The transmission efficiency of high precision ball screws is much higher than that of sliding screws, and the core advantage lies in the low resistance characteristic of rolling friction. However, in practice, friction loss may lead to decreased efficiency, increased heat generation and precision degradation. Through lubrication optimization, structural design improvement and material matching, the coefficient of friction can be stably controlled in the ideal range to extend the efficient operation cycle.
I. Lubrication Strategy: Reduce the friction resistance of contact surface
Grease Selection and Filling
High-viscosity grease is selected for low-speed and heavy-duty scenarios to resist heavy-duty extrusion through the oil film; while low-viscosity grease is used for high-speed and light-duty to reduce the stirring resistance and lower the temperature rise. The single filling quantity is 1/3-1/2 of the internal volume of the screw nut. Excessive or insufficient quantity will affect the effect of grease film, and quantitative injection through the nozzle can avoid human error. adopts grease with solid lubricant, under high temperature or vacuum environment, solid particles can fill the contact surface pits, reduce the friction coefficient and prolong the maintenance cycle.
Dynamic replenishment of lubricant
High-speed precision scenes are continuously supplied with lubricant through micro-flow oil drippers, together with the oil return tank to recycle the excess grease, avoiding the resistance caused by the over-thickness of the oil film and stabilizing the coefficient of friction. Oil-air lubrication mixes the lubricant with compressed air to form an oil mist, precisely controlling the thickness of the oil film, which reduces friction power consumption and pollution in high-speed scenes.
Structural design: optimizing contact state and force distribution
Matching of raceway and ball parameters
45° contact angle is used in general-purpose scenarios, 60° contact angle is used for purely axial loads, and 30° contact angle is used when radial loads account for a high percentage of the load, in order to optimize the coefficient of friction. When the radius of curvature of the raceway is 52%-53% of the diameter of the ball, the contact area is maximum and the coefficient of friction is low; if the curvature is too large, the contact area decreases and the friction rises. The screw pitch error is offset by the nut raceway correction, so that the contact point between the ball and raceway is evenly distributed, avoiding the localized force concentration which leads to the increase of friction coefficient.
Nut and screw gap control
The axial gap is eliminated by preloading the double nut spacer, and the friction coefficient rises slightly but the impact friction during reverse motion can be avoided. High-speed reciprocating motion scenario adopts a nut design with one end fixed and one end floating, which allows the screw to expand and contract freely due to heat and avoids fluctuation of friction resistance due to additional stress.
Smoothness of Circulation System
adopts internal circulation cannula type return to direction, which reduces centrifugal force and collision friction when the ball passes through, and lowers the fluctuation of friction coefficient. The lead of the screw is matched with the rotational speed, and when the lead is large, it is necessary to increase the diameter of the channel of the returner to reduce the resistance and control the friction coefficient rise in the high-speed section.
Third, the material and surface treatment: reduce the inherent friction coefficient
material combination
screw using SUJ2 bearing steel, ball selection of ceramic, can reduce the coefficient of friction, in high-speed scenarios have significant advantages. In food or medical equipment, 316 stainless steel is used for the screw and zirconia ceramics for the balls, combined with food-grade grease to resist corrosion and control the coefficient of friction.
Surface treatment
The surface of the screw raceway is chrome plated and then precision polished to reduce surface roughness, reduce the coefficient of friction and improve wear resistance. High temperature scenarios are coated with diamond-like coating to reduce the coefficient of friction and minimize the amount of wear.
Fourth, installation and maintenance: avoid additional friction sources
Coaxiality and parallelism control
The coaxiality error of the support bearings at both ends of the screw needs to be controlled in a reasonable range, calibrated by laser interferometer during the installation, to ensure that the screw is parallel to the guideway and to avoid the friction of off-loading. The nut holder and the worktable adopt elastic connection, which allows certain installation error and avoids rigid connection resulting in additional torque to increase the friction coefficient.
Pollutant isolation
Adopt shield or chip scraper to intercept dust, cutting fluid and other impurities to avoid friction coefficient increase and prolong the life of the screw. Regularly clean the inside of the nut to remove metal debris and avoid abrasive wear, and adding grease after cleaning can restore the coefficient of friction.
Five, friction state monitoring and early warning
real-time resistance monitoring through the torque sensor to achieve, resistance rises to a certain level when the trigger warning to avoid friction deterioration. monitor the surface temperature of the screw, rising temperature will lead to an increase in the coefficient of friction, and when it reaches the threshold value, it needs to be stopped and checked. Friction control of ball screws is a systematic project of "Lubrication - Design - Maintenance", through the synergy of multi-dimensions, it can maintain efficient operation, control the decay of transmission efficiency and prolong the service life.
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