Choosing the Right Hydraulic Fluid

by Brendan Casey

Most hydraulic systems will operate satisfactorily using a variety of fluids. These include multigrade engine oil, automatic transmission fluid and more conventional antiwear hydraulic oil. But which type of fluid is best for a particular application? While it is not possible to make one definitive recommendation that covers all types of hydraulic equipment in all applications, the following are some of the factors to consider when selecting a hydraulic fluid.

Multigrade or Monograde
Viscosity is the single most important factor when selecting a hydraulic fluid. It doesn’t matter how good the other properties of the oil are if the viscosity grade is not correctly matched to the operating temperature range of the hydraulic system. In this situation, maximum component life will not be achieved. Defining the correct fluid viscosity grade for a particular hydraulic system involves consideration of several interdependent variables. These include:

• starting viscosity at minimum ambient temperature
• maximum expected operating temperature, which is influenced by maximum ambient temperature
• permissible and optimum viscosity range for the system’s components

Typical minimum permissible and optimum viscosity values for different types of hydraulic components are shown in Figure 1.

If the hydraulic system is required to operate in freezing temperatures in winter and tropical conditions in summer, then it’s likely that multigrade oil will be required to maintain viscosity within permissible limits across a wide operating temperature range. If fluid viscosity can be maintained in the optimum range, typically 25 to 36 centistokes, the overall efficiency of the hydraulic system is maximized (less input power is given up to heat). This means that under certain conditions, the use of a multigrade can reduce the power consumption of the hydraulic system. For mobile hydraulic equipment users this translates to reduced fuel consumption.

There are some concerns when using multigrade fluids in hydraulic systems. The viscosity index (VI) improvers used to make multigrade oils can have a negative effect on the air separation properties of the oil.1 This is not ideal, particularly in mobile hydraulic systems which typically have a relatively small reservoir with correspondingly poor deaeration characteristics. The high shear rates and turbulent flow conditions often present in hydraulic systems destroy the molecular bonds of the VI improvers over time resulting in loss of viscosity. When selecting a high VI or multigrade fluid, it is recommended that the hydraulic component manufacturers’ minimum permissible viscosity values (Table 1) be increased by 30 percent to compensate for VI improver sheardown. This adjustment reduces the maximum permissible operating temperature that would otherwise be allowable with the selected oil and thereby provides a margin of safety for viscosity loss through VI improver shearing.

If the hydraulic system has a narrow operating temperature range and it is possible to maintain optimum fluid viscosity using a monograde oil, it is recommended not to use a multigrade for the reasons stated above.

Table 1. Typical Minimum Viscosity Values for

Hydraulic Components

Detergent or No Detergent

DIN 51524; HLP-D fluids are a class of antiwear hydraulic fluids that contain detergents and dispersants. The use of these fluids is approved by most major hydraulic component manufacturers. Detergent oils have the ability to emulsify water, and disperse and suspend other contaminants such as varnish and sludge. This keeps components free from deposits, however, it also means that contaminants do not settle out - they must be filtered out. These can be desirable properties in mobile hydraulic systems, which unlike industrial systems, have little opportunity for the settling and precipitation of contaminants at the reservoir, due to its relatively small volume.

The main concern with these fluids is that they have excellent water emulsifying ability, which means that if present, water is not separated out of the fluid. Water accelerates the aging of the oil, reduces lubricity and filterability, reduces seal life and leads to corrosion and cavitation. Emulsified water can be turned into steam at highly loaded parts of the system. These problems can be avoided by maintaining water content below the oil’s saturation point at operating temperature.

Antiwear or No Antiwear
The purpose of antiwear additives is to maintain lubrication under boundary conditions. The most common antiwear additive used in engine and hydraulic oil is zinc dialkyl dithiophosphate (ZDDP). The presence of ZDDP is not always seen as a positive, due to the fact that it can chemically break down and attack some metals, and reduce filterability. Stabilized ZDDP chemistry has largely overcome these shortcomings, making it an essential additive to the fluid used in any high-pressure, high-performance hydraulic system, such as those fitted with piston pumps and motors. A ZDDP concentration of at least 900 parts per million can be beneficial in mobile applications.

As far as hydraulic oil recommendations go, for commercial reasons relating to warranty, it is wise to follow the equipment manufacturer’s recommendations. However in some applications, the use of a different type of fluid to that originally specified by the equipment manufacturer may increase hydraulic system performance and reliability. Always discuss the application with a technical specialist from your oil supplier and the equipment manufacturer before switching to a different type of fluid.

Reference
1. Mannesmann Rexroth. “Mineral Oil-based Pressure Fluids for Vane Pumps, Radial Piston Pumps and Gear Pumps as Well as GM, GMRP, MCS, MCR, MR and MKM/MRM Motors.” (RE 07 075/07.98), p.2. 1998.

Link: http://www.machinerylubrication.com/article_detail.asp?articleid=702

Hydraulic Fluids Meet Increasing Operating Demands

by Debra Light, The Lubrizol Corporation

Today’s hydraulic fluids must last longer, provide greater protection and perform better than their predecessors. Construction equipment, injection molding machines, steel mills, forestry equipment and many other types of industrial and off-road operations rely on hydraulic fluids with special chemical properties to keep machinery running smoothly and lasting longer.

The mission of modern-day hydraulic fluids is to provide better performance in smaller, more efficient industrial and mobile equipment. The fluid must be able to accommodate higher speeds and survive higher operating pressures in equipment with smaller sump reservoirs. This of course also means higher temperatures, which expose the fluids to more severe conditions.

Not all hydraulic fluids are the same. Hydraulic fluids range in performance from basic to specialty products that deliver high-performance characteristics. The robustness and durability of the fluid, for example, will affect its overall performance in protecting equipment.

Specially formulated additive packages used in hydraulic fluids provide the required durability and performance retention. The Lubrizol Corporation developed a special durability testing protocol to demonstrate that the amount and type of antiwear additives and oxidation inhibitors – such as zinc dithiophosphate – along with corrosion and rust inhibitors, foam inhibitors, demulsibility additives and other chemical components, help hydraulic fluids deliver reliable performance under strenuous operating conditions.

Meeting Specs Not Enough
Recognizing that today’s hydraulic systems and fluids are crucial components of a healthy operation, managers want to know whether a particular fluid will continue to provide all the protection the user’s equipment requires.

Common questions asked by maintenance specialists in a wide range of industries include:
How long will the fluid continue to protect my equipment?
Will the product retain its demulsibility characteristics?
What is the expected oxidation life of the fluid?
Will the fluid attack my seals?
Can I perform fine filtration without loss of performance?

These questions arise from a variety of concerns. Additives are consumed at greater rates in today’s harsher environments. Systems are using less oil and are expected to last longer, making the fluid’s durability and retention of performance even more important. Quality control, especially in the area of fluid cleanliness, and precise formulating are also more critical than ever before.

Hydraulic fluid specifications are not enough to assure that the hydraulic fluid used in the operations will provide adequate protection over the desired timeframe. Specifications provide a basis for performance, but the reality in today’s environment is that equipment demands have increased, which has in turn increased the needs for the fluid to perform properly in much harsher conditions.

More Stringent Industry Testing Needed
So how does the lubrication industry measure or evaluate a hydraulic fluid’s ability to perform under harsher conditions if the specifications haven’t changed? The Lubrizol Corporation has modified a number of industry tests to answer this question. Results of these tests show that hydraulic fluids formulated with the proper zinc dithiophosphate for both antiwear and oxidation inhibition – along with additional oxidation inhibition technology – continue to be effective in protecting equipment, even in severe operating environments.

These proprietary tests were designed to reflect the immense challenges hydraulic fluids often face as they carry out their job. Higher pressures and higher temperatures increase the rate of oxidation and thermal stress the fluid must endure. In addition, the fluid spends less time in the smaller reservoirs of today’s equipment, which can cause a number of problems, such as not allowing air to escape, foam to break, water to separate, contaminants to settle out or the fluid to cool sufficiently.

Lubrizol’s tests measured the durability and retention of performance of hydraulic fluids under a variety of high-stress conditions.

Testing for Durability
The most direct way to assess durability is to extend the duration of the current standard pump tests and run the tests at higher temperatures. Standard industry pump tests, using Denison’s vane pump, the Eaton-Vickers vane pump and the Sundstrand piston pump, were chosen for this study.

Assessment/Requirement

All components are assessed visually for signs of

wear and discoloration at the end of test

Table 1. Denison Vane Pump

To determine retention of performance, the test fluids were saved and reevaluated in the standard ASTM bench tests typically used to evaluate hydraulic fluids. The Denison HF-0 specification (Table 1) was chosen because of its comprehensive nature in that it evaluates all aspects of a hydraulic fluid. The Sundstrand and Eaton/Vickers pump tests were designed to measure durability of the hydraulic fluid.

The Sundstrand piston pump was initially run under standard conditions with hydraulic fluid formulated with antiwear additives, and it passed all the criteria established for the test. A second Sundstrand pump was run under the same test, extended to 450 hours, or double the length of the standard test, and it also passed (Table 2).

Assessment/Requirement

All components are assessed visually for signs of wear

and discoloration at the end of test

Table 2. Sundstrand Piston Pump – Series 22

The Sundstrand pump test included one percent water contamination to further stress the fluid. Even with the added water, there was no evidence of any hydrolytic reactions that could cause the formation of precipitates. Contamination from precipitates leads to blocked valves and filter-plugging problems.

Another round of Sundstrand pump tests were run at an elevated temperature of 250°F. Because of the higher temperature, no water was added to this test, and all other conditions remained the same. This round of tests showed that the hydraulic fluid with the premium hydraulic additive package had the endurance to exceed the performance parameters of the Sundstrand piston pump without difficulty, despite the higher temperature and the extended length of the test.

Assessment/Requirement

Cam and ring weight loss not to exceed 90 mg

Table 3. Eaton Vickers 35VQ25 Vane Pump

The next phase of testing involved the Eaton-Vickers vane pump (Table 3), which ran for extended hours to determine the durability of the hydraulic fluid containing an antiwear additive package. After 1,000 hours, the pump was still below the weight loss limit for total ring and vane wear. Figure 1 illustrates how the hydraulic fluid was robust enough to exceed the parameters of the test by a large margin.

Figure 1. Eaton-Vickers 35VQ Vane Pump

Standard vs. Extended Length

The final phase of pump testing used the Denison vane pump under standard test conditions to generate stressed fluids needed for evaluating performance retention. The purpose of this test was to generate used fluid needed to complete the retention of performance portion of the study.

Performance Retention
Following testing in the Sundstrand piston pump, the Eaton-Vickers and Denison vane pumps, the used, stressed fluids were saved and then subjected to a number of the performance bench tests used to evaluate fresh hydraulic fluid. Testing included D943 oxidation testing, D4310 1,000-hour sludge tests, D2619 hydrolytic stability tests, D665 rust test, D892 foam test, and D1401 demulsibility test. This testing was conducted to determine whether the stressed fluid’s characteristics were still comparable to that of the new fluid. All of these tests were performed in accordance with established ASTM testing methods.

In the D943 oxidation test, after the additional stress of the Eaton-Vicker’s extended, 200-hour vane pump test, the premium hydraulic fluid provided superior oxidation life of almost 1,500 hours (Figure 2) to 2.0 acid number (AN), which still exceeds the minimum specification.

Figure 2. D943 Oxidation

Both the monograde and the multigrade used fluids passed the D2619 hydrolytic stability test, even after the oil was stressed from the pump test. They demonstrated only a 0.02 to 0.19 mg/cm2 weight loss – depending on the test phase – which are all below the 0.2 HF-0 fail limit. This ensures the additive package did not react with water under stressful operating conditions to form insoluble sludge, excess acidity or other adverse physical changes that could impede proper fluid performance.

The D4310, 1,000-hour sludge test is an important test in determining the sludging characteristics of today’s hydraulic fluids. Systems free of sludge are critical in meeting the demands for long service life, and the requirement for clean systems needed to prevent deposit build up in the various control valves or on the filter. Not all hydraulic additive packages offer the sludge resistance or thermal stability of the product used here (Figure 3) and not all hydraulic additive packages will provide those clean systems needed for long-life applications.

Figure 3. D4310 1,000 Hour Sludge

Lubrizol’s test data for thermal stability D2070 shows that both the monograde and the multigrade formulations were far below the 100 mg/100 mL sludge fail limit (Figure 4).

Figure 4. D943 Oxidation

The Denison filterability test is used as an indicator of the fluid’s performance under dry and wet conditions. Blocked filters can be a major maintenance issue for hydraulic fluids out in the field. The after pump results show that premium fluids, like these tested, retain their performance in this key Denison HF-0 test. Our study also showed that performance is retained in other key areas, for example rust protection, demulsibility, and antifoam characteristics (Table 4).

Table 4. Additional Test Results

The results of all the testing showed that even after the stress of the pump testing, the premium hydraulic fluids continued to provide superior oxidation life, hydrolytic stability, thermal stability, sludge prevention, filterability under both dry and wet conditions, rust protection, demulsibility and antifoam characteristics.
 

Figure 5. Filterability (Wet)

Going Beyond OEM Specs

The durability and retention of fluid performance can be evaluated beyond the requirements of OEM specifications by extending the length of time, or making the operating conditions of the tests more stressful. Bench testing of stressed fluids formulated with premium hydraulic additive technology demonstrates that these fluids are extremely durable and retain their performance quite well.

These studies also show that the special characteristics of premium hydraulic fluids enable them to surpass the designation of being mere commodities. They are specialty fluids that provide the endurance and retention of performance needed for long, low-maintenance equipment life in today’s more demanding environments.

Source:
www.machinerylubrication.com/article_detail.asp?articleid=753

Solving Hydraulic System Overheating Problems

by Brendan Casey

Overheating ranks No. 2 in the list of most common problems with hydraulic equipment. Unlike leaks, which rank No. 1, the causes of overheating and its remedies are often not well understood by maintenance personnel

Why do Hydraulic Systems Overheat?
Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat. A hydraulic system’s heat load is equal to the total power lost (PL) through inefficiencies and can be expressed as:

PLtotal = PLpump + PLvalves + PLplumbing + PLactuators

If the total input power lost to heat is greater than the heat dissipated, the hydraulic system will eventually overheat. Installed cooling capacity typically ranges between 25 and 40 percent of input power, depending on the type of hydraulic system.

Hydraulic Fluid Temperature
How hot is too hot? Hydraulic fluid temperatures above 180°F (82°C) damage most seal compounds and accelerate degradation of the oil. While the operation of any hydraulic system at temperatures above 180°F should be avoided, fluid temperature is too high when viscosity falls below the optimum value for the hydraulic system’s components. This can occur well below 180°F, depending on the fluid’s viscosity grade.

Maintaining Stable Hydraulic Fluid Temperature
To achieve stable fluid temperature, a hydraulic system’s capacity to dissipate heat must exceed its heat load. For example, a system with continuous input power of 100 kW and an efficiency of 80 percent needs to be capable of dissipating a heat load of at least 20 kW. Assuming this system has a designed cooling capacity of 25 kW, anything that increases heat load above 25 kW or reduces the cooling system’s capacity below 25 kW will cause the system to overheat.

Consider this example. I was recently asked to investigate and solve an overheating problem in a mobile application. The hydraulic system was comprised of a diesel-hydraulic power unit, which was being used to power a pipe-cutting saw. The saw was designed for sub-sea use and was connected to the hydraulic power unit on the surface via a 710-foot umbilical. The operating requirements for the saw were 24 GPM at 3,000 PSI.

The hydraulic power unit had a continuous power rating of 37 kW and was fitted with an air-blast heat exchanger. The exchanger was capable of dissipating 10 kW of heat under ambient conditions or 27 percent of available input power (10/37 x 100 = 27). The performance of all cooling circuit components were checked and found to be operating within design limits.

At this point it, was clear that the overheating problem was being caused by excessive heat load. Concerned about the length of the umbilical, I calculated its pressure drop. The theoretical pressure drop across 710 feet of ¾-inch pressure hose at 24 GPM is 800 PSI. The pressure drop across the same length of 1-inch return hose is 200 PSI. The theoretical heat load produced by the pressure drop across the umbilical of 1,000 PSI (800 + 200 = 1,000) was 10.35 kW. This meant that the heat load of the umbilical was 0.35 kW more than the heat dissipation capacity of the hydraulic system’s heat exchanger. This, when combined with the system’s normal heat load (inefficiencies) was causing the hydraulic system to overheat.

Beat the Heat
There are two ways to solve overheating problems in hydraulic systems: decrease heat load or increase heat dissipation.

Hydraulic systems dissipate heat through the reservoir. Therefore, check the reservoir fluid level and if low, fill to the correct level. Check that there are no obstructions to airflow around the reservoir, such as a buildup of dirt or debris.

Inspect the heat exchanger and ensure that the core is not blocked. The ability of the heat exchanger to dissipate heat is dependent on the flow-rate and temperature of both the hydraulic fluid and the cooling air or water circulating through the exchanger. Check the performance of all cooling circuit components and replace as necessary.

An infrared thermometer can be used to check the performance of a heat exchanger, provided the design flow-rate of hydraulic fluid through the exchanger is known. To do this, measure the temperature of the oil entering and exiting the exchanger and substitute the values in the following formula:

Where: kW = heat dissipation of exchanger in kilowatts
L/min = oil flow through the exchanger in liters per minute
T ºC = inlet oil temperature minus outlet oil temperature in Celsius

For example, if the measured temperature drop across the exchanger is 4ºC and the design oil flow-rate is 90 L/min, the exchanger is dissipating 10 kW of heat. Relating this to a system with a continuous input power of 100 kW, the exchanger is dissipating 10 percent of input power. If the system is overheating, it means that either there is a problem in the cooling circuit or the capacity of the exchanger is insufficient for the ambient operating conditions.

On the other hand, if the measured temperature drop across the exchanger is 10ºC and the design oil flow-rate is 90 L/min, the exchanger is dissipating 26 kW of heat. Relating this to a system with a continuous input power of 100 kW, the exchanger is dissipating 26 percent of input power. If the system is overheating, this means that the efficiency of the system has fallen below 74 percent.

Pressure Drop Means Heat
Where there is a pressure drop, heat is generated. This means that any component in the system that has abnormal, internal leakage will increase the heat load on the system and can cause the system to overheat. This could be anything from a cylinder that is leaking high-pressure fluid past its piston seal, to an incorrectly adjusted relief valve. Identify and change-out any heat-generating components.

A common cause of heat generation in closed center circuits is the setting of relief valves below, or too close to, the pressure setting of the variable-displacement pump’s pressure compensator. This prevents system pressure from reaching the setting of the pressure compensator. Instead of pump displacement reducing to zero, the pump continues to produce flow, which passes over the relief valve, generating heat. To prevent this problem in closed center circuits, the pressure setting of the relief valve(s) should be 250 PSI above the pressure setting of the pump’s pressure compensator (Figure 1).

Closed center circuit showing relief
valve (RV) setting 250 PSI above

the pressure compensator (PC)

setting of the variable pump (PV).

Figure 1

Continuing to operate a hydraulic system when the fluid is over-temperature is similar to operating an internal combustion engine with high coolant temperature. Damage is guaranteed. Therefore, whenever a hydraulic system starts to overheat, shut it down, identify the cause and fix it.

Reference:
http://www.machinerylubrication.com/article_detail.asp?articleid=680

The Negative Effects of Overpressurization on Hydraulic Components

by Brendan Casey

Power transmission in a hydraulic system is a product of flow and pressure. Therefore, the power rating of a hydraulic pump or actuator is a product of its ability to produce or absorb flow, as well as its maximum pressure rating. But what happens to hydraulic components if that pressure control device is screwed in an extra quarter turn and operating pressure exceeds design limits?

Even though the system won't necessarily blow up with a bang, damage can be caused by overpressurization that manifests itself in a number of ways, including:

• Loss of lubrication
• Valve plate separation
• Mechanical damage

 Figure 1. Separation of Cylinder Barrel and Valve Plate due to Overpressurization (Bosch Rexroth)

Loss of Lubrication
One variable that impacts the creation and maintenance of hydrodynamic films is load. Hydrostatic force and therefore load is the product of pressure and area (F = P x a). This means that increasing operating pressure increases the load on lubricated surfaces. If operating pressure exceeds design limits, loads can increase causing boundary lubrication and two-body abrasion to occur - all other things equal.

Cylinder Barrel and Valve Plate Separation
In axial and bent axis piston pump and motor designs, the cylinder barrel is hydrostatically loaded against the valve plate. To maintain full-film lubrication between the rotating cylinder barrel and the stationary valve plate, the hydrostatic force holding them in contact is offset by a hydrostatic force acting to separate the parts. This is achieved by making the effective area of half the total number of piston bores slightly larger than the effective area of the pressure kidney in the valve plate.

The higher the operating pressure, the higher the hydrostatic force holding the cylinder barrel in contact with the valve plate. However, if operating pressure exceeds design limits, the cylinder barrel will separate from the valve plate (Figure 1).

Design geometry prevents a perfect alignment of the opposing hydrostatic forces. This misalignment creates a twisting force (torque) on the cylinder barrel. During normal operation, torque is supported by the drive shaft (axial designs) or center pin (bent axis designs). If operating pressure exceeds design limits, the magnitude of the torque created causes elastic deformation of the drive shaft or center pin. This allows the cylinder barrel to tilt, bearing hard against the outlet side of the valve plate and separating from the inlet side. Once separation occurs, the lubricating film is lost and the resulting two-body abrasion damages (scores) the sliding surfaces of the cylinder barrel (Figure 2) and valve plate (Figure 3). Erosion of the kidney area of the valve plate can also occur as high-pressure fluid escapes into the case at high velocity. This surge of flow into the case can cause excessive case pressure, resulting in shaft seal failure.

Note that separation can also occur at operating pressures within design limits due to distortion (loss of flatness) of the valve plate, overspeeding or excessive wear of the cylinder barrel drive-spline in axial designs.

Figure 2. Scoring of Cylinder Barrel Caused by Separation

 Figure 3. Scoring of Valve Plate Caused by Separation

Mechanical Damage

Overpressurization can result in hose and seal failure, and mechanical failure of certain parts in pumps, motors, cylinders and valves. Excessive pressure in axial pump or motors has the same effect as excessive vacuum at the inlet. Both conditions put the piston-ball and slipper-pad socket in tension during intake (Figure 4), which can cause the piston retaining plate to buckle and/or the separation of the slipper from the piston, resulting in catastrophic failure.

In radial design motors, high case pressure can cause the pistons to lift off the cam while operating in 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 allows the motor to freewheel, resulting in uncontrolled machine movement.

 High pump or motor case pressure also results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, eventually resulting in leakage. 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.

Symptoms
Overpressurization can occur in operation without any detectable symptoms. Valve plate separation is often characterized by vibration or pulsation in the pressure line of the pump or motor combined with erratic machine movement as the cylinder barrel separates and re-couples to the valve plate.

Figure 4. Effect of Excessive Case Drain Pressure on Axial Design Piston

Causes

Faulty or incorrectly adjusted pressure control devices, such as pressure-relieving and reducing valves, and variable-pump pressure compensators cause overpressurization. Uncontrolled decompression, thermal expansion (in load-holding circuits) or back flow through check valves or logic elements can result in pressure transients (spikes). In systems experiencing rapid changes in load, the reaction time of the pressure control devices may not be fast enough to prevent pressure transients that exceed design limits. Problems associated with excessive case pressure can be avoided by connecting piston pump and motor case drain lines directly to the reservoir through dedicated penetrations.

Prevention
In most systems, examining the operation and adjustment of pressure control devices at regular intervals will prevent overpressurization.

To avoid problems associated with overpressurization of pump or motor cases, drain lines must not be connected to the return filter. Case drains should be returned to the reservoir through dedicated penetrations. These penetrations must be higher than the unit's case port and connected to a drop-pipe inside the reservoir that extends below minimum fluid level. 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.

Reference:
http://www.machinerylubrication.com/article_detail.asp?articleid=1006

Heating Things Up: What’s Next in Hydraulics?

by Brendan Casey

Constant progress is part of life. We may rail against it at times, but we can’t stop it. And, it manifests itself in many different ways.

In engineering, we’ve grown accustomed to machines and their component parts becoming stronger, lighter, cheaper, smaller, more powerful and more efficient.

Nowhere is this more evident than in the field of information technology. In fact, according to Moore’s Law – named after Gordon Moore, co-founder of Intel Corporation – the power of computer chips doubles every 18 months. At the same time, the cost of this extra computing power to the consumer decreases exponentially.

But as the cost of computing power to the consumer falls, the cost for manufacturers to fulfill Moore’s Law heads in the opposite direction. R&D, manufacturing and testing costs have steadily increased with each new generation of chips.

This has led to the definition of Moore’s Second Law, which states that the capital cost of semiconductor fabrication increases exponentially over time. This means progress comes at a cost – somewhere along the supply chain, at least.

The rapid and constant development we’ve seen in the IT sector is hard to match. But like most other areas of mechanical engineering, the hydraulics industry hasn’t been resting on its laurels, either.

Component operating pressures have increased steadily over the past 30 years, and this trend is forecast to continue. Seal and hose design has improved accordingly, as has oil additive and filtration technology. Advances in materials and manufacturing techniques mean that more efficient hydraulic components with higher power densities are available at lower cost.

In short, much progress has been made, most of it good for hydraulic equipment owners. But, it has come at a cost. Higher operating pressures and closer tolerances make the hydraulic machine more susceptible to wear and damage resulting from oil contamination and inadequate lubrication. This means proactive maintenance is essential for optimum reliability and performance of today’s hydraulic equipment.

But just as Moore’s Law is expected to hold true for another generation, so will the pursuit of performance gains by hydraulic equipment manufacturers. The question is: Which way from here?

To answer this question, it seems to me that some hydraulic equipment manufacturers are headed in the wrong direction. They are “dancing with the devil”, so to speak. Hopefully, my use of this metaphor will make sense once I explain.

A colleague who works for a major hydraulic component manufacturer recently told me his company’s original equipment manufacturer (OEM) customers are demanding components that can handle hydraulic system operating temperatures in excess of 110 degrees Celsius. This number is not for margin of safety; it’s for continuous operation.

But heat is the biggest enemy of a hydraulic system. It’s bigger than particle and water contamination these days due to the widespread understanding and adoption of modern filtration technologies.

And so, in my view, a hydraulic system running at 110°C is a hydraulic system from hell – figuratively speaking, of course. And so, to intentionally design such a system is like dancing with the devil. Let’s consider some of the reasons why.

Oil Life
According to Arrhenius’ Law, for every 10°C increase in temperature, the rate of reaction doubles. The chemical reactions we’re concerned with – in so far as hydraulic oil life is concerned – are oxidation (due to the presence of air) and hydrolysis (due to the presence of water). So the hotter the oil, the faster the rate of these reactions – and exponentially so.

By way of illustration, if you pour some cooking oil into a glass, it’ll take days, even weeks, before it darkens in color – a sign of oxidation. But pour the same amount of cooking oil into a frying pan – which gives the oil a large contact area with air – and then heat the heck out of it, and the oil will go black in a very short space of time.

When asked about the issue of oil life, my colleague from the hydraulic component manufacturer admitted that when lab-testing hydraulic components at temperatures of 110°C and above for extended periods, the oil had “turned black as ink and smelt unmentionable.”

Oil Viscosity
As I explained in my last column, the operating viscosity of hydraulic oil is crucial for both adequate lubrication and efficient power transmission.

The higher the maximum expected operating temperature, the wider the temperature operating window becomes. And the wider the temperature operating window, the more difficult it is to maintain oil viscosity within both allowable and optimum limits.

For example, consider a system with a cold-start temperature of 5°C and a maximum operating temperature of 110°C. To maintain viscosity between 800 centistokes at cold start and 25 centistokes at maximum operating temperature, it would require an ISO VG150 oil with a viscosity index of 2291. Just in case these numbers don’t mean much to you, this is not the type of hydraulic oil you can just call in and pick up from your local oil supplier.

Seal and Hose Life
Like everything else, the elastomers used to make hydraulic seals and hoses are improving all of the time. But oil temperatures above 82°C accelerate the degradation of most of these polymers. In fact, according to one seal manufacturer, operating temperatures 10°C above recommended limits can reduce seal life by 80 percent or more.

The constant cycle of heating and cooling – a process known as aging – is more severe when temperature extremes are greater. Aging causes these polymers to lose their elastic properties. This results in leaking seals and hoses.

False Progress
Increasing the temperature at which a hydraulic system is expected to operate does nothing to enhance machine performance. It only compounds a host of reliability issues which cannot be solved with current technologies. So, it’s not progress; it’s regress. This false progress will certainly come at a cost – to the hydraulic equipment owners who operate them.

Source
According to ISO Viscosity Grade calculator available at: http://www.mehf.com/2.A.4.b.4.htm
or
http://www.machinerylubrication.com/article_detail.asp?articleid=2019

Six Easy Steps to Maintain Your Hydraulic Equipment

by Brendan Casey

My wife often asks me why I still do consulting work. She wonders why I happily leave the comfort of my office to crawl all over hot, dirty, smelly hydraulic equipment.

For starters, I actually enjoy it. Secondly, it keeps me sharp. But perhaps most importantly, it keeps me in touch with the issues that hydraulic equipment users must grapple with.

One of the lessons I've learned over the years is that in the early stages of a consulting assignment, it is better to ask good questions rather than dispense good advice.

Pump Failures
A recent client had a series of catastrophic pump failures. These pumps were achieving less than half their expected service life. So naturally, the company wanted some answers. At our first meeting, the client opened proceedings with a brief history of the machine and an account of the events leading up to the failures. He then pushed a stack of oil analysis reports across the table.

Table 1. System Readings

Ask the Basic Questions
After taking notes on what I'd just been told, I fired off my first question: "What is the system's normal operating temperature?" The response was stunned silence. Client shrugs his shoulders.

"OK, what is the system's usual operating pressure range?" Blank look from client. "I don't know; we don't monitor either of those things."

At the end of this meeting, we walked through the control room. Both the operating pressure and temperature were displayed on the default PLC screen - albeit along with a lot of seemingly more important production information. Say no more.

But could you answer these two basic questions about the vital signs of your hydraulic equipment? If not, I strongly recommend you make the effort to get to know your hydraulic equipment better.

Gathering Information
This information is easy to collect. It can give valuable insight to the health of your equipment and is essential data if failure analysis is required. Here is how I recommend you accomplish this task:

First, you need an infrared thermometer, also called a heat gun. If you don't have one, you'll need to invest around 100 bucks to get one and then familiarize yourself on how to use it.

2. Next, using a permanent marker or paint stick, draw a small target on the hydraulic tank below minimum oil level and away from the cooler return. Label it 1. This marks the spot where you'll take your tank oil temperature readings.

By the way, the purpose of these targets is that regardless of who takes the temperature readings, they'll be taken from the same place each time.

If the system is a closed-circuit hydrostatic transmission, mark a convenient location on each leg of the transmission loop and number those locations as 2 and 3. Skip this step for open-circuit hydraulic systems.

Next, mark a target on the heat exchanger inlet and outlet and number these 4 and 5, respectively. This records the temperature drop across the cooler. The benefit of this step is that if the oil flow rate through the exchanger and the temperature drop across it are known, the actual heat rejection of the exchanger can be calculated.

And if the system is overheating, knowing the actual heat rejection of the exchanger can help determine whether the problem is the result of an increase in heat load (due to an increase in internal leakage, for example) or whether the problem lies in the cooling circuit itself.

For example, if a hydraulic system with an input power of 100 kilowatts is overheating, and the actual heat rejection of the exchanger is 30 kilowatts, then the efficiency of the system has fallen below 70 percent. Therefore, an increase in heat load is the likely cause. On the other hand, if the exchanger is rejecting only 10 kilowatts of heat (which in this example equates to 10 percent of input power), then it's likely there is a problem in the cooling circuit or there is insufficient installed cooling capacity.

Install a pressure gauge or transducer to record operating pressure if one is not already available. If the system is a closed-circuit hydrostatic transmission, install a similar device to record charge pressure.

With that accomplished, now draw up a table like Table 1 to record the date, time, ambient temperature, operating temperatures and operating pressure(s). Note that there is little point in recording the temperature across the heat exchanger if the fan or water pump isn't running. And, charge pressure is relevant only to closed-circuit hydrostatic transmissions.

In terms of compiling this data, it's a good idea to take readings on the hottest and coldest days of the year, and on a couple of average-temperature days in between. This provides a baseline of information. Beyond that, taking readings at regular intervals - each day or shift, for example - can provide early warning of impending problems. And if the system starts to give trouble, taking a set of readings will reveal if the machine is operating outside its normal parameters.

Source:
http://www.machinerylubrication.com/article_detail.asp?articleid=1396

The Seven Most Common Hydraulic Equipment Mistakes

by Brendan Casey

During my two decades working in the hydraulics industry, I’ve been in the fortunate position of being able to observe, and learn from, the mistakes and omissions hydraulics users make when maintaining their equipment.

Based on this long observation, here are the seven most common mistakes hydraulic equipment users make – so you can avoid them!

Mistake No. 1 – Changing the oil
There are only two conditions that mandate a hydraulic oil change: degradation of the base oil or depletion of the additive package. Because there are so many variables that determine the rate at which oil degrades and additives get used up, changing hydraulic oil based on hours in service, without any reference to the actual condition of the oil, is like shooting in the dark.

Given the current high price of oil, dumping oil which doesn’t need to be changed is the last thing you want to do. On the other hand, if you continue to operate with the base oil degraded or additives depleted, you compromise the service life of every other component in the hydraulic system. The only way to know when the oil needs to be changed is through oil analysis.

Mistake No. 2 – Changing the filters
A similar situation applies to hydraulic filters. If you change them based on schedule, you’re changing them either too early or too late. If you change them early, before all their dirt-holding capacity is used up, you’re wasting money on unnecessary filter changes. If you change them late, after the filter has gone on bypass, the increase in particles in the oil quietly reduces the service life of every component in the hydraulic system – costing a lot more in the long run.

The solution is to change your filters when all their dirt-holding capacity is used up, but before the bypass valve opens. This requires a mechanism to monitor the restriction to flow (pressure drop) across the filter element and alert you when this point is reached. A clogging indicator is the crudest form of this device. A better solution is continuous monitoring of pressure drop across the filter.

Mistake No. 3 – Running too hot
Few equipment owners or operators continue to operate an engine that is overheating. Unfortunately, the same cannot be said when the hydraulic system gets too hot. But like an engine, the fastest way to destroy hydraulic components, seals, hoses and the oil itself is high-temperature operation.

How hot is too hot for a hydraulic system? It depends mainly on the viscosity and viscosity index (rate of change in viscosity with temperature) of the oil, and the type of hydraulic components in the system.

As the oil’s temperature increases, its viscosity decreases. Therefore, a hydraulic system is operating too hot when it reaches the temperature at which oil viscosity falls below that required for adequate lubrication.

A vane pump requires a higher minimum viscosity than a piston pump, for example. This is why the type of components used in the system also influences its safe maximum operating temperature.

Apart from the issue of adequate lubrication, the importance of which cannot be overstated, operating temperatures above 82 degrees Celsius damage most seal and hose compounds and accelerate degradation of the oil. But for the reasons already explained, a hydraulic system can be running too hot well below this temperature.

Mistake No. 4 – Using the wrong oil
The oil is the most important component of any hydraulic system. Not only is hydraulic oil a lubricant, it is also the means by which power is transferred throughout the hydraulic system. It’s this dual role which makes viscosity the most important property of the oil, because it affects both machine performance and service life.

Oil viscosity largely determines the maximum and minimum oil temperatures within which the hydraulic system can safely operate. If you use oil with a viscosity that’s too high for the climate in which the machine must operate, the oil won’t flow properly or lubricate adequately during cold start. If you use oil with a viscosity too low for the prevailing climate, it won’t maintain the required minimum viscosity, and therefore adequate lubrication, on the hottest days of the year.

But that’s not the end of it. Within the allowable extremes of viscosity required for adequate lubrication, there is a narrower viscosity band where power losses are minimized. If operating oil viscosity is higher than ideal, more power is lost to fluid friction. If operating viscosity is lower than ideal, more power is lost to friction and internal leakage.

Using the wrong viscosity oil not only results in lubrication damage and premature failure of major components, it also increases power consumption (diesel or electricity) – two things you don’t want.

And despite what you might think, you won’t necessarily get the correct viscosity oil by blindly following the blanket recommendations of the machine manufacturer.

Mistake No. 5 – Wrong filter locations
Any filter is a good filter, right? Wrong! There are two hydraulic filter locations that do more harm than good and can rapidly destroy the very components they were installed to protect. These filter locations which should be avoided are the pump inlet and drain lines from the housings of piston pumps and motors.

This contradicts conventional wisdom: that it is necessary to have a strainer on the pump inlet to protect it from "trash". First, the pump draws its oil from a dedicated reservoir, not a garbage can. Second, if you believe it’s normal or acceptable for trash to get into the hydraulic tank, then you’re probably wasting your time reading this article.

If getting maximum pump life is your primary concern (and it should be), then it's far more important for the oil to freely and completely fill the pumping chambers during every intake than it is to protect the pump from nuts, bolts and 9/16-inch combination spanners. These pose no danger in a properly designed reservoir, where the pump inlet penetration is a least 2 inches off the bottom.

Research has shown that a restricted intake can reduce the service life of a gear pump by 56 percent. And, it’s worse for vane and piston pumps because these designs are less able to withstand the vacuum-induced forces caused by a restricted intake. Hydraulic pumps are not designed to "suck".

A different set of problems arises from filters installed on the drain lines of piston pumps and motors, but the result is the same as suction strainers. They can reduce service life and cause catastrophic failures in these high-priced components.

Mistake No. 6 – Believing hydraulic components are self-priming and self-lubricating
You wouldn’t start an engine without oil in the crankcase – not knowingly, anyway. And yet, I’ve seen the same thing happen to a lot of high-priced hydraulic components.

The fact is, if the right steps aren’t followed during initial start-up, hydraulic components can be seriously damaged. In some cases, they may work OK for a while, but the harm incurred at start-up then dooms them to premature failure.

There are two parts to getting this dilemma right: knowing what to do and remembering to do it. Not knowing what to do is one thing. However if you do know, but forget to do it, that’s soul-destroying. You can’t pat yourself on the back for filling the pump housing with clean oil when you forgot to open the intake isolation valve before starting the engine!

Mistake No. 7 – Not getting an education in hydraulics
The purpose of this article is to show that if you own, operate, repair or maintain hydraulic equipment and you aren’t aware of the latest hydraulic equipment maintenance practices, a lot of money can slip through your fingers.
 
Source: http://www.machinerylubrication.com/article_detail.asp?articleid=1650

How to Control Contamination From Hydraulic Hoses

by Brendan Casey

The need for hose replacement is a fairly common occurrence on hydraulic machines. Hydraulic hose fabrication is a big business with plenty of competition and more than a few cowboys running around. So if you own or are responsible for hydraulic equipment, where you source replacement hoses from, and how they're made, cleaned and stored - prior to installation on your machine, warrants your attention.

The hose fabrication process - or more specifically, the hose cutting process - introduces contamination in the form of metal particles from the hose's wire reinforcement and the cutting blade itself, and polymer dust from the hose's outer cover and inner tube.

The amount of contamination which enters the hose during cutting can be reduced by employing techniques such as using a wet cutting blade instead of a dry one, blowing clean air through the hose as it is being cut and/or using a vacuum extraction device. The latter two aren't very practical when cutting long lengths of hose from a roll or in a mobile hose-van situation.

Figure 1. Dennis Kemper, a Gates product application engineer, performs a hose cleanliness fluid flush at the company's Customer Solutions Center.

Therefore, the main focus must be on effectively removing this cutting residue - and any other contamination which might be present in the hose - prior to installation. The most efficient and, therefore, most popular way of doing this is by blowing a foam cleaning projectile through the hose using a special attachment connected to compressed air. If you are not familiar with this equipment, do a search on Google for "hydraulic hose projectile".

The manufacturers of these cleaning systems claim that hose cleanliness levels as good as ISO 4406 13/10 are achievable. But like most everything else, the results achieved depend on a number of variables, which include using a projectile of the correct diameter for the hose being cleaned, whether the projectile is used dry or wetted with solvent, and the number of shots fired. Generally, the higher the number of shots, the cleaner the hose assembly. Furthermore, if it is a new hose that's being cleaned, the projectile cleaning should be done before the ends are crimped on.

Figure 2. A pneumatic air gun propels a foam projective through the hydraulic hose.

Hose Horror Stories

Almost all hydraulic hose fabricators these days have and use hose cleaning projectiles; but how meticulous they are when doing it is another matter entirely. This means if you want to ensure you take delivery of hose assemblies to a certain standard of cleanliness, it's something you must specify and insist upon, as the following account from a heavy equipment mechanic demonstrates:

"I was changing some hoses on a Komatsu 300 HD for a customer, and he noticed me washing out a hose before I put it on. So, he asked: 'They clean 'em when they make 'em, don't they?' I said, 'Sure, but I like to check.' I took the caps off a new hose and washed it with solvent and emptied the contents into some paper towel as he watched. His response was 'holy (expletive).'"

And it's not just the standard of the cleaning which must be insisted upon. A few years back, I was at a client's premises when its hose supplier arrived to deliver a big supply of hose assemblies. When the pallet came off the truck, it was obvious to anyone with eyes that none of the hoses were capped to prevent contaminant ingression. And, the customer accepted them. Nuts. As soon as I saw what was going on, I advised my client to require all hoses be delivered with caps installed and not to accept them otherwise.

Figure 3. An illustration of a foam projective scrubbing the inside of a hose.

Abrasion and Bending

This sort of penny foolishness should not be tolerated from any hose fabricator. And, it's definitely not something to be laissez-faire about, either!

When it is time to install a replacement hose, other than ensuring it's clean, pay careful attention to routing, ensure all clamps are secure and tight, and if necessary, apply inexpensive polyethylene spiral wrap to protect the hose from abrasion.

Hydraulic hose manufacturers estimate that 80 percent of hose failures are attributable to external physical damage through pulling, kinking, crushing or abrasion of the hose. Abrasion caused by hoses rubbing against each other or surrounding surfaces is the most common type of damage.

Another cause of premature hose failure to be on the lookout for when replacing a hose is multi-plane bending. Bending a hydraulic hose in more than one plane results in twisting of its wire reinforcement. A twist of 5 degrees can reduce the service life of a high-pressure hydraulic hose by as much as 70 percent, and a 7 degree twist can result in a 90 percent reduction in service life.

Multi-plane bending is usually the result of poor hose-assembly selection and/or routing, but also can occur as a result of inadequate or insecure clamping where the hose is subjected to machine or actuator movement.

Paying attention to these often-overlooked details will not only ensure replacement hoses aren't responsible for contaminant ingression and possible collateral damage to the hydraulic system they become part of, they'll last the way they should, too!

Reference:
http://www.insidersecretstohydraulics.com/.
or
http://www.machinerylubrication.com/article_detail.asp?articleid=2350