Are you making these seven mistakes with your truck’s hydraulic system?

Optimal reliability of any hydraulic system requires a certain level of knowledge – and intervention ­– on the part of the end user. Because a truck’s hydraulic system typically only sees intermittent usage, it’s likely that it isn’t given much attention until it breaks down.

The reality is, the majority of hydraulic problems are caused by one of the following seven mistakes, at least one of which you are probably making right now. 

MISTAKE #1 – OIL CHANGES

There are only two conditions that necessitate a hydraulic oil change. They are:

- Degradation of the base oil.

- 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 by months or 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. The larger the reservoir, the more expensive this mistake.

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. That is the last thing you want to do.

Changing hydraulic oil on a fixed interval or hours-in-service is a bad idea for all but the smallest of hydraulic systems. The only way to know when the oil does need to be changed is through oil analysis.

You should be doing regular oil analysis on your truck’s engine. It is wise to include the hydraulics in your oil analysis program as well.

MISTAKE #2 – FILTER CHANGES

A similar situation applies to hydraulic filters. If you change them on interval or hours, you’re either changing them 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 other component in the hydraulic system, and that ends up costing a lot more in the long run.

The solution is to change filters when all of 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 to alert you when this point is reached.

A clogging indicator is the crudest form of this device. Although better than nothing, continuous monitoring of pressure drop across the filter using a differential pressure gauge is the better solution. Such gauges can be used to indicate the optimal time to change a filter element.

MISTAKE #3 - TOO HOT

There are not too many truck owners or drivers who would continue to operate an engine that was overheating. Unfortunately, the same can’t 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 high-temperature operation.

But how hot is too hot for a hydraulic system? It mainly depends on the hydraulic oil’s viscosity (how thick or thin the oil is) and its viscosity index (rate of change in viscosity with temperature), along with the type of hydraulic components in the system.

As the oil’s temperature increases, its viscosity decreases. A hydraulic system is operating too hot when it reaches the temperature at which oil viscosity falls below that required for adequate lubrication.

It should be noted that a vane pump requires a higher minimum viscosity than a piston pump. This is why the type of hydraulic components used also influences the system’s safe maximum operating temperature.

If your hydraulic system contains a vane pump, the minimum viscosity you should be looking to maintain is 25 centistokes. For mineral oils with a viscosity index of around 100, this equates to a maximum allowable operating temperature of 35 degrees C if you’re using ISO VG22 hydraulic oil or 65 degrees C for ISO VG68.

Apart from the lubrication issue – the importance of which can’t be overstated, operating temperatures above 82 degrees C 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 - 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 throughout the hydraulic system.

It’s this dual role which makes viscosity the most important property of the oil because it affects both system performance and service life.

Expanding on what was pointed out in Mistake #3 - Running too hot, the viscosity of the oil largely determines the maximum and minimum oil temperatures within which the hydraulic system can operate safely. 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 system has to operate in, the oil won’t flow properly or lubricate adequately during a 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 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 mechanical 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. These are two things you don’t want.

Now despite what you might think, you won’t always get this right by blindly following the manufacturer’s oil recommendation. The only way to be certain is to check that your truck’s actual TOW lies within the allowable TOW, and ideally, within the optimum TOW for the hydraulic oil you are using.

MISTAKE #5 - WRONG FILTER LOCATIONS

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 to-be-avoided filter locations are the:

- Pump inlet.

- Piston pump and motor case drain lines.

At this point, you may be shaking your head in disagreement. After all, this flies in the face of conventional wisdom that says you have to have a strainer on the pump inlet to protect it from “trash.”

Firstly, the pump draws its oil from a dedicated reservoir not a “garbage can.” Secondly, it’s wrong to believe that it’s normal or acceptable for trash to get into the hydraulic tank.

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 spanners. These pose no danger in a properly designed reservoir where the pump inlet penetration is a least 4” off the bottom.

Research has shown that a restricted intake can reduce the service life of a gear pump by 56 percent. It is 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 arise from filters installed on the drain lines of piston pumps and motors. However, 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 installed in a truck’s hydraulic system and you don’t get rid of them, there’s a good chance they’ll end up causing premature failures. If you’re still not convinced, or are nervous about discarding a filter the system manufacturer thought was wise to install in the first place, check with the pump or motor manufacturer.  

MISTAKE #6 – THE BELIEF THAT HYDRAULIC COMPONENTS ARE SELF-PRIMING AND SELF-LUBRICATING

You wouldn’t start an engine with no oil in the sump – not knowingly anyway. 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 start-up, hydraulic components can be seriously damaged. In some cases they may work okay for a while, but the harm done at start-up dooms them to premature failure.

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

There are two parts to getting this right:

- Know what to do.

- Remember to do it.

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 and engaging the PTO.

This sort of mistake is easily prevented by using a start-up procedure and checklist. Relying on memory, especially for important things, is not wise.

Even after 25-plus years 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 of what I need to do and the order I need to do it in. This simple technique eliminates all possibility of error.

MISTAKE #7 - NOT GETTING AN EDUCATION IN HYDRAULICS

If your truck has an onboard hydraulic system and you aren’t “clued up” on hydraulics, a lot of money can slip through your fingers. If you’d like to avoid the head-scratching and be “the guy” who knows hydraulic, keep your knowledge about the care and maintenance of hydraulic systems up-to-date.

There is considerable information available online, including whitepapers, reports, hydraulic and webcasts.

Brendan Casey is the founder of HydraulicSupermarket.com and the author of “The Hydraulic Maintenance Handbook,” “Insider Secrets to Hydraulics,” “Preventing Hydraulic Failures,” “The Hydraulic Troubleshooting Handbook,” “Hydraulics Made Easy,” “Advanced Hydraulic Control” and “The Definitive Guide to Hydraulic Troubleshooting.” A hydraulics specialist with an MBA, he has more than 25 years of experience in the design, maintenance and repair of mobile and industrial hydraulic equipment.