Energy Conservation

New Design for Hydraulic Power Units

Guest contributor:  Andreas Günder, Bosch Rexoth

Optimum power, less installation space: Thanks to new intellectual and design approaches, compact hydraulic power units increase the economic efficiency of machine tools.

Powerful force in a very confined space

In the production world, hydraulics are firmly established. Machine tool manufacturers appreciate hydraulics for their high power density, toughness and modular design. In the lower performance range up to 4 kW, however, there are also some challenges. Since the installation space is often limited, designers and technical purchasers are constantly looking for increasingly compact solutions.

Installation space is valuable

The demand for compact hydraulic drives is not only due to the structurally limited flexibility regarding extensions, modernization measures and refittings but also due to the requirements regarding acquisition costs and assembly times or structural extensions of the working space with given machine dimensions. In addition to the level of integration of the functions, energy efficiency often plays an important role as well. Last but not least, many manufacturers are following the miniaturization trend. If workpieces become increasingly smaller, the moved mass of the machine tool has to be decreased accordingly.

“Installation space eaters” hydraulic power units

To reduce the installation space, solution manufacturers can start mainly with the following components: hydraulic power unit and control cabinet. When considering this split, it becomes evident that compact power units which are also easy to integrate require completely new design approaches to eliminate all features which waste unnecessary space in the performance spectrum up to 4 kW and to ensure that the units are still compatible with many different machine designs.

Highly integrated design approaches

The features of such innovative design concepts according to the EU Eco-Design Directive 2009/125/EC for example include a tank which is optimized for efficient degassing and reduces the oil volume by up to 80 percent. A much more decisive factor for gaining space is, however, that all functions can actually be integrated in one small power unit – from an economic variable-speed drive for demand-based power output to sensor technology with filling level, temperature, pressure and filter contamination sensors to a completely wired frequency converter.

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Compact and ready for Industry 4.0

For the future viability of this approach with regard to Industry 4.0, a data interface is essential as well. Only with permanent condition monitoring can the operating conditions be optimized comfortably and relevant faults be detected early on. With this equipment, the user only has to connect the electric power, the data interface and the hydraulic supply during installation and the hydraulic power unit is ready for operation

New cooling with heatpipe

So-called heatpipes are considered to be a space-saving innovation regarding the cooling of hydraulic power units. Their high-performance passive thermal conduction allows for a further reduction of the frame size. The heatpipes absorb the thermal energy of frequency converter, motor and hydraulic oil and efficiently transfer it to a central heat sink such as e. g. cooling water…
This ensures an intelligently optimized thermal management within the hydraulic power unit and optimally utilizes the cooling power of the cooling water. There is no need for a separate hydraulic circuit for oil cooling. This reduces installation space, noise emissions, energy consumption and possibilities for leakage.

Heatpipe – Functional principle

Basically, a heatpipe consists of air-tightly sealed copper pipes with underpressure. Inside, there is a medium which transfers thermal energy. In the temperature range of hydraulic power units, the medium may be e.g. distilled water. The boiling temperature of the water is significantly reduced by the low pressure within the heatpipe, which means that a boiling or condensation process can already take place at low temperatures.

Functionality: If you dip the heatpipe for example in hot hydraulic oil, the thermal energy at the lower immersed part of the heatpipe is transferred to the water. The water exceeds the boiling point, evaporates and absorbs a large amount of thermal energy with low temperature difference (latent heat). The water steam rises to the upper part of the heat pipe which is cooled by e. g. a cooling element. Here, the water steam condensates and gives off the thermal energy to the cooling water. Thanks to the latent heatabsorption and dissipation, the thermal conductivity of heatpipes can be up to 1000 times higher than the thermal conductivity of copper pipes. Due to the high elasticity of the copper pipes, the heat pipe can be easily shaped. In this way, ideal heat paths can be formed inside the hydraulic power unit and the installation space can be considerably optimized. Similar application ranges with equal optimization potential can be found in computer technology. Here, the thermal energy in laptops caused by heat sources such as the CPU are transferred to the central cooling elements using heatpipes.


Plug & Play: no control cabinet

The frequency converter has a high potential for gaining installation space as well. If it has already been equipped with Multi-Ethernet interface for Sercos, Profinet and other standards by the manufacturer, machine and plant manufacturers are able to reduce the control cabinet requirement for the hydraulic unit by up to 100 percent. As a precondition, however, the sensor technology and the motor in the power unit have to be wired to the frequency converter in such a way that the frequency converter can control the hydraulic pressure autonomously. This means that the control cabinet can not only be designed with smaller dimensions. Sometimes it can even be completely omitted together with the corresponding installation effort and related sources of error.


Fully integrated small power units based on a completely innovative design approach for the performance range up to 4 kW provide machine and plant manufacturers with the advantages of hydraulic drives with very little space requirements. As an alternative to purely electrical solutions, the required energy can be converted into a linear movement in a precise and costeffective manner directly at the working area using a simple hydraulic cylinder. If sensor technology, frequency converter and data interface are integrated as well, users not only benefit from comprehensive condition monitoring but also from a significantly reduced control cabinet footprint or even from a design without control cabinet.
More information fully integrated power units:

Operating principle:


CMA/Flodyne/Hydradyne is an authorized Bosch Rexroth distributor in Illinois, Wisconsin, Iowa and Northern Indiana.

In addition to distribution, we design and fabricate complete engineered systems, including hydraulic power units, electrical control panels, pneumatic panels & aluminum framing. Our advanced components and system solutions are found in a wide variety of industrial applications such as wind energy, solar energy, process control and more.

Best Practices: Inconsequential Volume

Guest Contributor: SMC

The advent of manifold mounted, plug-in pneumatic valves has been a boon for machine builders. It allows them to mount complete valve packages in a safe and secure location on a machine. Using a D-sub connector, serial interface module, or similar single-point wiring system, all of the electrical control outputs can feed into one location on the manifold, greatly simplifying the wiring. Plumbing issues are reduced, since a single air pressure line can be used to feed a common pressure gallery. The same advantage applies to the common exhaust gallery. No longer would both a plumber and an electrician be required to replace a valve, since any valve can be replaced without disturbing electrical connections or plumbing lines.ex260_ported

Like most advances, there can be some downsides to this type of arrangement, depending upon the application. One of these revolves around considerations for energy conservation. Most circuits include two air lines extending from the valve to the actuator, one for extend and one for retract. Each time the valve cycles, the actuator is pressurized on one side or the other. Of course, the hose or tube that connects them must also be pressurized. This tubing volume is typically ignored as inconsequential when doing our sizing calculations, but should not always be overlooked.

The tubing volume offers no work at all…only a means of transferring the energy of the compressed air to the actuator. Inconsequential? Maybe. But depending upon the application, those volumes can be significant. Here is an example:

A packaging facility has corrugated boxes being erected at a mezzanine level and dropped down to the main floor where they are filled, sealed and palletized. The box drop-down employs gravity to lower the boxes, but incorporates a series of mechanical stops to sequence them as required. Each of the stops includes a pair of 2 inch bore x 1 inch stroke air cylinders to operate a brake that stops the box and releases it when needed. In all, four stops are included for each drop down, meaning eight actuators in total and four valves to control them.

The directional control valves are located at a central location on a single manifold, presenting a clean and compact package. However, the distance from manifold to actuator varies from 10  feet to 25 feet. In operation, each time a valve shifts, it exhausts one length of tubing and fills another. Given the small volume, we might still think this to be inconsequential, but let’s look at the numbers.

Assume ¼” OD tubing with an ID of approximately .180“. With the first station being 10 feet away from the valve, the volume of tubing going to the actuators is only 3.05 cubic inches. Since there are two lines from the valve to each actuator pair, the total volume is 6.10 cubic inches per complete cycle. To put this in perspective, each of the 2×1” cylinders has a total volume of 5-½ cubic inches. That’s the sum of both the extend and retract sides. Since there are two of them, the total cylinder volume is 11 cubic inches. The volume in the tubing is over half that of the actuators. Much of the energy developed at the compressor is being used to fill the tubing lines.

Let’s leave aside the potential problem with poor response times, and just look at the costs. Let’s also assume the working pressure to be 80 psi. By calculating the compression ratio to be 6.33, and multiplying that by the volume, the normalized volume becomes 38.6 standard cubic inches. If we have the cylinders being actuated five times a minute, we will be consuming 193 standard cubic inches per minute, or using more meaningful terms, 0.11 SCFM. That is just for filling the air tubing to the closest single station on the drop-down. How about the other 3 stops on the drop-down?

The chart below shows the air volume for each of the stations, with a total for all. Please note the total volume of the airlines extending out to the cylinders is 42.8 in³. When we calculate the total volume of all eight cylinders we get 44 in³. The volumes are almost identical. Now we have a situation where half the compressed air is consumed filling all the air lines.


If that single drop down operates 24/7 and the cost for electricity is $.10 per KWh, the annual cost to fill those lines is about $110, the same costs we incur to fill the cylinders. With multiple drop-downs, the costs are multiplied. Staying with our example, the facility has 50 packaging lines, for an annual cost of $5500.

While this may be viewed as an extreme example, think back on all the applications where long air feed lines were employed in order to have a clean valve package. Or consider the oversize tubing that was utilized just because it was handy. Many of us might figure that larger tubing is better anyway, and everything will run more efficiently. However, please take into account that the larger tubing may double the volume to be filled without adding any productive benefit at all.

What happens to the same example when 3/8 inch tubing is used instead of the ¼ inch? The tubing ID increases to .27 inches and the volumes increase by a factor of 2.28, as do the costs. Now the annual price tag has jumped to $250. Again…that is just to fill the tubing for one drop-down.

As a side note, there is also some potential for liquid moisture build-up in those long lines when an excess of air is left between the valve and the actuator, even when good air filtration and drying is in place. Problems can arise due to the adiabatic expansion that occurs with each exhaust cycle. Condensation may occur as the exhausting air temperature falls below that of the atmospheric dew point. With long air lines and many cycles, the moisture may not be completely exhausted and can accumulate in the lines. That accumulation will occur near the actuator ports and will eventually work its way into the actuator, shortening its life.

What’s the solution? Have the valves mounted on or near the actuators so that all or most of the air volume is used to actuate the cylinders. This will require a pressure line running the length of the drop-down with a connection to each valve. However, that pressure line represents energy capacitance that works in our favor. We will also have fewer connections, which will translate into fewer potential leaks. Of course, the downside is that electrical lines will have to be extended independently to each valve. However, with low voltage, low wattage coils, this probably is not that difficult. Another benefit to be gained is that electrical response times remain unaffected, while pneumatic response times improve dramatically.  The dollar savings is the reduction in air consumption by half. In our plant, that could be up to $5500.00 every year.

Reducing the costs for compressed air needs to be an ongoing project for any company that uses substantial amounts of air as part of the manufacturing process. There is a tendency to approach the issue at the compressor room and look for answers there. The resulting recommendations can be expensive as well as require a long ROI. However, dealing with demand side issues, as described here, offers an easy solution that can be incorporated at the OEM level with little additional cost. A case can even be made for modifying these types of applications in the field when machines come due for refurbishment. Addressing these “inconsequential” issues will offer significant savings on the bottom line. “Watch the pennies and the dollars take care of themselves”.

To learn more about Best Practices in Pneumatics, please visit

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CMA/Flodyne/Hydradyne is an authorized  SMC distributor in Illinois, Wisconsin, Iowa and Northern Indiana.

In addition to distribution, we design and fabricate complete engineered systems, including hydraulic power units, electrical control panels, pneumatic panels & aluminum framing. Our advanced components and system solutions are found in a wide variety of industrial applications such as wind energy, solar energy, process control and more.