Mistake No. 1 Changing the Oil
There are only two conditions that mandate a hydraulic oil change: degradation of the base oil or depletion of the additive package. Because there are so many variables that determine the rate at which oil degrades and additives get used up, changing hydraulic oil based on hours in service, without any reference to the actual condition of the oil, is like shooting in the dark.
Given the current high price of oil, dumping oil which doesn’t need to be changed is the last thing you want to do. On the other hand, if you continue to operate with the base oil degraded or additives depleted, you compromise the service life of every other component in the hydraulic system. The only way to know when the oil needs to be changed is through oil analysis.
Mistake No. 2 Changing the Filters
A similar situation applies to hydraulic filters. If you change them based on schedule, you’re changing them either too early or too late. If you change them early, before all their dirt-holding capacity is used up, you’re wasting money on unnecessary filter changes. If you change them late, after the filter has gone on bypass, the increase in particles in the oil quietly reduces the service life of every component in the hydraulic system costing a lot more in the long run.
The solution is to change your filters when all their dirt-holding capacity is used up, but before the bypass valve opens. This requires a mechanism to monitor the restriction to flow (pressure drop) across the filter element and alert you when this point is reached. A clogging indicator is the crudest form of this device. A better solution is continuous monitoring of pressure drop across the filter.
Mistake No. 3 Running too Hot
Few equipment owners or operators continue to operate an engine that is overheating. Unfortunately, the same cannot be said when the hydraulic system gets too hot. But like an engine, the fastest way to destroy hydraulic components, seals, hoses, and the oil itself is a high-temperature operation.
How hot is too hot for a hydraulic system? It depends mainly on the viscosity and viscosity index (rate of change in viscosity with temperature) of the oil, and the type of hydraulic components in the system.
As the oil’s temperature increases, its viscosity decreases. Therefore, a hydraulic system is operating too hot when it reaches the temperature at which oil viscosity falls below that required for adequate lubrication.
A vane pump requires a higher minimum viscosity than a piston pump, for example. This is why the type of components used in the system also influences its safe maximum operating temperature.
Apart from the issue of adequate lubrication, the importance of which cannot be overstated, operating temperatures above 82 degrees Celsius damage most seal and hose compounds and accelerate degradation of the oil. But for the reasons already explained, a hydraulic system can be running too hot well below this temperature.
Mistake No. 4 Using the Wrong Oil
The oil is the most important component of any hydraulic system. Not only is hydraulic oil a lubricant, it is also the means by which power is transferred throughout the hydraulic system. It’s this dual role which makes viscosity the most important property of the oil, because it affects both machine performance and service life.
Oil viscosity largely determines the maximum and minimum oil temperatures within which the hydraulic system can safely operate. If you use oil with a viscosity that’s too high for the climate in which the machine must operate, the oil won’t flow properly or lubricate adequately during cold start. If you use oil with a viscosity too low for the prevailing climate, it won’t maintain the required minimum viscosity, and therefore adequate lubrication, on the hottest days of the year.
But that’s not the end of it. Within the allowable extremes of viscosity required for adequate lubrication, there is a narrower viscosity band where power losses are minimized. If operating oil viscosity is higher than ideal, more power is lost to fluid friction. If operating viscosity is lower than ideal, more power is lost to friction and internal leakage.
Using the wrong viscosity oil not only results in lubrication damage and premature failure of major components, it also increases power consumption (diesel or electricity) two things you don’t want.
And despite what you might think, you won’t necessarily get the correct viscosity oil by blindly following the blanket recommendations of the machine manufacturer.
Mistake No. 5 Wrong Filter Locations
Any filter is a good filter, right? Wrong! There are two hydraulic filter locations that do more harm than good and can rapidly destroy the very components they were installed to protect. These filter locations which should be avoided are the pump inlet and drain lines from the housings of piston pumps and motors.
This contradicts conventional wisdom: that it is necessary to have a strainer on the pump inlet to protect it from “trash”. First, the pump draws its oil from a dedicated reservoir, not a garbage can. Second, if you believe it’s normal or acceptable for trash to get into the hydraulic tank, then you’re probably wasting your time reading this article.
If getting maximum pump life is your primary concern (and it should be), then it’s far more important for the oil to freely and completely fill the pumping chambers during every intake than it is to protect the pump from nuts, bolts and 9/16-inch combination spanners. These pose no danger in a properly designed reservoir where the pump inlet penetration is a least 2 inches off the bottom.
Research has shown that a restricted intake can reduce the service life of a gear pump by 56 percent. And, it’s worse for vane and piston pumps because these designs are less able to withstand the vacuum-induced forces caused by a restricted intake. Hydraulic pumps are not designed to “suck”.
A different set of problems arises from filters installed on the drain lines of piston pumps and motors, but the result is the same as suction strainers. They can reduce service life and cause catastrophic failures in these high-priced components.
Mistake No. 6 Believing Hydraulic Components are Self-Priming and Self-Lubricating
You wouldn’t start an engine without oil in the crankcase not knowingly, anyway. And yet, I’ve seen the same thing happen to a lot of high-priced hydraulic components.
The fact is, if the right steps aren’t followed during initial start-up, hydraulic components can be seriously damaged. In some cases, they may work OK for a while, but the harm incurred at start-up then dooms them to premature failure.
There are two parts to getting this dilemma right: knowing what to do and remembering to do it. Not knowing what to do is one thing. However if you do know, but forget to do it, that’s soul-destroying. You can’t pat yourself on the back for filling the pump housing with clean oil when you forgot to open the intake isolation valve before starting the engine!
Similarly, a sudden decrease in delta P could indicate a rupture in the element – something a clogging indicator will never alert you to.
Before I discuss this, let’s consider some of the advantages and disadvantages of common filter locations.
Locating filtering media in the pressure line provides maximum protection for components located immediately downstream. Filtration rates of two microns or less are possible, due to the positive pressure (in comparison to an intake line filter) available to force fluid through the media (in comparison to an intake line filter). Filter efficiency may be reduced by the presence of high flow velocities, and pressure and flow transients, which disturb trapped particles. The major disadvantage of pressure filtration is economic. Because the housings and elements (high-collapse type) must be designed to withstand system operating pressure, pressure filtration has the highest initial and ongoing cost.
The rationale for locating filtering media in the return line is this: if the reservoir, and the fluid it contains, start out clean and all air entering the reservoir and returning fluid is adequately filtered, then fluid cleanliness will be maintained. The other advantage of the return line as a filter location is that sufficient pressure is available to force fluid through fine media – typically 10 microns, but pressure is not high enough to complicate filter or housing design. This combined with relatively low flow velocity, means that a high degree of filtering efficiency can be achieved at an economical cost. For these reasons, return filtration is a feature of most hydraulic systems.
Off-line filtration enables continuous, multi-pass filtration at a controlled flow velocity and pressure drop, which results in high filtering efficiency. Filtration rates of two microns or less are possible, and water absorbent filters and heat exchangers can be included in the circuit for total fluid conditioning. Off-line filtration has a high initial cost, although this can often be justified on a life-of-machine cost basis.
Filter condition monitoring
Warning of filter-bypass is typically afforded by visual or electric clogging-indicators. These devices indicate when pressure drop (delta P) across the element is approaching the opening pressure of the bypass valve (where fitted). In the case of a return filter for example, if the bypass valve opens at a delta P of 3 Bar, the clogging indicator will typically switch at 2 Bar.
Advanced filter condition monitoring
Replacing standard clogging-indicators with differential pressure gauges or transducers enables continuous, condition monitoring of the filter element. This permits trending of fluid cleanliness against filter element pressure-drop, which may be used to optimize oil sample and filter change intervals. For example, the optimal change for a return filter in a particular system could be higher or lower than the clogging indicator switching pressure of 2 Bar.
Continuous monitoring of filter pressure drop can also provide early warning of component failures and element rupture. For example, if the delta P across a pressure filter suddenly increased from 1 to 3 Bar (all other things equal), this could be an indication of an imminent failure of a component upstream. Similarly, a sudden decrease in delta P could indicate a rupture in the element – something that a standard clogging indicator will not warn of.