The core advantage of recirculating ball screw and nut is precision transmission. Their positioning and stopping accuracy, efficiency, rigidity and service life can all be quantified by specific parameters, which are also the key indicators that need to be focused on in actual selection and application. Talking about precision transmission without referring to parameters is just empty talk. Combined with common parameters in practical applications, this article discusses the key points of precision transmission and how these parameters affect positioning and stopping performance.
1 Accuracy Class: Core Quantitative Indicator of Positioning and Stopping Accuracy
Accuracy class is the core quantitative indicator for measuring the positioning and stopping accuracy of ball screws. Internationally standardized into grades C0 to C10, with C0 being the highest and C10 the lowest. It should be selected according to application scenarios: C3 to C5 grades for precision machining (such as CNC machine tools and precision instruments), and C7 to C9 grades for general automation equipment (such as assembly lines and small manipulators).
There are three core measurement indicators of accuracy class: positioning accuracy, repeat positioning accuracy and lead accuracy. Positioning accuracy refers to the actual deviation of the screw from the starting point to the target position. The positioning accuracy of C3 grade screw is usually ≤ ±0.003mm/300mm, and that of C5 grade ≤ ±0.015mm/300mm. Repeat positioning accuracy refers to the deviation of returning to the same position multiple times, with C3 grade ≤ ±0.001mm and C5 grade ≤ ±0.005mm. This parameter directly determines the positioning consistency of equipment. For example, the ball screws of precision chip mounters must adopt C3 to C5 grades, otherwise component mounting offset will occur and affect the yield rate.
Lead accuracy refers to the deviation between the actual moving distance of the nut per revolution and the theoretical lead. The smaller the lead error, the higher the transmission accuracy. Common lead specifications include 5mm, 10mm and 20mm. For example, the BSM4020 screw has a lead of 20mm with a lead error ≤ ±0.015mm/300mm, which can meet the accuracy requirements of machine tool feed systems. It should be noted that installation deviation will affect accuracy. If the parallelism deviation between the screw and the guide rail exceeds 0.02mm/m, the positioning accuracy deviation will increase by 0.02mm/m. Therefore, laser interferometer calibration is required during installation, and error compensation parameters should be set to correct the deviation.
2 Transmission Efficiency: Important Reflection of Positioning and Stopping Smoothness
The transmission efficiency of recirculating ball screws is much higher than that of ordinary sliding screws, which is also one of its core advantages. Transmission efficiency is mainly determined by the friction coefficient between the balls and the raceway, usually ranging from 90% to 98%, while that of ordinary sliding screws is only 30% to 40%. The higher the transmission efficiency, the lower the energy consumption and heat generation of the equipment during operation, and the smoother the positioning and stopping, so as to avoid the thermal elongation of the screw caused by heat and affect the accuracy.
The key parameter affecting transmission efficiency is the friction coefficient. The friction coefficient between the balls and the raceway is usually 0.001 to 0.005, much lower than 0.1 to 0.2 of sliding screws, thanks to the rolling friction characteristics of the balls. In addition, the surface roughness of the balls, the grinding accuracy of the raceway and the lubrication state will also affect the transmission efficiency. Scratches on the ball surface, rough raceways or insufficient lubrication will lead to an increase in friction coefficient, a decrease in transmission efficiency, and even abnormal noise and jamming. For example, in high-speed operation scenarios (such as machine tool spindle feed), grease with high lubrication performance should be selected and replenished every 500 operating hours to ensure stable transmission efficiency.
3 Load Capacity: Basic Guarantee of Positioning and Stopping Stability
Load capacity refers to the maximum axial and radial loads that the ball screw and ball nut can bear. The core parameters are basic dynamic load rating (Ca) and basic static load rating (C0a). Basic dynamic load rating refers to the constant axial load that the screw can bear under the rated life (1 million revolutions), which is provided by the manufacturer through experiments and can also be estimated by the formula (internal circulation type: Ca = f1·f2·f3·Dw^1.3·Z^0.7·n^0.3), where Dw is the ball diameter, Z is the total number of balls, n is the number of thread turns, and f1 (material coefficient), f2 (contact angle coefficient) and f3 (accuracy coefficient) are selected according to actual conditions. Basic static load rating refers to the maximum static load to prevent plastic deformation between the balls and the raceway, and the formula is C0a = f0·Dw^2·Z·cosα, where f0 is the static load coefficient (2.8~3.5) and α is the contact angle (usually 45°).