1. Speed comparison:
In terms of speed, linear motors have a significant advantage, reaching speeds of up to 300 m/min and accelerations of 10g; ball screws, on the other hand, achieve speeds of 120 m/min and accelerations of 1.5g. Comparing speed and acceleration, linear motors have a considerable advantage, and their speed can be further improved once the heat generation issue is resolved, whereas the speed of a rotary servo motor combined with a ball screw is limited and difficult to increase significantly. In terms of dynamic response, linear motors also have an absolute advantage due to their faster response, wider speed range, and ability to reach maximum speed instantly upon startup and stop quickly during high-speed operation. The speed range can reach 1:10000.
2. Accuracy Comparison:
In terms of precision, linear motors, due to their simpler transmission mechanism and reduced interpolation lag, offer higher positioning accuracy, reproducibility accuracy, and absolute accuracy compared to a rotary servo motor + ball screw system, which is easier to implement through position detection feedback control. Linear motors can achieve a positioning accuracy of 0.1μm. A rotary servo motor + ball screw system can only achieve 2-5μm, but requires a lightweight transmission system across the entire closed-loop system (CNC-servo motor-backlashless coupling-thrust bearing-cooling system-high-precision rolling guide rail-nut seat-worktable) and high grating accuracy. To achieve higher stability, a rotary servo motor + ball screw system requires dual-axis drive. Linear motors, being high-heat-generating components, require strong cooling measures; therefore, achieving the same result with a linear motor requires a greater investment of resources.
3. Price Comparison:
In terms of price, linear motors are much more expensive, which is why they are not more widely used.
4. Energy consumption comparison:
Linear motors consume more than twice the energy of a rotary servo motor and ball screw when providing the same torque. Rotary servo motors and ball screws are energy-saving and power-enhancing transmission components. The reliability of linear motors is affected by the stability of the control system and has a significant impact on the surrounding environment. Effective magnetic shielding and protection measures must be taken to isolate the influence of strong magnetic fields on the rolling guide rails and to prevent the adsorption of iron filings and magnetic dust.
The following example will make it easier to understand some of the characteristics of linear motors and "rotary servo motor + ball screw":
A Japanese company's ultra-high-speed gantry machining center uses linear motors to drive the X and Y axes at a speed of V=120m/min. Why didn't this company use a rotary servo motor + ball screw combination? Because while the DN value of the Supers has increased from 70,000 to 150,000 and then to 220,000 for traditional ball screws, the inherent weakness of purely mechanical transmission limits the increase in linear speed, acceleration, and stroke range. If a Φ40×20mm product is used, vmax=110m/min, and since nmax=5500r/min is very high, the stroke range is clearly limited by the critical speed Nc. If a large-lead Φ40×40mm product is used, Vmax=220m/min, which obviously cannot meet the requirements for high positioning accuracy. Achieving a DN value of 220,000 reflects the company's design and manufacturing level. If we choose a Φ40×20 (double-headed) mm product and use it at n≈4000~5000r/min and V=80~100m/min, its safety, reliability, and service life will all exceed expectations. In fact, to date, there are no successful examples of using the SUPERS series drive in high-speed, high-precision CNC metal cutting machine tools (excluding CNC forming machine tools) with speeds V≥120m/min. In fact, the best application scenarios for "rotary servo motor + ball screw" are: mid-range high-speed CNC equipment and some high-end CNC equipment requiring V=40~100m/min, acceleration 0.8~1.5(2.0)g, and accuracy of P3 level or above.
Application Comparison:
In fact, while linear motors and "rotary servo motors + ball screws" each have their advantages, they also have their own weaknesses. Both have their optimal application ranges on CNC machine tools.
Linear motor drives have unique advantages in the following CNC equipment fields: (1) High-speed, ultra-high-speed, high-acceleration and large production batches, many movements requiring positioning, and frequent changes in speed and direction. For example, production lines in the automotive and IT industries, and the manufacturing of precision and complex molds. (2) Large, ultra-long stroke high-speed machining centers, and "hollowing out" machining of integral components with light alloys, thin walls, and high metal removal rates in the aerospace manufacturing industry. For example, the “HyperMach” machining center (46m) of CINCIATI in the United States; the “HYPERSONIC1400L ultra-high speed machining center” of MAZAK in Japan. (3) Requirements for high dynamic characteristics, servo-responsiveness at low and high speeds, and highly sensitive dynamic precision positioning. For example, the new generation of high-performance CNC electrical discharge machining tools, CNC ultra-precision machine tools, new generation CPC crankshaft grinders, cam grinders, CNC non-circular lathes, etc., represented by Sodick. (4) Light-load, high-speed special CNC equipment. For example, the “DML80 FineCutting” laser engraving and drilling machine of DMG in Germany, the “AXEL3015S” laser cutting machine of LVD in Belgium, and the “HyperCear510” high-speed laser processing machine of MAZAK.
The German company DMG is renowned for its mass production of various high-performance CNC equipment. It was an early adopter of linear motors in its servo feed systems, and their adoption rate is very high (all marked "Linear" after the machine tool model). The company offers three configurations for the two drive methods:
All axes are equipped with "high-speed" CNC equipment driven by linear motors. Examples include: DMC85VLinear, DMC75VLinear, DMC105VLinear, DMC60HLinear, DMC80HLinear, and DML80-FineCutting laser processing machine.
Hybrid drive type. For example, the DMF500Linear large vertical machining center with moving column is equipped with a linear motor on the X-axis (travel 5m) with V=100m/min; while the Y and Z axes use "rotary servo motor + ball screw" with V=60m/min.
These are "high-powered" machining centers where all axes are equipped with rotary servo motors and ball screws. For example, the DMC63H high-speed horizontal machining center has a speed of 80 m/min, an acceleration of 1 g, and a positioning accuracy of 0.008 mm. Other models include the DMC80H, DMC100H, DMC125H (duoBLOCK), and DMC60T.
The simultaneous use of both drive methods by the German company DMG demonstrates their respective advantages. Linear motors have significant room for improvement; as the technology matures, production volume increases, and costs decrease, their applications will become more widespread. However, considering energy conservation, emission reduction, and green manufacturing, as well as the inherent characteristics of the two structures, the "rotary servo motor + ball screw" drive still has a vast market potential. While linear motors will become the mainstream drive method in high-speed (ultra-high-speed) and high-end CNC equipment, the "rotary servo motor + ball screw" method will continue to maintain its dominant position in mid-range high-speed CNC equipment.
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