The management of a hydraulic system is concerned with all aspects from the initial design stage to its final disposal after completing a useful life. During the design phase the components and circuits that are selected for the system need to provide the performance that is required to meet the machine specification and have the necessary life to meet the expected duty cycle under the given environmental conditions. It is also important to ensure that the system is properly installed and commissioned and that appropriate maintenance and operational procedures are put into place by the machine builder/user.
Ultimately the major aspect of management is to achieve the most economic cost over the total lifetime of the equipment. This does not refer only to the cost of purchase because a higher cost system may provide a lower running cost so that in its lifetime the total cost may be less than that obtained with a system having a lower purchase cost. Reliability is a major issue because the level of this will reflect on the maintenance of the system and its operating cost in relation to the cost of machine ‘downtime’.
It may be preferable in some circumstances to use condition-monitoring techniques to determine loss of performance and predict component failure. This may be required where a high level of safety is demanded by the specification and often it may be necessary to perform some form of fault analysis for the system. All of these aspects will need to be considered as part of the systems management process.
The management of hydraulic systems is an important feature that needs to be considered during the design process, the level of which will depend on the ultimate use of the system. Clearly this involves a number of factors which include the complexity of the system, the machine duty cycle and its environment and the level of reliability required by the machine builder/user.
2 Aspects of systems management
In general systems management will need to:
The necessity and level of systems management needs to be determined. For example systems used for research test rigs, complex manufacturing machines and auxiliary drives on mobile plant will require quite different approaches. The objectives of the management system will, therefore, need to be defined. Likely objectives would include those concerned with:
The achievement of these objectives will be strongly related to the design of the system in relation to the machine duty cycle and its environment. To achieve high reliability it may be considered appropriate to measure major parameters that would include pressures, temperatures, valve positions, pump and motor displacements and flows for condition-monitoring purposes. This would allow predictive maintenance procedures to be used, which can avoid machine downtime due to component failures. It has been shown that reductions in the total cost of systems in their lifetime can be upwards of 10% by the use of condition-monitoring methods.
The premature failure of components and unreliable operation of hydraulic systems is often the result of inadequate contamination control, the incorporation of which should be part of the system design process. During installation of the system the levels of particulate contamination should be monitored using a particle counter and the system should be operated until these levels have reduced to the specified value. This process may require the filter to be changed in order that contaminants do not pass into sensitive components. When the contaminant levels have reached a satisfactory level the system should be drained and a new filter fitted.
Ideally the achievement of optimum system life and reliability is obtained by co-operation of the various manufacturers/suppliers involved in its manufacture and operation which is represented in Figure 1. The application of such a total cleanliness control programme will reduce the failure rate for systems as shown by the well-known 'Bath-tub' curve in Figure 2. This has particular effect on the number of failures at the beginning and end of the product life and consequently the overall life.
Figure 1 The Complete Partnership
Figure 2 The Bath-Tub Life Curve
5 Fault analysis
The evaluation of circuit faults needs to examine all of the system operating modes and states. However, circuit drawings are often complex, they are only able to show one operating state and, generally, they do not show component sizes, operating conditions and ratings.
Two commonly employed fault finding methods include Fault Tree Analysis and Failure Modes Effects Analysis (FMEA).
5.1 Fault Tree Analysis (FTA)
Fault trees are drawn to show possible causes of major malfunctions of the system that usually deals only with those failures that are considered to be most likely to occur, events of low probability being excluded.
This approach can be applied to the actuator circuit shown in Figure 3 for considering all the possible failures that will prevent movement of the actuator. Most of the causes of this failure are single events, which would include:
Figure 3 Valve Actuator System
Some faults require double failures to occur such as blockage of the return line filter AND the bypass valve jammed closed. This is normally an extremely unlikely situation but in some systems where safety interlocks are used these types of failures might need to be considered. A typical example is the circuits used for the control of hydraulic presses.
Thus a fault tree can be drawn by following the circuit from the component associated with the top event along the lines that lead either to the supply (pressure) or the return (tank), the top event itself may be linked to more basic faults. The event statements are linked through logic gates. OR gates require only one input to be available before the output event occurs. AND gates on the other hand require both input events to have occurred before the output event can happen.
In the simplest circuits, only OR gates are required, linking alternative fault events, e.g. there may be no flow from a directional control valve due to no input flow, OR failure of the pilot signal selecting the valve open position, OR valve jammed shut. In circuits with in-built redundancy, AND gates are also required, e.g. there is no flow in a specified line when there is no flow from the main pump supply AND no flow from the auxiliary pump supply.
5.2 Failure modes effects analysis (FMEA)
The fault tree analysis works from the top down in that a key malfunction is selected first and failures that can cause this are then established. The FMEA method works from the bottom up in that it determines the effect on the system of every failure mode of every component. This is a long and tedious procedure that is very suited to computerised methods.
An advantage that this method has over the FTA is that all failures are identified, some of which may have been missed in the FTA. However, identifying the relative importance of the failures is not part of the process and to reduce the amount of work involved the Pareto method is often used wherein only major malfunctions are considered, as in the FTA. When carrying out these analyses the fluid, the effects of the environment and the possibility of operator error must also be included.
If failure probability data is available, usually in terms of Mean Time Between Failures (MTBF), the reliability of the system can be established. However, such data for commercially available components does not usually exist and mostly this approach can only be applied to military projects and military/civil aircraft where it is necessary to perform a quantitative analysis of the system reliability.
Systems management involves some or all of the activities that have been discussed, which approach that is used depending on the requirements of the machine to which it is being applied. It may be considered on the basis of cost and/or safety to use condition-monitoring techniques for managing the operation of the machine. The parameters that can be monitored in fluid power applications include:
The most appropriate monitoring is determined by undertaking a careful examination of the failure modes and the available measurement methods. The priority methods would be those required to provide information on the most likely failures and then, depending on the importance of the system, other methods can be incorporated if necessary.
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