Have You Ever Really Got To Know Your Patient?

Proactive maintenance begets safety. A critical pathway to safety is world-class “total” hydraulic system maintenance. To that end, a vital component of my teachings is to teach engineers how to design maintenance friendly hydraulic systems. I also teach reliability engineers and maintenance planners how to take advantage of prudent design practices by developing world-class “total” proactive maintenance programs for hydraulic systems. The thrust of our collective work is to make it possible for our clients: maintenance personnel, to perform their work as safely as possible, and as effortlessly as possible.

In this article, I am going to address design and maintenance in the most sensitive part of a hydraulic system – a pump’s inlet.

Those of you that read my articles are probably saying, “here comes the bad news.” Regrettably, the hydraulics industry never fails to let me down in the “bad news” department; so here goes.

Here are some startling, but true, facts about the inlet side of hydraulic pumps:

Fact 1 - It’s a true statement “the lower a pump’s inlet restriction, the better.” However, the statement is meaningless without a “point-of-reference (POR)”. For example, if a pump manufacturer specifies that a pump’s MAXIMUM inlet restriction (under a given set of operating parameters), is 8” (20 cm) Hg., as I understand it, the pump manufacturer is saying that inlet restriction can be anywhere between 0” (0 cm) Hg and 8” (20 cm) Hg, but it cannot rise above 8” (20 cm) Hg.

It’s alarming how frequently this specification is misinterpreted by both engineers and mechanics. Both tell me that their interpretation of the specification is that if a pump’s inlet restriction remains below 8” (20 cm) Hg, a pump’s inlet restriction is hunky dory

Not so fast! This is where most engineers, and maintenance folks “blow it.” Upon start-up of any new pump the most vital operating parameter an engineer MUST immediately check is the pump’s inlet restriction to make certain it is within the pump manufacturer’s design specifications. The next most vital job, is to establish the pump’s “signature” inlet restriction.

For example, let’s say a pump manufacturer specifies that a given pump’s inlet restriction is 8” (20 cm) Hg MAXIMUM – the key word being “maximum.” The engineer installs a vacuum gauge at the pump’s inlet port, starts the pump, and observes the vacuum gauge. If the pump’s inlet restriction is higher than 8” (20 cm) Hg., it’s back to the drawing board. However, if the inlet restriction is, let’s say, 4” (10 cm) Hg., this value becomes the pump’s inlet restriction DNA for the rest of that pump’s life.

This post pump start up engineering function is vital because it:

a. Confirms that a pump’s inlet restriction meets the pump manufacturer’s specifications.

b. Confirm that the design of a hydraulic pump is satisfactory.

c.  Establishes the pump’s vital inlet restriction point-of-reference (POR) DNA

The pump’s vital POR MUST find its way into the respective machine’s service manual, so it can be used by reliability engineers to “trend” the respective pump’s inlet restriction throughout its life. It can also be used by maintenance mechanics to isolate problems at the inlet side of the pump.

Here’s are some examples of how reliability engineers will benefit from knowing a hydraulic pump’s inlet restriction POR:

1. Trend shows gradual rise in a pump’s inlet restriction above the pump’s POR:

Possible problems:

• Breather on hydraulic tank gradually deteriorating
• Screen in the hydraulic reservoir gradually deteriorating
• Increase in engine (pump) speed
• Flow adjusting screws on variable volume pump tampered with – increased flow.
• Incorrect oil
• Incorrect PTO drive ratio

2. Trend shows gradual decrease in a pump’s inlet restriction below the pump’s POR:

Possible problems:

• Air contamination of the oil - look for signs of “foam” in oil level sight glass. Oil complexion pale.
• Pump shaft seal leak – internal drain pumps only
• Flow adjusting screws on variable volume pump tampered with – decreased flow.
• Strainer not tightened correctly. Screws off due to vibration.
• Incorrect oil.

Here are examples of how a mechanic can benefit from knowing a hydraulic pump’s point of reference:

• If a hydraulic pump is making noise that is symptomatic of cavitation, the mechanic can use a vacuum gauge to determine the potential source of the problem. If the pump’s inlet restriction is higher than the POR, cavitation is suspect. On the other hand, if the inlet restriction is lower than the POR, pseudo-cavitation is suspect cause.

Case History: A mine in Utah ordered a hydraulic powered machine from a manufacturing company in California. It arrived in Utah via a low-bed transporter. In the process of driving the machine off the low-bed, the machine’s very expensive hydraulic pump suffered a catastrophic (fragment) failure. The manufacturer’s engineer told his client that the pump failed because the mechanic did not fill the pump housing with fluid before start-up. A new pump was airfreighted in from the manufacturer. The mine’s maintenance manager asked me to assist with the start-up of the new pump. I installed five diagnostic instruments on the pump prior to start-up, one of which was a vacuum gauge, which I installed at the pump’s inlet (as usual, this was no easy task because I had to search for the correct fittings to enable me to install the vacuum gauge).

The pump was an axial piston type. The maximum inlet restriction for a piston pump according to rule-of-thumb, is 4” (10 cm) Hg. The mechanic started up the diesel engine and slowly ramped up the engine speed. At 75% of engine speed the inlet restriction was a whopping 12” (30 cm) Hg. I indicated to the mechanic to shut the engine down. A long story short, the pump inlet had a serious design flaw. When I called the system’s design engineer, and politely explained the situation to him, he became defensive. He opined: “It was fine when we tested it at our plant prior to shipping.” I asked him what the pump’s inlet restriction was during the initial test. Why wasn’t I surprised when he confessed that he did not check it (but the machine’s service manual did recommend pump inlet restriction must be no more than 4” (10 cm) Hg.). In my humble opinion, had the engineer finished the task of designing the pump’s inlet, he would have seen that is was outside the pump manufacturer’s specifications. Why didn’t it fail in California, but failed in Utah? Good question! Strike one; Utah’s ambient temperature was 30oF lower than the temperature in sunny California. Strike 2; The machine’s current location was 5000 feet (1524 meters) above sea-level. The US$384K machine had to be shipped back to California so the engineer could do his job right the next time! Ouch!!! 

Fact 2 - Less than 5% of hydraulic system design engineers have, or know how to, determine a hydraulic pump’s inlet restriction point-of-reference (POR’s).

Fact 3 - Less than 1% of hydraulic system design engineers specify inexpensive adaptors (US$6.75) that would make it possible for assembly line workers, and/or maintenance technicians, to check a hydraulic pump’s vital inlet restriction on a routine maintenance schedule i.e., once every 30-days based on an 8-hour shift. The absence of non-invasive pressure and vacuum test devices literally forces maintenance personnel to ignore vital proactive maintenance procedures; expose themselves to jet-injection and burn injuries; expose hydraulic systems to airborne contamination; and, dump tens of thousands of gallons (liters) of hydraulic oil on mother earth. And people wonder why I’m cynical!!!!

Fact 4 - Machinery and equipment manufacturers are either not smart enough to know, or simply don’t care, that the vital pump inlet restriction specifications in their service manuals are invalid. The problem stems from the fact that pump specifications provided by the respective pump manufacturer are aimed at informing engineers, not maintenance personnel, about vital pump inlet specifications.

For example, the manufacturer of an axial piston pump specifies that a given pump’s inlet restriction must not exceed 4” Hg (mercury) at a given RPM, and oil temperature. It’s the design engineer’s responsibility to confirm the pump meets these specifications as soon as a pump is put into service. The problem is, less than 5% of design engineers perform this vital function, which means pump inlet restriction could conceivably exceed this critical parameter. The ignorance puts the pump on a path to inevitable premature failure.

Fact 5 - Now enter the manufacturer’s service manual. Manufacturer’s typically make photocopies of the respective pump manufacturer’s specifications and insert them in their service manuals. However, they typically never have reconciled the actual pump manufacturer’s specifications with empirical values, which makes the manufacturer’s specifications useless to the end user. In fact, it creates nothing but confusion.

If you have nothing better to day one day, call a machine manufacturer’s engineer and ask him/her for a given pump’s inlet restriction specifications. You will be instantly referred to page X of the machine's service manual where you will find a photocopy of the pump manufacturer’s pump inlet restriction specifications. Now, ask the engineer this million dollar questions: “I don’t want to know what the pump manufacturer’s inlet restriction specifications are, I want to know what you recorded prior to shipping the pump to me?" The most common response: “golly gee, I never checked it!”

Fact 6 - A hydraulic pump’s “living” inlet restriction specification is design dependent. That simply means that if two identical pumps are installed in different systems, their inlet restrictions will be different. It’s the design engineer’s responsibility to confirm that each pump’s inlet restriction meets the pump manufacturer’s specifications.

Conclusions:

1. Over the decades, I have wised up the fact that machinery and equipment manufacturers have no problem with the status quo. Untrained workers, poorly designed hydraulic systems, no oversight, and no accountability; it’s a license to steal.
2. Pump failure analysis data confirms that cavitation, and pseudo-cavitation, have caused, or contributed to, the demise of over 40% of open-loop pumps.
3. It is imperative for you to determine your hydraulic pump’s inlet restriction point-of-reference (POR), even if the designer ignored it - which is true in most cases. The specification is the only way for you determine the root cause of problems at the inlet side of your pump. If, for example, your axial piston pump’s POR is 3” (8 cm) Hg. Anytime your pump's inlet restriction rises, it is symptomatic of cavitation. On the other hand, if it decreases, it is symptomatic of pseudo-cavitation. Never ignore changes in pump inlet restriction, because whether the problem is cavitation, or pseudo-cavitation, the outcome will be the systemic demise of your entire hydraulic system. 
4. Untrained, and poorly trained, technicians contribute handsomely to corporate coffers because they generally know so little about how the inlet side of hydraulic pumps function that they ignore vital maintenance procedures, overlook critical problems, and ignore vital warning signs that ultimately lead to the complete destruction of pumps, and in many cases widespread systemic damage.

Here's what they typically don't know:

• A hydraulic does not "stand alone" for its performance. If a mechanic performs a pump flow test using only a flow meter, the test is inconclusive. Pump performance is affected by pump speed, and inlet restriction. Therefore a pump test is only valid if the test includes pump flow, pump speed (RPM), and pump inlet restriction.
• Hydraulic oil is “pushed” into and not “sucked” into pump.
• How to associate the operation at the inlet side of a hydraulic pump with atmospheric pressure
• How to associate reduced pump flow with problems at the inlet side of a pump.
• Pump cavitation “tears out” minute metal fragments, which are seeds that grow exponentially within a hydraulic system.
• How to take measures to insure the replacement pump they install in a hydraulic system does not fail for the same reason the original pump failed.
• How to differentiate between cavitation and pseudo-cavitation.
• Pseudo-cavitation is exponentially more harmful to a hydraulic system than cavitation.
• How to tell the difference between problems related to cavitation, and pseudo cavitation.
• Cause and effect of cavitation and pseudo cavitation.
• How to determine the inlet restriction point-of-reference (POR) for gear, vane and piston pumps.
• How cavitation and pseudo-cavitation effect the purpose of hydraulic oil.
• How to use a vacuum gauge.

Here's what they do know:

Is this problem bona fide “design oversight,” or is it a cunning way to generate tens of millions of windfall profits for an industry that has still yet to give birth to safety.

"Dumbing down" America's mechanics is a veritable “cash cow” for machinery and equipment manufacturers, because untrained workers don't have the experience and knowledge to call their bluff.

Regrettably, when untrained workers are injured or killed, their fate is simply written off as "collateral damage."

The fluid power industry rakes in hundreds of millions of dollars in profits compliments of untrained workers. Representatives from machinery and equipment manufacturers sit on the advisory boards of technical colleges to allegedly make sure mechanics are “dumbed down” by the system.

Takeaway from this article:

1. Make it a top priority to determine your pump’s inlet restriction point-of-reference (POR).

2. If it’s not within spec, and the cause is not maintenance related, contact the manufacturer immediately.

3. If you have lost pumps in the past due to cavitation, or pseudo-cavitation, file a warranty claim with the respective machine manufacturer.

4. Insist that the manufacturer update their specifications to reflect actual, rather than, make believe, pump inlet restriction.

The widespread ignorance about how the inlet side of a hydraulic pump functions, the causes and effects of cavitation, and pseudo cavitation, inspired me to design the brilliant MF200-CAV. See specs here https://www.fpti.org/pdf/FluidPower-MF200.pdf