development of a new pressure-compensator-valve in 2008, the … · functionalities of hydrodynamic...

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The 9th International Fluid Power Conference, 9. IFK, March 24-26, 2014, Aachen, Germany

Development of a New Pressure-Compensator-Valve for Hydrostatic-Hydrodynamic Journal Bearings

Dr.-Ing. Dirk Wehner*, Dipl.-Ing. Detlef Hammerschmidt**, Prof. Dr.-Ing. Jürgen Weber*** and Dipl.-Ing. Sabine Gold***

Hydrive Engineering GmbH, Neuhirschsteiner Str. 15, D-01594 Hirschstein, Germany* SKF Blohm+Voss Industries GmbH, Hermann-Blohm-Str. 5, D-20457 Hamburg, Germany**

TU Dresden, Institut für Fluidtechnik, D-01069 Dresden, Germany*** E-Mail: [email protected]

The main objective of the research project HYDROS /1/ was the development of a new hydrostatic bearing system with improved properties especially in terms of robustness, efficiency and space requirement. Beside the design of the bearing body with the bearing grooves, the choice and design of the inlet resistances for the lubricating oil supply was one of the main research focuses. A new simple inlet resistance based on the principle of a pressure compensator was invented during these investigations. The paper presents the functionality as well as the design process and discusses many advantages and its performance by experimental results.

Keywords: hydrostatic bearing, hydrodynamic bearing, inlet resistance, pressure compensator Target audience: marine applications, machine tool industry, renewable energies

1 Introduction

The bearing of rotating shafts can nowadays be realized by a large variety of solutions. Ball bearings are often used and available in many sizes. Heavy operating conditions like they occur to the bearings of propeller shafts in pod drives can lead to initial failures and very high maintenance costs. Conventional propeller shafts (stern tubes) are typically supported by journal bearings. They require more installation space but even offer advantages like higher robustness against load peaks and reduced maintenance effort.

Figure 1: Design and working principle of the new hydrostatic-hydrodynamic bearing


In 2008, the research project HYDROS /1/ started with the goal to replace the currently used ball bearings in pod drives by a new journal bearing system. That system shall offer a design space to be applicable in pod drives and enable the robustness of the conventional journal bearing for pod drives. The basic principle for the intended realization was a hydrostatic journal bearing with a new patented inlet resistance /2/. Several papers have been published that describe the different results of that extensive research work /3,4,5/.

Figure 1 illustrates the design and working principle of the finally realized journal bearing solution. It combines functionalities of hydrodynamic and hydrostatic bearings and is patented under /6/. The bearing body is equipped with a number of narrow bearing grooves. These grooves enable the oil supply for the shaft and realize the hydrostatic behaviour. When the hydrostatic power supply is turned off, the check valves get closed and the bearing can work in hydrodynamic mode. Therefore the required load pressure is induced by the rotation of the shaft. This concept offers several advantages:

• less space requirements in comparison to stern tube bearings

• high load capacities at zero shaft speed due to hydrostatic power supply

• pure dynamic mode at higher shaft speeds with no energy required

• robustness against load peaks due to check valves

• reduced maintenance effort

• good emergency operating features in case of a failure of the pressure supply

2 Concept for Hydraulic Power Supply

Hydrostatic bearings need a power supply that delivers pressurized lubricating oil to the bearing grooves. Therefore an inlet resistance is allocated to each groove that realizes a load dependent distribution of the oil. The higher the radial load the higher is the required load pressure between shaft and bushing. The flow rate through the inlet resistance depends on the load pressure and the actual gap height. The main task of the inlet resistances is to prevent a too small gap height on the load side. That means that the grooves on the load side (high load/groove pressure) require a higher flow rate and the grooves on the other side (low load/groove pressure) do not need any flow supply. The problem is that most of the well known hydraulic resistances have an opposite behaviour which makes them unsuitable for hydrostatic bearing supplies.

Figure 2: System of oil supplies for hydrostatic bearings

hydrostatic oil supply

resistance control displacement control

constant resistancevariable resistance

pressure dependent resistance

excentricity dependent resistance

electronically controlled resistance

- flow divider

- capillary tube valve

- PM flow controller /7/- LHS-patent /2/ - proportional control valve

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Figure 2 shows different possibilities for hydrostatic oil supply. The supply of the lubricating oil can be differentiated in resistance control and displacement control. The most widespread solution is the capillary tube valve as it is shown in Figure 3. It is simple and robust but the relatively long capillary tubes require extra design space. In addition to that, the flow rate through the unloaded grooves is higher than through the loaded grooves which leads to unnecessary energy losses and oil heating. The decreasing of the flow rate into the loaded grooves even leads to reduced gap heights. Thereby the load capacity of the entire bearing is limited.

Figure 3: Working principle of a capillary tube valve

The use of variable inlet resistances enables to improve bearing performance. These solutions are able to change their flow area depending on another state variable. In tool machines the hydrostatic supports are typically realized by a PM flow controller /7/ as it is shown in Figure 4. The groove pressure pG acts on an elastic metal membrane and changes the flow area of the second inlet resistance of the PM flow controller. The combination of the flow area and the pressure difference p0-pG leads to a flow characteristic as it is shown in Figure 4 (right). The non loaded grooves will be supplied by a limited flow QG0. Increasing groove pressures pG on the loaded side of the bearing lead to an increase of the inlet flow QG which can keep the gap height/excentricity close to constant. At a specified ratio of pG/p0 the inlet flow starts to decrease until pG=p0.

Figure 4: Working principle of a PM flow controller

The differing requirements between tool machine supports and propeller shaft bearings led to the new idea of a pressure dependent inlet resistance based on the idea of a pressure compensator. It is shown in Figure 5. A short and simple spool compares the groove pressure pG to a pre-set spring force. At low groove pressures pG the spool is closed and a very small flow area realizes a similar small flow into the groove. If the pressure pG exceeds the spring pre-load, the spool opens and realizes a larger flow area that even leads to increased flow QG into the loaded grooves. The maximum inlet flow QG is delivered within the medium range of the groove pressure pG. In comparison to the PM flow controller the spool of the new pressure compensator does not move permanently. The valve more acts like a pressure switch. Most of the operating time the spool is either in opened or closed state. Each point of the shown pressure-flow-characteristic can easily be adjusted by reproducible fabricable design parameters. The check valve realizes two functionalities: It prevents a negative inlet flow from the groove back to the pressure supply pipe and enables the bearing to operate in full hydrodynamic mode without any


pressure supply. High load peaks on the shaft can lead to groove pressures pG higher than the pressure supply p0. In that case the check valve allows the pressure pG to increase and to bear the high load for a short time.

Figure 5: Working principle of the new pressure-compensator-valve

Table 1 illustrates a comparison of the three introduced inlet resistances. Although the capillary tube valve presents a simple and robust solution, its functionality and efficiency does not match the performance requirements. The PM flow controller has a better performance and enables to save energy for the power supply. But it has significant disadvantages in terms of robustness and sensitivity. The cartridge design of the pressure compensator valve as well as the special flow characteristic make this solution a serious alternative for journal bearings with large shaft diameters.

criterion capillary tube valve PM flow controller pressure compensator valve

flow characteristic fix and linear load dependent, progressive

load dependent, root

temperature dependency high high low

contamination sensitivity low high low

modification of the supply pressure

trouble-free possible leads to modified flow characteristic

trouble-free possible

modification of the operating performance

only flow rate possible with restrictions individual possible

installation extra space flange area cartridge design with integrated check valve

robustness simple and robust sensitive robust with fail safe mode

costs very low higher low

Table 1: Comparison of the three introduced inlet resistances

3 Development and Design

A clear and precise requirements analysis delivers the main guidelines for the realisation of the detailed design. Figure 6 shows the realized design concept as well as one possible flow characteristic. A compact cartridge body

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houses all parts of the valve and makes it easy to assemble or to replace. The check valve connects the valve to the groove pressure port and is able to resist groove pressures up to 700 bar. The stroke of the spool determines the difference between minimum flow QGmin and maximum flow QGmax. The spool edges determine the flow area in opened and closed spool position. Simple design changes like the position of the end stops for the spring plate allow an easy adjustment of the flow characteristic and thereby an adaptation to different bearing sizes and requirements. The pre-compression of the spring defines the switching pressure pGopen whereas the spring rate determines the slope of the changeover between QGmin and QGmax. The ball-cone-connection between spring plate and spool ensures minimized friction forces.

Figure 6: Design concept and flow characteristic of the new pressure-compensator-valve

The development of the valve was attended by intensive simulation studies in order to prevent needless redesigns of physical prototypes. Figure 7 shows the simulation model of the valve which is implemented in the simulation software “SimulationX” by ITI GmbH. It represents a detailed description of the existing physical behaviour. It allows studying the influence of different design parameters as well as changing operating conditions. Even friction and leakage effects can be analyzed.

Figure 7: Simulation model and results of the pressure-compensator-valve

In order to keep the design as cheap as possible it does consciously not include any adjustability. Each deviation of relevant design parameters has an influence on the flow characteristics. One design question was whether the tolerance induced deviations can be accepted or not. Therefore a geometrical tolerance analysis was done and the

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switching pressure pGopen

system pressure p0

minimal flow QGmin

cartridge body

spring

spool

check valve

supply pressureport p0

groove pressureport pG

tank pressureport pT

stop screw

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relevant tolerance limits were considered in the simulation. Figure 7 shows the resulting flow characteristics of two worst case scenarios. Expecting that the tolerance limits will be caugth in rare cases the deviations can be accepted.

4 Experimental Investigations

Before the new prototypes are tested in the large bearing test stand it is necessary to validate the pressure flow characteristic of the valve itself. Figure 8 shows the curves of three different prototype measurements and compares them with the simulations. In Zone A can be seen that the real prototypes have a significant higher flow rate then it was expected by simulation. This behaviour could be found in each of the 18 manufactured prototypes. Measurement No.2 represents the average of all measured curves and should be compared to the simulation of nominal sizes. The chief cause of these deviations can be explained by incorrect assumptions of the simulation parameters of the hydraulic resistance (see RH_Spool in Figure 7). The rising and falling curves of the changeovers in Zone B give information about the spool friction. The simulated friction force of about 10 N can even be found in the measurements. Measurement No.2 shows a significant smaller friction force. In Zone C an effect can be found which is explained in the pictures beside the diagram. The measurement curve No.1 shows a bended changeover with a smoother slope. The asymmetric spring force can lead to a tilted spring plate position. While reaching the end stop first only one edge of the spring plate is in contact. After a short phase of alignment the spring plate is in full contact to the end stop and the changeover is finally succeeded.

Figure 8: Measured and simulated pressure-flow-characteristic of the pressure-compensator-valve

Although the expected flow rate was smaller than the realised one, the valves were tested in a large scale test stand /5/. Figure 9 shows an exemplary evaluation of the loading capacity of a bush bearing with a shaft diameter of 770 mm and a length of 420 mm. 18 narrow bearing grooves (No.1-18) are allocated asymmetrically in order to carry a higher load in main load direction. Each groove is connected either to a PM flow controller or to the new pressure-compensator-valve. Bearing gap heights h01 – h04 were measured at four positions. Groove pressures pG1 – pG18 were measured in each groove. The dotted green curves illustrating the results for the PM flow controller while the squared black curves show the pressure compensator performance.

The small diagram down right in figure 9 compares the flow characteristics of both valve solutions. The polar diagram up right in figure 9 illustrates the distribution of the groove pressures around the shaft for a constant load force of 1.3 MN acting in main load direction. In both solutions at that medium load situation the groove pressures do not exceed 75 bar. Due to the high flow rates of the pressure compensator at medium pressure levels the load carrying pressure distribution gets wider. That increases the load capability of the bearing.

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The left diagram in figure 9 shows the smallest gap height h01 and the resulting overall flow demand QGes for a quasistatic change of the load force FL. The shaft is centered at a gap height h01 of 0,3mm. Hence the shaft weight is compensated by a positive load force of about 0.25 MN. The overall flow demand QGes of the pressure compensator is due to the different flow characteristics to the PM flow controller significantly higher. The comparison of the gap heights shows that higher flow rates QGes lead also to higher gap heights at high load forces.

Figure 9: Measured load carrying behaviour of the new bearing system

5 Conclusion

During the HYDROS project a new journal bearing system has been developed. The combination of hydrostatic and hydrodynamic functionality enables new qualities. The main advantages are: reduced space requirements in comparison to stern tube bearings, high load capacities at zero shaft speed due to hydrostatic power supply, pure dynamic mode at higher shaft speeds with no energy required and robustness against load peaks due to check valves. In addition to that a new inlet resistance, the pressure compensator valve, was developed for the supply of the lubricating oil. It offers an optimal oil supply for high load capacities with minimal energy consumption. The functional advantages are added by design advantages. The cartridge design of the pressure compensator valve integrates the check valve and can easily be integrated within the bearing body.

The measurement results prove that the new bearing concept with the new pressure compensator valves is a real alternative to large scale ball bearings. The valve has to be optimized further. That includes the adjustment flow characteristic as well as endurance and peak pressure tests. The use of the simulation models enables a verified design of the valves and the whole bearing to a specific application.

Due to the achievable performance even other applications can be equipped with such a journal bearing. Heavy loads like they occur in wind generators or in tidal power plants may be carried safe and robust. Even the hydrostatic supports of large tool machines may be controlled by the new pressure compensator valve.


6 Acknowledgements

The presented research activities are part of the project “HYDROS - Lastgesteuertes hydrostatisches Lager für Pod-Antriebe“ (support code: 03SX245A). The authors would like to thank the German Federal Ministry of Economics and Technology and the Project Management Jülich for their financial support.

Nomenclature

Variable Description Unit

Supply Pressure [bar]

Groove Pressure [bar]

Tank Pressure [bar]

Gap Height [mm]

e Shaft Excentricity [mm]

Inlet Flow Rate into the Groove [l/min]

Overall Flow Rate for the Entire Bearing [l/min]

References

/1/ Research project “HYDROS – Lastgesteuertes hydrostatisches Radiallager für Pod-Antriebe”

funded by the German Federal Ministry of Economics and Technology, support code: 03SX245A

/2/ Ehluss, H.-G., Hydrostatisches Radiallager für Wellen-, Schaft- und Achslagerungen, patent specification DE 100 30 051 B4, DPMA, 2000

/3/ Gold, S., Weber, J., Wegmann, R., Brökel, K., New hydrostatic/hydrodynamic plain bearing concept for heavy duty applications, In:12th Scandinavian International Conference on Fluid Power, Tampere, Finnland, May 18-20, 2011

/4/ Gold, S., Weber, J. New Plain Bearing Concept for Support of the Propeller Shaft in Pod-Drives of Large Ships, In: 8th International Fluid Power Conference, 8. IFK, Dresden, Germany, Vol.1, pp. 237-249, March 26-28, 2012

/5/ Gold,S., Schneider, M., Weber, J. Entwicklung und Untersuchung eines neuartigen Gleitlagers für Propellerwellen in Pod-Antrieben, In: O+P Journal, Vol. 4/2013, pp. 16-26, 2013.

/6/ Wegmann, R., Radialgleitlager, patent specification DE 10 2009 012 398 A1, DPMA, 2009

/7/ Hyprostatik Schönfeld GmbH, PM-Regler und Strahlpumpe mit Führungsanwendungen, Technische Information, www.hyprostatik.de

development of a new pressure-compensator-valve in 2008, the … · functionalities of hydrodynamic...

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