Developing and Optimizing PM’s for Hydraulic Equipment
Brendan Casey, HydraulicSupermarket.com
IntroductionReliability Centered Maintenance (RCM) provides a framework that helps to ensure thatif a dollar is spent on improving reliability, that dollar is fully recovered – plus anacceptable return on investment. Placing preventative maintenance (PM) within the RCMframework ensures that the cost of PM tasks do not exceed the cost of the consequencesof failure. However, if the ‘new order’ of the maintenance department is to reduce andeventually eliminate the need for maintenance services, then merely aligning task costswith failure consequences is not enough. This paper outlines a preventative maintenanceprogram for hydraulic equipment that, where possible, reduces or eliminates the need forconstant investment in labor and materials – the fatal flaw of traditional PM activities.
Maintenance Strategy and ProcessReliability Centered Maintenance The primary objective of Reliability Centered Maintenance (RCM) is the economicoptimisation of machine reliability relative to organizational goals (Figure 1). RCMprovides a framework that helps to ensure that if a dollar is spent on improving reliability,that dollar is fully recovered – plus an acceptable return on investment. Whenintelligently applied, the RCM model aids in the development of economical, equipment-specific maintenance programs.
Condition-Based Maintenance Deployment of advanced maintenance tactics within the RCM framework typicallyinvolves the application of condition-based maintenance. Condition-based maintenanceencompasses proactive (PM) and predictive maintenance (PdM), the combined objectivesof which are machine life extension and early detection of faults and failures (Figure 2).Placing condition-based maintenance within the RCM framework ensures that the cost ofPM and PdM tasks do not exceed the cost of the consequences of failure.
Figure 2. Condition-based maintenance.
Maintenance Improvement According to Levitt (2003) and others, any equipment that requires periodic attention toprevent breakdown is a failure of design engineering. Under this premise, the ‘new order’of the maintenance department, is to reduce and eventually eliminate the need formaintenance services, i.e. to fix the equipment permanently. This being the case, thenmerely aligning PM task costs with failure consequences is not enough.
The objectives of the following preventative maintenance program are to:• provide reliable hydraulic assets; and • reduce or eliminate the need for constant investment in PM tasks.
PM Development and EliminationThe following six preventative maintenance routines are essential to gain maximummachine life and minimize the chances of premature failures and unscheduled downtimeof hydraulic equipment: 1. Maintain fluid temperature and viscosity within optimum limits; 2. Maintain fluid cleanliness; 3. Maintain hydraulic system settings to manufacturers' specifications; 4. Schedule component change-outs prior to failure; 5. Follow correct commissioning procedures; and 6. Conduct failure analysis. Maintaining Fluid Temperature and Viscosity within Optimum Limits Implementing this routine involves:1. Defining an appropriate fluid operating temperature and viscosity range for the
ambient temperature conditions in which the hydraulic system operates; 2. Selecting a fluid with a suitable viscosity grade and additive package; and3. Ensuring that both fluid temperature and viscosity are maintained within the limits
defined.
The viscosity of petroleum-based hydraulic fluid decreases as its temperature increasesand conversely, viscosity increases as temperature decreases. Limits for fluid viscosityand fluid temperature must therefore be considered simultaneously. Low fluid viscositycauses loss of lubricating film strength, which leads to boundary lubrication conditions,scuffing and adhesive wear. Excessively high fluid viscosity can result in damage tosystem components through cavitation.
Hydraulic fluid temperatures above 82°C (180ºF) damage most seal compounds andaccelerate degradation of the oil. A single over-temperature event of sufficient magnitudecan permanently damage all the seals in an entire hydraulic system, resulting in numerousleaks. The by-products of thermal degradation of the oil (soft particles) can causereliability problems such as valve-spool stiction and filter clogging.
Manufacturers of hydraulic components publish permissible and optimal viscosity values,which can vary according to the type and construction of the component. As a generalrule, operating viscosity should be maintained in the range of 100 to 10 centistokes (460to 80 SUS), however viscosities as high as 1000 centistokes (4600 SUS) are permissiblefor short periods at start up. Optimum operating efficiency is achieved with fluidviscosity in the range of 36 to 16 centistokes (170 to 80 SUS) and maximum bearing lifeis achieved with a minimum viscosity of 25 centistokes (120 SUS).
In order to determine the correct fluid viscosity grade for a particular application, it isnecessary to consider:
• starting viscosity at minimum ambient temperature;• maximum expected operating temperature, which is influenced by system
efficiency, installed cooling capacity and maximum ambient temperature; and • permissible and optimum viscosity range for individual components in a system.
For example, consider an application where the minimum ambient temperature is 15°C,maximum operating temperature is 75°C, the optimum viscosity range for the system’scomponents is between 36 and 16 centistokes and the permissible, intermittent viscosityrange is between 1000 and 10 centistokes.
Figure 3. Temperature/viscosity diagram for ISO viscosity grades. Note that thegradient of the lines shown will depend on the viscosity index of the particular fluidused.
From the temperature/viscosity diagram (Figure 3), it can be seen that to maintainviscosity above the minimum, optimum value of 16 centistokes at 75°C, an ISO VG68fluid is required. At a starting temperature of 15°C, the viscosity of VG68 fluid is 300centistokes, which is within the maximum permissible limit of 1000 centistokes at startup.
Having established the correct fluid viscosity grade, the next step is to define the fluidtemperature equivalents of the optimum and permissible viscosity values for the system’scomponents.
By referring back to the temperature/viscosity curve for VG68 fluid in Figure 3, it can beseen that the optimum viscosity range of between 36 and 16 centistokes will be achievedwith a fluid temperature range of between 55°C and 78°C. The minimum viscosity foroptimum bearing life of 25 centistokes will be achieved at a temperature of 65°C. Thepermissible, intermittent viscosity limits of 1000 and 10 centistokes equate to fluidtemperatures of 2°C and 95°C, respectively (See Table 1).
Viscosity Value cSt Temperature (VG68)Min. Permissible 10 95ºCMin. Optimum 16 78ºCOpt. Bearing Life 25 65ºCMax. Optimum 36 55ºCMax. Permissible 1000 2ºC
Table 1. Correlation of operating viscosity values with fluid temperature based onfluid viscosity grade.
Going back to our example, this means that with an ISO VG68 fluid with a VI similar tothat shown in Figure 3 in the system, the optimum operating temperature is 65°C.Maximum operating efficiency will be achieved by maintaining fluid temperature in therange of 55°C to 78°C. And if cold start conditions at or below 2°C are expected, it willbe necessary to pre-heat the fluid to avoid damage to system components. Intermittentfluid temperature in the hottest part of the system, which is usually the pump case, mustnot exceed 95°C.
Once an appropriate fluid operating temperature and viscosity range has been defined, thenext step is the selection of a quality hydraulic oil. If the system contains high-performance components, such as piston pumps or motors, an oil containing at least 900ppm (0.09% wt.) of the anti-wear additive ZDDP is recommended. The selection of an oilwith detersive/dispersive additives can be beneficial in mobile applications.
Having defined the parameters shown in Table 1 for a specific piece of hydraulicequipment, damage caused by high or low fluid temperature (low or high fluid viscosity)can be prevented, and recurring PM tasks in respect of this routine can be virtuallyeliminated, by installing fluid temperature monitoring instrumentation with alarms andshutdowns.
Recommended temperature alarm limits are: • Over-temperature alarm - temperature at which viscosity falls below minimum
optimum. • Over-temperature shutdown - temperature at which viscosity falls below minimum
permissible.• Under-temperature - temperature at which maximum, permissible viscosity is
exceeded.
In addition to the reliability gains that can be achieved through defining and maintainingoptimum fluid viscosity, continuous monitoring of hydraulic fluid temperature can revealproblems such as increased internal leakage, aeration and cavitation, making it aneffective and inexpensive condition-monitoring technique.
Maintaining Fluid Cleanliness Implementing this routine involves:1. Defining targets for particle and water contamination appropriate for the type of
hydraulic system;2. Monitoring the actual contamination levels against target levels; and 3. Instigating remedial action as necessary to maintain target cleanliness levels. Water in hydraulic fluid causes weakened lubricating film-strength, which leaves criticalsurfaces vulnerable to wear and corrosion. Water can react with additives in the oil toform corrosive by-products, which attack bearing metals. When selecting watercontamination targets, the type of hydraulic system and reliability objectives for theequipment need to be considered. Controlling water contamination below the oil’ssaturation point at operating temperature is recommended. If the system contains high-performance components, such as piston pumps or motors, a target of 100 ppm isdesirable.
Particles accelerate wear of hydraulic components. The rate at which damage occurs isdependent on the internal clearances of the components within the system, the size, shapeand quantity of particles in the fluid and operating pressure.
Some level of particle contamination is always present in hydraulic fluid, even in newfluid. The level of contamination, or conversely the level of cleanliness, consideredacceptable depends on the type of hydraulic system and reliability objectives for theequipment. Typical fluid cleanliness levels for different types of hydraulic systems,defined according to ISO and NAS standards, are shown in Figure 4. Note the correlationbetween fluid cleanliness level and the level of filtration in the system.
Figure 4. Recommended hydraulic fluid cleanliness and filtration levels.
Consider an application with a normal-pressure system and a target cleanliness level ofISO 16/13. According to Figure 4, a filtration level of 10-micron with an efficiency ofβ10 ≥ 100 is required to achieve a cleanliness level of ISO 16/13. This means that unlessthere is at least one filter in the system with a rating of β10 ≥ 100, it is unlikely that acleanliness level of 16/13 will be achieved. If an audit of the existing filters reveals thatthis level of filtration is not present somewhere in the system, then either the level offiltration must be upgraded or the target cleanliness level revised downward. In practice,it may be necessary to increase the efficiency of the filtering media (at target particlesize) if the contamination level is too high, or decrease efficiency if the fluid is cleanerthan required by the application, to optimize contamination control costs.
Having defined fluid contamination targets for a specific piece of hydraulic equipment,regular PM tasks in respect of monitoring and controlling contamination can beminimized through:• practical elimination of contaminant ingression; and• filter element condition-monitoring.
Eliminating Contaminant Ingression. Common points of contaminant ingress andmethods of exclusion are listed in Figure 5.
Filter Element Condition Monitoring. Return line filtration is a feature of mosthydraulic systems. Warning of filter-bypass is typically afforded by visual or electricclogging-indicators. These devices indicate when pressure drop across the element isapproaching the opening pressure of the bypass valve. If the bypass valve opens at apressure drop of 3 bar the clogging indicator will typically switch at 2 bar. Replacingthese indicators with pressure gauges or transducers enables continuous conditionmonitoring of the filter element. This enables trending of fluid cleanliness against filterelement pressure-drop, which can be used to optimize oil sample and filter changeintervals. Continuous monitoring of filter pressure drop can also provide early warning ofcomponent failures and element rupture. Monitoring of pressure and offline filterelements can be accomplished through the installation of differential pressure gauges ortransducers.
Install desiccant breathers or headspace bladders.
Wash down jets. Instruct operators to direct jets away from breathers.Coolant leaks. Substitute oil to water exchangers with oil to air.
Scheduled change-out of oil to water exchangers.Top-up fluid. Maintain high standards in the storage and handling
of lubricants.Soft particles Oil oxidation and additive
depletion by-products.Control air, water and hard particle contamination.Monitor fluid temperature and prevent hightemperature operation.
Hard particles Airborne (reservoir). Seal all reservoir penetrations and hatches. Install airbreather filtration of 3 microns or better. Installheadspace bladders in extreme environments.
Airborne (cylinders). Install rod protectors (bellows) to protect cylinder rodsand wiper seals from damage, and provide anadditional barrier to contaminant ingression
Top-up fluid. Maintain high standards in the storage and handlingof lubricants. Pre-filter all top-up fluid.
Figure 5. Common points of contaminant ingress and methods of exclusion.
Maintaining Hydraulic System Settings to Manufacturers' SpecificationsImplementing this routine involves:1. Developing a procedure for checking the operation and adjustment of the various
circuit protection devices installed in the hydraulic system; and2. Checking these settings at an appropriate service interval.
Faulty or incorrectly adjusted circuit protection devices can result in reduced machineperformance and cause damage to components through over-pressurization, cavitationand aeration. Over-pressurization occurs when the pressure developed in any part of ahydraulic circuit exceeds design limits. Over-pressurization can result in burst hoses,blown seals and catastrophic failure of hydraulic components.
Cavitation occurs when the volume of fluid demanded by any part of a hydraulic circuitexceeds the volume of fluid being supplied. This creates a partial vacuum within thecircuit, which causes the fluid to vaporize. Cavitation causes metal erosion, whichdamages hydraulic components and contaminates the hydraulic fluid. In some cases,cavitation can result in catastrophic failure of pumps and motors.
Aeration occurs when air contaminates the hydraulic fluid. Aeration can cause loss oflubrication resulting in scuffing, adhesive wear and localized heating of sliding surfacesand close fitting parts.
Machine manufacturers usually publish detailed instructions for checking and adjustinghydraulic system settings. This information typically includes a list of equipment requiredto carry out the checks. A set of pressure gauges is often the only equipment required. Ifthis information is not available for a specific piece of equipment, an appropriate
procedure needs to be developed. If necessary, consult a fluid power engineer forguidance.
Regular checking of hydraulic system settings not only ensures that the machine isoperating efficiently, but also gives early warning of faulty circuit protection devices -before they cause component failures. Settings should be checked during initialcommissioning and when a system is re-commissioned, following a component change-out or major maintenance work. The frequency of routine checks depends on the type ofsystem and reliability objectives for the equipment. A service interval of 2,000 hours isadequate for most systems.
Having defined an appropriate procedure and service interval for checking the settings ofa specific hydraulic system, labor and materials required to perform this task can beminimized through:• installation of pressure-test and other temporary connection points in relevant
locations of the hydraulic circuit; and• permanent installation of pressure and flow monitoring instrumentation, where
appropriate.
Scheduling Component Change-Outs Prior to FailureImplementing this routine involves:1. Determining the useful service life of each component in the hydraulic system using
historical and/or predictive data; and 2. Scheduling their change-out when useful service life has been achieved, rather than
running to failure.
This routine is particularly pertinent to rotating components, i.e. pumps and motors.When a pump or motor fails, large amounts of metallic particles are generated. Theseparticles circulate in the hydraulic fluid, often causing damage to other componentsbefore the system’s filters can remove them. In extreme cases, the contamination load canclog the filters, which results in unfiltered fluid being circulated through the system.
A pump or motor that fails in service is almost always more expensive to rebuild than onethat is removed from service in a pre-failed condition. A failure in service usually resultsin mechanical damage to internal parts of the component. As a consequence, parts thatwould otherwise have been serviceable have to be replaced. In extreme cases,components that would have been economical to repair become uneconomical to repair,increasing the cost of component replacement by as much as 50%.
In a condition-based maintenance environment, the decision to change-out a pump ormotor is usually based on deterioration in volumetric efficiency or remaining bearing life,whichever occurs first. Expected bearing life is influenced by a number of factors,including the type and construction of the component, installation arrangement, andcircuit design, operating load and duty-cycle. Once these variables are known, bearinglife can be predicted by calculation. This information is usually available from hydrauliccomponent manufacturers on request. To minimize the chances of hydraulic components
failing in service, predictive and/or historical data (where available) should be used toschedule change-outs.
Having defined the useful service life of the major components in a specific hydraulicsystem, PM task costs in respect of this routine are only required where component lifeextension beyond that expected is highly desirable. For example, where the componentreplacement cost is extremely high or machine availability for maintenance is limited.
Following Correct Commissioning ProceduresImplementing this routine involves:1. Developing equipment-specific commissioning procedures; and2. Training maintenance personnel to effectively carry out these procedures.
Incorrect commissioning during start-up can result in damage to hydraulic componentsthrough cavitation, aeration and inadequate lubrication. In many cases, this damage willnot show itself until the component fails hundreds or even thousands of service hoursafter the event.
A common misconception among maintenance personnel with limited training inhydraulics is that because oil circulates through hydraulic components in operation, nospecial attention is required during installation beyond fitting the component andconnecting its hoses. This is not the case.
Following correct commissioning procedures ensures that hydraulic components arecommissioned properly during installation or when a system is re-started aftermaintenance. Improper commissioning is one of the most common causes of ‘infantmortality’ in hydraulic equipment.
To prevent component damage during initial start-up, obtain the machine manufacturer’scommissioning procedures and train maintenance personnel to carry them out effectively.If this information is not available for a specific piece of equipment, appropriateprocedures need to be developed. If necessary, consult a fluid power engineer forguidance.
Having developed commissioning procedures for a specific piece of hydraulic equipmentand provided the necessary staff training, ongoing costs associated with this routine arelimited to training of new maintenance employees.
Conducting Failure AnalysisImplementing this procedure involves setting up a system that ensures:1. All failed components are submitted for analysis; and2. Feedback is adequately reported in a timely manner.
Root cause failure analysis is an essential element of any preventative maintenanceprogram. The logic for this is simple - if a failure occurs and the cause of failure is notidentified and rectified immediately, then the replacement component is likely to suffer asimilar fate.
The objective of a preventative maintenance program is to reduce the occurrence ofpremature component failures and unscheduled downtime. Even with the bestpreventative maintenance program, a premature failure can still occur. Manufacturingdefects, circuit design faults and operator abuse are typical causes.
When a failure does occur, it is essential that a thorough analysis be conducted in order todetermine the root cause. Establishing the cause of failure enables remedial action to betaken to prevent similar failures. Conducting failure analysis on hydraulic equipment is aspecialized task that requires a detailed understanding of hydraulic circuits, theconstruction of hydraulic components and their modes of failure.
Reputable hydraulic repair shops can usually provide this service, which in most caseswill be included in their price to rebuild the component. If a new component has beenfitted and the failed component is not going to be repaired, failure analysis should still becarried out. If necessary, consult a specialist in this area.
Once it is set up, there are no ongoing PM task costs in respect of a failure analysisprogram.
ConclusionTraditional PM activities require a constant investment in labor and materials. Aligningthe cost of these activities with the cost of the consequences of failure ensures that taskcosts do not exceed failure costs. However, the economic reality of global competitiondemands that continuous improvement is the new maintenance goal. This paper advancesan approach to the preventative maintenance of hydraulic equipment that providesreliable assets and reduces or eliminates the costs associated with recurring PM tasks.
About the Author: Brendan Casey has more than 16 years experience in themaintenance, repair and overhaul of mobile and industrial hydraulic equipment. For moreinformation on reducing the operating cost and increasing the uptime of your hydraulicequipment, visit his Web site: www.InsiderSecretsToHydraulics.com
