Modern water hydraulics—the new energy-transmission technology in fluid power

G.H. Lim*, P.S.K. Chua, Y.B. He

Nanyang Technological University, School of Mechanical and Production Engineering,

50 Nanyang Avenue, Singapore 639798, Singapore

Accepted 1 February 2003

Abstract

With the increasing demand for an environmentally-friendly fluid medium in the fluid-power industries, recent advances in water hydraulics technology have sparked renewedinterest in the application of water, instead of oil, as the energy-transmission medium. Thispaper introduces the history of water hydraulics and its present research. The advantages and

disadvantages of water as an energy-transmission medium are discussed. A water-hydraulicsystem in Nanyang Technological University is introduced.# 2003 Elsevier Ltd. All rights reserved.

Keywords: Energy transmission medium; Water hydraulics; Oil hydraulics; Fluid power

1. History of water hydraulics

In 220 B.C., Archimedes, a Greek physicist, put forward the Hydraulic Law (theprinciple of buoyancy) and invented the Archimedean Screw, a device for raisingwater [1]. The first water pump was invented in 200 B.C. [2,3]. In 100 B.C., water-powered wheels appeared in China. Du Yu of China invented the chain mills drivenby waterwheels between 265 and 420 A.D.. The first water-hydraulic press, inventedby Bramah, was granted a British patent in 1795. In 1906, Williams and Janneycoined in the idea of replacing water-based fluid by oil, thus avoiding corrosion,lubrication and freezing as well as leakage problems, at high temperatures, involvingwater. The following years saw a tremendous increase in the application of oil

Applied Energy 76 (2003) 239–246

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0306-2619/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

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* Corresponding author. Fax: +65-6791-1859.

E-mail address: mghlim@ntu.edu.sg (G.H. Lim).

hydraulics. The turning point came in 1994, when fresh interest was revived in waterhydraulics. This time round, with the advances in materials and designs, tap-waterhydraulics was made possible. Water hydraulics technology, after being tucked awayfor almost half a century, has since been taken seriously. With more water-hydrauliccomponent suppliers emerging in the world, the complete water-hydraulic systemhas become a reality [4]. Fig. 1 shows the applications of water and oil hydraulics.

2. Current research and development of water hydraulics

Water hydraulics were overtaken by oil hydraulics in the early part of the lastcentury in terms of research effort and industrial applications [5]. In recent years,with developments in materials and machining, it is becoming possible to producewater hydraulic components that are made of lubrication-free, anticorrosive mate-rials and achieve very close tolerances to reduce internal leakages due to the lowviscosity of water [6,7]. Growing concern about environmental issues has led torenewed interest in water hydraulics. Because water is non-toxic, environmentallyfriendly and readily available, many industries are steadily turning to water-hydraulic systems to replace their oil-hydraulic counterparts.Many companies are involved in water hydraulics. These include Danfoss, Hytar

OY, SPX Fluid Power (former Fenner Fluid Power), Hauhinco Trading, ElwoodCorporation, Hunt Valve Company, Schrupp Inc., the Oilgear Company, HainzlIndustriesysteme GmbH & CoKG, Ebara Research Co. Ltd, Kawasaki HeavyIndustry Ltd., Kayaba Industry Co. Ltd., Koganei Co., Komatsu Ltd., MitsubishiHeavy Industries Ltd., Moog Japan Ltd., Nabco Ltd., Nachi Co., SMC Co., TokyoPrecision Instrument Ltd., Yuken Kogyo Co. Ltd., and so on [5,8–17].

Fig. 1. Past applications of water and oil hydraulics.

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Institutions interested in modern water-hydraulics are listed as: Institute ofHydraulics and Automation, Tampere University of Technology, Finland, Linkop-ing University, Sweden, Department of Control and Engineering Design, TechnicalUniversity of Denmark, University of Wales, Cardiff (where a team led by ProfessorJohn Watton has explored extensively condition monitoring of oil hydraulic equip-ment with possible extensions to water hydraulics), Fluid Power Center, Universityof Bath, Tohoku University (Professor S. Hayashi) and many other universities inJapan, the State Key Laboratory of Fluid Power Transmission and Control, Zhe-jiang University, China, Huazhong University of Science and Technology, China,and School of Mechanical and Production Engineering, Nanyang TechnologicalUniversity, Singapore.Organizations such as FPNI [18], NFPA [19], BFPA [20], VDMA [21], and

JHPS [22] are involved in water hydraulics. Some international conferences haveoccured in water hydraulics. These include SICFP (Scandinavian InternationalConference on Fluid Power), Bath International Fluid Power Workshop, AachenConferences on Fluid Power Technology, ICFP (International Conference on FluidPower Transmission and Control), and JHPS International Symposium on FluidPower.Applications of water hydraulics can be found in nuclear engineering, coal mines,

steel foundries, desalination plants, fire-fighting, food/beverage production andplastics moulding. There is an increasing demand for environmental-friendly fluidsin farming, forestry, chemical, pharmaceutical, food processing and the paperindustries, where fluid leakages and spillages are a major concern.At present, oil hydraulics is superior to water hydraulics in engineering properties

such as efficiency and service lives of elements. One reason is that technologicalefforts required for water hydraulics are far less than those for oil hydraulics. Aninitial hurdle for the application of water hydraulics is the inaugural equipment cost.A water hydraulic system can cost between 30 and 200% more than the oil-basedsystem [3]. However, taking an overall financial view, a water-based system shouldprove to be a viable alternative in the long term. Its low fluid purchase and storagecosts, lower operational costs, lower disposal costs, reduced insurance premiums,and better health and environmental compliance make it a sound and logicalattraction. Water-hydraulic systems are usually made of very high-quality materialsand fabricated with precision machining, which enable them to normally last longerthan oil-hydraulic components.

3. Advantages and disadvantages of water as the pressure medium

What makes water hydraulics advanced principally is that water is hygienic,environmentally friendly, non-flammable, non-toxic, readily available and cost-effective. The thermal conductivity of water is 4–5 times that of mineral oil. So watersystems tend to require less cooling capacity. Water contains much less air insolution that may affect the rigidity of hydraulic systems. And water has anadvantage for faster operation and control of hydraulic systems.

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The main disadvantage of pure water hydraulics is the low viscosity. The leakagein water hydraulic elements is about 30 times larger than that for oil if the geome-tries of the leakage paths are the same. The kinematic viscosity of oil at 50 �C (thenormal operating temperature for oil) is 23 cSt while the kinematic viscosity of waterat 40 �C (the normal operating temperature for water) is 0.66 cSt [23]. The kinematicviscosity of oil at operating temperatures is about 30 times that of water. Hence, wemust prepare better surfaces of water hydraulic components to reduce leakages.High-precision machining is undoubtedly the basis of the new water hydraulics. Ahigher filtration grade is needed because of the narrower gaps used with waterhydraulics. Narrower gaps in water-hydraulic components may cause coulomb-fric-tion and wear. The low viscosity of water also makes water hydraulics inherentlynoisier than oil hydraulics [24].Ordinary water has a corrosive effect on many metals because it contains

dissolved gases such as oxygen, chlorine and others. Thus, the use of corro-sion-resistant materials such as stainless steel, special bronze, anodized alumi-nium, polymers, and ceramics, is required. The hardness of the water mustnot be too great (5–10�d), and the pH acidity coefficient must be in therange of 6–7 [25]. The restricted range of operating temperature should be 5–50 �C to avoid problems caused by freezing or vaporization. The vapourpressure for water is 120 mbar at 50 �C, which is higher by approximately 7orders of magnitude that for mineral oil (approximately 10�5 mbar). In order toavoid cavitation and erosion, the operating pressures in water hydraulics are oftenkept lower than for mineral oil.Corrosion-resistant materials (i.e. stainless steel, aluminum, copper alloys, poly-

mers, and ceramics), better surface and closer tolerances for the materials, operatingtemperature 5–50 �C, and water quality control (i.e. hardness, pH acidity and bac-teria) are the basic requirements for water hydraulics.

4. The water hydraulic system in NTU

The modern water hydraulic system installed in the School of Mechanical andProduction Engineering, Nanyang Technological University (NTU), shown inFig. 2, was acquired from a European manufacturer. The system set-up compriseda water-hydraulic system, a loading structure for the cylinder and the motor, sev-eral sensors, a signal conditioner, National Instrument’s AT-MIO-16L-9 dataacquisition card, and a PC installed with LabVIEW as the interfacing software.The dead weights were fabricated with 40 pieces of 10 kg for the cylinder loading.The weights were mounted incrementally on the water hydraulic cylinder set in thevertical position. Similarly, the water-hydraulic motor is loaded by coupling it toanother load motor, with the loading adjusted by throttling the load motor of the oilhydraulic line.The study on the modern water-hydraulic system in NTU was focused on online

condition monitoring and fault diagnosis of water-hydraulic actuators. Fig. 3 showsthe schematic set-up of the whole system that comprises a water hydraulic system, a

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data acquisition system and a personal computer with software for signal processingand analysis. LabVIEW software was used to acquire and process experimentaldata. This process includes averaging, digital filtering, windowing, FFT and graphpresentation. Based on their vibration and flow-response signatures, the behavioursof two tap-water hydraulic components, a hydraulic cylinder and a reciprocatingmotor, were investigated. The investigation includes a piston seal well, rod seal wearand piston wear of the cylinder, capstan/slipper shoes wear of the motor, lining wearand fatigue crack of the thrust plate of the water hydraulic motor. Also, the waterhydraulic cylinder was introduced to an oil system and an oil hydraulic cylinder wasintroduced to a water system in order to investigate the performance characteristicsof actuators in water and oil media.Experimental results show higher vibrations in hydraulic actuators with the water

medium compared to those with the oil medium and higher inlet flow responses inthe water-hydraulic system than that in oil hydraulic system when other conditionsare identical. Vibration signature analyses and flow responses are effective waysto do online condition monitoring and fault diagnoses of modern water-hydraulicsystems.

Fig. 2. The modern water hydraulic system in NTU.

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Fig. 3. Schematic set-up of the modern water hydraulic system in NTU for on-line condition

monitoring.

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5. Conclusions

This paper presented the application of water hydraulics as a revived and renewedenergy transmission technology. With new materials and advanced machining tech-nology being introduced to engineers almost everyday, better product design cap-ability, enforcement of legislation and an increasing demand for water-hydraulicequipment, the reliability and cost-effectiveness for water hydraulics has great pro-spects. The study on the modern water-hydraulic system in NTU Singapore dis-covered some different characteristics of water and oil hydraulics for on-linecondition monitoring and fault diagnoses.

Acknowledgements

The authors would like to express their sincere thanks to Alfred Tan Cheng Hockfor his contribution to this study.

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