System-oriented thinking and analysis skills are required to keep your industrial hydraulics operations in good working order.
By Carl Dyke, CD Industrial Group Inc.
In a fluid power system, the source energy enters at one central point, just like an electrical system. The energy is distributed through control mechanisms and conductors to the final motion devices, just like in many electrical systems. Like an electrical relay, a hydraulic directional valve is a block with protruding conductors. An industrial hydraulic system is often viewed as analogous to an electrical system, and there are indeed many similarities.
The assumption made by some maintainers, that the unseen medium traveling within hydraulic lines needs little attention—and that changes in component or system performance are unlikely or infrequent—is often based on a comparison, and not necessarily a correct one, to electrical systems. A hydraulic system is much more mechanically and biologically active than an electrical system.
While making sure that electrons can flow freely does require some electrical system maintenance work, the electrons themselves don’t require care. Electrical terminals must be kept tight, but the conductors themselves need little attention. By comparison, hydraulic hoses and tubes are under significant mechanical stresses both inside and out, at nearly all times. The hydraulic fluid moving within the conductors can easily degrade, too.
Maintenance as a discipline
A hydraulic system is not often purchased independently in the industrial plant environment. Rather, hydraulic systems come to exist on a factory floor as a part of a complex machine or system that was purchased to manufacture a product or complete a process. As a result, hydraulic system maintenance is often not developed as a specific discipline. The machine manufacturer may provide some hydraulics maintenance notes within the pages of the equipment manual. Often times, those notes contain little more than how to fill and commission the hydraulic system, and not how to maintain it on an ongoing basis. Maximizing the asset life, and specifically the life of the hydraulic system, is not a common topic in machinery manuals.
Once a new machine or manufacturing line is up and running, the electrically driven hydraulic pumps start up along with other electrical motor driven functions on the machine. The directional valves with hoses connected to the cylinder and hydraulic motors remain in place in the same way an electrical relay remains mounted in a controls cabinet with its associated wiring. It is easy for the machine owner to assume all is well and overlook the development of a maintenance program for the hydraulic system.
If viewing a hydraulic system as an inert set of components as perhaps some electrical systems are viewed leads to the incorrect approach to system maintenance, how then should a hydraulic system be maintained?
Continuous observation of the system and logging basic performance data is a valuable discipline to develop. Many preventative maintenance work orders in the factory environment ask for little more than to “check” the pump and to “check” the cylinders. If a maintainer finds that those components are still in place and appear to be performing their designated function, the check boxes on the form are ticked. Has good observation work been performed? No, it has not. Is it the maintenance department’s key responsibility to make sure that major machine components are still in place and functioning? I would argue that these tasks fall to the machine operators.
The maintenance department is tasked with providing a certain type of care. Assuming that a machine or system was properly designed for the use it receives, and that the machine was commissioned correctly, the maintenance department is there to help achieve reliable machine performance and the maximum possible machine life.
If no fluid is leaking noticeably from a hydraulic system, and if machine functions appear normal, many would assume that there is little maintenance work to do. Hydraulic failures in a factory are infrequent for many systems. “Our hydraulic systems rarely break down or malfunction” is a common statement made by many that I visit in the mills and factories of North America. It is a very telling statement with positive and also negative aspects. What we learn is that the hydraulic systems do, in fact, break down. Not every single breakdown and malfunction can be avoided with maintenance. If very little true maintenance is done, however, it stands to reason that the breakdowns that do occur may indeed be tied to a lack of specific activities. Many breakdowns that result in lost production time can be detected early and thus avoided or planned for, with careful observation and data recording.
Firefighters who go door-to-door in a community, providing fire prevention information and asking if you have recently changed your smoke detector battery, are engaged in fire prevention work. Many would argue that this activity is nowhere near as exciting as putting out a fire. Few citizens however, need a complete analysis of the economics of this fire-prevention work to know that the high costs of a building fire may include the lost lives of the building occupants, the risked lives of the firefighters and many other capital and operational expenses.
Maintaining a hydraulic system similarly lacks the particular type of excitement of fighting a fire. A reliable system that lasts for many years is the only reward. Yet this excitement and reward is tied to profitability and possibly, business survival. What other type of excitement is or should be desired?
The daily routine of checking system and subcircuit pressures, cylinder cycle times and also system and component temperatures is a valuable one. A spreadsheet can easily serve for record keeping and for graphical analysis. Daily or weekly fluctuations in the values may be allowable for any number of reasons. The value of the activity, however, becomes clear when a lasting trend starts and the values are seen to climb or drop from the norm. These activities fall under an approach referred to as condition-based maintenance.
A cylinder or valve that runs consistently warmer than usual may indicate the development of an internal leak. This internal leakage may correlate with a cylinder cycle time change as well. An increase in the overall system temperature may correlate with a change to the observed maximum system pressure. Pressure control valves or pump controls may need to be adjusted. This is maintenance by careful monitoring of the internal performance of the system.
A cylinder used in a pressing or product forming function may have its maximum force controlled by a pressure-reducing valve. Routinely checking this valve setting is important, as this type of valve may fail open—and allow excessive pressure at the cylinder. Discovering any change early may help save the cylinder from excessive wear and stress.
Routinely checking pressure values, temperatures and cycle times as described above is uncommon and yet necessary in my experience. You cannot see inside a hydraulic system, so one needs to observe very carefully from the outside and make use of testing instruments. Regular data gathering gives you the needed insight for systems analysis.
How can you achieve this data gathering? The use of one, high-quality, digital pressure gauge and the installation of many quick-connect test points enables the pressure measurement work. Test point fittings are common on all sub-circuits for many brands of mobile machine hydraulic systems. These inexpensive fittings should be installed in industrial plant hydraulic systems as well.
The same digital pressure gauge may also be capable of reading partial vacuum values. This makes it the correct tool to routinely measure the pressure at the pump inlet. A pump inlet pressure reading that has moved deeper into the vacuum range from its commissioned value is the best way to detect a tank strainer that is starting to become plugged or a collapsed suction hose with its potential to cause cavitation. The pump manufacturer will provide you with a maximum (most negative) vacuum value that is acceptable for your pump model.
Measuring temperatures can be as simple as recording the tank thermometer reading. An overheating system can have a number of causes and should be investigated as soon as possible. Your monthly hydraulic oil analysis report indicates whether the viscosity is still correct. Oil that is too thin will not adequately lubricate pump components. This leads to overheating and shortened pump life. Many components can be checked with a handheld contact thermometer or with a laser pointed temperature gun. Infrared thermography cameras have become affordable and common for many production plants. Their advanced temperature measuring capabilities can be put to work for both electrical and hydraulic system maintenance.
Keep track of cylinder cycle times and hydraulic motor speeds to detect any change in flow rate early. A cylinder that has slowed down, however slightly, indicates that either an internal leak is developing or that a pump has reduced its displacement. Flow measurement instruments are not commonly installed in hydraulic circuits for permanent use, but they are certainly the correct instruments to use for many diagnostic and system commissioning activities. For basic daily maintenance monitoring, flow issues can be detected by timing a cylinder with a stopwatch. If your plant machinery has cylinder position sensors connected to programmable controllers, then the task of tracking cycle times can be automated.
The use of filters to remove particle contaminants from hydraulic fluids is a common topic among hydraulic maintenance practitioners. It is well documented that the presence of hard particles within a hydraulic fluid is a cause for component wear and performance degradation, and a possible cause for sudden component failure. This is especially true for valves. Valves either jam in the open position, the closed position or somewhere between those two points. The source or cause of a valve jamming or seizing is often a solid contaminant such as silica, a metal particle or rust.
Have your hydraulic fluid analyzed by a lab to detect the presence of water and to compare the size and quantity of particle contaminants with the maximum allowed by the manufacturer of your pumps and valves. Improve the quality of filters used along with your filling practices to bring the contamination levels down to acceptable levels. Share the fluid cleanliness level required by the component manufacturers along with the monthly or quarterly particle count on the lab reports with all maintainers. This practice helps to drive awareness along with better maintenance. Many undesired particles enter a hydraulic system via a poor-quality tank breather. High quality micronic breathers containing a desiccant can help to strip solid particles entering the tank and also control moisture.
If a filter has only a basic, inexpensive, pop-up style indicator to alert a maintainer when the filter needs to be changed, valuable opportunities to observe the system have been lost. Filter housings can be equipped with sensitive differential pressure gauges or sensors. These instruments can indicate the very beginning of a sudden run up in contamination.
This article serves only as an overview of some major issues in industrial hydraulic system maintenance and to illustrate the needed systems-thinking. Many of the maintenance challenges for a hydraulic system happen on the inside where they are not readily observed. Measuring and then controlling what some would regard as very subtle properties yields the best long-term benefit.
Like a biological system, each industrial hydraulic system may need its own unique maintenance emphasis. Hydraulics will certainly require more attention than most electrical systems but similarly require strong analysis skills to achieve the best results.
CD Industrial Group Inc.
Filed Under: Fluid Power Basics, Slider