Why Hydraulic Systems Crash - Hydraulic Oil Temperature

My series of articles titled "Why Hydraulic Systems Crash," have a common thread, all the problems I explore stem from poor design. Design engineers have ignored the problems my articles relate to for so long that I am forced to believe that it’s a case of planned obsolescence.

In my first article, I addressed pump inlet restriction. The previous article was about pump/motor case pressure. In this article, I am going to address the issue of oil temperature.

Have you noticed how machinery and equipment manufacturers are quick to blame "dirt and heat" for the root cause of almost all premature hydraulic system failures? The blame for the problems is usually shouldered by the owners of hydraulic systems. However, what most owners and operators of hydraulic systems don’t seem to realize is that it’s not poor maintenance that is to blame for the problem, it’s poor hydraulic design that is the root cause of the problem. Let me explain.

When the internal combustion engine was first developed, it was so unreliable that engine design engineers felt it was important enough the put separate engine oil pressure, and coolant temperature gauges, in front of the driver; even though over 98% of drivers had no clue about engine oil pressure, or coolant temperature. I am going to guess the gauges helped vehicle owners save tens of thousands of dollars because, even though they knew little or nothing about either system, they knew if oil pressure went down, or coolant temperature went up, it was time to shut the engine off.

Today’s engines are so reliable that there is arguably no reason for a driver to be concerned about engine oil pressure, or coolant temperature. However, you will be hard pressed to find a vehicle that is not equipped with engine oil pressure, and oil temperature warning lights. The notoriously reliable diesel engines that power hydraulic systems are equipped with coolant temperature, and oil pressure warning lights, which are in view of the operator. If oil pressure, and engine coolant temperature, continue to be monitored on today’s highly reliable engine’s is it out of necessity, or because it’s “traditional?”

Hydraulic systems are in many respects similar to engines with respect to pressure and temperature. However, unlike engines, it’s evident that design engineers didn’t think it was necessary for operators to know a hydraulic system’s prevailing oil temperature, or system pressure. This makes no sense, because the root cause of most hydraulic system failures stem from overheating, and low, or excessive, pressure.

It is common to hear people opine that "engineers don't think out of the box." When it comes to hydraulics, my take on the situation is a little different. I believe the long standing design problems with hydraulic systems don’t stem from the fact that engineers don't think out of the box, it’s because they don’t think out of “tradition."

There is overwhelming evidence to support the fact that America’s universities don’t teach engineering students three most vital elements of hydraulic system design, which are; safety, maintenance, and troubleshooting. The problem is exacerbated by the fact that most design engineers have no hands-on experience. Hydraulic system design would take on a whole new meaning if design engineers faced the wrath of hydraulic oil under pressure; flow tested a hydraulic pump in situ; or simply went through the motions of executing proactive maintenance on a hydraulic system. If universities don't teach engineers key elements of hydraulic system design, which includes: safety, maintenance, and how to test the volumetric performance of hydraulic components in situ, isn't it safe to conclude that "all of the above" will be absent in hydraulic system design?  

Here’s what I mean by my assertion that “engineers don’t think out of tradition:”

Let’s say an engineer graduates from XYZ University, and winds up in the design department of a company that manufactures, for example, front-end loaders.

The “apprentice” engineer is interned with an engineer that has decades of hydraulic system design experience under his/her belt. The problem is, neither the seasoned engineer, nor the intern typically have the skillsets needed to design a safe, and maintenance friendly hydraulic system. The intern will learn poor system design related to safety and maintenance, from his/her mentor, and will, like his/her mentor before him/her, carry the tradition of poor design forward for who knows how long. The problem stems from the birth of fluid power design as we know it today.

Now let's turn to the issue of heat. An engineer apparently calculates the heat load of a given hydraulic system. From the heat load results, the engineer designs the respective hydraulic system’s cooling system. That's the good news. Now to the bad news. I have been teaching hydraulic system design, from the point of view of safety, maintenance and troubleshooting, for more than three decades, and in that time, I have never met an engineer that compared calculated heat dissipation against actual heat dissipation. Moreover, I have never seen reference to, in a service manual, a hydraulic system’s point-of-reference (POR) for the temperature differential across the ports of an oil cooler.

The same is true for the people that maintain hydraulic systems. I have never met a mechanic that either checks the temperature differential across a cooler on a routine maintenance schedule, nor have I seen a routine maintenance schedule with a specification for an oil cooler’s temperature differential point-of-reference.

This means we can add hydraulic oil temperature to the list of a hydraulic system’s vital operating parameters that both design engineers and mechanics ignore. And owners and operators of hydraulic systems wonder why hydraulic systems are unsafe, unreliable and prohibitively expensive to maintain!

It gets worse! Contrary to popular belief the responsibility of monitoring hydraulic oil temperature should be vested in the vehicle operator, and NOT the mechanic. Hydraulic system designers obviously disagree. Over 95% of hydraulic systems do not have oil temperature gauges for the machine operator’s consumption. Most don’t even make it easy for mechanics to view the temperature.

Take stationary hydraulic power units for example. Clients pay hundreds and thousands of dollars for hydraulic power units, which they depend on for the safety of their workers, and production. When it comes to equipping a power unit with an oil temperature gauge many designers reach down into the bottom of the barrel.

Have you ever seen those chintzy oil level/oil temperature gauges that look like something you would get at a carnival if you hit an elephant on the bum with a banjo? The temperature gauge; if you can see it, consists of a glass, mercury-filled thermometer from the 40’s, and 50’s. Many of these cannot be seen by mechanics, because they may be hidden between the reservoir and an adjacent wall.

Now, let's turn our attention to mobile equipment. The most common cooler type employed on mobile equipment is the air/oil design. Since the cooler typically shares the same cooling fan as the engine's radiator, the oil cooler is mounted directly behind the radiator.

This situation epitomizes my point about poor system design. With no means for the operator to observe vital oil temperature, and literally no way for a mechanic to check the oil temperature, the only way to know if the hydraulic oil in a mobile machine is excessive is when the system suffers a catastrophic failure, or it simply bursts into flames. 

It’s a win, win for machinery and equipment manufacturers. Poorly trained design engineers, “dumbed down” mechanics and machine operators, equals guaranteed income for the life of the system.

Here’s what you need to do if you want to stop the bleeding:

1. Accept the fact that hydraulic oil temperature is the machine operator’s responsibility; NOT the mechanics. The earliest signs of problems within a hydraulic system are indicated by a rise in temperature. Early detection means an end to systemic catastrophic failure.
2. It is vital for machine operators and mechanics to know each hydraulic system's normal operating temperature. It is also vital for mechanics to know each oil coolers temperature differential point-of-reference (POR).
3. Temperature is the single most effective operating parameter that can be relied upon to determine the overall health of a hydraulic system. It also can be used to determine if an operator is "abusing" a machine. Let me hasten to add, "abuse" can stem from poor training.
4. A well designed hydraulic system's operating temperature should be approximately 60oF (16oC) above ambient temperature (rule-of-thumb).
5. For each 20oF (-6.6oC) rise in oil temperature above approximately 140oF (60oC) oil life is reduced by 50% (rule-of-thumb).
6. Hydraulic oil temperature must be observed by the operator daily. Changes in temperature must be reported to the supervisor immediately.
7. Temperature differential across a cooler must be checked every 30-days based on an 8-hour shift.
8. WARNING - knowingly operating a hydraulic system that is overheating is highly irresponsible. An overheating hydraulic system can fail unexpectedly, which could lead to an accident that can cause severe injury, death and/or substantial property damage.
9. In most hydraulic systems, the oil temperature sensors are installed in the wrong location; another piece of evidence that supports my “tradition” theory. You will find most oil temperature sensors in a hydraulic reservoir. The correct for an oil sensor is at the inlet side of the oil-cooler. This is the actual operating temperature of a hydraulic system. If, for example, an engineer does not separate (with baffles) a pump's inlet from the system's return line, it may cause the system's return flow to circulate back into the pump's inlet, thereby negating vital oil "dwell" time. If the oil sensor is in the reservoir, it will not alert anyone to the problem, because the overheating problem will be in the system. With respect to a closed-loop system engineers get it wrong a staggering 99.9% of the time. They put the temperature sensors in the tank, when they MUST be in the closed loop.
10. Don’t rely on hydraulic system design engineers to design hydraulic systems that are safe, and maintenance friendly. They usually don’t have the knowledge or experience to do the job properly. You need to develop your own specifications, which designers must meet if they want your business. If you want to stop the bleeding, TELL, don't ASK, machinery and equipment manufacturers to install oil temperature, and oil pressure gauges on EVERY machine's instrument panel - no exceptions! They must be installed if for no other reason than safety.
11. When you take delivery of a new machine it is imperative to establish the oil cooler’s temperature differential point-of-reference (POR) immediately. Again, this should have been the design engineer’s responsibility. The POR must be established while the hydraulic system is cycling under full-load, otherwise it’s pointless. Don’t be at all surprised if you find there isn’t a temperature differential. Poorly trained hydraulic system design engineers typically don’t do post design tests to check actual operating parameters against design parameters. Accordingly, there is a distinct possibility the air/oil cooler on your machine should be sitting on your mantelpiece at home, rather than occupying space on your machine. An air/oil cooler is typically constructed with two vertical tubes – inlet and discharge, with numerous parallel tubes but-welded into them. If the air/oil cooler is mounted above the oil reservoir, and the return oil flow enters and exits the cooler at its base, there is no incentive for the cooler to fill with oil (slide 1). In fact, the hotter the oil, the less effective the cooler is.

The correct configuration for the oil flow through a cooler, which is mounted above the oil level in a hydraulic reservoir, is depicted in illustration (slide 2).

Case history: After attending my workshop, a mechanic decided to "get to know his patient." His company owned a drilling rig that had an operating temperature (hydraulic oil) of 220oF (104oC). According to the manufacturer this highly unacceptable operating temperature was "normal." When he executed the test to determine the cooler's temperature differential point-of-reference (POR) he found it was minimal. He discovered that the cooler was mounted above the oil reservoir, which means it didn’t work from day one! Further proof of how little most design engineers know about the actual operating parameters of the hydraulic systems they design.

Conclusion-

Just as well over 80% of the people that work on hydraulic systems are untrained. Moreover, the few that are, attended courses that all but ignored the vital factors that cause the early demise of so many hydraulic systems. Owners and operators of hydraulic systems have no idea about how badly they are getting fleeced by machinery and equipment manufacturers. Pumps are failing prematurely due to problems at the inlet. Pumps and motors are failing prematurely due to excessive case pressure. Moreover, oil overheating, and high/low system pressure cause widespread systemic damage to hydraulic systems. These design deficiencies cost owners and operators of hydraulic systems tens of millions of dollars in losses due to premature component failures, unplanned downtime, unnecessary overtime, hiring manufacturer's technicians' who themselves are invariably untrained, exorbitant airfreight costs, etc. With the profound ignorance that pervades the fluid power industry, it's easy for machinery and equipment manufacturers to make your untrained mechanic, and/or machine operator, the industry's scapegoats.

Untrained mechanics are perpetual "revenue generators" for machinery and equipment manufacturers. They know it, and that's why they are completely satisfied with the status quo! I challenge you to convince me otherwise!

If you want to learn hydraulic safety, how to maintain hydraulic systems, and how to SAFELY execute tests to determine the volumetric performance of ALL hydraulic components, you only have one choice – my workshops. No, I won't insult your intelligence, or compromise your safety, by attempting to convince you that a "bucket" is a suitable alternative for a flow meter. Remember, if you can’t afford to pay to attend my workshop, and you have a strong desire to be an exceptional hydraulic technician, let me know. I will never deny anyone the opportunity to learn to be safe while working on hydraulic systems based on their financial status.  

In my next article, I will discuss how badly people with hydrostatic transmission are getting screwed because of poor design practices.