Assessing Five Key Areas of Modern Hydraulic Power Units

Any manufacturing facility that uses hydraulics powered by conventional fixed-speed hydraulic power units (HPUs) has engineers and personnel who know all too well how large, noisy, and inefficient older HPUs can be. In the past, such features were simply an accepted part of the manufacturing environment. But now, advances in engineering and design have led to new variable-speed power units that are smaller, quieter, and more efficient, intended for use in a wide variety of applications, and can directly replace traditional hydraulic systems.

Like other types of variable-speed, electrically driven equipment, variable-speed HPUs are significantly more energy-efficient than their fixed-speed counterparts. In addition, with built-in sensors, diagnostics, and cloud capabilities, they can be easily connected to an IoT environment to deliver valuable productivity and predictive maintenance data. Altogether, these benefits make modern hydraulic power units an attractive alternative to traditional units for energy efficiency, reduced cooling requirements, lower noise levels and increased reliability.

For engineers that specify new equipment or are charged with updating or replacing older HPUs, variable-speed systems provide more efficient options to consider before choosing where to invest.

When replacing or retrofitting an HPU, manufacturing engineers are advised to consider some key metrics that differentiate modern hydraulic system design from conventional systems: size, noise, energy efficiency, connectivity, and total cost of ownership.

1. Size: The overall size of an HPU is determined largely by the size of its hydraulic fluid reservoir. For traditional HPUs, the generally accepted rule is to size the reservoir a minimum of three to five times the maximum pump flow, to allow for degassing (time enough for the oil to sit in the reservoir and air bubbles to rise to the surface). For example, to achieve a max flow of 150 L using this old rule requires an approximately 600-L reservoir, which occupies a considerable amount of space within the unit's footprint.

Conversely, some modern HPUs use alternative methods of degassing that reduce the required amount of reservoir oil to a 1:1 ratio. More efficient manifold block designs, along with optimized reservoir shapes, allow for smaller tanks. That technique improves flow characteristics and better integrates component functions. The resulting manifold is more compact and lighter than a conventional manifold and can be manufactured in a variety of shapes to fit into tight spaces.

This is combined with a special "J-shaped" reservoir that allows for passive degassing of any bubbles in the oil. As a result, a unit that produces a max flow of 150 liters per minute only needs a reservoir capacity of 150 liters, or 75 percent less reservoir capacity than a traditional HPU. In terms of space on the plant floor, that can translate into a 0.5 m2 footprint, versus a typical 2 m2 footprint, and an 80 percent reduction in weight. The manufacturing plant would only need to supply and maintain one-fourth as much hydraulic oil in this example.

2. Noise: By nature, manufacturing facilities are considered noisy and usually require special personal protective equipment to block out that noise. But what is considered an acceptable amount of noise is not universally defined. In a steel manufacturing plant, for example, an HPU could operate in total silence and still not reduce the overall noise on the plant floor. But in other types of facilities, such as automotive manufacturing or rubber molding with assembly lines and long machine run times, quiet operation is desirable to maintain a pleasant working environment. Noise can also be a consideration due to the vibrations produced by sound waves, which can also have a negative impact on nearby equipment and processes.

For environments where reducing sound is important, variable-speed hydraulics is making notable headway, operating at around 10 dB quieter than conventional HPUs in the 7.5 kW to 30 kW range. Considering every three decibels equals a twofold increase in noise volume, this represents a significant improvement in noise reduction. Lowering the system noise to 75 dB is considered conversation level, which means two co-workers could stand next to the HPU and communicate with each other without wearing earplugs.

How is this noise reduction accomplished? First, by operating at variable speeds, modern units are naturally quieter, as they are not demanding their full power 100 percent of the time but instead only delivering the power needed at the moment. Second, modern systems can be constructed of materials that dampen sound and minimize vibrations by using design strategies like a liquid-cooled motor, compact arrangement of components, unitary housing, and built-in sound-insulating mats that are not features in conventional power units. For example, to eliminate the damping bearings that transmit noise to the reservoir tank, Bosch Rexroth attached its CytroBox motor pump group to a polymer-concrete foundation that absorbs vibrations.

Takeaway: Recent advances in HPU designs have resulted in noise levels as low as 75 dB (conversation level), which is a significant leap in sound reduction.

3. Energy Efficiency: A fixed-speed HPU operates at 100 percent motor speed, 100 percent of the time, regardless of whether the application requires it or not. Any energy that is not being used to do work is converted to heat, and simply turning a motor and pump at 1,800 rpm with no flow from the system requires several kW of wasted energy. This method of operation creates many subsequent problems, including higher energy costs and excess heat production that must be controlled using cooling or other dissipation methods (requiring even more energy).

Variable-speed systems create harmony by using electronic speed control of a motor and pump so that the energy output is adjusted to match the demands and conditions of the operation. By reducing the speed 40 percent to 70 percent under partial-load conditions and accounting for no-load conditions, variable-speed HPUs have demonstrated energy savings up to 80 percent when compared to their fixed-speed counterparts. Lower, more controlled operating speeds also reduce the unit's heat output, allowing it to run cooler and reduce or eliminate the need for additional cooling measures, as well as their associated costs.

Takeaway: When compared to equally sized fixed-speed HPUs, variable-speed HPUs run cooler and are up to 80 percent more energy-efficient.

4. Connectivity: Many manufacturers struggle with decisions between replacing outdated equipment or retrofitting older components to bring them into the modern age of IoT. Although incorporating data-gathering capabilities is essential to reducing maintenance costs, improving productivity and protecting uptime, deciding how to do that can be overwhelming.

Whether adding sensors to your older HPU or opting for a new plug-and-play model, the data that can be collected is invaluable to critical manufawcturing functions such as productivity, troubleshooting and predictive maintenance. With a modern HPU, access to cloudbased diagnostics and data analytics tools also streamlines workflows and reduces the demand on personnel (especially less-experienced employees) to commission new equipment or capture critical data to troubleshoot equipment in person.

Some companies like Bosch Rexroth offer sensor package options that include case drain flow/temperature, particle counter, energy consumption, pump damage and other monitoring parameters, with real-time accesscess to performance and usage reports 24/7 via dashboards on a mobile device.

With some older HPUs, connectivity is still an option, but it is one that can come with many challenges and costs. In these cases, it is important to calculate these costs and compare them to the cost of replacement with a modern, i4.O-ready HPU.

Takeaway: Consider how your operation might benefit from HPU data and compare the cost of retrofitting an older HPU with data sensors to the cost of replacing it with a new plug-and-play model equipped with an integrated sensor package and open interfaces.

5. Total Cost of Ownership: The belief that nothing costs less than the equipment a manufacturer already owns is misleading, and good data often demonstrates that the opposite is closer to the truth. While many older, fixed-speed HPUs can be outfitted with sensors and adapted for modern manufacturing and IoT, their energy usage remains inefficient and unchanged. They also still take up the same large amount of space on the factory floor, and they are still noisy. Comparatively, investing in a new, compact variable-speed HPU like the CytroBox can immediately reduce noise, free up space on the manufacturing floor for other profit-producing equipment and lower energy costs by operating more efficiently.

Takeaway: Modern variable-speed HPUs may be more of an investment upfront, but manufacturers with high energy costs driven by fixed-speed HPUs will see more value and a faster return on their investment with energy savings.

Choosing to maintain older fixed-speed HPUs or investing in new variable-speed HPUs depends on many factors within a manufacturing environment, including capital expenditures, lean maintenance goals, and carbon-reducing initiatives. New HPUs should be considered for any operation that is seeking IoT-ready solutions that lower energy costs and reduce noise, as well as open up space on the factory floor.

This article is written by Tyler Stahl, Application Engineer, Bosch Rexroth Corp. (Bethlehem, PA). For more info visit here .

This article first appeared in the June, 2022 issue of Motion Design Magazine.