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STUDY ON SLIPPERS FOR RAW WATER HYDRAULIC AXIAL PISTON
PUMPS AND MOTORS'
1. Abstract
As concerns for environmental compatibility, safetyand hygienic requirements, hydraulic systems using rawwater as pressure medium become more and moreattractive. A raw water hydraulic system has many advantages over conventional oil hydraulic systems,such as: environmentally compatibility, no pollution and no fire hazard, elimination of expensive oils with the associated problems of contamination, maintenance,storage, handling, elimination of the return hose, and elimination of health hazards, etc. But as a hydraulic medium, raw water has also drawbacks, such as very low viscosity, high vapor pressure, lack of lubrication and chemically active nature, etc., that all tend to promote problems of corrosion, wear, cavitation erosion, leakage, and so on, those problems must be overcome in the design and development of the raw water hydraulic components, especially the selection of materials and design of structure for the friction pairs in raw water hydraulic pumps and motors.
The slipper/swashplate pair is one of the very important friction pairs in axial piston pumps and motors. Under the lubrication of raw water. materials for slipper/swashplate combination should have superior mechanical strength, low friction, resistance to corrosion, abrasion, cavitation erosion and sliding wear, and the structure of the slipper should be suitable for the characteristics of raw water and materials used
Based on authois researches, a new slipper foraxial piston pumps and motors is introduced in this paper. From tests with the slipper/swashplate lest rig, it shows that the new type slipper combined with appropriate swashplate has good tribological characteristics. This slipper can operate with raw water, even sea water. The successful study on slippers will
provide good helps for design and development of raw water hydraulic axial piston pumps and motors with higher performance.
2. Introduction
The use of water as hydraulic fluid is not new. The first hydraulic machine as a means of transmitting not only energy but also control signals is raw water hydraulic press that manufactured by Joseph Bramah in 1795, which marks the beginning of the age of modern applied hydraulics. After that almost 100 years raw water was used as hydraulic medium, until the beginning of 20th century the mineral oil began to take the place of raw water. Because of the much better lubrication and anti-corrosi on properbes of mineral oil as compared with raw water and the development of oil resistant sealing materials, oil hydraulics boosted quickly and raw water hydraulics was left behind, gradually acquiring the image of an obsolete technology.
As concerns over safe or environmental issues a number of other hydraulic fluids have been developed over the past decades, such as synthetic base fluids, water base fluids, and vegetable oil base fluids, etc. These fluids have their own advantages and a certain range of applications. For a given application, engineers can make the hydraulic system have satisfactory performance by careful hydraulic fluid choice and system design. However, these hydraulic fluids also have its own shortcomings,so me shortcomings of these fluids are inherent and fatal. No one of these hydraulic fluids can meet the requirements of fire resistance and environmentally protection at the same time, and most of these fluids are very expensive.
The re-emergence of using raw water as hydraulic fluid is mainly because of the following reasons.
(1) Environmental Protection Requirement
Without strict adherence to specific control techniques, it is nearly impossible to eliminate the threat of leakage in hydraulic systems. Industry observers believe that as much as 85% of all hydraulic fluids eventually leave their systems through slow leaks, catastrophic line breaks, or failures of fittings and seals. The leakage of hydraulic fluids with some toxic chemical additives not only make working conditions messy and unsafe, but also can not be biodegradable and have
potential threat to the environment. Raw water has very good environmental compatibility and no pollution; water in the event of leakage eventually evaporates without leaving greasy or dirty residuals needs of applications for water hydraulics. The need of new applications is the intrinsic force of re-emergence and development of water hydraulics
(2) Requirement of Safety
Mineral oil are flammable, its leakage or spilling from systems may be result in a fire when in or near heat sources. Water is not flammable and does not present a fire hazard. In some areas, such as steel and glass production, iron making and foundry, injection molding, die casting, nuclear power, coal mining, etc. the requirement of safety is prime, so raw water hydraulics may be the best choice in the near future
(3) Hygienic Requirement
In some fields, such as gold mining, food and medicine processing, water supply industry, etc , where escaping of oil or chemical additives from hydraulic systems can smear the product and damage the quality of manufactured product. Under these conditions, raw water is an ideal pressure medium.
(4) Economy Requirement
Water is available everywhere, needing no purchase, transport, storage, maintenance and disposal costs, so using water instead of mineral oil and other hydraulic fluids may offer tremendous economy. In ambient water environment, the system using raw water as hydraulic fluid can be designed as an open circuit.That means the return lines and water reservoir can be
eliminated, further more the heater and the cooler of system are no longer needed, so the volume and weight of hydraulic equipment will reduced and the efficiency of system increased. If used underwater, such as in marine engineering, ocean exploration engineering, subsea equipment, underwater operation tools and robots, etc., water hydraulics can automatically counterbalance the
water hydrostatic head, this is very important for improving the efficiency and performance of the
hydraulic systems.
(5) Need of New Applications
Along with the developments of modern science and technologies, many areas with special requirements provide many new opportunities of applications for water hydraulics. Such as fusion reactor equipment in nuclear power engineering, marine equipment and robots in ocean exploration engineering, etc，where the fire resistance, hygienic requirement, economy and environmental protection are all considered. Because of higher power density and smaller size of hydraulics than that of other power transmissions, there have urgent needs of applications for water hydraulics.The need of new applications is the intrinsic force of re-emergence and development of water hydraulics.
3. Material Selection
The slipper/swashplate pair is one of the very important friction pairs in axial piston pumps and motors. Under the lubrication of raw water,ingenious material selection is necessary.
In a typical axial swashplate is stationary piston pump or motor, the swashplate is stationary ; the swashplate angle with the shaft centerline determines the length of the piston stroke (see Fig. 1)
When using raw water as lubricant,according to rawwater's chemically active nature, low viscositylubrication and high vapor pressure,more problem should be considered.
Firstly, raw water especially seawater is more aggressive , a large number of ions in seawater result in that seawater have much higher electric conductivity and may lead to electrochemistry erosion. The materials used for slipper and swashplate inevitably suffer aggressive corrosion from raw water , and the materials used for traditional oil hydraulic components will not be suitable here.
Secondly, the kinematic viscosity of water at 50℃ is approximately 0.55cs ,which is less than 1/30 of the viscosity of a typical mineral oil fluid. The very low viscosity must increase the difficulty of developing hydrodynamic film between slipper and swashplate,and the very small change of water’s viscosity with pressure means that elastohydrodynamic lubrication with hard materials is unlikely to occur. Hydraulic fluids with additives that aid in the boundary lubrication of contacting surface permit metal-on-metal contact without large amounts of surface damage or wear. Raw water contains on such additives and its lubrication is very poor. The poor lubrication of water is clearly one of its major dtawbacks, and will lead to increase contact fraction and possibly wear of contacting conponents. Indeed it is well documented that wear was the greatest problem experienced by the early users of water based fluids in axial piston pumps. Researches indicate that the elastohydrodynamic film of water is about 0.1μm thick at 3m/s and 20-60℃, the elastohydtodynamic film of HWBF is about 0.15μm thick at 0.19~1.27m/s and 23.5℃. So there are risks as high dry friction and strong wear between slipper and swashplate under the lubrication of raw water.
Thirdly, the vapor pressure of water is much higher than that of mineral oil, which means that water boils or vaporizes much easier. Due to water’s high vapor pressure and high velocity flow caused be water’s low viscosity, the slipper and swashplate will be exposed to strong and fast cavitation erosion.
So material selection for slipper/swashplate pair should depend on the operation conditions and the special properties of raw water. Materials for slipper/swashplate combination should have superior mechanical strength, low friction, resistance to aggressive corrosion, cavitation erosion,abrasion,and sliding wear, and should be easy available and cost low.
4.Structure Design
The structure design of slipper is very important as well as the material selection.
The use of polymers as the material of slipper will derive a new problem-how to make the slipper wrap on the ball-shaped head of piston. Probably the solution is to use corrosion resistant metals combined with polymers. The slipper can be divided into two parts. One is slipper pad made of polymer, directly contact on the swashplate, its main function is making the slipper have good tribological properties. The other is shoe made of corrosion resistant alloy, its main function is making the slipper wrap on the ball-shaped head of piston. This structure can take the advantages of both metal’s ductility and polymer’s good tribological property(see Fig.2).
The main static forces exerted on slipper are show in Fig.3. The force F acted on slipper from piston head includes hydrostatic force Fp on the piston end, spring force Fs, inertia force Fa of piston and slipper, and friction force Ff between piston and cylinder bore as formula:
The reaction force N from swashplate can be divided into two components, component Nh of hydrostatic bearing force and component Nm of mechanical surface pressure,as following:
N=Nh+Nm
The relationship of N and F is:
F=N·cosα
5.Conclusions
Based on author’s researches, some conclusions about material selection and structure design of slipper under lubrication of raw water can be summarized as following:
(1) Slipper made of polymers combined with swashplate made of corrosion resistant alloys has superior tribological properties.
(2) Cotton fiber-filled phenolics can be used to manufacture slipper for a certain application range of operation pressure below 14MPa.
(3) The spring force exerted on slipper should be strong enough to keep about 0.1~0.2MPa contacting pressuer on the surfaces of slipper and swashplate.
Farther tests are underway, and ceramics with good toughness will be introduced to manufacture slipper and swashplate. The experiences of study on slippers will provide good helps for design and development of raw water hydraulic axial piston pumps and motors.
References
[1] Joseph L. Foszcz, Hydraulic Fluid Choices, Plant Engineering, (8),1996,68.
[2]Li Zhuangyun, et al, Development of Hydraulic Pump to Operate with Raw water, Proceedings of ASME Fluid Engineering Division Summer Meeting, Washington DC, USA, June 21-25,1998.
[4]吳仁榮，水潤滑滑動軸承的設(shè)計(jì)計(jì)算，機(jī)電設(shè)備，（6），1997,30.
[5]許耀銘，油膜理論與液壓泵和馬達(dá)的摩擦副設(shè)計(jì)，北京；機(jī)械工業(yè)出版社，1987.
關(guān)于自然水液壓軸向活塞泵和電動機(jī)的滑動部分的研究
1.摘要
基于環(huán)境和諧、安全和衛(wèi)生需要的考慮，液壓系統(tǒng)使用自然水作為壓力介質(zhì)這種現(xiàn)象變得越來越廣泛。一個自然水液壓系統(tǒng)相對于普通的油液壓系統(tǒng)有很多優(yōu)點(diǎn)，例如：環(huán)境和諧，沒有污染，沒有火的危害，與昂貴的油相關(guān)的危害、維護(hù)、貯存、管理問題的消除，返回塑料管的消除和健康危害的消除等等。但是作為液壓系統(tǒng)的介質(zhì)，自然水也有一些缺點(diǎn)，例如非常低的粘性，很高的水蒸氣壓力，缺少潤滑和化學(xué)性自然等等，都加劇了腐蝕、磨損、氣蝕侵蝕、泄露等問題，這些問題必須在設(shè)計(jì)和自然水液壓元件的發(fā)展，尤其是材料和自然水液壓泵和電動機(jī)的摩擦部分的結(jié)構(gòu)設(shè)計(jì)中被克服。
滑動部分/旋轉(zhuǎn)羅盤部分是軸向活塞泵和電動機(jī)中的重要摩擦部分。在自然水的潤滑下，滑動部分和旋轉(zhuǎn)羅盤結(jié)合部分的材料應(yīng)該較好的機(jī)械強(qiáng)度、低摩擦力、腐蝕的抵抗性、磨損、氣蝕侵蝕和滑動摩擦，并且滑動部分的結(jié)構(gòu)應(yīng)該對于自然水和使用材料的特性是適合的。
根據(jù)作者的研究，一種新型的用于軸向活塞泵和電動機(jī)上的滑塊會在這篇文章中介紹。從滑塊/旋轉(zhuǎn)羅盤部分試驗(yàn)中表明這種新型的與合適旋轉(zhuǎn)羅盤結(jié)合的滑塊有著非常好的摩擦特性。這種滑塊能夠用自然水潤滑，甚至是海水也可以。這個關(guān)于滑塊的研究將會為自然水液壓軸向活塞泵和電動機(jī)的更好發(fā)展提供好的幫助。
2.介紹
水用來作為液壓流體已經(jīng)不是新技術(shù)了。作為轉(zhuǎn)換能源和控制信號的一種方式，第一臺液壓機(jī)器就是自然水液體壓力，并且在1795年由杰瑟夫大量生產(chǎn)，同時也標(biāo)志著現(xiàn)代液壓時代的開始。在那之后幾乎100年自然水被用作液壓介質(zhì)，直到20世紀(jì)的初期，油介質(zhì)開始取代水介質(zhì)。和自然水相比，油介質(zhì)有更好的潤滑和礦物油的抗腐蝕性，并且油抗性密封材料，油液壓裝置發(fā)展的很快，自然水液壓裝置就落后了，漸漸成為一種過時的技術(shù)了。
基于在安全和環(huán)境問題方面的考慮，許多其他液壓流體已經(jīng)在過去的幾十年發(fā)展了，例如合成來源液體、水來源液體和蔬菜油來源液體等等。這些液體有著它們各自的優(yōu)點(diǎn)和確定的用途。對于一種給定的用途，工程師能夠通過合理的液壓流體選擇和系統(tǒng)設(shè)計(jì)來使液壓系統(tǒng)滿足這個條件。然而，這些液壓流體也存在著自身的缺點(diǎn)，其中的某些缺點(diǎn)是內(nèi)在的并且是重大的。這些液壓流體中沒有一種能同時滿足防火性和環(huán)保性的要求，同時大部分液體是很昂貴的。
使用自然水作為液壓流體主要由于以下幾個原因：
（1）環(huán)境保護(hù)的需要
如果對于特殊控制技術(shù)沒有嚴(yán)格的遵守，評估液壓系統(tǒng)中的泄露威脅是不可能的。工業(yè)觀察員相信所有液壓流體中85%的流體最后都是通過緩慢泄露、嚴(yán)重的連接破損或者設(shè)備和密封的故障而離開液壓系統(tǒng)的。液壓流體和一些有毒的化學(xué)添加劑的泄露不僅使工作條件混亂和不安全，同時也不能生物降解和對環(huán)境存在潛在的危害。自然水擁有很好的環(huán)境適應(yīng)性并且沒有污染；水在泄露后也會消失而沒有油膩和臟的殘留物。
（2）安全的需要
礦物油是可燃的，當(dāng)周圍有熱源的時候它的泄露可能會導(dǎo)致起火。水是不可燃的，所以不會存在起火隱患。在一些區(qū)域，例如鋼鐵和玻璃生產(chǎn)，鋼鐵制造和鑄造，注射模具，拉模鑄造，核動力，采煤等等，對于安全的要求是主要的，因此自然水液壓可能會成為將來的最好選擇。
（3）液壓的需要
在一些領(lǐng)域，例如金礦開采，事物和藥品生產(chǎn)，水提供工業(yè)等等，這些由于油或者化學(xué)添加劑從液壓系統(tǒng)中泄露而弄臟產(chǎn)品或是損害大批產(chǎn)品的質(zhì)量，在這些條件下，自然水是一種理想的壓力介質(zhì)。
（4）經(jīng)濟(jì)的需要
水在很多地方都是存在的，不需要購買、運(yùn)輸、貯存、保持和清理成本，因此使用水代替礦物油和其他液壓流體可以提供極大的經(jīng)濟(jì)性。在包圍的水環(huán)境中，系統(tǒng)使用自然水作為液壓流體能夠被設(shè)計(jì)成開式循環(huán)。那意味著返回線和水儲藏可以消除，加熱和冷卻系統(tǒng)也不再需要了，因此液壓設(shè)備的容積和重量也會減少而系統(tǒng)的效率會增加。如果水下作業(yè)，例如在海運(yùn)的工程上，海洋探索工程，海底設(shè)備，水下操作工具和機(jī)器人等等，水液壓能夠自動的抵消水靜力的前端，這對于提高效率和液壓系統(tǒng)的性能是很重要的。
（5）新應(yīng)用的需要
隨著現(xiàn)代科學(xué)和技術(shù)的發(fā)展，許多有著特殊需求的領(lǐng)域?yàn)樗簤禾峁┝撕芏噙\(yùn)用的機(jī)遇。例如在核電工程中的核聚變反應(yīng)裝置，在海洋探索工程中的海運(yùn)設(shè)備和機(jī)器人等等，這些地方需要考慮抗燃性，清潔的設(shè)備，經(jīng)濟(jì)性和環(huán)保等方面。和其它的能源轉(zhuǎn)換相比，由于液壓系統(tǒng)有較高的力密度和較小的尺寸，所以對于水液壓的應(yīng)用是迫切需要的。新應(yīng)用的需要是水液壓的出現(xiàn)和發(fā)展的內(nèi)在反應(yīng)。
3．材料選擇
滑動部分/旋轉(zhuǎn)羅盤部分是軸向活塞泵和電動機(jī)中的重要摩擦部分。在自然水的潤滑下，巧妙地選擇材料是有必要的。
在一個典型的軸向活塞泵或電動機(jī)中，旋轉(zhuǎn)羅盤固定不動的，旋轉(zhuǎn)羅盤與軸線的角度決定了活塞盤的長度。
當(dāng)使用自然水作為潤滑劑時，由于自然水的化學(xué)活性自然，低粘性，潤滑性差和較高的蒸汽壓力，所以需要考慮很多問題。
首先，自然水，尤其海水是活性的，海水中的大量離子導(dǎo)致海水有很強(qiáng)的導(dǎo)電性并且會導(dǎo)致電化學(xué)腐蝕。用在滑塊和旋轉(zhuǎn)羅盤上的材料會不可避免地遭到自然水的活性腐蝕，并且用于傳統(tǒng)油液壓器件也不適合用在這里。
第二點(diǎn)，水的運(yùn)動粘性在50℃時接近0.55cs，少于典型礦物油液體粘性的1/30。過低的粘性一定會增加發(fā)展滑塊和旋轉(zhuǎn)羅盤之間水力薄層的困難程度，并且水的粘性伴隨著壓力的很小變化意味著硬質(zhì)材料的流體動力潤滑是不容易發(fā)生的。帶有添加劑的液壓流體使得金屬與金屬表面之間沒有大量表面破壞或是磨損。自然水不包含如此多的添加劑并且它的潤滑性是很差的。水的潤滑性差很明顯是它的一個主要缺點(diǎn)，并且會導(dǎo)致接觸、破碎、和接觸器件之間摩擦的加劇。事實(shí)上摩擦是早期用水作為軸向活塞泵的流體的使用者遇到的最大問題。研究表明水的流體彈性動力薄層在3m/s和20~60℃時大約是0.1μm厚，HWBF的流體彈性動力薄層在0.19~1.27m/s和23.5℃時大約是0.15μm厚。因此在自然水潤滑下，滑塊和旋轉(zhuǎn)羅盤之間有干摩擦和強(qiáng)烈摩擦?xí)r會存在危險。
第三點(diǎn)，水的蒸汽壓力要比礦物油的高很多，這意味著水沸騰或是蒸發(fā)會更加容易。由于水的高蒸汽壓力和高速流動性由水的粘性地決定的，所以滑塊和旋轉(zhuǎn)羅盤會受到強(qiáng)烈和快速的氣穴氣蝕損害。
因此，對于滑塊和旋轉(zhuǎn)羅盤部分材料的選擇應(yīng)該依據(jù)工作條件和自然水的特殊性質(zhì)?；瑝K和旋轉(zhuǎn)羅盤之間的連接部分應(yīng)該有很好的機(jī)械強(qiáng)度、低摩擦、抗腐蝕性、和氣穴氣蝕、磨損和滑動摩擦，并且應(yīng)該有很好的實(shí)用性和低成本。
4.結(jié)構(gòu)設(shè)計(jì)
滑塊的結(jié)構(gòu)設(shè)計(jì)和材料的選擇是同樣重要的。
用在滑塊上的高分子材料會產(chǎn)生一個新的問題，就是怎樣使滑塊包圍住活塞前端的球形部分。其中一個解決辦法就是使用抗腐蝕材料與高分子材料連接。滑塊可以分成兩部分。一部分就是由高分子材料制成的滑塊墊，直接與旋轉(zhuǎn)羅盤相連，他的主要的功能就是使滑塊有很好的摩擦性能。另一部分是由抗腐蝕的合金材料制成的鞋，它的主要功能是使滑塊在活塞的球形頭上摩擦。這個結(jié)構(gòu)能夠很好的利用金屬的展延性和高分子材料的摩擦性。（見圖2）
在滑塊上的主要的靜力在圖3中有表示。滑塊上的力F來自活塞頭，包括活塞尾端的液體壓力Fp，反彈力Fs, 活塞和滑塊的慣性力Fa,還有活塞與圓孔之間的摩擦力Ff。
來自旋轉(zhuǎn)羅盤的反作用力N能夠被分成兩部分液壓軸承力Nh和機(jī)械表面壓力Nm，如下所示：N=Nh+Nm
N和F的關(guān)系是：F=N·cosα
5.結(jié)論
根據(jù)作者的研究，一些關(guān)于滑塊在自然水潤滑下的材料選擇和結(jié)構(gòu)設(shè)計(jì)結(jié)論可以總結(jié)為如下幾條：
（1） 用高分子材料制作成的滑塊與用抗腐蝕性合金制成的旋轉(zhuǎn)羅盤結(jié)合有很好的摩擦性。
（2） 在滑塊的大量制造中棉布纖維填充的酚醛塑料被使用，目的是使工作壓力低于14MPa。
（3） 在滑塊上的反彈力應(yīng)該足夠強(qiáng)以確?；瑝K和旋轉(zhuǎn)羅盤表面上的連接壓力在0.1~0.2MPa。
除此之外還有一些實(shí)驗(yàn)需要進(jìn)行，并且擁有良好韌性的陶瓷材料也會被引進(jìn)用來大量制造滑塊和旋轉(zhuǎn)羅盤。在滑塊研究上的經(jīng)驗(yàn)將會為自然水液壓軸向活塞泵和電動機(jī)的設(shè)計(jì)和發(fā)展提供很大的幫助。
參考文獻(xiàn)：
[1] Joseph L. Foszcz, Hydraulic Fluid Choices, Plant Engineering, (8),1996,68.
[2]Li Zhuangyun, et al, Development of Hydraulic Pump to Operate with Raw water, Proceedings of ASME Fluid Engineering Division Summer Meeting, Washington DC, USA, June 21-25,1998.
[4]吳仁榮，水潤滑滑動軸承的設(shè)計(jì)計(jì)算，機(jī)電設(shè)備，（6），1997,30.
[5]許耀銘，油膜理論與液壓泵和馬達(dá)的摩擦副設(shè)計(jì)，北京；機(jī)械工業(yè)出版社，1987.