The importance of hydraulic equipment maintenance

Over the last two and a half years, I have received a lot of feedback from my Newsletter readers. One question that pops up regularly is: "Why do you give away so much valuable information in your Newsletter and on your web sites?"

I've spent the better part of 16 years working in and running hydraulic repair shops i.e. rebuilding hydraulic components. During this time I kept seeing the same pattern: Failed hydraulic component comes into shop along with concerned customer who wants to know why it has failed after a relatively short time in service. Based on what I saw after tear-down, I would explain the cause of failure - for example, high contamination levels, wrong oil viscosity, high temperature operation, cavitation, faulty circuit protection devices and so on. Customer leaves thousands of dollars poorer with rebuilt component and a hard-learned lesson on hydraulic equipment maintenance.

For as long as there are hydraulic equipment owners, mechanics and maintenance people out there who believe that hydraulics don't require any special kind of attention, this cycle will continue. In an effort to bridge the knowledge gap on what needs to be done to get maximum life from hydraulic components, I have written Insider Secrets to Hydraulics and it's sequel, Preventing Hydraulic Failures.

Over the past 30 years, the performance, sophistication and operating pressures of hydraulic equipment have increased significantly. This is particularly true in the case of mobile hydraulic equipment. As a result, modern hydraulic equipment is not only more expensive to fix when it breaks, proactive maintenance is imperative to maximize service life and minimize operating costs. It's not realistic to expect (as many equipment owners do) to run a hydraulic machine for 10,000 hours, without checking anything more than the fluid level, and not have any problems.

Six routines must be followed in order to minimize the chances of your hydraulic equipment suffering costly, premature component failures and unscheduled downtime:

Maintain fluid cleanliness;
Maintain fluid temperature and viscosity within optimum limits;
Maintain hydraulic system settings to manufacturers' specifications;
Schedule component change-outs before they fail;
Follow correct commissioning procedures; and
Conduct failure analysis.

An effective, proactive maintenance program requires time, effort and some expense to implement. But it is cost-effective. The investment is quickly recovered through savings as a result of improved machine performance, increased component life, increased fluid life, reduced downtime and fewer repairs.

So if you own, operate or maintain hydraulic equipment, are serious about minimizing your running costs and your current maintenance practices are unsophisticated or non-existent, start implementing a proactive maintenance program today. Reading my 'Inside Hydraulics' Newsletter is certainly a step in the right direction.

ABOUT THE AUTHOR: Brendan Casey has more than 18 years
experience in the maintenance, repair and overhaul of
mobile and industrial hydraulic equipment. For more
information on reducing the operating cost and increasing
the up-time of your hydraulic equipment, visit his
web site: http://www.InsiderSecretsToHydraulics.com

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Hydraulic motors - how dry starts damage them

I was asked recently to conduct failure analysis on a hydraulic motor that was the subject of a warranty claim. The motor had failed after only 500 hours in service, some 7,000 hours short of its expected service life.

Inspection revealed that the motor's bearings had failed through inadequate lubrication, as a result of the hydraulic motor being started with insufficient fluid in its case (housing).

A common misconception among maintenance personnel with limited training in hydraulics, is that because oil circulates through hydraulic components in operation, no special attention is required during installation, beyond fitting the component and connecting its hoses. Nothing could be further from the truth.

After this hydraulic motor was installed, its case should have been filled with clean hydraulic oil prior to start-up. Starting a piston-type motor or pump without doing so, is similar to starting an internal combustion engine with no oil in the crankcase - premature failure is pretty much guaranteed.

Some of you may be thinking that the case should fill with hydraulic fluid through internal leakage. In most cases it will, but not before the motor or pump has been damaged. In many cases, this damage may not show itself until the component fails prematurely, hundreds or even thousands of service hours after the event.

In this particular example the warranty claim was rejected on the basis of improper commissioning and the customer was lumbered with an expensive repair bill.

How can this type of failure be prevented?

This example highlights the importance of following proper commissioning procedures when installing hydraulic components. As in this example, if the case of the hydraulic motor had been filled with fluid prior to start-up, the failure of this motor and the significant expense of its repair could have been prevented.

Editor's note: for more information on hydraulic failures and how to prevent them, read Preventing Hydraulic Failures.

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Hydraulic system troubleshooting - check the easy things first

In Part II of Insider Secrets to Hydraulics, I outline a logical approach to hydraulic system troubleshooting that begins with checking and eliminating the easy things first. The benefits of this approach are clearly illustrated by a troubleshooting situation I was involved in recently.

The machine in question had a complex hydraulic system, the heart of which comprised two engines driving ten hydraulic pumps. Six of the pumps were variable displacement pumps and four of these had electronic horsepower control.

The symptoms of the problem were slow cycle times in combination with lug-down of the engines (loss of engine rpm). The machine had just been fitted with a new set of pumps.

The diagnosis of the mechanic in charge was that the hydraulic system was tuned above the power curve of the engines i.e. the hydraulics were demanding more power than the engines could produce, resulting in lug-down of the engines and therefore slow cycle times. The other possible explanation of course, was that the engines were not producing their rated horsepower.

Due to the complexity of the hydraulic system, I knew that it would take around four hours to run a complete system check and tune-up. So in order to eliminate the easy things first, when I arrived on site I inquired about the condition of the engines and their service history. The mechanic in charge not only assured me that the engines were in top shape, he was adamant that this was a "hydraulic" problem.

Four hours later, after running a complete check of the hydraulic system without finding anything significant, I was not surprised that the problem remained unchanged. After a lengthy discussion, I managed to convince the mechanic in charge to change the fuel filters and air cleaner elements on both engines.

This fixed the problem. It turned out that a bad batch of fuel had caused premature clogging of the engine fuel filters, which were preventing the engines from developing their rated horsepower.

If the relatively simple task of changing the engine fuel filters had been carried out when the problem was first noticed, an expensive service call and four hours of downtime could have been avoided.

Learn more about hydraulic troubleshooting techniques.

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Adding hydraulic oil - without the dirt

Hydraulic fluid straight from the drum, has a typical cleanliness level of ISO 4406 21/18.

A 25 GPM pump operating continuously in hydraulic oil at this cleanliness level will circulate 3,500 pounds of dirt to the hydraulic system's components each year!

To add hydraulic oil, and not the dirt, always filter new oil prior to use in a hydraulic system.

This can be accomplished by pumping the oil into the hydraulic reservoir through the system's return filter. The easiest way to do this is to install a tee in the return line and attach a quick-connector to the branch of this tee. Attach the other half of the quick-connector to the discharge hose of a drum pump.

When hydraulic oil needs to be added to the reservoir, the drum pump is coupled to the return line and the oil is pumped into the reservoir through the return filter. As well as filtering the oil, spills are avoided and the ingress of external contamination is prevented.

The benefits of carrying out this simple modification are well worth the minor cost involved.

Learn more about contamination and hydraulic oil cleanliness.

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Hydraulic pump and motor case drains - filter with caution

One of our readers wrote to me recently with the following question: "We have recently been involved in designing and building a hydraulic machine. The system has three, separate circuits each with an axial-piston pump and a common reservoir. Case drain filtration was included to reduce the possibility of cross contamination if a failure occurs. After contacting the pump manufacturer I was led to believe that it isn't the norm, but if the pressure drop across the filter is kept to less than 30 PSI it will be OK. This just forces filter maintenance. What filter beta or micron rating should be used?"

Installing filters on piston pump and motor case drain lines can result in excessive case pressure, which causes seal failure and mechanical damage.

Seal failure

High case pressure results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, which eventually results in leakage past the seal. If case pressure exceeds the shaft seal's design limits, instantaneous failure can occur. The subsequent loss of oil from the case can result in damage through inadequate lubrication.

Mechanical damage

The effect of high case pressure on axial piston pumps is the same as excessive vacuum at the pump inlet. Both conditions put the piston-ball and slipper-pad socket in tension during inlet (see below). This can cause buckling of the piston retaining plate and/or separation of the slipper from the piston, resulting in catastrophic failure.

High case pressure can cause the pistons of radial piston motors to be lifted off the cam. This can occur in operation during the outlet cycle. The pistons are then hammered back onto the cam during inlet, destroying the motor. If residual case pressure remains high when the motor is stopped, loss of contact between the pistons and cam can allow the motor to freewheel, resulting in uncontrolled machine movement.

To avoid these problems, pump and motor case drain lines should be returned to the reservoir through dedicated penetrations. These penetrations must be higher than the unit's case port and be connected to a drop-pipe inside the reservoir that extends below minimum fluid level. For the reasons outlined above, filters are not recommended on case drain lines. While this does allow a small percentage of fluid to return to the reservoir unfiltered, in most applications the contamination risk is low and can be effectively managed using oil analysis and other condition-based maintenance practices.

Filter with caution

If a filter is fitted to a pump or motor drain line, I recommend a 125-micron screen, grossly oversized for the maximum expected flow rate. The filter housing must incorporate a bypass valve with an opening pressure lower than the maximum, allowable case pressure for the particular component (typically 5-15 PSIG). Installing a gauge or transducer upstream of the filter for monitoring case pressure is also advisable. To learn more about the failure modes of hydraulic components and how to avoid them, read Preventing Hydraulic Failures available here.