The clearance of ball screw directly affects the transmission precision and stability of the equipment, and it is crucial to master the technical details of the adjustment. The following from the gap causes, adjustment methods, tools, etc., for you to analyze the key points of the ball screw gap adjustment.
First, the causes of ball screw gap
Manufacturing and assembly errors: ball screw in the production process, threaded raceway, ball dimensional accuracy there is a certain tolerance, assembly, if the nut and screw coaxiality, parallelism is not up to standard, will produce the initial gap. For example, a batch of ball screws due to insufficient grinding accuracy of the raceway, the axial clearance after assembly exceeds 0.03mm, resulting in machine tool processing thread pitch error.
Wear and fatigue: After long-term use, the contact surface between the ball and the raceway will be worn out due to friction, the diameter of the ball becomes smaller, and the raceway is dented. For example, in the high-frequency reciprocating motion automated production line, the ball screw runs more than 5000 hours per year, and after 2 years, the wear of the ball increases the gap from 0.01mm to 0.05mm, and the positioning accuracy of the equipment decreases significantly.
Thermal deformation effect: When the equipment is running, the ball screw generates thermal expansion due to friction heat or environmental temperature change. If the installation is not reserved enough thermal expansion gap, the thermal expansion of the screw elongation, cooling contraction gap will occur. A heat treatment equipment in the screw, due to the working temperature of 200 ℃, after cooling the axial clearance increased by 0.02mm.
Second, the type of ball screw gap and detection
gap type
axial clearance: refers to the nut relative to the screw along the axial movement of the free amount, directly affecting the positioning accuracy of the equipment and reverse transmission accuracy. Such as in the CNC machine tools, axial clearance is too large, will lead to the processing of workpiece size error increases, the contour accuracy deterioration.
Radial clearance: the clearance between the nut and the screw in the radial direction affects the rotary accuracy and stability of the screw. Excessive radial clearance will make the screw rotate with shaking, cause vibration and noise, and shorten the service life of the screw and bearings.
Detection method
Micrometer detection method: the micrometer is fixed in the machine bed and other stable position, so that the head against the end of the nut, manually move the nut back and forth to observe the micrometer pointer oscillation range. For example, in the ordinary lathe to detect the axial clearance of the screw, micrometer reading difference is the amount of axial clearance, if the reading fluctuations of more than 0.02mm, the need for gap adjustment.
Sensor detection method: the use of high-precision displacement sensors, through the data acquisition system to monitor the relative displacement of the nut and the screw in real time. This method is highly accurate, up to the micron level, commonly used in high-precision equipment gap detection, such as semiconductor manufacturing equipment in the ball screw detection.
Third, the ball screw gap adjustment method
gasket adjustment method
operation steps: set up a set of different thicknesses of gaskets (thickness range 0.01 - 0.1mm) between the nut holder and the nut. Loosen the nut holder fixing bolt, increase or decrease the thickness of the shims to adjust the relative position of the nut and the screw to eliminate the gap. For example, if the axial clearance is detected to be 0.03mm, increase the thickness of shims by 0.03mm, retighten the bolts, and then use a micrometer to check until the clearance meets the requirements (generally controlled between 0.005 - 0.01mm).
Scenario: Suitable for small and medium-sized equipment, simple structure, low cost, but the adjustment accuracy is limited by the thickness specification of the gasket, and the gasket is prone to plastic deformation after repeated adjustments, affecting the adjustment effect.
Thread gap adjustment method
Operation steps: one end of the nut is processed with external threads, and the gap is adjusted by its matching lock nut and adjusting nut. First loosen the lock nut, turn the adjusting nut so that the nut produces axial displacement, compression or stretching of the ball and the raceway between the contact preload, thereby eliminating the gap. After the adjustment is completed, tighten the lock nut to prevent the adjusting nut from loosening. For example, on small precision engraving machines, the axial clearance can be reduced from 0.02mm to 0.008mm by rotating the adjusting nut.
Features: easy to adjust, can be fine-tuned during the operation of the equipment, but requires high accuracy in thread machining, otherwise it will affect the stability and accuracy of the adjustment.
Differential Gap Adjustment Method
Procedure: A gear with a difference of 1 tooth is machined on the flange of each of the two nuts and meshes with the corresponding inner ring. By turning the two nuts at the same time with the same number of teeth (e.g. turning 1 tooth), the relative displacement of the nuts will be realized to adjust the backlash. Assuming a screw pitch of 10 mm, the displacement of one tooth is about 0.01 mm, allowing precise adjustment of the gap.
Advantage: High adjustment precision, up to micron level, and stable gap after adjustment, not easy to change, commonly used in high-precision equipment, such as coordinate boring machine, machining centers, etc., but the structure is complex, the cost is higher.
Preloading Nut Method
Operation Procedure: Use two nuts, and by applying preload force to make them contact with the raceways on both sides of the screw respectively. The preload force is applied in the form of spring preload, hydraulic preload, and so on. Take spring preload as an example, a compression spring is installed between the two nuts, and the spring force causes relative displacement of the nuts to realize preload. Such as in the automated assembly line screw, the use of spring preload nut, can effectively eliminate the axial clearance, improve the rigidity and precision of the transmission.
Note: The size of the preload force should be reasonably selected according to the load and working conditions of the screw; if the preload force is too large, it will increase the frictional resistance, reduce the transmission efficiency, and accelerate the wear of the balls and raceways; if the preload force is too small, it will not be able to effectively eliminate the gap.
Fourth, gap adjustment tools and auxiliary equipment
Measuring tools: In addition to micrometers, displacement sensors, you also need to use a plug rule (for detecting the thickness of the gasket or gap size), vernier calipers (measuring the nut, the relevant size of the screw), etc., to ensure that the adjustment process of dimensional accuracy control.
Installation tools: torque wrenches are used to accurately control the tightening torque of nut holders, bolts, etc., so as to avoid tilting of the nut due to uneven torque, which affects the effect of gap adjustment; tools such as hydraulic pullers can assist in the dismantling and mounting of nuts, bearings and other components, so as to prevent damage to the parts.
Cleaning and lubrication materials: anhydrous ethanol is used to clean the oil and impurities on the surface of the screw and nut; suitable grease (e.g. lithium grease, molybdenum disulphide grease) is coated on the raceway of the screw after the adjustment is completed, to reduce friction, lower wear and prolong the service life of the screw.
V. Test and Maintenance after Gap Adjustment
Accuracy Test: After the adjustment is completed, use the micrometer or sensor to test the axial and radial gaps of the ball screws again to ensure that they meet the requirements of the equipment. At the same time, through the actual operation of the equipment, test the machining accuracy or positioning accuracy is back to normal. For example, process standard test pieces on the CNC lathe, measure the dimensional accuracy and surface roughness of the test pieces, and verify the gap adjustment effect.
Operation monitoring: After the equipment is put into use, regularly monitor the operation status of the screw, including temperature, vibration, noise and other operating conditions. If the temperature rises abnormally (more than 10℃ above the normal working temperature), vibration increases, abnormal noise, etc., it may be that the gap adjustment is improper or the screw has other faults, and should be shut down in time for inspection.
Regular maintenance: according to the requirements of the equipment manual, lubricate, clean and check the gap of the ball screw regularly. Generally every 1000 - 2000 hours of operation, need to re-lubricate grease; every 6 - 12 months, a comprehensive inspection and adjustment of the screw gap, to ensure that the equipment always maintains good operational precision and stability.
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