What are the specific requirements for the thread quality of self-locking tuning screws?
The thread quality of the self-locking tuning screw directly determines the self-locking reliability, tuning smoothness and service life. The core requirements focus on the four dimensions of tooth profile integrity, dimensional accuracy, surface quality and meshing suitability. The specific standards are as follows:
A, tooth type and structural integrity requirements
Accurate tooth profile: it must conform to the corresponding standards (such as ISO 68, UNEF, etc.), the error of tooth profile angle (such as 60) shall be ≤ 1, and the arc radius of tooth top and tooth bottom shall meet the design requirements (no sharp edges or excessive arcs).
No structural defects: the thread has no broken teeth, missing teeth or broken teeth, no obvious scratches and pits on the tooth side, and the terminal transition thread (lead section) is smooth, without burrs or steps, so as to avoid jamming during tuning.
Self-locking structure is intact: if it is a locking thread (such as nylon locking ring, embossed thread and variable tooth thread), the locking structure is not damaged or deformed (such as nylon ring not falling off or cracking, and embossed teeth not lodging).
Second, the core size accuracy requirements
Accuracy of pitch diameter: the pitch diameter tolerance of thread is controlled at IT6-IT8 level (for example, the pitch diameter tolerance of M3 screw is ±0.015~±0.025mm), and the difference between single pitch diameter and virtual pitch diameter is ≤0.015mm to ensure tight meshing.
Pitch accuracy: pitch error ≤ 0.01 mm per 25mm length, and cumulative pitch error (total length) ≤±0.03mm to avoid inaccurate tuning stroke or self-locking failure caused by pitch deviation.
Small diameter and large diameter: the tolerance of small diameter is IT7-IT9, and the tolerance of large diameter is IT8-IT10, so as to ensure that the meshing clearance with the mating threaded hole (or nut) is reasonable (usually the clearance is ≤0.03mm) without looseness or over-tightening.
Third, the surface quality and performance requirements
Surface roughness: the roughness Ra of thread flank and crest is less than 0.8 μ m, and the surface is smooth without burr and scale, which reduces the friction resistance during meshing and ensures smooth tuning.
Surface hardness: after heat treatment, the surface hardness of the thread meets the requirements (such as HRC35~45), and the hardness uniformity deviation is ≤3HRC to avoid deformation or wear in use.
Surface treatment quality: the coating (such as zinc plating, nickel plating and passivation) is uniform, the thickness tolerance is 1 μ m (8~12μm for conventional coating), there is no missing plating, peeling and bubbles, and the adhesion of the coating is up to standard (no shedding after 100-grid test).
Four, the meshing and self-locking function adaptation requirements
Smoothness of meshing: When assembling with the matching threaded hole (or nut), there is no jamming and obvious resistance fluctuation in manual rotation, and the torque is even in the tuning process.
Stability of self-locking torque: the self-locking torque should be within the set range (for example, M3 screw 5 ~ 15 ncm), and the torque fluctuation of the same batch is ≤ 20%, and there is no looseness after locking.
Wear resistance: after tuning for a specified number of times (≥1000 times), the thread wear is ≤0.01mm, the tooth profile has no obvious deformation, and the self-locking performance has no attenuation.
Locking reliability: after vibration test (10~50Hz, lasting 30min), there is no loosening of the screw and no obvious increase of the thread engagement clearance.
V. Additional requirements for special scenes
Micro-thread (≤M2): the tooth profile tolerance is tightened, the pitch diameter error is ≤±0.008mm, and the surface roughness Ra is ≤ 0.4 μ m, which is suitable for precision assembly scenes.
Use in high temperature environment: the thread surface treatment needs high temperature resistance (such as ceramic coating and high temperature passivation) to avoid coating failure or thread sticking at high temperature.
Use in corrosive environment: stainless steel 316 or anticorrosive coating is selected, so the thread has no risk of corrosion and the meshing performance is not affected by corrosion.
