What are the requirements for the accuracy of self-locking tuning screws used in aerospace communication?
The self-locking tuning screw in the field of aerospace communication should not only adapt to the precise tuning function of communication equipment, but also meet the high reliability of aerospace scenes. Its machining accuracy should cover many core dimensions such as thread, key dimensions, surface quality and self-locking structure, and should meet the special aerospace standards such as SAE and ASTM. The specific requirements are as follows:
Thread accuracy
Thread is the core of self-locking tuning screw to realize locking and tuning functions, and the precision requirement is extremely strict. For example, the thread grade will be specified in SAE related standards, such as 3A grade, which is suitable for aerospace precision assembly scenarios. The pitch error should be controlled at ≤0.001mm/25mm, and the tooth profile half-angle error should be ≤ 10', so as to ensure the smoothness and tuning accuracy of the screw. In addition, the tolerance of major diameter, middle diameter and minor diameter of common aerospace thread series such as UNJF should be controlled in micron level, for example, the tolerance of major diameter of some specifications of screws should be ≤0.001 inch (0.025mm) to avoid the influence of thread size deviation on torque stability during tuning. At the same time, the internal thread roughness is often required to reach 63μ in Ra, and the external thread roughness is 125μ in Ra, so as to reduce the friction interference during thread matching.
Critical dimension accuracy
The accuracy of the overall size of the screw directly affects the assembly adaptability and tuning effect, and the dimensional tolerance of each key part is mostly controlled at micron level. Take the aerospace self-locking screw made of titanium alloy as an example, its holding diameter should be controlled within 0.2485-0.2495 inch, and the deviation is only 0.001 inch; The fastening length should be in the range of 0.740-0.760 inch, and the head diameter should be in the range of 0.449-0.507 inch. In addition, the details such as the angle of the head countersunk head also have strict requirements, for example, some specifications need to be controlled at 99.0-101.0 to prevent the assembly from not fitting tightly due to angle deviation, which will affect the force transmission during tuning. Generally, the dimensional tolerance of the whole machining should reach the level of ±0.001mm, and the consistency deviation in mass production should not exceed ±0.001mm.
Accuracy of self-locking structure
The accuracy of self-locking structure determines the stability of screw locking performance, which is the key to avoid tuning failure caused by equipment vibration in space communication scenes. For the self-locking structure with locking keys, the inserts with internal thread sizes of 0.1900 and 0.2500 need to be equipped with two locking keys with an interval of 180, and the sizes of 0.3125 and above need four locking keys with an interval of 90. The height and thickness tolerances of the locking keys should be ≤0.002mm, and the surfaces of the locking keys should be free from burrs and deformation, so as to ensure that the matching structure can be accurately embedded when locking. If the self-locking structure is glued, the thickness of the adhesive layer should be accurately controlled at 0.05-0.15mm, and the adhesive layer should cover the effective contact area of the self-locking thread evenly, so as not to block the through hole or affect the thread engagement, and ensure the stability of the self-locking torque.
Surface and geometric accuracy
On the one hand, the surface should be free from any defects that affect the performance, and cracks should be checked by fluorescent penetrant inspection (sensitivity level 2), and at the same time, no burr, scratch, lack of materials and other problems should be guaranteed, and the maximum chamfer of the edge should not exceed 0.015 inch to prevent the precise interface of communication equipment from being scratched during assembly. On the other hand, the geometric tolerance should be strictly controlled, and the straightness error of the screw should be less than or equal to ≤0.0005mm/m/m, so as to prevent the jamming during screwing and affect the tuning operation; The coaxiality error between the head and the screw should be ≤0.001mm, so as to avoid the deviation of tuning accuracy caused by uneven stress after assembly. In addition, the thermal error in machining should be controlled at ≤0.0005mm, and the vibration error should be ≤0.0003mm to avoid geometric deformation caused by environmental factors.
Batch consistency accuracy
Aerospace communication equipment is mostly assembled in batches, and the accuracy and consistency of screws are very important. In mass production, the consistency deviation of key dimensions should be ≤±0.001mm, and each batch of products should be sampled and tested by CMM and other equipment. At the same time, it is necessary to carry out batch testing of self-locking performance, simulate vibration, temperature change and other working conditions in the aerospace environment, detect the torque change of screws at different tightening angles, ensure that the fluctuation range of self-locking torque of the same batch of products is ≤5%, and avoid some equipment tuning failure or screw loosening due to individual differences.
