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Energy efficiency of spindle units in modern manufacture

Опубликовано в Результаты исследований

Fedorynenko D., Doctor of Sc.

Sapon S., PhD 

Boyko S., PhD 

Kosmach O., PhD 

Chernihiv national university of technology, Chernihiv, Ukraine

 

Energy consumption reduction and energy wastage eliminating are among of the main goals in European Union. [1]. Today, only the minor part of consumed energy is used for actual value-adding process, for example modern manufacture in Europe is characterized by large unnecessary energy use in industrial sector (20-50 %) [2]. In the conditions of constant price the rise on energy carriers and rigid competition on commodity production markets increase of energy efficiency, productivity of machine-building manufacture, decrease in production cost is one of the actual global problems. The problem of effective use of power resources, increase of manufacture productivity is directly connected with machining process on machine tools and decisively influences machine-building production cost [3]. Nowadays only some modern machine tools provides accurate data on the energy their products require on all cycles of manufacture. Unfortunately, today in most case neither the tool machine manufacturers nor their customers don’t have high energy efficiency of machines and production lines.

According to the data of the works [4-6] great influence on current consumption in the course of processing is made by  spindle unit (about 30 %) and auxiliary machine tool units (to 60 %) from general expenses on electric power. In the same places was specified that power consumption by  spindle unit and power consumption for lubricant oil inlet for its lubrication and cooling is varying in the range of 50 - 70 % depending on processing conditions on CNC machine. Energy efficiency of the main motion drive will decrease if it works below rated power. On the other hand, maximum allowed power of spindle unit limits the speed of cutting and productivity of machining process.

At high-speed cutting of materials there are problems of quality maintenance of detail machining; vibration and heat stability, energy efficiency of technological machining system; work resource of guide ways, ball screws and spindle bearings [4, 7-9]. Maintenance of these indicators depends greatly on bearings of spindle unit. Application of drive bearings for specific speed increase of spindle unit has certain restrictions as balls are the source of vibrations [10] which increase proportionally to the square of angular speed of spindle running. Besides, when rotational speed is higher than 150,000 rpm, lifetime of rolling elements of bearings decreases significantly and can be as short as 6-8 weeks [11]. In the works [12–14] was stated that hydrostatic bearings, which together with increase of machining reliability and productivity allow providing of exclusively high indicators of detail machining quality which compared with other types of spindle supports, are the most perspective type of spindle bearings for realization of high-speed machining. Only hydrostatic spindle combines ultra-high accuracy and high effectiveness in vibrations suppression.

It is commonly known that energy efficiency of spindle unite will decrease at growth of the cutting speed which is caused by increase of mechanical and hydraulic energy friction losses. According to the data [15] it is established that power losses in high-speed (it is more 100,000 rpm) spindle knots with hydrostatic supports increase in 3-4 times in comparison to low-speed ones (to 4,000 rpm) and make about 30 % from spindle drive power.

Thus, there is a contradiction in technological machining system between simultaneous maintenance of high indicators of productivity and energy efficiency of machining processes on machine tools. Overcoming of the specified contradiction is an actual problem of machine-building manufactures, the solution of which will allow to raise essentially energy efficiency of machining processes and to reduce production cost.

Development of new technology of cutting of difficult-to-machine materials on machine tools is expected on the basis of use of energy-efficient designs of adaptive hydrostatic spindle bearings which work with insignificant power friction losses (no more than 10 % from cutting power) simultaneously providing speed factor indicator − to 2.5 million n×d.

The disadvantage of most existing spindle units is that the bearing pockets form does not allow obtaining the full effect of hydrodynamic bearing capacity, thus reducing its speed factor [16, 17].

To increase the efficiency of machining processes was suggested improvements adjustable radial hydrostatic bearing pockets by performing spherical bearing for which was supplied the compressed fluid through adjustable valves, thus it allows to increase bearing rapidity [18].

Performance of spherical bearing pockets can reduce the energy loss caused by abrupt change in shape of the flow resistance, which reduces energy losses to the viscous friction with increasing of rotor speed.

The basic component of adjustable radial bearing of dynamic and hydrostatic bearing is the housing. In holes of housing with tension is set sleeve with two conical bands on the inner surface that interact with conical elastic bands of hydrostatic bushing. Hydrostatic bushing made of longitudinal grooves system of curved profile, between which formed five pockets. In addition, bushing has the guide sleeve and belt tongue that orient it in the case. Axially stationary sleeve fixed by flange.

Adjustable radial dynamic and hydrostatic bearing works as follows. The working fluid pressure supplied through choke and throttle washer pockets to hydrostatic bushings, where the carrier layer fluid support the rotor in certain position. Drainage of the pockets is due to jumper that acts as throttling element and by changing of the radial clearance can to regulate the stiffness of the bearing and fluid flow. Adjust the size of the radial clearance in the bearing perform by rotating of nut in the end of the housing, which provides axial displacement of hydrostatic sleeve. It is in contact with the internal conical bands of conical surfaces bushings that deformed and reducing or increasing the clearance between the rotor and jumper of hydrostatic elastic sleeve.

When using the hydrostatic bearing mode supply compressed fluid through adjustable valves spending to five pockets of hydrostatic bearing sleeve, i.e. adjustable valves spending establishing on permanent capacity. If you exceed the speed of rotation of the rotor certain limit, which is set by the program, microprocessor controller gives the command to stop the flow of oil into pockets that are fed through the valves by overlapping. In order to cool the working fluid at working in high speed through the oil valve enters to the lower bearing pocket. Thus, this bearing works in the hydrodynamic lubrication regime.

The use of adjustable valves of energy consumption for bearing supply allow to reduce supply in the working bearing at increasing of rotor speed, thus such bearing turns to the hydrodynamic mode, thus reducing of the energy losses as at viscous friction and for the flow of the working fluid. This provides increase of rapidity and reduce of bearing power consumption that associated with moving friction surfaces.

When using a hydrostatic bearing mode bearing is compressing fluid through adjustable consumption valves to five hydrostatic bearing pockets.

In comparison to existing analogues of spindle fluid bearings application of new designs of fluid hybrid bearings with special low-viscous lubrication will allow to raise power efficiency of machining on metal-cutting machine tools within 30 %.

Application of mechatronic control systems of spindle position and special oil lubrication and cooling systems will allow carrying out effectively both rough and finish work with difficult-to-machine materials, providing radial error motion to 0.1 micrometer and possibility of smooth change radial stiffness in limits to 300 N/micrometer depending on load and speed of spindle running.

 

References

 

  1. Directive 2005/32/EC of the european parliament and of the council. – Official Journal of the European Union, 2005. – pp. 191/29 – 191/58.
  2. Energy  and  eco-efficiency  of  machine  tools,  processes  and  handling  equipment. – ENEPLAN Report. – 2011. – 119 p.
  3. Gutowski T., Dahmus J., Thiriez A. Electrical Energy Requirements for Manufacturing Processes, 13th CIRP International Conference on Life Cycle Engineering, Leuven, 2006.
  4. Electrical Energy Requirements for Manufacturing Processes. – 2011. – Website: http://www.heidenhain.us/enews/stories_1011/MTmain.php. (10.2011)
  5. HMC Increases Energy Efficiency . – 2015. – Website: http:// http://www.mmsonline.com/products/hmc-increases-energy-efficiency.
  6. Energy efficiency in machine tools. Bosch Rexroth AG Technical review. BRC/MKT1; K-HP. 2014.
  7. King R. Handbook of High-Speed Machining Technology.– Springer, 1985. – 472 p.
  8. EC - 7th Framework Programme. Challenge 6: ICT for Mobility, Environmental Sustainability and Energy Efficiency. Deliverable D3.3: “Design for energy efficiency”. Estomad Project. 2012.
  9. The application of High Speed Machining. – Website: http://www2.coromant.sandvik.com/coromant/pdf/dm_articles/hsm6_12.pdf.
  10. MORI, M. High-Speed Tapered Roller Bearing for Machine Tool Main Spindles / M. MORI, T. KOBAYASHI // NTN Technical review. – No.74. – 2006. – pp. 16–23.
  11. Elka Precision. – Website:http://www.elkaprecision.com/hydrostatic_spindles.html
  12. Kane, R. Surface Self-Compensated Hydrostatic Bearings. Thesis for a Doctor’s of Philosophy Degree in Mechanical Engineering: Massachusetts Institute of Technology, 1999. – 138 p.
  13. Zelinski, P. High speeds spindle of high power on hydrostatical supports / Modern machine shop. – 2001. – vol. 73. – No 11. – pp. 104 – 108.
  14. HYDROSTATIC BEARING SYSTEMS. -  2011. - Website:http://www.zollern.de/fileadmin/Upload_Konzernseite/Downloads/Brochueren/Casting_and_Forging/ZOL-HydrostatLager11-engl-z525.pdf
  15. Over 30 years experience in High Speed Cutting. – 2005. –  Website:http://www.ibag.se/download/Catalogue.pdf
  16. Hydrostatic spindel . – Website: http://hyprostatik.de/fileadmin/inhalte/pdfs/hydrostatic_spindles.pdf
  17. CBN Hydrostatic Grinding Spindle Systems. – 2012. –Website: http://www.tacrockford.com/pdf/240.520/240520-cbn-hydrostatic-grinding-spindle-system.pdf (2012.11.01)
  18. Patent №89288 | Authors: Fedorynenko D., Habibulina A., Sapon S. | Published: 10.04.2014 | MPC: F16C 32/06
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