Ball screw high-speed operation of the invisible threshold: critical speed analysis and breakthrough program
I. Critical speed: the "warning line" of high-speed transmission
Ball Screws have a critical threshold value when rotating at high speed -- critical speed. When the rotational speed is close to or exceeds this value, the screw will be due to its own uneven mass distribution and structural vibration characteristics, triggering violent lateral shaking. This vibration leads to increased ball and rail impacts, which can not only cause accuracy to plummet from the micron level to the ten-micron level, but can also dramatically shorten the life of the screw and even cause equipment failure. For example, in high-speed machining centers, if the speed of the screw exceeds the critical value, the machining surface may appear obvious vibration pattern, the scrap rate increased significantly.
Second, the core factors affecting the critical speed
(1) the length and thickness of the screw
The longer the screw, the smaller the diameter, the lower the critical speed. This is like a thin bamboo pole, the longer the easier to shake.
(2) Rigidity of support
The more rigid the support at both ends of the Screw, the higher the critical speed. Simply put:
The support method with one end fixed and one end hanging is the most unstable and has the lowest critical speed;
Both ends are firmly fixed in the most rigid way, under the same conditions, the critical speed can be 5-10 times higher than the former.
(3) manufacturing accuracy and material properties
Insufficient processing accuracy (such as the screw surface is not smooth, internal stress residue) or poor material rigidity, will make the critical speed reduction. For example, the use of carbon fiber reinforced aluminum alloy material screw, lighter than traditional steel, more rigid, critical speed can be increased by more than 40%.
Third, how to determine whether it is close to the critical speed?
(1) Vibration monitoring method
Observe the vibration change when the screw is running through the laser vibration meter or touching by hand:
When running at normal low speed, the vibration is smooth without obvious shaking;
Close to the critical speed, the vibration will suddenly intensify, and even abnormal noise (such as high-frequency "whistling") can be heard.
(2) Speed - Accuracy test
Gradually increase the speed of the screw, while testing the positioning accuracy:
If the accuracy suddenly deteriorates (e.g. the original ±0.005 mm positioning error increases to ±0.05 mm), it means that the critical speed has been approached or exceeded.
IV. Practical strategies to break through the critical speed
(1) Strengthen the support structure
Upgrade bearings: Adopt aerostatic bearings or magnetic levitation bearings to support the screws in a non-contact manner, which reduces mechanical wear and improves rigidity at the same time. For example, aerostatic bearings can form a 10-micron-thick air film, doubling the critical speed.
Fixed design at both ends: Ensures that the screws are firmly mounted at both ends, reducing the length of overhang, which is especially suitable for long-stroke equipment.
(2) Optimization of screw structure
Thickening or variable cross-section design: Increase the diameter in the middle part of the screw that is easy to vibrate to improve the overall stiffness. For example, thickening the middle part of the screw from 30 mm to 40 mm can increase the critical speed by 30%.
Hollow and lightweight: Use hollow structure or carbon fiber material to reduce the rotational inertia and enhance damping (reduce vibration amplitude).
(3) Advanced Manufacturing and Balancing Process
Precision dynamic balancing process: professional equipment is used to adjust the dynamic balancing of the screw to keep the unevenness at a very low level (e.g., below 5 grams/mm/kg) to avoid eccentric vibration when rotating.
High-precision processing: Ensure the surface roughness (e.g. Ra≤0.4 micron) and coaxiality (radial runout＜0.005 mm) of the screws to reduce the friction and impact during operation.
(4) Active vibration control
Intelligent vibration avoidance algorithm: Through real-time monitoring of speed and vibration data by the control system, the motor acceleration and deceleration strategy is automatically adjusted to skip the critical speed interval. For example, when approaching the critical speed, let the equipment briefly reduce speed through.
Piezoelectric ceramic vibration suppression: Piezoelectric ceramic sheet is installed in the support seat, when the vibration is detected, the ceramic sheet immediately generates reverse vibration to offset the resonance, and the precision control can be up to 0.001 millimeter level.
V. Practical application cases
(1) high-speed CNC machine tool upgrading
A factory's gantry milling machine uses a 1.5-meter-long traditional screw, with a critical speed of 15,000 rpm and a maximum feed speed of 45 meters/minute. By replacing the fixed support at both ends, thickening the screw diameter and adopting aerostatic bearings, the critical speed was increased to 30,000 rpm, the feed speed exceeded 70 m/min, the machining efficiency was increased by more than 50%, and the vibration noise was reduced by 15 dB.
(2) Aviation servo mechanism optimization
The screw in an aviation equipment needs to run at high speed in a vacuum environment, and the original design critical speed is 20,000 revolutions / min, which cannot meet the demand. By adopting hollow titanium alloy screw + magnetic levitation support, the critical speed exceeds 50,000 revolutions / minute, and the vibration amplitude is controlled within 5 m / s ², successfully passing the extreme environment test.
Contact Us
📞 Tel: +86-8613116375959
📧 Email: 741097243@qq.com
🌐 Official website: https://www.automation-js.com/
