The Top 2 Causes of Hydraulic Vane Pump Failure and How to Avoid them

One of our members wrote to me recently regarding the following problem:

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“Recently, we bought a used hydraulic power unit (15HP electric motor directly coupled to a vane pump). A high-pitched, clicking noise is generated when the unit runs. We have checked the following:

–We thought it was a motor bearing, so we detached pump from motor, no noise heard.
–Pressure line was connected to tank line (to simulate low pressure < 100 psi), very little noise heard.
–As pressure is increased, noise gets louder and louder, very intolerable.
–Measured current draw of electric motor – no overload.

What do you think could be causing the excessive noise?”

Given that the symptoms described above are consistent with a restriction at the pump inlet, I inquired if there was a suction filter in the circuit. Our reader replied:

“The system has a 40 micron suction filter but I have not checked it because I have to drain the oil and take off the access hatch to get to the filter.”

The restriction caused by a suction filter, which increases at low oil temperatures (high viscosity) and as the element clogs, increases the chances of a partial vacuum developing at the pump inlet. Excessive vacuum at the pump inlet causes cavitation erosion and mechanical damage.

Cavitation Erosion

When a partial vacuum develops in the pump intake line, the decrease in absolute pressure results in the formation of gas and/or vapor bubbles within the oil. When these bubbles are exposed to elevated pressures at the pump outlet they implode violently. When bubbles collapse in proximity to a metal surface, erosion occurs. Cavitation erosion contaminates the hydraulic oil and damages critical surfaces.

Mechanical Damage

When a partial vacuum develops at the pump inlet, the mechanical forces induced by the vacuum itself can cause catastrophic failure. In vane pump designs, the vanes must extend from their retracted position in the rotor during inlet. As this happens, fluid from the pump inlet fills the void in the rotor created by the extending vane. If excessive vacuum exists at the pump inlet – it will act at the base of the vane. This causes the vanes to lose contact with the cam ring during inlet, and they are then hammered back onto the cam ring as pressurized fluid acts on the base of the vane during outlet (figure 1). The impact damages the vane tips and cam ring, leading rapidly to catastrophic failure.

Figure 1. Vane pump section (Bosch Rexroth Corp).

The intolerable noise our reader is referring to is symptomatic of cavitation bubble collapse and possibly, the vanes being hammered against the cam ring. Both of these conditions are intensified by increasing system pressure.

The solution to our reader’s problem is simple: replace the suction filter or better still, discard it completely. If suction filtration must be installed, follow these precautions to prevent pump damage:

A filter located outside of the reservoir is preferable to a suction strainer. The inconvenience of servicing a filter located inside the reservoir is a common reason why suction strainers go unserviced – until after the pump fails.

–If a suction strainer is installed, opt for 250 microns rather than the more common 150 microns.

–The filter should be grossly oversized for the pump’s flow rate to ensure that pressure drop is minimized, even under the most adverse conditions.

–Regardless of the type of filter employed, it must incorporate a bypass valve to prevent the element from creating a pressure drop that exceeds the safe vacuum limit of the pump.

–A gauge or transducer should be installed downstream of the filter to enable continuous monitoring of absolute pressure at the pump inlet.

If hydraulic systems are part of your operations, then it’s highly likely you will one day need to troubleshoot and repair a vane-type pump or motor. While today’s products are robust, reliable and highly efficient, they are still mechanical devices and subject to eventual failure due to normal wear and, more often, abuse.

When vane pumps fail, there is almost always a proximate event — an overload, a leak, or some such circumstance that can be pointed to as the cause of the failure.

These obvious, visible causes of failure are often simply the last step in a process that has been going on for months or years. The truth is that more than 80% of all failures in vane-type pumps and motors can be traced back to a single cause, and that cause is dirty hydraulic fluid.

The first tip I’m going to offer is simply this — keep the fluid clean.

Your filtration plan is the first line of defence against failure of all your hydraulic components, not just vane-type pumps and motors. That’s right, you need to develop and implement a filtration plan to keep your fluids clean. The plan should include cleanliness targets appropriate to the system and application environment, and detailed procedures for maintaining them.

If 80% of all hydraulic failures are caused by dirty fluid, then fluid cleanliness should be every bit as much a part of a system design as the selection of the pump, valves, actuators and bearings. Unfortunately, when some system designers select a filter, they look no further than a filter manufacturer’s catalogue, with little regard for the particular system’s total requirements.

Proper selection and placement of contamination-control devices in a system to attain the targeted cleanliness eliminates the root cause of up to 80% of hydraulic system failures. Additionally, a focus on system cleanliness assures the user of a cost-effective approach to contamination control that allows the price of the filters and elements to be quickly recovered by savings from improved performance, increased component life, increased oil life, increased uptime and fewer repairs.

The intricacies of designing and maintaining effective filtration systems are beyond the scope of this article. Fortunately, there are a number of good references available from filtration, fluid and equipment suppliers. One such manual is Eaton’s Guide to Systemic Contamination Control, available for free download at http://hydraulics.eaton.com/products/ filtration.htm.

The first step in troubleshooting dirty fluids is understanding where the contamination comes from. There are four basic sources:

* Contaminated new oil

* Built-in contamination

* Externally sourced contamination

* Internally generated contamination.

Here are some essential procedures to follow regarding dirty fluids.

Don’t assume new fluid is clean. Never assume that hydraulic fluids are clean simply because they are new, and never install hydraulic fluid that you have not filtered yourself immediately before it’s placed in your equipment. A portable transfer cart equipped with high-efficiency filters is very cheap insurance.

Even if it’s new, clean the machine yourself: A new or rebuilt machine typically has been flushed to remove any contaminants introduced during the manufacturing process, but regardless of how conscientious the machine builder might have been, you still need to ‘run-in’ any new or rebuilt equipment with no load applied — and aggressively filter the fluid while you’re doing it.

Good housekeeping pays big dividends: Most hydraulic systems don’t live in clean-room environments, so you need to be especially careful about keeping environmental contamination out of your equipment. There are four major places where dirt will gain access to your system:

* Reservoir breathers and vents

* Access plates

* Shaft and cylinder seals

* Any and all components opened during maintenance.

What you don’t see can be the worst of all: Actually, the most destructive contamination in nearly any system is minute metal particles that come off the pump’s internal components. If you don’t remove them very quickly, one particle becomes two particles, two become four, four become eight and soon your precision hydraulic components begin to self-destruct.

Troubleshooting

At the risk of being accused of beating a dead horse, let me say once more that 80% of all hydraulic system problems are directly traceable to contaminated fluids. So it makes sense to begin troubleshooting any system failure by learning to identify the kind damage dirty fluids produce.

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Marked or ‘frosted’ vanes (Figure 1), grooved shaft seal diameters (Figure 2), or a ‘chopped’ ring (Figure 3) are all classic signs of dirty fluid. You know the drill: find the source of the contamination and eliminate it before you repair the pump or motor, flush the system, and put it back into operation. Then get even more aggressive about your filtration program.

Cavitation damage

The major cause of component damage that isn’t related to dirty fluids is cavitation. This occurs in two different ways, but the results are the same.

Cavitation is essentially the process of bubbles forming in the fluid and then imploding. When a bubble implodes, it generates extremely high temperatures, high enough to weaken and even melt the metal parts of your system. Repeat the process tens of millions of times, and the result is a seriously eroded surface and a non-functional component.

The first source of cavitation is air leaking into the fluid. If this continues, the result is the kind of catastrophic damage seen in Figures 4 and 5. Typical causes include:

* Suction line allowing inlet of air

* Shaft seals worn, allowing ingestion of air

* Reservoir problems

* Low oil level

* Poor baffling

* Reservoir inlet too high

* Reservoir too small

* Unsuitable fluid.

The second form of cavitation comes from exerting high forces on the fluid while it’s moving through the system. In this case, the fluid is literally ripped apart to form voids with a high internal vacuum. The implosion of these voids can be extremely destructive, as illustrated in Figure 6. Typical causes of damage include:

* Excessive inlet velocity

* Poor reservoir and/or system design

* Clogged inlet strainer and filters

* Restricted fluid flow

* High water content in oil

* A pump that is running over the recommended rpm.

Erosion damage

Erosion damage can look a lot like cavitation damage, as shown by Figure 7, but it occurs when a high-speed oil stream propels contaminant particles against a surface. Conceptually, it can be thought of as a form of sandblasting using extremely small particles.

Erosion damage usually occurs at metering edges or critical surfaces, and tends to be less severe and extensive than cavitation damage. An aggressively applied filtration plan is your best defence against erosion damage, although using a fluid above the recommended ISO level can also produce erosion.

Catastrophic damage

Catastrophic failures are easy to spot because something is normally destroyed. Typical causes include over-pressurization, air locks, misalignment, improper modifications, component misapplications and incorrect assembly procedures. Examples are shown in Figures 8, 9 and 10.

Don’t forget, however, that a catastrop
hic failure may be the final result of a long-term problem traceable to dirty fluids. Your filtration plan is your best insurance against these failures, too.

The key to avoiding catastrophic failure is the same as the key to avoiding all of the other failures that may befall a hydraulic system. Keep the fluid clean. Do that and you will find hydraulic troubleshooting and repair taking up a very small part of your workday.

Kyle Janssen is a vane pump engineer with Eaton Hydraulics Operations, Eaton Corp., Eden Prairie, Minn.

OVERHAULING A VANE PUMP

Vane-type pumps are easily overhauled in the field without any special tools. The process begins by disconnecting the power source and removing the bolts holding the cover plate to the pump housing. Be sure to mark the pump body and cover so they can be reassembled in the proper orientation.

Slide the cartridge out of the housing and place it on a bench.

Scribe a line across the outer surface of the cartridge kit to provide a reference for parts during assembly.

Place the cartridge on a flat surface (outlet support plate down) and remove the two socket head screws.

Slide the inlet support plate and seal packs off the cartridge.

With the outlet support plate pointing up, slide the outlet support plate and seal packs off the cartridge; do not allow the flex side plate to slide with the support plate. Move the flex side plate off centre just enough to lift up and away without sliding.

Remove the cam ring from the rotor and vanes. Locate the arrow stamped into the rotor periphery. Remove the vanes and inserts in order, starting at the arrow. Keep them in order for inspection.

Do not remove cartridge-locating pins from the inlet support plate unless they are damaged. The pins are of a drive-loc type and can be difficult to remove.

ASSEMBLY

Reverse the disassembly sequence, noting the following points.

Coat all parts except seals and back-up rings with clean hydraulic fluid. Use small amounts of petroleum jelly to hold the O-rings in place.

All sharp edges on a new cartridge kit should be stoned prior to installation.

The O.D. of all component parts of the cartridge kit must be in line with each other or the cover cannot be installed.

Check rotor for bind by inserting an index finger through the shaft opening of the inlet support plate. Hold the cartridge kit in a horizontal shaft position and lift the rotor with the finger. The rotor should move freely back and forth within the cartridge. If the rotor binds, open the kit, clean and stone all possible areas of bind, then reassemble using the aforementioned procedure. The rotor must move freely within the cartridge when assembled.

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