SFE precision ground ball screw As a high-precision transmission component, its radial runout is a key indicator of operational stability and accuracy maintenance ability, which directly affects the performance of equipment in precision machining and automation control. From the definition of the nature of the impact on the system, and then to control standards, need to be understood from a multi-dimensional depth.
First, the basic definition and measurement of the essence of
SFE precision ground ball screw of radial runout, refers to the screw shaft in the process of rotation, its cylindrical surface (or threaded raceway and ball contact with the working surface) relative to the theoretical axis of rotation in the radial direction (perpendicular to the direction of the axis of the screw) generated by the amount of cyclical offset. This offset is manifested as fluctuations in radial displacement at fixed measurement points (e.g., journals, nut fit segments) when the screw is rotating, and is usually measured in microns (μm) by micrometers, laser interferometers, and other equipment at specific rotational speeds (e.g., 100-300 r/min).
In terms of physical nature, radial runout is the deviation of the actual center of rotation of the screw from the ideal geometric center, and consists of two categories: "circular runout" (maximum deviation within a single cross-section) and "full runout" (deviation over the full length of the axial direction). For example, in the SFE1610 model ball screw, if the radial circular runout at the journal is measured to be 5 μm, it means that the maximum radial offset of this cross-section does not exceed 5 μm when it is rotated.
Second, the causes and influencing factors
The generation of radial runout is the result of the accumulation of a variety of errors in the manufacturing, assembly and use process, the core influencing factors include.
1. Manufacturing accuracy errors
Screw shaft roundness, cylindricity error: If the shaft body outer circle processing there is ellipticity (long axis and short axis difference > 3μm), or cylindricity overshoot (full-length diameter change > 5μm), the rotation is bound to produce radial offset.
Processing deviation of threaded raceway: the guide error of raceway helix and tooth angle deviation will lead to periodic change of the position of contact point of ball and raceway, which indirectly triggers radial runout;
Material uniformity: if there is internal segregation or forging defects in the material of the screw shaft (e.g., SUJ2 bearing steel), a trace deformation may be generated after heat treatment, which will form the hidden source of runout.
2. Assembly and installation errors
bearing mounting coaxiality: support screw bearing housing and screw axis if not concentric (coaxiality error > 0.01mm), will force the screw in the rotation of the "eccentric" rotation, amplifying the radial runout;
nut and screw fit Gap: If the concentricity deviation of the inner raceway of the nut and the raceway of the screw is too large, it will generate additional radial force during transmission and aggravate the runout.
3. Deteriorating factors during use
After long-term operation, the uneven wear of the ball and raceway will cause the contact track to deviate, so that the radial runout increases with the increase in life.
Under heavy loads or shock loads, the screw shaft may produce a small amount of bending deformation, especially in the slender screw with L/D ratio > 20, which is easy to trigger persistent radial runout due to deflection.
Third, the multi-dimensional impact on equipment performance Radial
runout overshoot will have a knock-on effect on the drive system of SFE Ball Screws, especially in high-precision scenarios.
Decrease in positioning accuracy and repeatability
Radial runout causes additional radial force when the nut moves in the axial direction, making the nut running trajectory "serpentine" deviation. For example, in a precision coordinate boring machine, if the radial full runout of the SFE Ball Screw reaches 10μm, it will increase the positioning error of the table by 3-5μm, and it will not be able to meet the machining requirement of ±2μm.
1. Vibration and noise amplification
The periodic radial force generated by the radial runout excites the resonance of the screw-nut system, especially at high speeds (e.g., >3000r/min), where the vibration frequency is superimposed on the intrinsic frequency of the equipment, creating significant noise (>75dB). In the semiconductor packaging equipment, this vibration will lead to welding head positioning offset, reducing the chip welding yield.
2. Accelerated component wear and life decay
beating leads to uneven distribution of contact stress between the ball and raceway (local stress can be elevated by 20% -30%), so that premature fatigue pitting on the raceway surface. For example, SFE ball screws with excessive radial runout may have their service life shortened from the designed 10,000 hours to less than 6,000 hours under rated load.
3. Servo System Response Lag
The additional radial force generated by radial runout increases the running resistance of the nut, resulting in the deviation of the actual output torque of the servo motor from the commanded torque, and the response delay increases (e.g., from 0.5ms to 1.2ms), which affects the accuracy of dynamic trajectory followers (e.g., interpolation error in curved surface machining).
Fourth, the control standard and precision level adaptation
Different precision levels of SFE precision ground ball screw, the allowable value of radial runout has a clear specification, need to be selected according to the needs of the equipment:
| Accuracy class | Radial circular runout (at journal, μm) | Radial full runout (1000mm length, μm) | Typical Application Scenarios |
| C3 | ≤3 | ≤8 | Semiconductor wafer processing equipment |
| C5 | ≤5 | ≤12 | Precision grinding machines, coordinate boring machines |
| C7 | ≤8 | ≤20 | General CNC machine tools, automated production lines |
For example, the SFE2020 ball screw used for optical lens processing needs to use C3 level, and its radial runout is controlled within 3μm to ensure that the face shape accuracy of lens grinding is ≤0.5μm; whereas, the C7 level of ordinary machine tool feed system can meet the basic processing requirements.
Five, the key points of concern under special working conditions
In extreme working conditions, the control of radial runout needs to be strengthened.
High-speed scenarios (speed>3000r/min): the need for dynamic balancing of the screw correction (residual unevenness of ≤ 2g・mm/kg), while the use of high-precision bearings with preload (such as angular contact ball bearing set), to reduce the centrifugal force on the amplification of radial runout.
Long stroke equipment (screw length > 2m): it is necessary to add a support seat in the middle to control the radial runout increment caused by deflection (≤2μm per meter length increment).
Corrosive environment: when stainless steel (such as 316 screws) is selected, surface grinding accuracy should be ensured to avoid runout deviation caused by uneven thickness of plating.
Summarize
The radial runout of SFE precision ground ball screw is the core index to measure its manufacturing accuracy and assembly quality, which is directly related to the positioning accuracy, operation stability and life of the equipment. By clarifying the requirements of precision level, controlling the manufacturing and assembly errors, and optimizing the adaptation of working conditions, the radial runout can be limited to the permissible range, providing a reliable guarantee for the precision drive system. In practical applications, it is necessary to choose the appropriate SFE Ball Screw model and control program, taking into account the equipment accuracy requirements (e.g. ±1μm vs ±5μm) and cost balance.
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