High precision ball screw is a rotary motion into linear motion, or linear motion into rotary motion of efficient transmission devices, its working principle is based on rolling friction instead of sliding friction, the following is a detailed analysis:
First, the core structure of the composition of
screw shaft
surface machining with spiral raceway, raceway cross-section shape is mostly Gothic arch or round, determines the contact angle of the ball and raceway (common 45 °), affecting the axial stiffness and load carrying capacity.
The material is usually high carbon chromium bearing steel (e.g. GCr15) or alloy structural steel (e.g. 42CrMo), hardened and tempered to HRC 58-62 to ensure wear resistance and fatigue resistance.
The nut
also has a spiral raceway inside, which forms a closed space with the raceway of the screw shaft to accommodate the balls.
The nut is fitted with a reversing device (e.g. cannulated, end-capped or internal recirculating) that guides the ball through the nut in a circular motion.
Balls
are used as rolling elements, usually in bearing steel (GCr15) or ceramic materials (Si₃N₄), with diameters ranging from 1mm to 10mm.
The number and size of balls directly affects the load carrying capacity, e.g.:
for light duty applications (e.g. 3C equipment): 2-3 mm diameter balls in small quantities;
for heavy duty applications (e.g. machine tools): 5-10 mm diameter balls in large quantities with a double-nut preloaded structure.
End caps and seals
The end caps are used to fix the reversing device, and the seals (e.g. lip seals) prevent the entry of foreign objects and maintain internal lubrication.
Second, the principle of work in detail
Movement conversion mechanism
Rotary → linear: when the screw shaft rotates, the ball rolls between the nut and the raceway of the screw, and due to the role of the reverse device, the ball keeps circulating, pushing the nut to do linear movement along the axial direction of the screw.
Linear → rotation: when the nut is subjected to external force to make linear movement, the ball drives the screw shaft to rotate, realizing the reverse conversion of movement.
Rolling Friction Advantage
The coefficient of friction of traditional sliding screw is about 0.1-0.2, while the coefficient of friction of ball screw is only 0.002-0.005.
The low friction reduces energy loss and heat generation, and high speed transmission can be realized (line speed can be more than 100m/min).
Preload and stiffness enhancement
Double nut preload: Preload force is generated by relative displacement of two nuts, eliminating axial clearance, so that the ball and raceway are under compression without load, and axial stiffness is enhanced by 2-3 times.
Selection of preload force:
Light load high precision: preload force is 1/30-1/20 of rated dynamic load;
Heavy load high rigidity: preload force is 1/10-1/5 of rated dynamic load.
Third, the key performance parameters
lead (Lead)
Definition: every one-week rotation of the screw, the nut straight-line distance. Commonly, the lead is 2mm, 5mm, 10mm and so on.
Influence: the larger the lead, the faster the linear movement speed, but the positioning accuracy may be reduced; on the contrary, the smaller the lead, the higher the accuracy but the speed is limited.
Accuracy level
The international standard (ISO 3408) divides the accuracy of ball screws into C0-C10 level, with C0 level being the highest and C10 level being the lowest.
Main indicators:
Positioning accuracy (±μm/300mm): for example, C5 grade is ±18μm/300mm;
Repeat positioning accuracy (±μm): for example, C3 grade is ±5μm.
Rated Load
Dynamic Load (Ca): The maximum load that a Ball Screw can bear at a certain rotational speed and L10 life (1,000,000 revolutions).
For example:
For a Ball Screw with 20mm diameter and 5mm lead, the dynamic load is about 8kN.
Static load (Coa): The load that causes permanent deformation of the ball and raceway up to one ten-thousandth of the diameter of the ball, which is usually 2-3 times of the dynamic load.
Typical application scenarios
CNC machine tools
are used for X, Y, Z-axis feeding system to realize fast and accurate positioning of tools. For example:
High-speed machining centers require ball screws with a lead of 20-32mm and an accuracy of C3 or higher to meet the needs of high feed speeds (60m/min) and micron-level accuracy.
Industrial robots
Joint drives and linear axis drives require a balance between high accuracy and dynamic response. For example:
The wrist joints of 6-axis robots utilize small lead (2-5mm), large lead angle ball screws to enhance motion flexibility.
Semiconductor equipment
Lithography and wafer transfer systems require high precision. For example, the table positioning of lithography machine requires C0 grade ball screws with ±0.1μm positioning accuracy, coupled with air-bearing guides to achieve sub-micron motion control.
Aerospace
satellite antenna pointing mechanism, aircraft landing gear retraction system. For example:
The solar sail drive of a certain model of satellite adopts lightweight titanium alloy ball screws, which maintains a transmission efficiency of >90% under the environment of - 55℃~+125℃.
May, and other transmission methods of comparison
| project | Ball screws | Trapezoidal lead screw | Timing belt drive |
| Transmission efficiency | 90%-96% | 20%-40% | 95%-98% |
| Positioning accuracy | Micron-scale | Millimeter | Millimeter |
| velocity | 高速(≤100m/min) | Low speed (≤15m/min) | Medium and high speed (≤50m/min) |
| Load capacity | Medium to high loads | Low to medium loads | Low loads |
| Maintenance costs | High (requires regular lubrication) | Low (low lubrication requirements) | Medium (need to be tensioned) |
Sixth, the technology development trend of
high-speed: through the optimization of raceway design, the use of ceramic ball (low density, low coefficient of thermal expansion), to achieve line speed > 120m/min.
High precision: combined with laser interferometer closed-loop feedback, positioning accuracy to the sub-micron level (± 0.05μm) breakthrough.
Intelligent: built-in strain sensors to monitor load in real time, combined with AI algorithms to predict fatigue life, to achieve predictive maintenance.
Lightweight: carbon fiber composite nut and hollow screw technology to reduce the inertia of the moving parts and improve dynamic response.
Through the above principles and applications, it can be seen that cnc machine lead screw has become the core component of modern precision machinery transmission by virtue of its high efficiency, high precision and high rigidity.
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