The maintenance of hydraulic systems
The criteria for monitoring the efficiency and operational conditions of hydraulic sistems have evolved in recent years to the point of modifying reference parameters for the definition of the fitness of the system together with the paradigms for maintenance. There is still considerable room for improving relations between design and maintenance, that is, the capability of engineering mechanical parts already built for facilitating maintenance
As is the case for the subsystems of all mobile machinery, hydraulic devices and systems are subjected to change in conduct over time. The phenomena of usual wear, reciprocal adjustments of the parts in motion and modification of hydraulic oil characteristics are the origins of the natural shift in performance away from what was ensured when the system was first put in service. Maintenance is the only instrument capable of guaranteeing the over time the performance the components and the systems were designed to provide.
Throughout the past maintenance was divided between ordinary and extraordinary activities. The former are the set of steps normally taken at specified intervals over time, steps which must be taken for maintenance to keep the system working, whereas the latter are reactions usually carried out following a breakdown, performance and other losses traditionally considered programmable.
The scenario has been radically changed over recent years with the modification of the reference parameters for defining the suitability of the system for its purpose and changes made to the maintenance paradigms. The move has been to predictive maintenance, the ongoing monitoring of operating conditions, and toward design for maintenance. This change has come about slowly but has been driven by two fundamental points: a different approach to guaranteed safety and performance and making available the most sophisticated, reliable and low cost sensors and electronics.
Maintenance and design
An Advanced Technology Program (ATP) promoted by the National Institute of Standards and Technology (NIST) was held in Atlanta Georgia, U.S.A. in 1998 under the title Condition-Based Maintenance (CBM). At the time, the discussion brought to light the three principle barriers to the generalized application of the concept: the inability to accurately and reliability estimate the residual life of a component or a machine (prediction); the inability to continuously monitor a machine (sensorization); the inability of maintenance systems to learn and identify significant breakdowns and recommend the corrective action to take (logic synthesis).
In the same setting, the innovations capable of removing the existing barriers were identified as: predictive capability, sensorization and reliable and economic monitoring systems, improvements in logic capabilities for the analyses of control instruments, mainly software. Nearly twenty years on the main requirements have not changed substantially but the instruments for meeting these needs have been remarkably improved. This is the case especially for the issues of sensorization and the analytic ability of software systems for machines which, in the current scenario, is extremely different from what was foreseen at the end of the last century.
Without entering into the technological details of the way this may have been, or will be, applied to machinery and hydraulic systems, which still make up a fundamental part of these issues, the main question of logic involves the close collaboration of design and maintenance where there is still ample room for improvement. Design is beginning to recognize the close collaboration between these two phases, and has already done so in some cases, and more in general acknowledge the absolute need to introduce maintenance to the mandatory requirements of design from the very beginning of the synthesis of systems processes.
At least in general, maintainence work for maintaining performance, especially from the point of view of safety, is still widely underestimated. The drive behind regulatory and legislative measures may remedy this shortcoming but what appears as a more effective solution would be grass roots efforts to raise awareness, especially among of end-users, that correct maintenance is not a useless addition to costs but an essential factor for the efficient and safe use of their equipment.
Regulatory requirements
For the earthmoving machinery sector, the European Union norm EN 474-1 dedicates an entire sector, 5.22, to the maintanence of machinery. However, the expectation of this norm is simply that of carrying out maintenance operations in compliance with the manufacturer’s instructions. There is no mention of the conceptual maintenance framework or especially of the link between the need for maintenance of the functional features and systems the machine must possess. The very real and demanding feature of the norm is found buried in the norm and in recent years has led to a truly revolutionary paradigm in the procedure for the analysis of machinery onboard systems, analyses of function and consequential safety functions.
The safety functions evaluation procedure requires a set of activities, all complicated and all associated with the concept of maintenance within pre-set levels, of the function each component, system and machine must perform and requires careful evaluations of the effects of loss or degrading shown in functioning. The traditional and most efficient instrument for this task is the FMECA (Failure Modes Effects and Criticality Analysis), a step up from the more limited FMEA, for more general Risk Management activities. This is the only approach for gaining an understanding of the interconnection network present at all levels for design for the maintenance of hydraulic systems which now account for all cases of electro-hydro-mechanical systems.
Without getting into the worth of security functions analyses, which involve the application of a system of complicated and articulated norms, attention must be trained on the fact that maintenance is and must be a factor which cannot be eliminated from these analyses. The responsibility for analyses falls on the manufacturer, with repercussions on suppliers in the case of oursourcing, for identifying measures to take, and on the end-user or anyone responsible for carrying them maintenance. The space available in this text does not allow the explanation of details but it should be enough to present the need for a thorough study of the regulatory side as well to make it possible to come to terms with the techniques for suppliers.
Functional safety also opens new scenarios for the possibility of designing new components based on the principle of intrinsic safety to allow management of breakdown conditions without the total loss of safety functions, even with a reduction of performance. In this way the concepts of predictive and reactive maintenance could be brought together without penalizing the ability of the machine to operate and do so safely.
Perspectives
The prospectives opening onto a scenario of intelligent maintenance of systems have certainly been activated. The inability to perform continuous monitoring has been overcome thanks to the availability of components with integrated electronics capable of interacting with the machine’s IT system to manage predictive and adaptive maintenance strategies whereas sensorization techniques make it increasingly feasible to apply a strategy enabling the components themselves to become sensors for monitoring they own states. Reliable MEMS sensors and nanotechnology are now available at accessible prices to make it possible to integrate control and monitoring in ways which could not be imagined only a few years ago. Another consideration is the fact that most of the losses of function can be traced to the contamination or deterioration of the hydraulic fluid as the first cause.
New sensorization techniques now open methods for applying chemical analyses to liquid or gasseous phases for evaluating the state of fluid deterioration for moving on to control and replacement at pre-set intervals for maintenance on condition based on statistics. This, for example, is the case of a monitoring system based on the integration of sensors applied to liquid and gasseous phases being developed by C.N.R.-IMEM, National Research Council Institute of Materials for Electronics and Magnetism and C.N.R.-IMAMOTER, but there is no lack of other examples of commercial systems already available on the market. An equally important factor certain to come up for serious development in the coming years is assistance for maintenance procedures to be made available by the use of enhanced instruments for diagnosis and repairs. The idea is to have a mobile device transmitting sensorization data on a machine for identifying the conditions of a present or imminent breakdown and then recommend the correct steps to take, superimposing visual imaging information on the real system for dealing with the management of the maintenance programming to guide the operator to the correct solution. And this is not science fiction; pilot examples have been completed and the potential is enormous for guaranteeing that repairs are correct as well as for utilizing the information to cope with strategies for the management of the maintenance programming.
Conclusions
The need for a different and more integrated approach to maintenance strategies of electro-hydro-mechanical subsystems is absolutely essential in modern design. The changeover is certainly the result of the pressure of regulatory norms as well as the natural evolution of technology and is likely to gain momentum from the progressive changes on the side of contracts for the sale of goods providing guarantees of functions of the machine and performance. In this changed scenario the efficient operation of the machine at peak performance will be the premier criterion and carry more weight in design and maintenance strategies, especially in predictivie maintainence. To fulfill this purpose the technologies at the service of systems for the transmission of power via fluids are many and intersect with IT, electronic, chemical-physical, the science of materials and system disciplines. Adequate knowledge of the strategies and suitable instruments available for management will, in the coming years, become the factors capable of ensuring the advantages in competition required to maintain market shares.
