The ball screw pair with mature manufacturing technology and industrial scale has a history of more than 100 years since its birth, and the comparison table of old and new codes of domestic bearings and imported bearings (57) has a history of 46 years in China. With the development of mainframe, the functions of its products are constantly expanding and improving. From the initial "AC-LM energy-saving drive" (before 1960s) to "precise positioning" (after 1970s), and then from "large lead and quick drive" (1980s) to "precise high-speed drive" (mid-1990s), the development of "speed" and "precision" can be said to keep pace with the times.
When the ball screw and the ball nut are relatively displaced, the acceleration is ≥ 1g, the linear speed is ≥60m/min, or the DN value is ≥ 120000(DN value is defined as the product of the nominal diameter of the ball screw pair do(mm) and the relative rotational speed of the ball screw and the ball nut n(r/min)), and the accuracy is P3 (GB/min).
Because of the excellent rolling friction characteristics and high transmission efficiency (η=90~95%) of the ball screw pair, and the fast and flexible transmission function of the comparison table (58) between the old and new codes of domestic bearings and imported bearings, it has favorable conditions for speeding up. There are three ways to speed up: (1) improve the lead Pn, which is what we often call a large lead (or super-large lead) ball screw pair. Because the increase of Pn is not conducive to the improvement of lead accuracy, the static stiffness of the feed system decreases rapidly, so it is only suitable for fast driving occasions with low requirements on positioning accuracy. (2) Considering the accuracy, speed and dynamic characteristics, increase Pn and N (rotational speed) and the number of thread heads appropriately. Due to the limitation of critical speed Nc, it is not advisable to pursue high speed. (3) Innovating in product structure and transmission mode, such as changing the main drive of lead screw into the main drive of nut; Change single drive into double drive; Hollow screw, small nut diameter, light rolling element, etc.
The appearance of PHS-BS originated in the middle and late 1990s. The heat treatment classification and hardness test method of SKF bearings were the first to introduce HMC series for high-speed CNC machine tools, with diameters of φ 36 ~ 55 mm, Pn= 16~32mm, Vmax= 1 16m/min, acceleration 1.3g, and so on. In recent years, world-renowned ball screw manufacturers have successively introduced a number of silent PHS-BS products with DN ≥ 150000(220000), V≥ 120m/min(200m/min) and acceleration ≥ 1.5g, such as BSS and S3 of NSK Company of Japan.
The PHS-BS research was first conducted by Beijing Machine Tool Research Institute in China, and the GSZ2000 high-speed ball screw pair comprehensive measuring instrument was developed in the Ninth Five-Year Plan. Subsequently, Bote Company in Jining, Shandong Province also cooperated with Shandong University of Technology to successfully develop BTJS-03 high-speed ball screw pair measuring instrument. At present, the level of PHS-BS in China is: Vmax≥60m/min(80m/min), acceleration ≥ 0.4g, accuracy ≥P3, DN ≥ 140000.
Fig. 2 PHS-BS is used to realize fast feed and high-precision positioning in the three axes of μ 1000 series vertical machining center of Precision Electromechanical Company of Beijing Machine Tool Research Institute. To sum up, the new generation PHS-BS has the following characteristics:
(1) optimized the design of the circulating reversing device. A new method to solve the problem that the main bearing expansion gap of NSK bearing ball mill is too small. Experimental research shows that the cyclic reversing device is a key link that directly affects the fluency and dynamic characteristics, vibration and noise of PHS-BS ball, and restricts the improvement of DN value. In the early PHS-BS, thick-walled cut-in bead pipe was adopted, so that the extension direction of the spiral line of the ball nut was butted with the bead pipe. In recent years, a new internal circulation structure has become popular. It is equipped with "end plug" return devices at both ends of the spiral line of the ball nut, so that the DN value reaches 200,000, the noise is reduced by 6-7dB(A), and the radial size of the nut is reduced by about 30%.
(2) Optimize the ball chain structure. In order to reduce the centrifugal force of the roller when it rotates at high speed, small diameter balls, Si3N4 ceramic balls or DS modified balls are used. In order to prevent the rolling elements from colliding, squeezing and rubbing at high speed, an isolator with lubricating function made of special engineering plastics is added to the ball chain, which effectively reduces the temperature rise and noise and increases the stability of the rolling elements.
(3) In order to reduce the influence of "wedge effect" on the stability of the roller when it runs at high speed, the geometric parameters of the internal and external thread raceway are optimized, which reduces the roughness of the raceway surface, or modifies the surface to improve the friction characteristics.
(4) making the ball screw hollow, and performing strong cooling from the inside of the ball screw. Some enterprises also arrange damping rods in the inner holes to suppress the vibration at high speed. HIWIN's "Cool Type I" series also strongly cools ball nuts.
(5) German Rexroth, INA, Domestic Bearings, Imported Bearings In recent years, the comparison table of new and old specifications, dimensions and parameters (60) Japanese THK and other companies have introduced high-speed linear drive devices with ball nuts directly driven by AC motors.
(6) The lead screw and nut are driven by two AC motors respectively. Superposition of their rotational speed and rotational direction can double the speed and realize micro-feed.
(7) Make full use of the function of ball screw pair to complete synchronous motion, and adopt the driving mode of double motors and double screws to improve the stability of servo feed system at high speed and improve kinetic energy characteristics, such as HPC-800HP high-speed horizontal machining center of Cincinnati Lamb Company in the United States (see Figure 3). In addition, there are DMC63H of German DMG, DHP-80-5x of Swiss DIXI Company, A55E of Japanese Mu Ye Company, DHSC500 of China Dalian Machine Tool Group, NJ-5HMC40 of Ningjiang Machine Tool Group, etc. It is also driven by double motors and double lead screws.
Fig. 3 The X, Y and Z axes of HPC-800HP horizontal high-speed machining center of Lamb Company in Cincinnati, USA are driven by double PHS-BS, with V=80m/min and acceleration1.5g. The stability of fast feed is greatly improved by the double drive of the picture frame structure.
According to the investigation and follow-up at CIMT International Machine Tool Exhibition, it can be seen that the new method to solve the problem of too small expansion gap of NSK bearing steel ball milling main bearing is still the mainstream in mid-range high-speed CNC machine tools, and PHS-BS is also very common in some high-end high-speed CNC machine tools.
The two stars appeared on the same stage, each with its own bright spots.
German DMG company is famous for mass production of various high-performance numerical control equipment. Its servo feed system adopts AC-LM earlier, and the adoption rate is also very high (marked "linear" after the machine tool model). The company has three types of AC-LM and PHS-BS configurations:
All shafts are equipped with "fast" CNC equipment driven by AC-LM. Comparison table of domestic bearings and imported bearings (6 1). For example: DMC85V linear, DMC75V linear, DMC 105V linear, DMC60H linear, DMC80H linear and DML 80- precision cutting laser processing machine, etc.
Hybrid drive type. For example: DMF500 large vertical machining center with linear moving column, with AC-LM(5m stroke) for X axis, v =100m/min; On the other hand, PHS-BS is used for Y and Z axes, and V=60m/min. In addition, CTV250, CTX300, CTX420, DMC 104V, DMF220F and DMF360F all belong to the hybrid drive type.
All shafts are equipped with "powerful" machining centers driven by PHS-BS. For example, in DMC63H high-speed horizontal machining center, the X, Y and Z axes are all driven by PHS-BS (φ 50, Pn=35), with V=80m/min, acceleration 1g and positioning accuracy of 0.008mm In addition, there are DMC80H, DMC 100H and DMC/kloc.
AC-LM and PHS-BS have been adopted by DMG Company, and they have appeared on the same stage in their leading products. The diversity of driving modes is also manifested in the same model and series of high-speed CNC equipment in the same enterprise, and different driving modes are configured according to different use occasions. For example, Grob BZ500 is equipped with PHS-BS, while BZ500L is equipped with AC-LM. The MA-400HA model of otaru iron works in Japan is equipped with PHS-BS, and the MAC-Star-400 model is equipped with AC-LM. Specht63 series of Hueller-Hille in Germany is equipped with PHS-BS, Specht 500L series is equipped with AC-LM and so on.
DMG and some companies only configure AC-LM on the long-stroke x-axis. The comparison table (62) between the old and new codes of domestic bearings and imported bearings aims to improve the dynamic characteristics and positioning accuracy of the machine tool, at the same time, minimize the non-processing time and improve the production efficiency, while other shafts are still driven by PHS-BS, thus making the machine tool more attractive to users. This "hybrid drive" mode is the optimal combination of "functions" of AC-LM and PHS-BS, and its goal is to meet the personalized needs of users to the greatest extent and pursue the best cost performance in innovation. This design concept can really bring tangible benefits to users. The "hybrid" model objectively reflects the diversity of market demand and reflects the highlights of the two stars.
In fact, although AC-LM and PHS-BS have their own advantages, they also have their own weaknesses. Table 1 compares AC-LM and PHS-BS, from which we can see that both of them have their best application ranges in CNC machine tools.
Table 1 comparison between AC linear motor and precision high-speed ball screw pair
AC motor drive has unique advantages in the following fields of numerical control equipment:
(1) High-speed, ultra-high-speed, high acceleration, large production batch, many motions to be positioned, and frequent changes in speed and direction. For example, the production line of automobile industry and IT industry, and the manufacture of precision and complex molds.
(2) Large-scale, ultra-long-stroke high-speed machining center, "hollowing out" machining of light alloy, thin wall and high metal removal rate in aerospace manufacturing industry. For example, the "Hyper Mach" machining center (46m) of Cincinnati Company in the United States; The "Hypersonic 1400L Ultra-high Speed Machining Center" of MAZAK Company in Japan (see Figure 4) adopts AC-LM in the X and Y axes, and the fast-forward speed is 120m/min, so that the whole aluminum block can be hollowed out into airplane parts (PHS-BS is still adopted in the Z axis).
Fig. 4 hypersonic 1400L gantry machining center of MAZAK company in Japan. X-axis and Y-axis are driven by linear motors, with V= 120m/min.
(3) It requires high dynamic characteristics, low-speed and high-speed follow-up, and high-sensitivity dynamic and accurate positioning. For example, a new generation of high-performance CNC electric machining machine tools, CNC ultra-precision machine tools, a new generation of CPC crankshaft grinder, cam grinder, CNC non-circular lathe and so on. Represented by Sodik.
(4) Special numerical control equipment with light load and high speed. For example, German DMG's "DML80 Fine Cutting" laser engraving and punching machine, Belgian LVD's "AXEL30 15S" laser cutting machine, Madzak's "Hyper Cear5 10" high-speed laser processing machine and so on.
Let's analyze the best application of PHS-BS. Although the DN value of PHS-BS has experienced a speed-up process from 70,000 to 654.38+0.5 million and then to 200,000 (220,000), it can be seen from Table 65,438+0 that the improvement of linear speed, acceleration and stroke range is always limited due to the weakness of pure mechanical transmission. Take the new generation BSS and S3 series of NSK Company in Japan as an example, and the DN value introduced in its product samples is 220,000. If φ40×20mm products are selected, Vmax= 1 10m/min. Because the speed of NMAX = 5500rpm is very high, due to the limitation of critical speed Nc, the travel range is obviously not too long. If the product with large lead Φ φ40×40mm is used, Vmax=220m/min, which obviously cannot meet the occasion of high positioning accuracy. The DN value of 220,000 reflects the design and manufacturing level of the enterprise from one side. If φ40×20 (double-ended) mm products are selected and used at n≈4000~5000 rpm and V=80~ 100m/min, their safety, reliability and working life can be higher than expected. In fact, up to now, there is no successful example of PHS-BS driving on high-speed and high-grade CNC metal cutting machine tools (except CNC forming machine tools) that require fast-forward speed V≥20m/min.
According to the investigation and analysis, the best application occasions of PHS-BS are: medium and high-grade high-speed numerical control equipment and some high-grade numerical control equipment with V=40~ 100m/min, acceleration of 0.8~ 1.5(2.0)g, accuracy above P3 level and travel range within 3m.
Thoughts on revitalization and development
Recently, in "Several Opinions of the State Council on Accelerating the Revitalization of Equipment Manufacturing Industry", it was put forward: "Develop large-scale, precision and high-speed CNC equipment and CNC systems and functional components, change the current situation that large-scale and high-precision CNC machine tools rely on imports, and meet the needs of the development of machinery, aerospace and other industries." AC-LM and PHS-BS are very important linear drive functional components in large, precise, especially high-speed numerical control equipment. The development of high-speed machining functional parts, high-speed driving technology and precision machining technology is listed as the key technology in the "Special Plan for the Development of CNC Machine Tools" (draft) presided over by the National Development and Reform Commission. Obviously, it is an urgent task to realize the localization and industrialization of main functional components such as CNC system, motorized spindle, AC-LM and PHS-BS as soon as possible to revitalize large, precise and high-speed CNC equipment in China.
(1) It is urgent to accelerate the revitalization. In China, the development of these two kinds of functional components to achieve high-speed driving and accurate positioning lags behind the market demand, and there is an obvious gap with overseas. Take PHS-BS as an example. When the second and third generation products have been launched overseas, China is still in the initial stage of the first generation product development of PHS-BS, and it has not been commercialized. As for AC-LM, which is used to equip high-speed and high-end CNC machine tools, there is no enterprise with strong technical strength and mass production conditions to provide products to the market, and it has not yet stepped out of the research stage of "academic mode". We should be soberly aware that it is urgent to accelerate the development of AC-LM and PHS-BS, which is the urgent desire of the localization and industrialization of CNC equipment in China and the need of the times to enhance China's comprehensive national strength.
(2) Complementary advantages and common development. Although AC-LM and PHS-BS develop in parallel, different countries have different national conditions, different enterprises have their own advantages and disadvantages, and the recognition and adoption rate of the two driving modes overseas are also different. However, China's national conditions are: vast territory, abundant resources, large population and abundant labor resources. Starting from China's national conditions, taking Scientific Outlook on Development and new industrialization as the guiding ideology, we should plan the development of AC-LM and PHS-BS in an all-round way, which should not be neglected and be indispensable. Even if AC-LM technology is mature in the future, the output will go up and the cost will come down. From the perspective of energy saving and green manufacturing, PHS-BS drivers still have a broad market space. While AC-LM will become the mainstream driving mode in high-speed (ultra-high-speed) and high-grade CNC equipment, PHS-BS will continue to maintain the mainstream position in mid-range high-speed CNC equipment. However, if PHS-BS blindly pursues high indicators and "recklessly fights" within the advantages of AC-LM, it may not be worth the candle.
(3) Scientific development focuses on the foundation. The reasons for the slow development of PHS-BS in China are analyzed. The basic structure of rolling SKF bearings mainly lies in the fact that enterprises have long neglected the input of basic theory and performance test, and their development potential is insufficient. Faced with the problems exposed after speeding up, they don't know where to start solving them. Due to our lack of in-depth research on the basic theories such as friction theory of rolling screw pair, friction characteristics and fretting at high speed, motion mechanism of rolling body under different working conditions, vibration and noise, torque characteristics and dynamic and static stiffness characteristics, the process of speeding up is restricted. However, AC-LM still has a lot of basic work to do in all-digital control technology, heat dissipation, magnetic isolation, electromagnetic interference, zero mode, increasing thrust, reducing energy consumption, modular components and so on. The research results of universities and research institutes in China should be transformed into productive forces as soon as possible, and advanced scientific and technological achievements and continuous innovation in practice should be used to promote the localization and industrialization of AC-LM and PHS-BS.
(4) orderly development and correct orientation. It is necessary to make an in-depth investigation and calm analysis of the CNC equipment market in China. In numerical control equipment, what are high speed and ultra-high speed? How much is the high-speed metal cutting machine and the forming machine respectively? What is the ratio of light load to heavy load? According to the prediction of relevant people, by 20 10, about 20% of CNC machine tools in the world will adopt AC-LM. Experience shows that the prediction of quantity is often not accurate enough. If this 20% refers to all CNC machines, what about high-speed CNC machines? Does China also account for 20%? Our analysis of the market should avoid subjective inference and blind optimism.
Whether developing AC-LM or PHS-BS, we should draw lessons from the development of some mechanical and electrical products in the past to avoid blindly launching, low-level duplication and wasting resources. At the same time, the related functional components matched with AC-LM and PHS-BS should be developed simultaneously, such as high-speed, high-rigidity precision rolling linear guide pair, braking device and protection device. Address: http://www.nskfag.org/news/201104 _ 36435.html.