The efficiency of hydraulic systems and fluids is dependent on the operating conditions, the stress on the system and the specific application. In any given year fluid power systems consume 2.25 to 3.0 quadrillion BTUs, which includes approximately 1.7 quadrillion for industrial applications, 1.2 for mobile and 0.1 for aerospace. That all sounds like there’s a lot of fluid power hard at work. Yet the kicker is, the average efficiency of these systems is only 21% though not to be confused with the 80 to 90% efficiency of a ‛single’ hydraulic pump. So, the question is, could the optimization of hydraulic fluids improve efficiencies, thereby reducing energy consumption?
Hydraulics is one of the most powerful, and perhaps contrary to the 21% stated above, one of the most efficient methods to move things. In fact, the only mechanisms stronger are certain types of magnets, such as neodymium or iron nitride magnets and magnets are not used to power the bucket on a 14-ton capacity dump truck.
Hydraulics are like pneumatic systems in function. Both hydraulics and pneumatics use fluids to operate. Unlike pneumatics which use gasses, hydraulics systems are powered by hydraulic fluids and are capable of exerting extreme loads of up to 10,000 PSI, versus a pneumatics system of approximately 100 PSI.
A unique advantage of fluid power is that torque or force can remain constant. This is exemplified in fluid power transmission, where high torque can be maintained at low speed. Hydraulic motors generate high torque even when operating at low rotational speeds, which is not the case with electric motors. A fluid power system can provide effective control of speed, force, torque and direction by using simple control valves. Plus, they can fit into smaller spaces than equivalent mechanical or electrical drives. In fact, with their higher horsepower-to-weight ratio the typical hydraulic motor will weigh less than its electric equivalent.
New technological advancements are increasing efficiency of hydraulic components and systems in reducing downtime and energy consumption while facilitating the efficient operation of heavy construction, agricultural equipment, industrial machines and even the lifting of Tower bridge. 
Efficiency in Hydraulics
A hydraulic pump converts mechanical power into hydraulic energy (aka hydrostatic energy such as pressure and flow). The power of the flow generated by the pump will overcome the pressure of the load at the pump’s outlet.
To determine the mechanical/hydraulic efficiency of a pump, divide the theoretical torque by the actual torque required to drive it. In a simple example, a pump operating at 1,000 HP and generating 800 HP of hydraulic power is 80% efficient.
To determine volumetric efficiency, establish the pressure of the pump and divide its actual flow by its theoretical flow. To calculate theoretical flow, multiply the pump's driven speed by its displacement per revolution. For instance, if the pump is being driven at 1,000 RPM and has a displacement of 100 cc/revolution, then its theoretical flow is 100 liters/minute. Comparatively, actual flow must be measured with a flow meter. If the pump shows a flow of 90 liters/minute at 207 bar when measured by the meter, then the pump has a volumetric efficiency of 90%. The flow measured by the flow meter is irrelevant unless there’s reference to theoretical flow.
However, the condition of a hydraulic pump ascertained by its volumetric efficiency is affected by any wear or damage that’s causing an increase in internal leakage.
Hydraulic Fluid
A hydraulic system is only as good as its fluid, and there are many factures that go into determining the best fluid for an efficient system. Recommendations can be all over the board.
When SAE 10W engine oil is specified, 10W hydraulic oil is the recommended choice for heavy duty off-highway hydraulic systems. On the other hand, Caterpillar has its own line of fluids. Its TO-4 fluids are specifically formulated SAE 10, 30 and 50 oils that meet Caterpillar's lubricant specifications for their latest transmission and drive trains. Komatsu often recommends its hydraulic zinc-based anti-wear fluid.
The bottom line: a higher viscosity suggests a thicker fluid, causing slower flow. The viscosity of a liquid or semisolid changes with temperature, protecting and maintaining the operability of the system. Therefore, the viscosity index is the datum for the ability of hydraulic oil to avoid becoming thinner at elevated temperatures.
Increased Efficiencies in Hydraulic Systems would not be Possible without Advancements in Hydraulic Fluid
A system is most effective when using a maximum efficiency hydraulic fluid, which demonstrates a high viscosity index (typically 80 to 110) and maintains good shear stability.
The hydraulic viscosity index is the change in viscosity of hydraulic oil as temperature changes. Viscosity decreases with an increase in temperature. Since hydraulic fluid should maintain a high (or even very high) viscosity index, the fluid temperature should not increase beyond its manufacturers’ recommendations.
Viscosity index is typically classified into four categories:
- Low (up to 35)
- Medium (35-80)
- High (80-110)
- Very high (above 110)
Viscosity is the fluid's resistance to flow. If the viscosity is too high, the fluid will become thicker as the viscosity rises and causes a greater resistance to flow. However, if the fluid’s viscosity index is too low the fluid can freeze and may fully impede the movement of fluid through the system. The viscosity of hydraulic fluid is measured in Centistokes (cSt). As an example of temperature, the oil in a hydraulic system of construction equipment typically ranges from 30 to 60°C.
Mineral-Based Hydraulic Fluids and Additives
The addition of specific additives can improve the viscosity index of mineral-based hydraulic oils. Hydraulic oils with a high viscosity index provide protection from wear when hydraulic systems and pumps are at high temperatures and also improves performance at low temperatures.
Mobile construction and even agricultural equipment operate year-round and use multi-grade or multi-viscosity hydraulic fluids. This is because the ambient temperatures where they are used varies widely and is typically outside the range of a single grade conventional fluid. Various outdoor equipment experiences a wide range of temperatures from the frigid winters in Sweden to the hot summers in the South of France. Equipment such as:
- Balers
- Excavators
- Snowplows
- Tractors
- Municipal wast trucks
- Combines
- Hydraulic cranes
- Sprayers
- Wheel loaders
- Utility lift trucks
Viscosity modifiers (VM) and pour point depressants (PPD) in multi-grade hydraulic fluids contain a hydraulic performance additive. PPDs control wax crystallization to ensure lubricants continue to flow in cold temperatures, while VMs raise hydraulic fluid viscosity index so fluid viscosity is constant over a wide temperature range. Multi-grade high performance fluids provide superior operating efficiency and wear protection, as well as a hydraulic system’s smooth response in extreme cold or high humidity and heat.
Choosing the right VM for multi-grade hydraulic fluids is important to the overall performance system which impacts:
- Ability to seperate water
- Filter of fluids
- Fluidity at low temperature
- Performance characteristics of high-quality hydraulic fluids
- Increase in Viscosity Index
- Resistance to viscosity loss
- Shear stability
Hydraulic fluids serve many important functions, such as helping in the transfer of energy, acting as a medium for power transmission, lubricating and reducing leaks from the clean transport of fluid throughout the system. In Additionally, hydraulic fluids cool equipment and move contaminants out of the lines and into filters. There are tremendous economic advantages of today’s advanced fluid technology. Hydraulic systems are more simple than mechanical and electrical systems. They are also safer, more economical to operate and much easier to maintain due to fewer moving parts.
Conclusion
Hydraulic systems convert mechanical power to fluid power by rotating a pump’s input shaft. Control valves then direct flow to actuators using motors and cylinders to change the fluid power back to mechanical force, which generating high-power densities that power equipment. The most common applications for hydraulic systems are in outdoor heavy equipment, including agriculture, construction, waste reduction, mining, and utility equipment, as well as heavy industrial machines, marine/offshore equipment and even aerospace. Any application requiring heavy lifting, hauling, bending, pressing, farming and control of an aircraft utilizes hydraulics at the core of its operation.
The life blood of a hydraulic system is the fluids that drive it. Hydraulic fluids provide energy transmission that facilitates the work the equipment must complete. Additionally, these fluids furnish lubrication, help control contamination and provide heat transfer to prevent motors and pumps from overheating. The proper viscosity of the fluid is imperative for smooth operation. A high, excessive viscosity may cause inadequate mechanical efficiency at low temperatures. Conversely, if oil temperature increases, viscosity will decrease and reduce volumetric efficiency, causing overheating and excessive wear on components. It's best to refer to the documentation provided by the component manufacturers for hydraulic fluid recommendations.
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