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Unearthing Efficiency


Unearthing Efficiency

Maximized power output, faster system response and reliability of performance are key trends for hydraulic systems in off-highway equipment, and machines that deliver on these trends must stand up to resulting higher pressures and loads. In addition, operators are seeking out hydraulic fluids that can function in an array of operating conditions, two presenters said at the Society of Tribologists and Lubrication Engineers annual meeting in Minneapolis.

To keep up, lubricant manufacturers must provide formulations that can decrease unscheduled maintenance and repairs and increase productivity, minimizing the total cost of ownership to the end user, said Shubhamita Basu, technology manager at additive manufacturer Lubrizol.

Our goal would be [to formulate] a fluid that does not just increase productivity, but also maintains durability, she told attendees at the industry gathering in May.

Operators frequently seek out hydraulic oils with high viscosity indices to deliver fuel efficiency and ensure equipment is protected in any kind of weather. But as Basu emphasized throughout her presentation, a high viscosity index is not the only driver of fuel efficiency.

Formulations should ideally be tailored to construction equipment that operates in a range of temperatures and under different loads. These include low-viscosity fluids that can flow in cold-start conditions and high-viscosity fluids with balanced viscometrics that make them suitable as all-season oils, Lucas Camposo, technical service manager-Central & South America at Evonik Oil Additives, noted in his presentation.

A fluid must have a low enough viscosity under cold-start conditions to prevent fluid flow problems which would lead to poor efficiency through energy losses and sluggish operation.However, that same fluid needs to have sufficient viscosity at operating temperatures to provide the power to do effective and efficient work, Camposo later told LubesnGreases.

Lubrizols Basu expanded: When we talk about efficient hydraulic fluids, for the most part, we are actually talking about multigrade fluids with V.I. greater than 140, which are formulated with viscosity modifiers to minimize the change in viscosity at high or low temperatures.

Efficiency boils down to the power you are getting out from whatever power [energy or fuel] youre putting in, said Basu. Because there isnt a corporate average fuel economy standard like there is for on-road light-duty vehicles, efficiency in hydraulic equipment has to be measured in other ways, such as productivity, energy or fuel consumption per unit of time, work done per amount of fuel and temperature reduction, she elaborated.

Keeping a Balance

Both presenters highlighted that formulators must strike a balance between volumetric efficiency and mechanical efficiency. Camposo explained that volumetric efficiency is reduced when there are power losses resulting from viscosity that is too low, while mechanical efficiency is reduced due to power losses from viscosity that is too high.

Hydro-mechanical efficiency is dependent on frictional losses and viscous drag.Volumetric efficiency is related to flow losses due mostly to internal leakage, he expanded.

In a hydraulic pump, for example, volumetric efficiency decreases as the fluid becomes thinner at higher temperatures and is therefore more susceptible to internal leaks, said Lubrizols Basu. Leaks dont necessarily mean there will be liquid on the floor where you can see it, as systems also have internal leaks, she elaborated to LubesnGreases.

Lets say a fluid is too thin. When it comes out, that changes the pressure inside and can lead to cavitation. And if you have too much air in the fluid, it can change the density and reduce the amount of power transmitted, Basu added.

Mechanical efficiency, on the other hand, increases for thinner fluids, which can flow faster without much internal resistance. Additionally, friction, pumping and circuit design can lead to excess flow or pressure that affects mechanical efficiency, which can also lead to losses in total hydraulic system efficiency, Basu noted.

Thinner viscosity is good for mechanical efficiency, and thicker viscosity is good for volumetric efficiency, Basu summarized. But she clarified that looking at just the efficiency of a hydraulic pump to determine the total efficiency of a hydraulic system can be misleading, as there are numerous other pumps, motors, hoses, filters valves and seal fittings that can affect total system efficiency.

Shear Stability

To keep mechanical efficiency and volumetric efficiency balanced, the idea is to formulate a fluid with high V.I., but it also should be shear stable, commented Camposo.

Evonik conducted a field test using two hydraulic excavators, a dump truck, a wheel loader and an on-site truck scale at a construction site in Germany. The company tested how oils performed under truck loading and digging operations at 90- and 180-degree swivel angles at four temperature ranges (50-60, 60-70, 70-80 and 80-90 degrees Celsius) to determine productivity through tons moved per hour and efficiency through tons moved per liter of fuel consumed, Camposo detailed in his presentation.

The test sequence was A-B-A under constant conditions, Camposo said, meaning the tests were performed first with the reference fluid, then with the test fluid and again with reference fluid.

A total of 17 hydraulic fluids of different viscosity grades, viscosity indices and shear stability were evaluated in the performance demonstration. Evonik showed the results of two high-viscosity ISO 46 oils-a 162 V.I. fluid with poor shear stability and a 186 V.I. fluid with high shear stability-that were tested and compared against a reference fluid with 103 V.I. Both fluids were formulated using Evoniks polyalkylmethacrylate viscosity index improvers.

Camposo explained that the companys V.I. improvers coil and uncoil as the temperature of the fluid changes. At high temperatures, the additives expand to increase viscosity in a fluid, while at cold temperatures they coil up tightly, adding very little viscosity to the fluid. An efficient fluid is one that adds viscosity at high temperature and [reduces] viscosity at low temperature, he pointed out.

Regarding shear stability, Camposo said a V.I. improver made with low molecular weight polymer was used. These small polymers will experience less shear stress and will preserve optimum viscosity during operation. So for maximum efficiency, it is important to select a V.I. improver that is of the proper size to minimize shearing under the equipment operating conditions, he noted.

The results of the field test showed that the fluid with poor shear stability had no efficiency improvement in the digging phase and only a 3 percent improvement in truck loading over the low-V.I. reference oil, while the fluid with high shear stability showed an efficiency improvement of 19 percent in digging and of 13 percent in truck loading.

Productivity improved significantly with the more shear stable fluid, with a 20 percent gain in digging operation and 16 percent in truck loading, compared to 1 percent and 4 percent, respectively, for its less shear stable counterpart, Camposo noted.

Measuring Efficiency

In the absence of an industry standard, hydraulic efficiency is often measured by looking at pump efficiency, determining total system efficiency by mimicking real-world operation under controlled test conditions, and field evaluation of light- and heavy-duty cycles, Lubrizols Basu noted.

To investigate the effects of viscosity index on fuel efficiency, Lubrizol researchers selected two ISO 46 multigrade candidate oils blended with different viscosity modifiers but with the same base oil and additive packages. These were tested alongside a monograde reference oil with a V.I. of 106. The first oil had a V.I. of 151, while the second oil had a V.I. of 170.

The fluids were tested in four ways: using the KRL tapered roller bearing test (DIN 51350-6 or CEC L45-A-99) to measure shear stability, a Mini Traction Machine to measure traction coefficient, a rig assembled at Lubrizol to measure total system efficiency and, finally, a controlled vehicle efficiency field test.

Lubrizol ran the KRL test for 200 hours to measure the durability of the oils over a longer period of time, well above its standard runtime of 20 hours. The results showed that the second oil lost more than 15 percent of its viscosity at 100 degrees C after 50 hours, while the first oil maintained its viscosity up to 150 hours. Essentially, candidate 1 provides better durability over time, which was not evident from the V.I., Basu commented.

Next, traction coefficient, which is often a good indicator of fuel efficiency performance, was measured over variable slide-to-roll ratio, starting at pure rolling and moving to 50 percent slide-to-roll ratio. Candidate 1, with the lower V.I., actually has the better traction coefficient over candidate 2, which wont be predictable looking at the V.I. only, Basu expanded.

Real-world Conditions

The rig that Lubrizol developed to measure total hydraulic system efficiency mimicked the hydrostatic drive system in mobile hydraulic equipment and was instrumented to measure the efficiency of each component, including pumps, motors, lines, filters and coolers.

We dont want to formulate for one part of the system. The reason why we designed the rig was to look at the efficiency as a whole, not just for one component, she emphasized.

Results showed that candidate 2 had higher pump efficiency than candidate 1; however, candidate 1 had much higher motor efficiency. The lower V.I. candidate 1 showed much higher total system efficiency than the higher V.I. candidate 2, said Basu.

Lastly, Lubrizol conducted a controlled vehicle test using a back-hoe loader to measure brake-specific fuel consumption. The company tested the two candidate oils efficiency gains in cycles involving loaded rear lifts, front laden lifts, front unladen lifts and a mixed-duty cycle. Candidate 2, with the 170 V.I., consistently showed less efficiency than candidate 1 in all four duty cycle types.

Lubrizol added in a third oil with a V.I. of 185, making it the highest-V.I. candidate fluid, and tested it alongside the two candidate oils in a mixed-duty cycle. The higher-V.I. oil did not increase efficiency at all. Only candidate 1, with the 151 V.I., improved efficiency in the controlled test, said Basu.

Viscosity index is only one part of the [efficiency] story. It has a lot more into it, and all of that needs to be considered altogether in order to come to a conclusion, she stressed.

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