulic equipment – and even in nature, to fish fins as they swim through the water. Fins may heal, hydraulic equipment and machinery don’t!
    Cavitation cannot always be seen, but a demonstration here at Eaton’s training centre in Ohio, shows an extreme example of the effects of excessive air in hydraulic fluid. CavitationAs well as the presence of foam, other symptoms of air contamination are:

    • Excessive or abnormal noise – often caused by aeration or cavitation in the hydraulic system. Aeration is caused when air is introduced to hydraulic fluid and noise is created when those fluids are compressed. Cavitation occurs when the hydraulic fluid levels run low and air makes its way into the system instead.
    • High pressure – often caused when hoses and pipework have too small a diameter to transfer fluids in high quantities, meaning that air blockages inevitably occur. Draining the system and fitting larger hoses can rectify the problem.

    As mentioned above, hydraulic fluid contains up to 12% dissolved air by volume. This dissolved air can come out of the hydraulic fluid under certain conditions resulting in entrained air; this process is known as gaseous cavitation.
    When fluid temperature increases or static pressure decreases, the air solubility is reduced and bubbles can form within the fluid. A decrease in static pressure and subsequent release of dissolved air can occur at the pump inlet, because of:

    • Clogged suction strainers or inlet filters;
    • Undersized or clogged reservoir breather;
    • Restricted intake line;
    • Turbulence caused by intake line isolation valves;
    • Poorly designed inlet;
    • Excessive vertical distance between the pump intake and minimum fluid level.

    Other causes of decreased static pressure can also be caused by changes in fluid velocity through the system, flow transients and incorrectly adjusted or faulty anti-cavitation or load control valves.
    Entrained air can also be caused by external ingestion. Like gaseous cavitation, this often occurs at the pump as a result of: loose intake-line clamps or fittings, porous intake lines, low reservoir fluid level or a faulty shaft seal on the pump. It can also occur due to faulty or incorrectly adjusted load control valves, leading to air being drawn past the gland of double-acting cylinders, and return fluid plunging into the reservoir (drop-pipes extending below minimum fluid level should be fitted to all return penetrations).
    As always, prevention is better than cure and proactive hydraulic systems and equipment maintenance will prevent the occurrence of most air contamination problems.

    Producing quiet, hydraulically-actuated machines and systems requires more than just quiet hydraulic components. Meeting the stringent sound-level specifications of today’s industrial hydraulic systems and machines takes careful engineering.

    The hydraulic pump should be considered first. It not only produces sound directly but generates vibrations and fluid pulsations. These react with other machine parts which produce more sound.

    Hydraulic pump selection:  hydraulic pumps generate more acoustic energy per unit of hydraulic power by running at high speed rather than at low. For this reason, a hydraulic pump should operate at 1200rpm whenever sound is critical. Below 3000 PSI, the trade-off between pressure and hydraulic pump size for a given drive power has little effect on noise, so you are free to select any combination of these factors that otherwise meet your needs.

    Mechanical isolation: To meet lower sound level limits, the hydraulic pump should be mechanically isolated from the rest of the machine or hydraulic system using anti-vibration mountings. All connections to the hydraulic pump must be made with flexible hose. Flexible hose will often reduce noise even where anti-vibration mountings are not used. It prevents vibrations from reaching other lines and hydraulic components to keep them from becoming sound sources. In long lengths, this hose is, itself, a good sound generator so only short lengths should be used. For long runs, use solid pipes with short hoses at the ends. All long lines must be supported every metre or so, preferably with clamps providing vibration damping. Lines must not contact panels that are good sounding boards. Where they pass through such panels, allow sufficient clearance to prevent direct contact; never use bulkhead fittings in such cases.

    Acoustic isolation: The greatest sound level reductions are attained with the hydraulic pump acoustically as well as mechanically isolated. This requires that the hydraulic pump be completely enclosed in a non-porous shell weighing at least 10 kg per square metre of surface. No openings can be tolerated and all joints must be sealed with resilient gaskets or mouldings. Grommets of rubber or other soft material should be used to close openings around piping and to prevent mechanical contact between the enclosure and piping.

    Fluids: The condition of the hydraulic fluid being pumped is also important in controlling sound. Fluid viscosity, temperature and vacuum by themselves have no effect on sound levels. It is important to control them, however, to prevent the formation of entrained air or vapour bubbles that can double sound levels and reduce pump life.

    A combination of high fluid temperature and inlet vacuum generates cavitation bubbles. However, at low temperatures, a high viscosity fluid in a very long suction line can also produce sufficient vacuum to cause cavitation. Important methods of suppressing bubble formation include: using short runs or large diameter inlet lines; keeping the reservoir elevation close to or above that of the pump; using low pressure-drop inlet filters that signal when they are producing high vacuums and need changing; and, providing adequate fluid controls. These are all good hydraulic practices that become increasingly important where you must achieve low sound levels.

    Reservoirs: Reservoirs enable entrained bubbles to be released. These can come from sources other than the hydraulic pump inlet and are usually present in the fluid returning to the reservoir. Low reservoir temperatures reduce the rate of bubble escape and may result in incomplete release. As pointed out earlier, high temperatures promote bubble formation. The best balance between these two alternatives is achieved by maintaining the temperature of oil leaving the reservoir in the range of 120-150 degrees F and the temperature of water-based fluids between 100-120 degrees F.

    A simple reservoir has to be large to allow complete bubble release. By providing baffles to guide the fluid through a circuitous path and by locating return and pump inlet lines as far apart as possible, a reservoir holding between two to three minutes of maximum pump flow can be adequate.

    Best practise when fitting hydraulic tubing and hydraulic hoses:

    • Don’t take heavy cuts on thin wall tubing with a tubing cutter. Use light cuts to prevent deformation of the tube end. If the tube end is out or round, a greater possibility of a poor connection exists;
    • Ream tubing only for removal of burrs. DO NOT over ream tubing as it can weaken the connection;
    • Do not allow chips to accumulate in the tubing – they can be difficult to remove after bending;
    • Follow the manufacturers recommendations on the use of flaring tools. Don’t over-tighten the feed screw handle on a compression type flaring tool. Improper use of a tool can cause washout and/or splitting of the flare connection;
    • Bend tubing instead of cutting and using a fitting. This reduces pressure drop and minimises system losses. The minimum radius of a tubing bend should be at least three times the inside diameter of the tube. Larger bends are preferred;
    • Sketch the optimum tubing route before beginning the bending process. Be sure to use tubing with the proper temper to prevent wrinkles and flattened bends;
    • Most flares are made by hand or power tools that swage the tube end over a split die. The standard flare angle is 37 degrees from the centre-line. For best results, heavy wall tubing should be cut, deburred, and flared and bent using power equipment.

    Explore our Resource Library for more information: