Seven Deadly Hydraulic System Sins

Seven Deadly Hydraulic System Sins
Seven Deadly Hydraulic System Sins
Seven Deadly Hydraulic System Sins

Optimal maintenance and reliability of hydraulic equipment requires a certain level of knowledge (and intervention) on the part of the end user. Unfortunately, this knowledge is often lacking.

As a result, hydraulic machines may be unfairly branded as unreliable and expensive to operate. With that in mind, here are seven common mistakes hydraulics users make—at least one of which you are probably making right now.

Mistake 1 – Not analyzing oil

Only two conditions necessitate changing hydraulic oil: degradation of the base oil or depletion of the additive package. Because so many variables 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 shooting in the dark.

Given the current high price of oil, dumping oil that doesn’t need to be changed is a big waste of money. The larger the reservoir, the more expensive this mistake becomes.

On the other hand, if you continue to operate with the base oil degraded or additives depleted, you compromise the service life of every component in the hydraulic system. That’s the last thing you want to do.

As you can see, changing hydraulic oil on a fixed number of hours in service is a bad idea for all but the smallest of hydraulic systems. The only way to know when the oil needs to be changed is through oil analysis.

Mistake 2 – Not monitoring filters
A similar situation applies to hydraulic filters. If you change them based on hours of use, 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 resulting 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 ideal is to change filters when all their dirt-holding capacity is used up, but before the bypass valve opens. That requires a mechanism to monitor flow restriction to flow (pressure drop) across the filter element and alert you when the ideal changing point is reached.

A clogging indicator is the crudest form of this device. Although such an indicator is better than using nothing, the better solution is a using a differential pressure gauge or transducer that continuously monitors pressure drop across the filter.

Mistake 3 - Running too hot
Few equipment owners or operators would continue to run an engine that was overheating. Unfortunately, the same can’t be said when the hydraulic system gets too hot. But as with an engine, the fastest way to destroy hydraulic components, seals, hoses, and the oil itself is high-temperature operation.

But how hot is too hot for a hydraulic system? It depends mainly on the viscosity and the 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 goes up, its viscosity goes down. A hydraulic system is operating too hot when it reaches the temperature at which oil viscosity falls below the level needed for adequate lubrication.

A vane pump requires a higher minimum viscosity than a piston pump, for example. That’s why the type of hydraulic components used in a system affects its safe maximum operating temperature.

If your hydraulic system contains a vane pump, the minimum viscosity you should be looking to maintain is 25 centistokes (cSt or mm2/s). For mineral oils with a viscosity index of about 100, this equates to a maximum allowable operating temperature of 35° C (95° F) if you’re using ISO VG22 oil, or 65° C (149° F) for ISO VG68.

Apart from the lubrication issue—the importance of which can’t be overstated—operating temperatures above 82° C (179° F) damage most seal and hose compounds and accelerate degradation of the oil. But for the reasons explained previously, a hydraulic system can be running too hot well below this temperature.

Mistake 4 – Using the wrong oil

The oil is the most important component of any hydraulic system. Not only is hydraulic oil a lubricant, it’s also the means by which power is transferred. This dual role makes viscosity an oil’s most important property. It affects both machine performance and service life.

Expanding on what I said about Mistake 3, the viscosity of the oil largely determines a hydraulic system’s safe maximum and minimumoil temperatures. This is sometimes referred to as the temperature operating window (TOW).

If you use oil with a viscosity that’s too high for the climate the machine has to operate in, the oil won’t flow properly or lubricate adequately during cold start. If you use oil with a viscosity that’s too low for the prevailing climate, it won’t maintain the required minimum viscosity, and therefore adequate lubrication, on the hottest days.

That’s not the end of it. Within the allowable extremes of viscosity required for adequate lubrication, there’s a narrower viscosity band where power losses are minimized. If the oil’s operating viscosity is higher than ideal, more power is lost to fluid friction. If operating viscosity is lower than ideal, more power is lost to mechanical friction and internal leakage.

So using oil of the wrong viscosity not only results in lubrication damage and premature failure of major components, it also increases power consumption (diesel or electric)—two things you don’t want.

You won’t always get viscosity right by blindly following the machine manufacturer’s oil recommendation. The only way to be certain is to check that your machine’s actual temperature operating window lies within the allowable TOW, and, ideally, within the optimum TOW for the oil you’re using.

Mistake 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 that can rapidly destroy the very components they were installed to protect.

These to-be-avoided filter locations are the pump inlet, and the piston pump and motor case drain lines. At this point, it wouldn’t surprise me if you’re shaking your head in disagreement. After all, this flies in the face of the conventional wisdom that says you have to have a strainer on the pump inlet to protect it from “trash.”

Well firstly, the pump draws its oil from a dedicated reservoir, not a garbage can. Secondly, 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 cycle than it is to protect the pump from nuts, bolts, and 9/16” combination wrenches, which pose no danger in a properly designed reservoir that has the pump inlet penetration at least four inches off the bottom.

Research has shown that a restricted intake can reduce the service life of a gear pump by 56%. It’s even 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 expensive components.

If these filters are fitted to any of your hydraulic machines and you don’t get rid of them, there’s a good chance they’ll end up causing a premature failure.

If you’re still not convinced or are nervous about discarding a filter the machine manufacturer thought was wise to install in the first place, check with the pump or motor manufacturer. If you do manage to find a hydraulic pump or motor manufacturer who recommends the use of suction strainers and/or conventional depth filters on case drain lines, please write to me, care of Lift and Access.


Mistake 6 – Believing in self-priming and self-lubricating

You wouldn’t knowingly start an engine without oil in the crankcase. Yet I’ve seen what amounts to the same thing happen to a lot of pricey hydraulic components.

Fact is, if the right steps aren’t followed during initial startup, hydraulic components can be seriously damaged. In some cases, they may work OK for a while, but the harm done at startup dooms them to premature failure.

You’d be amazed at the number of these types of failures that wrongly end up as warranty claims by hydraulic equipment owners. It’s frustrating for everyone concerned because they’re totally preventable.

There are two parts to getting this right: knowing what to do and remembering to do it. If you don’t know what to do, that’s one thing. If, however, you do know but forget to do it, well 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.

This sort of mistake is easily prevented by using a startup procedure and checklist. I don’t know about you, but these days I don’t like relying too much on memory, not for important stuff anyway.

So even after 20-plus years of working on hydraulic equipment, I would never attempt to commission or re-commission a hydraulic system without having a piece of paper to remind me what I need to do and the order in which to do it. This simple technique eliminates all possibility of error.


Mistake 7 – No hydraulic education

As I hope this article has shown, if you own, operate, repair, or maintain hydraulic equipment and you aren’t “clued up” on hydraulics, a lot of money can slip through your fingers.

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