The smart way to troubleshoot hydraulics

Aug. 18, 2006
Plant managers, building engineers and project managers have an inherent stake in the productivity and health of their hydraulic equipment. When there’s a recurrent or chronic problem, it’s imperative to have background information or, better yet, a strong suspicion of what’s wrong before the service providers, inside or outside the company, are called.

Plant managers, building engineers and project managers have an inherent stake in the productivity and health of their hydraulic equipment. When there’s a recurrent or chronic problem, it’s imperative to have background information or, better yet, a strong suspicion of what’s wrong before the service providers, inside or outside the company, are called.

You don’t have to be a technocrat to analyze the current and past performance, maintenance information and failure data. Probable cause and direction determination isn’t a lengthy, time-consuming endeavor -- it doesn’t take long to narrow and focus the effort put forth by the troubleshooters. In addition, it’s smart business to be an informed consumer.

Too often, machine repair adopts the shotgun approach to troubleshooting. I was involved in a machine repair fiasco when a storage and retrieval machine that ran back and forth on a rail wouldn’t move. I was called in after two days of intense shotgun troubleshooting resulted in replacing the logic backplane, the 480-VAC motor and all the associated AC controls. The only thing left for them to replace was the wiring harness, which they were eyeing with great suspicion. Why no one pulled out a meter and looked for a more likely cause I’ll never know. It turned out that mechanical failure of an interlock contact in the forward circuit was keeping both forward and reverse contactors from energizing.

On the hydraulic side, it’s a cavitation problem that comes to mind. A pump on a machine was making a horrendous noise even after it had been replaced. The machine wasn’t functioning properly and the symptoms were the noise and the machine slowing to a crawl. A little common sense here may have avoided the expense and downtime associated with the pump replacement.

Cavitation is a vapor pressure phenomenon that produces tiny bubbles in the fluid, usually at the business end of a hydraulic pump. The bubbles implode and pit the metal impeller vanes, gears or pump housing. The result is premature wear that can occur in a very short time. Two possibilities should jump out at you right away: low fluid in the reservoir or a restriction in the fluid path that starves the pump. In this case, the intake strainer was partially blocked with debris (a piece of a rag) and the pump wasn’t getting the flow it required.

Diagnostic ports

I believe very firmly in watching and listening to a machine when it’s healthy because you’ll be receptive to it telling you a lot when it’s sick. If your job function doesn’t allow you the luxury of taking some time to be around your healthy machines, do the next best thing and talk to those who spend all their time with them. Operators with or without mechanical knowledge usually can give you volumes of data and direct observations that contain links and clues to the problems at hand. In addition, the day-to-day mechanical crew can provide a wealth of valuable information. Also, I’m a huge fan of test ports that accept a pressure gauge to permit monitoring key locations in the hydraulic circuit.

Ask your service team if the machines have enough test ports. If the answer is negative, add them during the next routine shutdown to provide valuable information in the unlikely event of a system failure. Then, gather pressure readings from each port during production and record the results along with temperature.

The following list of suspects suggests some common hydraulic problems and their root causes. Using them might help discover a solution. At the very least, they might provoke thought and direction that will lead you to a happy ending.

Suspect 1: A precipitating event

The first step in any troubleshooting routine is analyzing the last thing(s) that happened to the system before it began to fail. There’s almost always a single event that’s connected in some way and leads to the failure source. It might appear unrelated, it might have a fairly significant time lapse, but it can be proven to be directly linked to the failure. Look for a seemingly benign maintenance procedure, part replacement, preventive maintenance procedure, factory upgrade or the like. Ask the operator, maintenance personnel or anyone else who might have been involved with the equipment. Record the events for analysis, and document to the best of everyone’s recollection and from maintenance records exactly when the events occurred.

Retreat to a quiet area with your findings and look for an event or a trend. It might reveal a directional clue or perhaps a mistake that could have led to an inadvertent problem. Perhaps the replaced component wasn’t the correct part. Compare part numbers on the old and the new part. Perhaps the installer used Teflon tape (a definite no-no for hydraulic systems). Kinked hoses, pinched hoses or kinked lines are other possibilities. Verify that pressure readings on the machine gauges are what the OEM intended. The adjustments on the machine may have been manipulated incorrectly. Set-up procedures might have been skipped or performed improperly.

Suspect 2: Filtration

Look for signs of improper or inadequate filtering. Check the OEM’s filter change schedule and ensure that the recommended filter type was installed. The filter change might have introduced contaminants. Dirt or machine debris, including product or product waste, may have entered the system. Verify the filter rating for flow and filtering particle size meets OEM specifications. Verify that the strainer is clean. If someone added fluid to the reservoir, confirm that it was the correct fluid, not just something that was in the shop at the time.

Suspect 3: Contamination

Think about contamination. Foreign matter may have entered during an inspection or fluid check. Anytime the reservoir fill cap is removed, there’s a high risk of contamination from the machine environment or from simple carelessness. Teflon tape can be a real villain if a small piece enters the system. It can prevent a spool from shifting or block an orifice. Everyone assumes the passages in a hydraulic system are gargantuan, but this isn’t always the case. Inside of a large valve are small fluid passageways for logic or spool shift assistance.

Check the breather for plugging if the reservoir is open to atmosphere (most are). Reduced air flow here can cause lots of problems. Ensure that someone didn’t remove the filter medium and leave it open to the environment. Contamination could have been sucked into the system. My central air conditioning is hampered every year by cottonwood trees about half a mile away. The condenser draws cool air from every side and exhausts it out the top, but cottonwood seeds restrict the air flow.

Suspect 4: Heat

Watch out for heat. A general guideline is to keep the reservoir fluid temperature between 120°F and 150°F. The sight glass on the reservoir frequently includes a thermometer for checking fluid temperature. Hydraulic fluid begins to deteriorate at elevated temperatures and loses its lubricating quality, which, in turn, leads to more heat, which is the beginning of a death spiral.

Short component life, fluid life, inadequate lubricating properties are side effects of heat generation. Common causes of heat are improper valve, cylinder, pump or reservoir size. A reservoir that’s too small can’t dissipate system heat fast enough. Confirm that the heat exchanger is big enough and that it isn’t fouled. Someone could have thrown a rag over the cooling fins, partially blocking air flow. If the heat exchanger is water cooled, make sure the water is flowing.

If the problem is chronic, it might suggest improper sizing. If the problem is new, determine what changed to cause the difference.

Suspect 5: Components

Finally, think about the right component for the application. Using oversized components won’t really hurt your design, except for cost because device size is usually proportional to the price, which can become a limiting factor along with the physical size. For example, using a 1,000-amp disconnect for a toaster just doesn’t make a lot of sense.

In hydraulics, however, dissipating bypass energy in a grossly oversized pump can produce a tremendous amount of heat, getting us into the situation described above. But, sizing a pump correctly, then using undersized tubing also generates system heat. This problem usually isn’t an issue for existing machinery that has functioned well for a period of time, as the OEM has probably sized the components correctly. It becomes an issue when replacement parts are needed, but the original part isn’t available or is obsolete. Then the choice is up to the discretion of the installer, qualified or not.

Machine awareness, failure analysis and a structured approach to failure analysis will always prevail in problem-solving. And, most often, this powerful trio can do it at less expense and with less invasive procedures (which frequently lead to more failures) than alternative approaches. That’s the approach we use on car repairs, so why not use it on your machinery problems, too?

Steven P. Thomsen, P.E., is director of engineering services at PRW Associates Architects + Engineers in Rochester, N.Y. contact him at [email protected] and (585) 760-5333 x 102.

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