How to repair miniature ball screws?
Micro ball screw is widely used in medical equipment, precision instruments, semiconductor equipment and other micro transmission systems due to its high precision and small size. Due to long-term high-frequency, small load movement, or due to improper installation, insufficient lubrication and other factors, miniature ball screws are prone to wear and tear, hysteresis, precision degradation and other problems. Timely and professional repair of damaged miniature ball screws can not only reduce the cost of equipment replacement, but also ensure the continuous and stable operation of the system. This article will systematically introduce the repair process and key technologies of micro ball screw.
First, micro ball screw common damage types and causes of analysis
Before repairing, it is necessary to clarify the types of damage and causes of micro ball screw, to provide accurate basis for the repair program.
1. raceway and ball wear
This is the most common form of damage. micro ball screw raceway (screw raceway and nut raceway) and ball size is small (ball diameter is usually only 1-3mm), long-term high-speed operation, the contact stress is concentrated, easy to surface scratches, pits or fatigue spalling. The causes are grease failure (such as drying, pollution), operating environment dust intrusion, or load over the rated value (miniature screw rated dynamic load is usually ≤ 5kN).
2. Thread clearance is too large
manifested in the screw and nut axial runout increased, transmission accuracy decreased (positioning error of more than ± 0.005mm). Mainly due to uneven wear and tear of the raceway after long-term use, or assembly of the preload is not properly adjusted, commonly used in frequent start-stop conditions (such as laboratory automation pipetting equipment).
3. Screw bending deformation
miniature screw diameter in 3-10mm, weak rigidity, if the installation of radial bias load (such as screw and drive motor coaxiality deviation > 0.02mm), or transportation, storage, collision, easy to produce a small bending (straightness error > 0.01mm/m), resulting in increased vibration and noise during operation.
4. Ball circulation system failure
micro ball screw circulation channel (such as end cap type, tube type) due to the size of the delicate, easy to due to foreign body stagnation, circulation parts wear and tear caused by ball jamming or fall off. Commonly found in dusty environments (such as optical instrument lens focusing mechanism), or overfilling of grease caused by channel blockage.
Second, the assessment and preparatory work before repair
(one) Damage assessment
Appearance inspection: use a magnifying glass (10-20 times) to observe the surface of the screw whether there are cracks, corrosion, whether there is obvious spalling of the raceway; check whether there is oil leakage at the end of the nut, whether the circulating parts are intact.
Accuracy test: use micrometer to measure axial clearance (normal should be ≤ 0.003mm), rotate the screw to observe the needle jumping amplitude; through the laser micrometer to detect the straightness of the screw, record the position of the bending peak; manually rotate the screw, feel whether the resistance is uniform, whether there is any jamming or strange sound.
Core judgment criteria: If the raceway spalling area exceeds 10%, the screw bending amount > 0.02mm/m, or the thread tooth type is seriously damaged, it is recommended to replace the new product; slight wear, gap exceeding the standard or small deformation can be repaired.
(twe) Tools and materials preparation
Special tools: micro bearing dismantler (clamping force adjustable, to avoid damage to the screw), precision grinding paste (particle size W5-W10, for raceway repair), torque wrench (range 0-5N・m, to control the preload force), micrometer (accuracy of 0.001mm), V-type iron and magnetic table base (calibration of straightness).
Consumables: special micro ball screw grease (such as polyurea grease, dropping point ≥ 120 ℃), stainless steel ball (accuracy class G2 or above, diameter tolerance ± 0.001mm), wear-resistant coating (for repair of small scratches), anhydrous alcohol (clean parts).
Environmental requirements: Repair needs to be carried out on a clean bench (dustproof grade ≥ ISO 8), the temperature is controlled at 20 ± 2 ℃, to avoid the measurement error caused by temperature difference.
Specific restoration steps
(one) Disassembly and cleaning
Nut disassembly: fix the miniature ball screw on the V-iron, use special tools to slowly spin out the nut, record the number of balls and the arrangement during the process (it is recommended to take pictures for the archive), to prevent the balls from being scattered and lost.
Component disassembly: If the nut is a detachable type, carefully remove the circulation end cap or tube, take out all the balls and store them in a clean container.
Deep cleaning: use anhydrous alcohol to rinse the screw, nut and ball, remove residual grease and impurities; use a soft brush to clean up the raceway and circulating channel to ensure that there is no foreign matter adhering to the last blow-dry with compressed air.
(twe) Targeted repair operation
1. Raceway and ball repair
For slight scratches on the surface of raceway, apply a small amount of W10 grinding paste, use special fixture to hold the nut on the screw and grind it back and forth at low speed (speed ≤5mm/s), clean it once every 5 minutes until the scratches disappear, and finally replace it with W5 grinding paste to polish it.
If there is wear on the surface of the ball, it is necessary to replace the new ball with the same specification (the new ball needs to be cleaned with alcohol and dried), to ensure that the difference in the diameter of all the balls is ≤0.0005mm.
2. Screw bending correction
will be bent on the screw frame in equal height V-type iron, with a magnetic table seat fixed percentage meter, the probe against the middle position of the screw, slowly rotate the screw to find the highest point of bending.
Tap the bent part with a copper hammer (or use a precision press to apply tiny pressure), and re-measure the straightness after each correction until the error is ≤0.01mm/m, to avoid over-correction leading to screw breakage.
3. Gap Adjustment and Preload Repair
For single-nut screws with excessive gap, the preload can be adjusted by adding shims (thickness 0.001-0.003mm) or replacing the thickened balls (diameter increased by 0.002-0.005mm), so that the axial gap is restored to ≤0.003mm.
Double-nut structure can be adjusted by adjusting the preload screw between the nuts, with a torque wrench to monitor the preload torque (micro-screw preload torque is usually 0.1-0.5N・m), to ensure that both sides of the force is uniform.
4. Recirculation System Repair
If the recirculation end cap is worn and causes ball jamming, use a precision file to slightly trim the end cap guiding corners or replace with a new end cap of the same type.
If there is any deformation of the tube circulation channel, use a special rod to calibrate the arc of the tube to ensure that the ball passes through smoothly and without obstruction.
(three) Assembly and lubrication
Re-assembly: Install the ball and circulating parts in the reverse order of disassembly, and apply a small amount of grease inside the raceway to assist the positioning of the ball, to ensure that the ball is uniformly distributed without any jamming phenomenon.
Lubrication and filling: After the assembly is completed, fill the special grease into the oil filling hole of the nut (the filling amount is 1/3-1/2 of the volume of the nut), and slowly rotate the screw so that the grease is evenly distributed on the surface of the raceway and the balls.
Fourth, the test and verification after repair
1. Accuracy review: Measure the axial clearance again with micrometer to ensure that it is ≤0.003mm; detect the straightness of the screw operation by laser interferometer, and the error should be controlled within the range of equipment requirements (usually ≤0.01mm/m).
2. Running test: manually rotate the screw, feel the resistance uniformity and no jamming; in the simulated load conditions (load 50% of the rated load) run 1000 cycles, monitor the running noise (should be ≤ 50dB) and temperature rise (≤ 8 ℃), no anomalies is qualified.
3. Marking record: Mark the date of repair, repair content and testing data on the end of the repaired screw, so as to facilitate the follow-up maintenance tracking.
Fifth. Repair notes and maintenance recommendations
1. Precision control: micro ball screw parts are tiny, the repair process should avoid excessive force leading to deformation or damage to the parts, all measuring tools need to be calibrated (calibration cycle ≤ 1 year).
2. Material matching: the replacement of ball, grease and other consumables should be matched with the prototype, and it is forbidden to mix parts with different specifications to avoid aggravating the wear.
3. Regular maintenance: the repaired screws need to shorten the maintenance cycle, and it is recommended to check the lubrication condition once every 500 hours of operation, replenish or replace the lubricating grease in time, and keep the operating environment clean.
In summary, the repair of micro ball screw needs to be combined with precision testing and fine operation, for different types of damage to take targeted measures. Through the scientific repair process, it can effectively restore its transmission accuracy and service life, and provide protection for the stable operation of micro-precision equipment. If the repair is still unable to meet the precision requirements, it is necessary to replace the new product in time to avoid affecting the performance of the whole machine.
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