The ball screw consists of a screw, nut, balls and a ball return device . Its main function is to convert rotational motion into linear motion. It is widely used in precision equipment, especially CNC equipment. The pulse signal sent by the control system controls the rotation of the servo motor, and then the precision ball screw converts the rotational motion of the servo motor into the linear motion of the worktable, thereby realizing the processing trajectory of the workpiece. control. Therefore, the assembly process and accuracy testing are very strict, and the technical requirements for assembly workers are also high.
1. Fault phenomenon
A CNC equipment company passed the static accuracy inspection after the precision ball screw of the CNC lathe was assembled . However, after the test run or after a period of use, the axial runout of the precision ball screw was seriously out of tolerance, which affected the machining accuracy of the machine tool. The equipment repair rate is 43%, which brings huge economic losses and reputational losses to the company.
2. Failure Cause Analysis
The precision of the precision ball screw does not meet the requirements
The ball screw has been precision inspected when it was put into storage , so the quality problem can be ruled out. However, slender shaft parts (machine tool screws, etc.) should be placed vertically during storage to prevent gravity deformation from affecting the accuracy. The ball screw is placed vertically in a special packaging box and passed the static inspection after assembly, so this problem can be ruled out.
Improper assembly damages the accuracy of the screw
Through understanding , we found that the assembly workers were familiar with the precision ball screw assembly and were extremely careful during assembly, completely following the assembly process, so this possibility was ruled out.
The machine tool travel exceeds the limit , the workbench collides, and the precision is damaged
Overtravel occurs when operating the machine tool , which can easily lead to machine tool collision. However, it is understood that there is no problem with the travel limit switch of the machine tool, and there is no feedback from users in this regard.
The detection method is incorrect, resulting in excessive accuracy.
The inspector uses a dial indicator with a magnetic gauge base and uses the workbench linear guide slide as the detection benchmark. The detection method and detection benchmark also meet the requirements.
The assembly process is incorrect, causing assembly errors
The original assembly steps are : first install the linear guide, use the linear guide slide as the reference to calibrate the lead screw, and then use the mounting holes of the support seats at both ends of the lead screw as fixed threaded holes. After fixing the lead screw with screws, use the magnetic base to fix the micrometer on the linear guide slide, measure the radial runout of the lead screw, side busbar and lead screw, and ensure that the parallelism and runout of the lead screw are within the required range. Re-match the support seat positioning taper pin , and re-check it after installing the taper pin . The processing methods used in each step of the assembly are basically correct, the detection benchmark is consistent with the process benchmark, the accuracy of the detection gauge can also meet the requirements, and the measured elements are also correct. There is no problem with the entire assembly process, but through on-site observation, it was found that the following problems still exist in the assembly process:
When using a hand drill to reame a hole, due to the large reaming allowance , strong vibration, unstable control of both hands, and excessive swing of the reamer, the cone hole is larger at the top and smaller at the bottom after reaming , and the taper is not the required 1:50. The coloring method was used to check and found that the contact position between the cone pin and the cone hole was incorrect, and the position was close to the small end, which could not guarantee good contact accuracy. It was easy to loosen when subjected to alternating stress and vibration, resulting in a decrease in positioning accuracy.
After machine reaming, there is no margin for fine reaming , and no manual fine reaming is performed. The fit of the conical pin relies on the friction generated by the interference of the mating surface to position. Since the surface roughness after machine reaming reduces the actual interference, the friction is insufficient, the self-locking performance is reduced, and it is easy to loosen when subjected to alternating stress and vibration, resulting in a decrease in positioning accuracy.
The hole diameter is not strictly controlled during reaming, and the taper pin is not used for trial fitting during the reaming process . During assembly, a copper rod is used to tap it in gently. Since sufficient interference is not guaranteed, the friction after assembly is too small; when subjected to load and vibration, the taper pin is easily loosened and loses its positioning function, resulting in poor motion accuracy of the workbench.
No dust prevention was performed during the drilling and reaming process. Although the screw pair is equipped with a dust ring, cast iron dust is very fine and easily adheres to the surface of the screw. If a small amount of dust enters the transmission surface of the screw pair , the grinding effect of the dust will aggravate the wear of the screw pair , resulting in a decrease in the accuracy of the screw and a shortened service life.
3. Troubleshooting
After rough reaming with a machine, a fine reaming margin must be left before fine reaming by hand
This method can correct the taper error after machine reaming , reduce the surface roughness value of the hole wall, improve the contact accuracy of the mating surface, and ensure sufficient friction torque.
When reaming holes, use taper pins to test fit them in time.
The purpose is to facilitate the control of the reaming hole diameter. The hole diameter is qualified when the taper pin can be inserted by hand to about 80% to 85% of the pin length ; after cleaning the iron filings, the taper pin should be coated with engine oil and then driven in with a copper rod until it is flush with the workpiece surface to ensure sufficient friction torque and prevent it from loosening under load during use.
Do a good job of dust prevention on the surface of the screw
During the drilling, reaming and tapping process , the surface of the screw should be dust-proofed. Wrap the exposed part of the screw with a clean cloth to prevent dust from entering the transmission surface of the screw pair , so as to extend the service life of the precision components and improve the assembly accuracy.
In the fields of machine tools and industrial robots for manufacturing automobiles, mobile phones, computers, etc., LCD and semiconductor production equipment, injection molding machines for producing plastic parts, etc., ball screws play an important role as core parts in product manufacturing plant equipment.
