Load simulation loading device and tightening torque design method for hub clamping screws

Abstract

The present invention discloses a load simulation loading device and a tightening torque design method for a hub clamping screw, comprising a loading disk, a hub central piece arranged on the loading disk, a main reducer rotor shaft and a hub clamping structure for connecting the hub central piece and the main reducer rotor shaft, the hub clamping structure comprising a large clamping nut, a lower conical ring, an upper conical ring, a load-bearing plate, an anti-loosening plate, a hub clamping screw and an anti-loosening plate fixing bolt, four vertical loading channels with a fillet phase of 90°, two first horizontal loading channels arranged perpendicular to each other and two second horizontal loading channels arranged parallel to each other are arranged on the loading disk, which can accurately simulate the fatigue test load applied to the main reducer rotor shaft, has a simple structure and is easy to operate, and through the calculation of the number of hub clamping screws and the tightening torque, the number of hub clamping screws and the tightening torque value on the main reducer rotor of the helicopter transmission system can be accurately designed, thereby improving the reliability of the connection structure.

Description

Technical Field

[0001] The present invention relates to the technical field of helicopter transmission systems, and in particular to a load simulation device for fatigue testing of a rotor shaft in a main reducer of a helicopter transmission system. Furthermore, the present invention relates to a method for designing the tightening torque of a hub compression screw utilizing the load simulation device. Background Art

[0002] The helicopter transmission system is one of the three major moving parts of the helicopter. The hub of the typical main reducer rotor shaft is assembled as follows: Figure 1 and Figure 2 As shown, during the flight of the helicopter, the engine power transmitted by the main reducer is transferred to the rotor through the main reducer rotor shaft 3, driving the rotor to generate the rotor aerodynamic load of the helicopter flight. The rotor aerodynamic load is transmitted to the main reducer rotor shaft 3 through the hub centerpiece 2, and the main reducer finally transmits the rotor aerodynamic load to the fuselage platform.

[0003] In order to improve the connection reliability between the main reducer rotor shaft 3 and the rotor hub central part 2, a circle of evenly distributed hub clamping screws 8 are usually used to generate sufficient axial clamping load. At the same time, in order to prevent the disappearance or increase of the tightening torque of the hub clamping screws 8, causing the clamping nut 1 to produce a large circumferential rotation, causing the rotor shaft hub clamping structure to be damaged during the inspection period, resulting in irreparable losses and consequences, two grooves are designed at the top of the main reducer rotor shaft 3, and an anti-loosening plate 7 with two claws is designed to be assembled to the clamping nut 1. The anti-loosening plate 7 is assembled on the clamping nut 1 through a circle of evenly distributed fixing bolts 9, and the claws of the anti-loosening plate 7 are assembled into the groove of the main reducer rotor shaft 3, thereby preventing the clamping nut 1 and the main reducer rotor shaft 3 from rotating relative to each other.

[0004] The number and tightening force of the hub clamping screws 8 are determined by the designer based on design requirements, reference to similar engineering designs, and experience to determine the structural scheme; the clamping load is converted based on the diameter, number, and tightening torque of the hub clamping screws 8, and a simple comparative analysis is performed between the clamping load of the clamping screws and the rotor lift load transmitted by the main reducer rotor shaft.

[0005] Due to the lack of a theoretical design method for the number and tightening torque of hub clamping screws 8, a design drawing on existing practical experience was developed to design the fastening structure between a certain type of main reducer rotor shaft 3 and the rotor hub centerpiece 2. The design included 12 hub clamping screws 8, each measuring MJ12×1.25, with a tightening torque of 30.4 to 35.5 Nm. Furthermore, the anti-loosening plate 7 was designed with six anti-loosening plate fixing bolts 9, each measuring MJ8×1.25, with a tightening torque of 25 to 30 Nm. During fatigue testing of the main reducer rotor shaft, the tightening torque of the hub clamping screws 8 disappeared, causing the large clamping nut 1 to rotate circumferentially, resulting in the fracture of the anti-loosening plate 7. The grooves and large clamping nut 1 of the main reducer rotor shaft 3 also wore, and the lower conical ring 6 showed severe wear. Subsequently, the failure of circumferential rotation of the clamping nut 1 was solved by replacing the anti-twist plate, shortening the inspection cycle of the fatigue test, and increasing the tightening torque of the hub clamping screw 8. However, there was a lack of systematic theoretical methods to guide the design of the number and tightening torque of the hub clamping screws, resulting in a large design risk. Summary of the Invention

[0006] The present invention provides a load simulation loading device for fatigue testing of a rotor shaft of a main reducer of a helicopter transmission system and a method for designing the tightening torque of a hub clamping screw, so as to solve the technical problem that the setting of the diameter, number and tightening torque of the existing hub clamping screws is highly dependent on existing practical experience and lacks a systematic theoretical method to guide the design of the diameter, number and tightening torque value of the hub clamping screws, resulting in a large design risk.

[0007] According to one aspect of the present invention, a load simulation loading device for fatigue test of a main reducer rotor shaft of a helicopter transmission system is provided, comprising a loading disk, a hub central piece arranged on the loading disk, a main reducer rotor shaft and a hub clamping structure for connecting the hub central piece and the main reducer rotor shaft, the hub clamping structure comprising a clamping large nut, a lower conical ring, an upper conical ring, a bearing plate, an anti-loosening plate, a hub clamping screw and an anti-loosening plate fixing bolt, four vertical loading channels with a fillet phase of 90°, two first horizontal loading channels arranged perpendicular to each other and two second horizontal loading channels arranged parallel to each other are arranged on the loading disk, the vertical loading channels are used to simulate lift loads and bending moment loads applied by the rotor, the first horizontal loading channel is used to simulate shear loads applied by the rotor, and the second horizontal loading channel is used to simulate torque loads applied by the rotor.

[0008] According to another aspect of the present invention, a method for designing the tightening torque of a hub compression screw is provided, which utilizes the above-mentioned load simulation loading device and includes the following steps:

[0009] S1. Set the initial number and tightening torque of the hub clamping screws;

[0010] S2. Determine the maximum torque load Mz that the hub clamping structure of the main reducer rotor shaft can withstand. max , calculate the limit load of the main reducer rotor shaft based on the power spectrum of the main reducer and the load spectrum of the rotor, calculate the maximum fatigue test load of the main reducer rotor shaft based on the load spectrum of the rotor, and the larger value of the limit load and the maximum fatigue test load of the main reducer rotor shaft is the maximum torque load Mz that the hub clamping structure can withstand max ;

[0011] S3. Calculate the friction torque M generated by the friction force Fμ of the hub pressing structure Fμ ;

[0012] S4. Verify whether the number and tightening torque of the hub clamping screws of the main reducer rotor shaft meet the use requirements. If not, increase the number and / or tightening torque of the hub clamping screws and repeat steps S1 to S3.

[0013] Furthermore, in step S2, calculating the limit load specifically includes the following steps:

[0014] S211. Assume that the center O of the coordinate system for the rotor aerodynamic loads is located at the center of the rotor hub, that the coordinate axis OZ is upwardly positive along the main reducer rotor axis, that the coordinate axis OX is located in the longitudinal plane of the fuselage and is perpendicular to the coordinate axis OZ and points toward the tail, and that the coordinate axis OY is determined using the right-hand rule. Calculate the lift load Fz, longitudinal force load Fx, lateral force load Fy, rolling moment load Mx, pitching moment load My, and torque load Mz of the limiting loads, respectively.

[0015] S212. Calculate the torque load Mz in the limit load. Based on the power spectrum of the main reducer, select the maximum power state in the power spectrum to obtain the steady-state maximum power value transmitted by the main reducer rotor shaft. Combined with the minimum rotor speed allowed within the operating range of the helicopter rotor, calculate the maximum steady-state torque value Mz0 of the main reducer rotor shaft during operation. Set the maximum torque fluctuation coefficient of the limit load to 2.5, and obtain the torque value of the main reducer rotor shaft limit load Mz = 2.5×

[0016] Mz0;

[0017] S213. Calculate the lift load Fz in the limit load. Based on the rotor load spectrum, select the maximum static lift load value Fz0 in the load spectrum. Set the maximum lift load fluctuation coefficient of the limit load to 1.5 to obtain the lift load value of the main reducer rotor shaft limit load: Fz = 1.5 × Fz0.

[0018] S214. Calculate the shear load Fxy in the limit load. Based on the rotor load spectrum, convert the longitudinal force load Fx and the lateral force load Fy in the load spectrum into a comprehensive shear load F0. Select the maximum comprehensive shear load value F0. Set the maximum shear load fluctuation coefficient of the limit load to 1.5 to obtain the shear load value of the main reducer rotor shaft limit load: Fxy = 1.5 × F0.

[0019] S215. Calculate the bending moment load Mxy in the limit load. According to the load spectrum of the rotor, convert the rolling moment load Mx and the pitching moment load My of the load spectrum into the comprehensive bending moment load M0. Select the maximum comprehensive bending moment load value M0. Set the maximum bending moment load fluctuation coefficient of the limit load to 1.5 to obtain the bending moment load value Mxy = 1.5 × M0 of the main reducer rotor shaft limit load.

[0020] Furthermore, in step S2, calculating the fatigue test load specifically includes the following steps:

[0021] S221. Determine the shear load Fxy' and bending moment load Mxy' in the fatigue test load, conduct fatigue testing on the main reducer rotor shaft, determine the first-stage fatigue test load based on the load spectrum of the main reducer rotor shaft, and after successfully completing the first-stage fatigue test load and the preset number of test cycles, select a preset shaft section between the hub center and the bearing support of the main reducer rotor shaft to evaluate the fatigue life of the main reducer rotor shaft. Using theoretical mechanics methods, calculate the fatigue test load applied to the hub center of the main reducer rotor shaft. The axial force load Fz', bending moment load Mxy' and torque load Mz' of the shaft section are obtained. Then, the bending moment load Mxy' is used as the fatigue life load. Combined with the SN shape curve of the main reducer rotor shaft material, the fatigue limit and the life cycle number corresponding to the bending moment load Mxy' after the main reducer rotor shaft completes the first-stage fatigue test are calculated. Using Miner's cumulative damage law, the fatigue damage caused by the first-stage fatigue test of the main reducer rotor shaft is calculated based on the ratio of the test cycle number and the life cycle number of the first-stage fatigue test of the main reducer rotor shaft.

[0022] S222. Determine the lift load Fz' and torque load Mz' in the fatigue test load. Based on the load spectrum of the main reducer rotor shaft, select the maximum lift load Fz' and torque load Mz' in each state of the load spectrum. Taking into account the actual situation during helicopter flight, use the maximum lift load Fz' in each state of the load spectrum as the static lift load in the fatigue test load, and select the maximum torque load Mz' in each state of the load spectrum as the static torque load in the fatigue test load.

[0023] Furthermore, step S221 further includes the following steps:

[0024] S2211. When fatigue failure of the main reducer rotor shaft requires multiple fatigue test loads, the lift load Fz' and torque load Mz' of each fatigue test load level remain unchanged, and only the load levels of the rotational shear load Fxy' and rotational bending moment load Mxy' are increased. The load increase ratio for each load level is 10%-20%.

[0025] S2212. According to the fatigue life index of the main reducer rotor shaft, complete the first-stage fatigue test, pre-assess the fatigue life of the main reducer rotor shaft to be 500 hours, complete the second-stage fatigue test, pre-assess the fatigue life of the main reducer rotor shaft to be 2000 hours, complete all multi-stage fatigue tests, and the main reducer rotor shaft reaches the fatigue life index. Combined with the SN shape curve of the main reducer rotor shaft material, calculate the fatigue limit of the main reducer rotor shaft after completing the multi-stage fatigue test and the life cycle number corresponding to the bending moment load Mxy'. Apply Miner's cumulative damage law, and based on the ratio of the test cycle number and the life cycle number of the multi-stage fatigue test of the main reducer rotor shaft, calculate the fatigue damage caused by the multi-stage fatigue test of the main reducer rotor shaft.

[0026] Furthermore, when conducting fatigue testing on the main reducer rotor shaft, the test load loading scheme includes the following steps:

[0027] S231, lift load Fz' of fatigue test, the static load in lift load Fz' is applied evenly to four vertical loading channels with a fillet phase of 90°, and the lift load Fz' applied by the four vertical loading channels remains unchanged during the test;

[0028] S232. Fatigue test shear load Fxy': A dynamic load in the shear load Fxy' is applied as a fluctuating load that varies according to a cosine law to two mutually perpendicular first horizontal loading channels. During the test, the combined shear load F0' of the two mutually perpendicular first horizontal loading channels remains constant, and the direction of the combined shear load F0' rotates 360° about the axis of the main reducer rotor shaft.

[0029] S233. Fatigue test bending moment load Mxy': The dynamic load in the bending moment load Mxy' is applied as a fluctuating load that varies according to the cosine law to four vertical loading channels with a 90° fillet phase. During the test, the combined bending moment load M0' of the four vertical loading channels remains unchanged, and the direction of the combined bending moment load M0' rotates 360° around the axis of the main reducer rotor shaft.

[0030] S234. The torque load Mz' of the fatigue test has opposite load directions on the two second horizontal loading channels. The static load in the torque load Mz' is first applied to the two second horizontal loading channels arranged parallel to each other. Then, the dynamic load fluctuating according to the cosine law is applied to the two second horizontal loading channels. The torque load value generated by the load fluctuation is equal to the dynamic load of the torque load Mz'. The period of the static load fluctuation and the dynamic load fluctuation of the torque load Mz' is the same as the shear load Fz'.

[0031] The phase change cycle is the same as that of the bending moment load Mxy'.

[0032] Furthermore, in step S3, calculating the friction force Fμ of the hub pressing structure specifically includes the following steps:

[0033] The nonlinear finite element method is used to calculate the torque M generated by the friction force Fμ of the hub pressing structure. Fμ , a finite element model was established in the finite element software ANSYS. The geometric model includes all parts of the rotor shaft hub. According to the actual assembly situation, the contact surface between the tapered ring, the anti-loosening plate, the hub center piece and the main reducer rotor shaft is established as a "Fiction" contact unit. The friction coefficient is set to 0.2. Then, the clamping load Qp of the hub clamping screw and the limit load and maximum fatigue test load are applied. The torque M generated by the friction force Fμ under the limit load and maximum fatigue test load is obtained through the support and reaction load solver of the post-processing module of the finite element software ANSYS. Fμ .

[0034] Furthermore, the number of test cycles of the first level fatigue test load is 200,000 to 500,000 times.

[0035] Furthermore, the torque fluctuation coefficient is set to 10%, and 10%×Mz′ is taken as the dynamic torque load in the fatigue test load.

[0036] Furthermore, in step S4, the determination conditions for verifying whether the number and tightening torque of the hub clamping screws of the main reducer rotor shaft meet the use requirements are as follows:

[0037]

[0038] Where: Fb is the residual compressive load borne by the half-circle bearing plate, in Newtons;

[0039] F Z1 is the axial force load generated by the rotor bending moment load at the radius of the load center of the bearing plate, in Newtons;

[0040] F Z2 is the lift load evenly distributed to the four vertical loading channels, in Newtons;

[0041] FZ3 The unit is Newton, which is the compression load generated by the hub compression screw borne by the half-circle bearing plate;

[0042] M Fμ is the friction torque generated by the compression load Qp of the hub compression screw, in N·m;

[0043] Mz max It is the larger value of the torque load Mz in the limit load and the torque load Mz' in the maximum fatigue test load, and the unit is N·m.

[0044] The present invention has the following beneficial effects:

[0045] The load simulation loading device of the present invention applies lift load and bending moment load through four vertical loading channels with a rounded angle phase of 90 degrees, applies shear load through two first horizontal loading channels, and applies torque load through two second horizontal loading channels. It can accurately simulate the fatigue test load applied to the rotor shaft of the main reducer, has a simple structure and is easy to operate, so as to facilitate the calculation of the number of hub tightening screws and the tightening torque.

[0046] In addition to the above-described objects, features and advantages, the present invention has other objects, features and advantages. The present invention will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0048] Figure 1 This is a schematic diagram of the structure of the hub assembly of a typical main reducer rotor shaft;

[0049] Figure 2 This is a top view of the hub assembly of the rotor shaft of the main reducer;

[0050] Figure 3 It is a structural schematic diagram of a load simulation loading device according to a preferred embodiment of the present invention;

[0051] Figure 4 It is a top view of the load simulation loading device of the preferred embodiment of the present invention.

[0052] Legend:

[0053] 1. Tightening large nut; 2. Hub center piece; 3. Main reducer rotor shaft; 4. Lower conical ring; 5. Upper conical ring; 6. Load-bearing plate; 7. Anti-loosening plate; 8. Hub tightening screw; 9. Anti-loosening plate fixing bolt; 100. Loading plate; 101. Vertical loading channel; 102. First horizontal loading channel; 103. Second horizontal loading channel. DETAILED DESCRIPTION

[0054] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered below.

[0055] like Figure 1 As shown, the load simulation loading device of this embodiment includes a loading disk 100, a hub central part 2 arranged on the loading disk 100, a main reducer rotor shaft 3 and a hub clamping structure for connecting the hub central part 2 and the main reducer rotor shaft 3, the hub clamping structure includes a clamping large nut 1, a lower conical ring 4, an upper conical ring 5, a bearing plate 6, an anti-loosening plate 7, a hub clamping screw 8 and an anti-loosening plate fixing bolt 9, and the loading disk 100 is provided with four vertical loading channels 101 with a fillet phase of 90°, two first horizontal loading channels 102 arranged perpendicular to each other and two second horizontal loading channels 103 arranged parallel to each other, the vertical loading channel 101 is used to simulate the lift load and bending moment load applied by the rotor, the first horizontal loading channel 102 is used to simulate the shear load applied by the rotor, and the second horizontal loading channel 103 is used to simulate the torque load applied by the rotor.

[0056] The load simulation device of this embodiment applies lift and bending loads via four vertical loading channels 101 with 90° angles, shear loads via two first horizontal loading channels 102, and torque loads via two second horizontal loading channels 103. This device accurately simulates the fatigue test loads of the main reducer rotor shaft 3. Its simple structure and easy operation facilitate calculation of the number and tightening torque of hub clamping screws 8.

[0057] This embodiment provides a method for designing the tightening torque of a hub compression screw, which includes the following steps:

[0058] S1. Setting the initial number and tightening torque of the hub clamping screws 8;

[0059] S2. Determine the maximum torque load Mz that the hub clamping structure of the main reducer rotor shaft can withstand. max, calculate the limit load of the main reducer rotor shaft based on the power spectrum of the main reducer and the load spectrum of the rotor, calculate the maximum fatigue test load of the main reducer rotor shaft based on the load spectrum of the rotor, and the larger value of the limit load and the maximum fatigue test load of the main reducer rotor shaft is the maximum torque load Mz that the hub clamping structure can withstand max ;

[0060] S3. Calculate the friction force F of the hub pressing structure μ The friction torque M generated Fμ ;

[0061] S4. Verify whether the number and tightening torque of the hub clamping screws 8 of the main reducer rotor shaft meet the use requirements. If not, increase the number and / or tightening torque of the hub clamping screws 8 and repeat steps S1 to S3.

[0062] In order to calculate the maximum load that the hub clamping structure of the main reducer rotor shaft 3 can bear, taking into account the fatigue life design index of the main reducer rotor shaft 3, the main reducer rotor shaft 3 needs to calculate the limit load of static strength design (the maximum load occurring during the use of the helicopter) and the fatigue load of fatigue strength design at the beginning of design.

[0063] Usually, when designing a helicopter, the host formulates the power spectrum of the main reducer and the load spectrum of the rotor according to the helicopter's operating environment and flight mission. When designing the main reducer rotor shaft 3, it is necessary to calculate the maximum load that the hub clamping structure can bear based on the power spectrum of the main reducer and the load spectrum of the rotor. When performing static strength design, the limiting load of the main reducer rotor shaft 3 is calculated based on the power spectrum of the main reducer and the load spectrum of the rotor. When performing fatigue strength design, the maximum fatigue test load of the main reducer rotor shaft is calculated based on the load spectrum of the rotor.

[0064] In step S2, calculating the limit load includes the following steps:

[0065] S211. The rotor aerodynamic load generated by the blades is transmitted to the main reducer rotor shaft 3 through the hub clamping structure. The load item of the limiting load is the same as the rotor aerodynamic load. The components of the helicopter rotor aerodynamic load include lift load Fz, longitudinal force load Fx, lateral force load Fy, rolling moment load Mx, pitching moment load My and torque load Mz. Among them, the center O of the coordinate system of the rotor aerodynamic load is located at the center of the rotor hub, the coordinate axis OZ is positive along the main reducer rotor shaft, the coordinate axis OX is located in the longitudinal plane of the fuselage, perpendicular to the coordinate axis OZ and points to the tail, and the coordinate axis OY is determined by the right-hand rule.

[0066] S212. Torque load Mz in the limit load: Based on the power spectrum of the main reducer, select the maximum power state in the power spectrum to obtain the steady-state maximum power value transmitted by the main reducer rotor shaft. Combined with the minimum rotor speed allowed within the operating range of the helicopter rotor, calculate the maximum steady-state torque value Mz0 that occurs during the operation of the main reducer rotor shaft. Set the maximum torque fluctuation coefficient of the limit load to 2.5 to obtain the torque value Mz = 2.5 × Mz0 of the main reducer rotor shaft limit load.

[0067] S213. Lift load Fz in the limit load: Based on the rotor load spectrum, select the maximum static lift load value Fz0 in the load spectrum, set the maximum lift load fluctuation coefficient of the limit load to 1.5, and obtain the lift load value Fz = 1.5 × Fz0 of the main reducer rotor shaft limit load.

[0068] S214. Shear load Fxy in the limit load: Based on the rotor load spectrum, convert the longitudinal force load Fx and the lateral force load Fy in the load spectrum into a comprehensive shear load F0. Select the maximum comprehensive shear load value F0, and set the maximum shear load fluctuation coefficient of the limit load to 1.5. The shear load value Fxy of the main reducer rotor shaft limit load is obtained as Fxy = 1.5 × F0. Considering that the main reducer rotor shaft continues to rotate during operation, while the direction of the shear load Fxy remains unchanged, the shear load Fxy in the limit load is actually the rotational shear load Fxy of the main reducer rotor shaft.

[0069] S215. Bending moment load Mxy in the limit load: Based on the rotor load spectrum, convert the rolling moment load Mx and pitching moment load My in the load spectrum into a comprehensive bending moment load Mxy. Select the maximum comprehensive bending moment load value M0, set the maximum bending moment load fluctuation coefficient of the limit load to 1.5, and obtain the bending moment load value Mxy = 1.5 × M0 for the main reducer rotor shaft limit load. Considering that the main reducer rotor shaft continues to rotate during operation, while the direction of the bending moment load Mxy remains unchanged, the bending moment load Mxy in the limit load is actually the bending moment load Mxy of the main reducer rotor shaft.

[0070] In step S2, calculating the fatigue test load includes the following steps:

[0071] S221. The purpose of fatigue test is to obtain the fatigue resistance of the main reducer rotor shaft. When calculating the fatigue test load, it is necessary to clarify the load type that causes fatigue damage to the main reducer rotor shaft. For the fatigue test of the main reducer rotor shaft, the fatigue characteristic loads are the rotational shear load Fxy' and the rotational bending moment load Mxy', while the lift load Fz' and the torque load Mz' are the background loads of the main reducer rotor shaft.

[0072] Rotational shear load Fxy' and rotational bending moment load Mxy' in fatigue test load: Fatigue test of main reducer rotor shaft is carried out. The present invention adopts the method of step-by-step load lifting. First, the first-stage fatigue test load is determined according to the load spectrum of main reducer rotor shaft. Assuming that the first-stage fatigue test load and the appropriate number of test cycles (usually the number of test cycles is 300,000 times) are successfully completed, a suitable shaft section section is selected from the shaft section between the hub center and the bearing support of the main reducer rotor shaft to evaluate the fatigue life of the main reducer rotor shaft. The method of theoretical mechanics is adopted according to the main reducer rotor shaft hub. The centrally applied fatigue test load can be used to calculate the axial force load Fz', bending moment load Mxy', and torque load Mz' of the shaft section. The bending moment load Mxy' is then used as the fatigue life load. Combined with the SN shape curve of the main reducer rotor shaft material, the fatigue limit and the number of cycles corresponding to the bending moment load Mxy' after the first-stage fatigue test are calculated. Using Miner's cumulative damage law, the fatigue damage caused by the first-stage fatigue test of the main reducer rotor shaft is calculated based on the ratio of the number of test cycles to the number of cycles in the life cycle. If the main reducer rotor shaft fails after the first-stage fatigue test, the fatigue damage of the main reducer rotor shaft is 1. Fatigue failure of the main reducer rotor shaft requires multiple fatigue test loads. For each fatigue test load, the lift load Fz' and torque load Mz' remain unchanged, while only the load levels of the rotational shear load Fxy' and rotational bending moment load Mxy' are increased. The load increase ratio for each load level is 10%-20%. According to the fatigue life index of the main reducer rotor shaft, the first-level fatigue test was completed, and the fatigue life of the main reducer rotor shaft was pre-assessed to be 500 hours. The second-level fatigue test was completed, and the fatigue life of the main reducer rotor shaft was pre-assessed to be 2000 hours. After completing all multi-level fatigue tests, the main reducer rotor shaft met the fatigue life index.

[0073] The present invention adopts the metal material fatigue four-parameter S-N curve combined with Miner's cumulative damage law to evaluate the fatigue life value of the main reducer upper casing.

[0074] The basic expression of the four-parameter S-N curve of metal material fatigue is as follows:

[0075]

[0076] Where: S is the dynamic load of the symmetrical cycle, unit is Newton;

[0077] S L is the fatigue limit, which is equal to the fatigue strength at a life of 109 cycles, in Newtons;

[0078] N is the number of fatigue life cycles corresponding to the load level S;

[0079] A, C, B—material constants.

[0080] Since the fatigue test load of the main reducer rotor shaft has multiple load levels, in order to comprehensively consider the influence of each load level, it is necessary to convert each load level in the multi-level fatigue test load into an equivalent symmetric cyclic load (the static load of the symmetric cyclic load is zero, only dynamic load) using the Sorderberg method at the comprehensive bending moment load cycle (the load cycle includes static load and dynamic load) of the selected main reducer rotor shaft fatigue life calculation section. The Sorderberg conversion formula is as follows:

[0081] σ -1 =σ a / (1-σ m / σ s ) where: -1 is the equivalent symmetrical cyclic load in Newtons;

[0082] σ a is the dynamic value of the test load cycle, in Newtons;

[0083] σ m is the static value of the test load cycle, in Newtons;

[0084] σ s is the yield load in Newtons.

[0085] Symmetrical cyclic load σ -1 The four-parameter S-N curve of metal material fatigue can be substituted and, combined with the number of test cycles of each level of load of the main reducer rotor shaft, the fatigue limit of the main reducer rotor shaft under each level of test load can be solved.

[0086] According to Miner's cumulative damage law, a component is considered to have lost its ability to function normally when the cumulative damage at each load level equals 1. Because the fatigue test of the main reducer rotor shaft involves multiple load levels, the fatigue limit calculated using the four-parameter S-N curve for metal material fatigue will produce damage at each load level of the main reducer rotor shaft load spectrum. When the cumulative damage of the load spectrum reaches a specified value, the fatigue limit obtained from the main reducer rotor shaft fatigue test can be assessed as reasonable, and the fatigue life of the main reducer rotor shaft has reached the specified life indicator. This allows the maximum load of the main reducer rotor shaft fatigue test to be determined.

[0087] The calculation formula of Miner's cumulative damage law is as follows:

[0088]

[0089] The calculation formula for fatigue life is as follows:

[0090]

[0091] Where: n i is the actual number of use cycles of the i-th level load within unit time T;

[0092] N i is the life cycle number of the part when only the i-th level load is applied, which is calculated through the SN curve;

[0093] D is the cumulative damage of the parts.

[0094] S222. Calculating the lift load Fz' and torque load Mz' in the fatigue test load includes the following steps: Since the lift load Fz' and torque load Mz' are the background loads of the fatigue test load of the main reducer rotor shaft, the lift load Fz' and torque load Mz' with the largest values ​​in each state in the load spectrum are selected based on the load spectrum of the main reducer rotor shaft. Taking into account the actual situation during helicopter flight, the lift load Fz' with the largest value in each state in the load spectrum is used as the static lift load in the fatigue test load. The torque load Mz' with the largest value in each state in the load spectrum is selected as the static torque load in the fatigue test load. Taking into account the torque fluctuation coefficient of 10%, 10%×Mz' is taken as the dynamic torque load in the fatigue test load.

[0095] When conducting fatigue testing on the main reducer rotor shaft, the test load loading scheme includes the following steps:

[0096] S231. During the fatigue test, the static load in the lift load Fz' is applied evenly to the four vertical loading channels 101 on the loading plate 100 with a fillet phase of 90 degrees. The lift load Fz' applied by the four vertical loading channels 101 remains unchanged during the test.

[0097] S232. During the fatigue test, a dynamic load in the shear load Fxy' is applied as a fluctuating load that varies according to a cosine law to two mutually perpendicular first horizontal loading channels 102 on the loading plate 102. During the test, the combined shear load F0' of the two mutually perpendicular first horizontal loading channels 102 remains constant, and the direction of the combined shear load F0' rotates 360° about the axis of the main reducer rotor shaft.

[0098] S233. During the fatigue test, a dynamic load within the bending moment load Mxy' is applied as a fluctuating load that varies according to a cosine law to four vertical loading channels 101 on the loading plate 100, each with a 90° fillet phase. During the test, the combined bending moment load M0' on the four vertical loading channels 101 remains constant, and the direction of the combined bending moment load M0' rotates 360° around the axis of the main reducer rotor shaft.

[0099] S234. When carrying out fatigue test, the load directions of the two second horizontal loading channels 103 are opposite. The static load in the torque load Mz' is first applied to the two second horizontal loading channels 103 arranged parallel to each other on the loading disk 100, and then the fluctuating load of the dynamic load that changes according to the cosine law is applied to the two second horizontal loading channels 103. The torque load value generated by the load fluctuation is equal to the dynamic load of the torque load Mz'. The period of static load fluctuation and dynamic load fluctuation of the torque load Mz' is the same as the period of phase change of the shear load Fz' and the bending moment load Mxy'.

[0100] In step S3, the calculation of the friction force Fμ of the main reducer rotor shaft hub clamping structure includes the following steps: due to the pre-tightening force of a circle of hub clamping screws 8, friction exists between the upper conical ring 5, the lower conical ring 4, the anti-loosening plate 7, the hub center piece 2 and the main reducer rotor shaft 3 of the main reducer rotor shaft. Various friction forces will produce the effect of offsetting the torque of the main reducer rotor shaft. After calculating the torque load of the limit load and the maximum fatigue test load of the main reducer rotor shaft, it is necessary to calculate the torque generated by the friction force of the hub clamping structure. The present invention proposes a nonlinear finite element method to calculate the friction torque M generated by the friction force Fμ. Fμ A finite element model was established in the finite element software ANSYS. The geometric model included all parts of the main reducer rotor shaft hub. According to the actual assembly situation, a "Fiction" contact unit was established on the contact surface between the upper conical ring 5, the lower conical ring 4, the anti-loosening plate 7, the hub center piece 2 and the main reducer rotor shaft 3. The friction coefficient was set to 0.2. Then, the clamping load Qp of the hub clamping screw 8 and the limit load and maximum fatigue test load were applied. The friction torque M generated by the friction force Fμ under the limit load and maximum fatigue test load was obtained through the support and reaction load solver of the post-processing module of the finite element software ANSYS. Fμ Through analysis and verification, the present invention proposes that the friction torque M generated by the friction force Fμ Fμ To reach the limit load, maximum fatigue test load of 1 / 3 times the maximum torque load value Mz max , that is, the friction force Fμ generated by the pre-tightening force of the hub clamping screw 8 needs to meet the above conditions.

[0101] In step S4, when verifying whether the diameter, number and tightening torque of the hub clamping screws 8 of the main reducer rotor shaft meet the use requirements, the following steps are specifically included:

[0102] S41. Design loads and load transfer paths

[0103] S411、Rotor aerodynamic loads

[0104] The components of a helicopter rotor aerodynamic loads include lift load Fz, longitudinal force load Fx, lateral force load Fy, rolling moment load Mx, pitching moment load My, and torque load Mz. The center O of the rotor aerodynamic load coordinate system is located at the center of the rotor hub. Coordinate axis OZ is positive upward along the main reducer rotor shaft. Coordinate axis OX lies in the longitudinal plane of the fuselage, perpendicular to coordinate axis OZ and pointing toward the tail. Coordinate axis OY is determined using the right-hand rule. During the design and testing of the main reducer rotor shaft, the hub clamping structure must withstand the rotor aerodynamic load limit and the maximum load of the fatigue test.

[0105] S412, assembly load

[0106] The assembly load of the hub pressing structure comes from the preload generated by the tightening torque of the hub pressing screw 8 and the preload generated by the tightening torque of the anti-loosening plate fixing bolt 9.

[0107] S413, load transfer path

[0108] The hub clamping structure is the only load transfer path for the rotor aerodynamic load to be transmitted to the rotor shaft of the main reducer. Therefore, the high reliability of the hub clamping structure must be ensured to avoid design risks.

[0109] The assembly load is derived from the preload generated by the tightening torque of the hub clamping screw 8 and the preload generated by the tightening torque of the anti-loosening plate fixing bolt 9.

[0110] During the design phase, the design loads that determine the diameter, number, and tightening torque of hub hold-down screws 8 are the rotor aerodynamic limit load and the maximum hub load for fatigue testing. The rotor aerodynamic limit load is a design input provided by the main engine and can be used directly. However, the maximum hub load for fatigue testing requires an estimate of the maximum fatigue test load based on the fatigue life design specifications, based on the main drive rotor shaft design load spectrum.

[0111] For the assembly structure of a certain type of main reducer rotor shaft hub (see Figure 1), the aerodynamic load of the rotor comes from the rotor blades, and the aerodynamic load on the blades is transmitted to the main reducer rotor shaft 3 through the hub central part 2, among which, most of the aerodynamic torque load of the rotor is transmitted to the main reducer rotor shaft 3 through the spline, and a small part of the aerodynamic torque load of the rotor is transmitted to the main reducer rotor shaft 3 through the friction force of the contact surface of the upper conical ring 5 and the lower conical ring 4; the lift load Fz, longitudinal force load Fx, lateral force load Fy, rolling moment load Mx and pitching moment load My of the rotor are transmitted to the main reducer rotor shaft 3 through the assembled upper conical ring 5 and lower conical ring 4; the assembly load comes from the tightening torque of the hub clamping screw 8 The axial compression load generated by the hub clamping screw 8 acts on the clamping nut 1 and the bearing plate 6, among which the axial tensile load borne by the clamping nut 1 is transmitted to the main reducer rotor shaft 3 through the thread, and the bearing plate 6 transmits the axial compression load to the upper conical ring 5, and the upper conical ring 5 transmits the axial compression load to the lower conical ring 4 through the hub central part 2, and the lower conical ring 4 transmits the axial compression load to the main reducer rotor shaft 3 through the conical surface, finally realizing the load balance of the main reducer rotor shaft 3; the axial preload generated by the tightening torque of the anti-loosening plate fixing bolt 9 acts on the clamping nut 1, ensuring that the anti-loosening plate 7 and the clamping nut 1 do not separate under normal working conditions.

[0112] S414, Design Load

[0113] Considering that the main reducer rotor shaft 3 maintains the rated speed during the operation of the rotor, the rotor aerodynamic shear force and bending moment load act on the main reducer rotor shaft 3 in the form of rotating shear force and bending moment load. Figure 1 ), the limiting load of the main reducer rotor shaft is shown in Table 1.

[0114] Table 1 Limit loads on the rotor shaft of a certain type of main reducer

[0115] Lift load Fz(N) Rotational shear load F(N) Rotational bending moment load M (Nm) Torque load Mz (Nm) 102900 7546 74069 25008

[0116] According to the design load spectrum of the main reducer rotor shaft, the maximum load of the main reducer rotor shaft fatigue test was estimated according to the fatigue life design index. Since the fatigue characteristic loads of the main reducer rotor shaft are the shear force and bending moment loads of the rotor load, the maximum shear force and bending moment loads of the main reducer rotor shaft fatigue test were calculated according to the S-N shape curve parameters of the main reducer rotor shaft material and a fatigue life of 15,000 hours. The maximum lift and torque loads of the fatigue test were selected from the maximum load in the design load spectrum, among which the load fluctuation coefficient of the torque load was considered to be 10%. The specific loads are shown in Table 2.

[0117] Table 2 Maximum load of fatigue test on rotor shaft of a certain type of main reducer

[0118] Load Item Static load Dynamic loads Lift load Fz(N) 61000 0 Rotational shear load F(N) 0 11161 Rotational bending moment load M (Nm) 0 91635 Torque load Mz (Nm) 17500 1750

[0119] The basic equation of the SN shape curve is as follows:

[0120] S=S L [H+A×(N+C) -B ]

[0121] in:

[0122] S is the dynamic load or stress;

[0123] S L is the fatigue limit of the material, corresponding to a stress level with a cycle number of not less than 107;

[0124] N is the number of life cycles;

[0125] A, B, C, and H are fatigue performance constants.

[0126] In the early stage of design, the diameter, number and tightening torque of the main reducer rotor shaft hub clamping screw 8 and the anti-loosening plate fixing bolt 9 are determined based on the main reducer rotor shaft hub structure and experience. According to the tightening torque value, the preload of the main reducer rotor shaft hub clamping screw 8 and the anti-loosening plate fixing bolt 9 is converted. The tightening torque calculation formula of the clamping screw 8 and the anti-loosening plate fixing bolt 9 is as follows:

[0127]

[0128] Where: T is the tightening torque, Nm; Qp is the preload, unit is Newton;

[0129] Ψ is the thread lead angle;

[0130] ρ' is the equivalent friction angle of the thread pair, usually ρ' = arctg1.155f, where f is the friction coefficient of the thread pair;

[0131] d2 is the thread pitch diameter, in mm;

[0132] f e The friction coefficient of the combined contact surfaces of the nut and the connected part or washer;

[0133] D1 is the outer diameter of the nut bearing surface, in mm;

[0134] d0 is the diameter of the connected part or gasket hole, in mm;

[0135] The above formula can also be rewritten as

[0136] T=kQ p d

[0137] Where k is the tightening torque coefficient.

[0138]

[0139] In order to check whether the tightening torque value of the screw is appropriate, it is necessary to check the strength of the thread. The calculation formula for the thread strength of the main reducer rotor shaft 3, the hub clamping screw 8, the anti-loosening plate fixing bolt 9 and the clamping nut 1 is as follows:

[0140]

[0141] Where, τ is the thread shear strength, unit is MPa;

[0142] F is the axial load of the thread, in Newtons;

[0143] S is the safety factor, which is 1.5;

[0144] b is the thread root width, for common threads, b = 0.87 × P, where P is the pitch in mm;

[0145] z is the number of rotations, z = L / P, L is the rotation length;

[0146] k Z In order to consider the coefficient of uneven force on each thread tooth, k is taken Z =0.625.

[0147]

[0148] Where, σw is the bending strength of the thread, in MPa;

[0149] F is the axial load of the thread, in Newtons;

[0150] S is the safety factor, which is 1.5;

[0151] h is the working height of the thread, in mm;

[0152] b is the thread root width, for common threads, b = 0.87 × P, where P is the pitch in mm;

[0153] z is the number of rotations, z = L / P, L is the rotation length;

[0154] k Z In order to consider the coefficient of uneven force on each thread tooth, k is taken Z =0.625.

[0155] S42, determine the number and tightening torque of the hub clamping screws 8

[0156] Considering that the main reducer rotor shaft maintains its rated speed during rotor operation, it is subject to rotational shear and bending moment loads. To simulate the loads on the main reducer rotor shaft, the aforementioned helicopter transmission system main reducer rotor shaft fatigue test load simulation loading device was utilized during the main reducer rotor shaft fatigue test.

[0157] According to the test load loading scheme of the main reducer rotor shaft fatigue test, the reason why the tightening torque of the hub clamping screw 8 disappears or increases is that during the test, the axial clamping load of the bottom of the hub clamping screw 8 and the bearing plate changes. When there is a separation gap between the contact surfaces, the large clamping nut rotates under the fluctuation of the torque load. If the thread direction of the large clamping nut is the same as or opposite to the rotation direction of the rotational shear force and bending moment load, the tightening torque of the hub clamping screw 8 disappears or increases.

[0158] Through the load path transfer analysis of the hub clamping structure, the upper conical ring 5 and the lower conical ring 4 are the key paths for transmitting the rotor load to the main reducer rotor shaft 3. They not only transmit all lift loads, shear loads and bending moment loads, but also need to transmit part of the torque load. The upper conical ring 5 in contact with the bearing plate 6 significantly affects the tightening torque of the hub clamping screw 8.

[0159] Taking the load on the bearing plate 6 and the upper conical ring 5 as the research object, the axial load is as follows:

[0160] The vertical upward axial load generated by the lift load (four vertical loading channels 101); the vertical downward axial compression load Qp generated by the hub clamping screw 8 (one circle of hub clamping screws 8); the vertical axial load generated by the bending moment load (four vertical loading channels 101).

[0161] According to the stress analysis of the bearing plate, considering that the phase of the rotational bending moment changes during the fatigue test, in order to simplify the analysis model, the moment when the phase direction of the rotational bending moment is perpendicular to the cross-section evenly divided by the hub clamping screws 8 of the bearing plate is extracted (when the number of hub clamping screws 8 is odd, there is a hub clamping screw 8 located at the cross-section position; when the number of hub clamping screws 8 is even, the number of hub clamping screws 8 on both sides of the cross-section is the same). At this time, among the rotational bending moments applied by the four vertical loading channels, two vertical loading channels apply vertical The upward axial force load is applied by the two vertical loading channels, and the vertical downward axial force load is applied by the two vertical loading channels; at the same time, considering the lift load applied by the four vertical loading channels, the axial force load applied by the two vertical loading channels on one side is reduced, while the axial force load applied by the two vertical loading channels on the other side is increased; for the hub clamping screw 8, when the external load transmitted by the half-circle conical ring is in the same direction as the clamping load, the tightening torque of the hub clamping screw 8 in contact with the bearing plate 6 changes, and at the same time, under the action of the torque load, the clamping nut 1 rotates circumferentially.

[0162] Based on the above theoretical analysis and experimental verification, the present invention proposes the following criteria for determining whether the tightening torque value of the hub clamping screw 8 does not change:

[0163]

[0164] Where: Fb is the residual compressive load borne by the half-circle bearing plate, in Newtons;

[0165] F Z1 is the axial force load generated by the rotor bending moment load at the radius of the load center of the bearing plate, in Newtons, where Mxy is the rotor bending moment load, in Nm, D is the diameter of the load-bearing plate at the load center, in mm;

[0166] F Z2 is the lift load evenly distributed to the four vertical loading channels, in Newtons, where Fz is the rotor lift load, in Newtons;

[0167] F Z3 is the compression load generated by the hub compression screw 8 borne by the half-circle bearing plate, in Newtons. When the number n of hub compression screws 8 is an odd number, When the number n of the hub tightening screws 8 is an even number, Qp is the compression load of a single hub compression screw 8, in Newtons;

[0168] M Fμ is the friction torque generated by the compression load Qp of the hub compression screw 8;

[0169] Mz maxis the larger value of the torque load Mz in the limit load and the torque load Mz' in the maximum fatigue test load;

[0170] When the number and tightening torque of the hub clamping screws 8 meet the judgment criteria, and the strength of the threads at each location of the hub structure meets the design, it meets the usage requirements; if the number and tightening torque of the hub clamping screws 8 do not meet the judgment criteria, increase the number and / or tightening torque of the hub clamping screws 8, repeat steps S1 to S3, and then judge again until it meets the usage requirements.

[0171] In this embodiment, the calibration data is shown in Table 3.

[0172] Table 3 Tightening torque verification data of the hub clamping screw of a certain type of main reducer rotor shaft

[0173]

[0174]

[0175] For the assembly structure of a certain type of main reducer rotor shaft hub, 12 MJ12×1.25 hub clamping screws 8 are initially set, and the tightening torque value of each hub clamping screw 8 is 35.5Nm.

[0176] According to the limit load of the main reducer rotor shaft 3, the residual clamping load of the half-circle bearing plate is calculated to be -113863N. According to the maximum fatigue test load of the main reducer rotor shaft 3, the residual clamping load of the half-circle bearing plate is calculated to be -216515N. This shows that the tightening torque value of the hub clamping screw 8 is too small, resulting in circumferential rotation of the clamping nut. Therefore, the anti-loosening plate is found to be broken, and the groove and the clamping nut on the main reducer rotor shaft are worn.

[0177] The tightening torque of each hub clamping screw 8 is adjusted to 80 Nm according to the calculated value, and the thread strength at each location of the hub structure meets the design requirements.

[0178] According to the limit load of the main reducer rotor shaft 3, the residual clamping load of the half-circle bearing plate is calculated to be 110761N. According to the maximum fatigue test load of the main reducer rotor shaft 3, the residual clamping load of the half-circle bearing plate is calculated to be 8109N. The test verified that the tightening torque value of the hub clamping screw 8 did not change, and the clamping nut 1 did not rotate circumferentially.

[0179] The tightening torque design method of the hub clamping screw 8 of the present invention realizes the precise design of the diameter, number and tightening torque value of the hub clamping screw 8 on the main reducer rotor 3 of the helicopter transmission system. It can solve the problem that the tightening torque of the hub clamping screw 8 may disappear or increase during the field scientific research test flight and strength test of the main reducer rotor shaft 3, resulting in abnormal assembly status of the main reducer rotor shaft 3 and the rotor hub central part 2, micro-wear in the contact area, and the risk of damage to parts in the assembly area. It can improve the reliability of the connection structure.

[0180] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for designing the tightening torque of a hub compression screw, characterized in that: A load simulation loading device is used, the load simulation loading device comprising a loading plate (100), a hub central piece (2) arranged on the loading plate (100), a main reducer rotor shaft (3) and a hub clamping structure for connecting the hub central piece (2) and the main reducer rotor shaft (3), the hub clamping structure comprising a clamping large nut (1), a lower conical ring (4), an upper conical ring (5), a bearing plate (6), an anti-loosening plate (7), a hub clamping screw (8) and an anti-loosening plate fixing bolt (9), the The loading disk (100) is provided with four vertical loading channels (101) with a fillet phase of 90 degrees, two first horizontal loading channels (102) arranged perpendicular to each other, and two second horizontal loading channels (103) arranged parallel to each other. The vertical loading channels (101) are used to simulate the lift load and bending moment load applied by the rotor, the first horizontal loading channels (102) are used to simulate the shear load applied by the rotor, and the second horizontal loading channels (103) are used to simulate the torque load applied by the rotor. The method comprises the following steps: S1. Set the initial number and tightening torque of the hub compression screws; S2. Determine the maximum torque load Mz that the hub clamping structure of the main reducer rotor shaft can withstand. max , calculate the limit load of the main reducer rotor shaft based on the power spectrum of the main reducer and the load spectrum of the rotor, calculate the maximum fatigue test load of the main reducer rotor shaft based on the load spectrum of the rotor, the larger value of the torque load between the limit load and the maximum fatigue test load of the main reducer rotor shaft is the maximum torque load Mz that the hub clamping structure can withstand max ; S3. Calculate the friction torque M generated by the friction force Fμ of the hub pressing structure Fμ ; S4. Verify whether the number and tightening torque of the hub clamping screws of the main reducer rotor shaft meet the use requirements. If not, increase the number and / or tightening torque of the hub clamping screws and repeat steps S1 to S3. In step S4, the determination conditions for verifying whether the number and tightening torque of the hub clamping screws of the main reducer rotor shaft meet the use requirements are as follows: ； Where: Fb is the residual compressive load borne by the half-circle bearing plate, in Newtons; F Z1 is the axial force load generated by the rotor bending moment load at the radius of the load center of the bearing plate, in Newtons; F Z2 is the lift load evenly distributed to the four vertical loading channels, in Newtons; F Z3 The unit is Newton, which is the compression load generated by the hub compression screw borne by the half-circle bearing plate; M Fμ is the friction torque generated by the compression load Qp of the hub compression screw, in N·m; Mz max It is the larger value of the torque load Mz in the limit load and the torque load Mz' in the maximum fatigue test load, and the unit is N·m.

2. The method for designing the tightening torque of the hub compression screw according to claim 1, characterized in that: In step S2, calculating the limit load specifically includes the following steps: S211. Assume that the center O of the coordinate system for the rotor aerodynamic loads is located at the center of the rotor hub, that the coordinate axis OZ is upwardly positive along the main reducer rotor axis, that the coordinate axis OX is located in the longitudinal plane of the fuselage and is perpendicular to the coordinate axis OZ and points toward the tail, and that the coordinate axis OY is determined using the right-hand rule. Calculate the lift load Fz, longitudinal force load Fx, lateral force load Fy, rolling moment load Mx, pitching moment load My, and torque load Mz of the limiting loads, respectively. S212. Calculate the torque load Mz within the limit load. Based on the power spectrum of the main reducer, select the maximum power state in the power spectrum to obtain the steady-state maximum power value transmitted by the main reducer rotor shaft. Combined with the minimum rotor speed allowed within the operating range of the helicopter rotor, calculate the maximum steady-state torque value Mz0 of the main reducer rotor shaft during operation. Set the maximum torque fluctuation coefficient of the limit load to 2.5 to obtain the torque value of the main reducer rotor shaft limit load: Mz=2.5×Mz0. S213. Calculate the lift load Fz in the limit load. Based on the rotor load spectrum, select the maximum static lift load value Fz0 in the load spectrum. Set the maximum lift load fluctuation coefficient of the limit load to 1.5 to obtain the lift load value of the main reducer rotor shaft limit load Fz=1.5×Fz0. S214. Calculate the shear load Fxy in the limit load. Based on the rotor load spectrum, convert the longitudinal force load Fx and the lateral force load Fy in the load spectrum into a comprehensive shear load F0. Select the maximum comprehensive shear load value F0. Set the maximum shear load fluctuation coefficient of the limit load to 1.5 to obtain the shear load value of the main reducer rotor shaft limit load: Fxy = 1.5 × F0. S215. Calculate the bending moment load Mxy in the limit load. Based on the load spectrum of the rotor, convert the rolling moment load Mx and the pitching moment load My of the load spectrum into the comprehensive bending moment load M0. Select the maximum comprehensive bending moment load value M0. Set the maximum bending moment load fluctuation coefficient of the limit load to 1.5 to obtain the bending moment load value Mxy=1.5×M0 of the main reducer rotor shaft limit load.

3. The method for designing the tightening torque of the hub compression screw according to claim 2, characterized in that: In step S2, calculating the fatigue test load specifically includes the following steps: S221. Determine the shear load Fxy' and bending moment load Mxy' in the fatigue test load, conduct fatigue testing on the main reducer rotor shaft, determine the first-stage fatigue test load based on the load spectrum of the main reducer rotor shaft, and after successfully completing the first-stage fatigue test load and the preset number of test cycles, select a preset shaft section between the hub center and the bearing support of the main reducer rotor shaft to evaluate the fatigue life of the main reducer rotor shaft. Using theoretical mechanics methods, calculate the fatigue test load applied to the hub center of the main reducer rotor shaft. The axial force load Fz', bending moment load Mxy' and torque load Mz' of the shaft section are obtained. Then, the bending moment load Mxy' is used as the fatigue life load. Combined with the SN shape curve of the main reducer rotor shaft material, the fatigue limit and the life cycle number corresponding to the bending moment load Mxy' after the main reducer rotor shaft completes the first-stage fatigue test are calculated. Using Miner's cumulative damage law, the fatigue damage caused by the first-stage fatigue test of the main reducer rotor shaft is calculated based on the ratio of the test cycle number and the life cycle number of the first-stage fatigue test of the main reducer rotor shaft. S222. Determine the lift load Fz' and torque load Mz' in the fatigue test load. Based on the load spectrum of the main reducer rotor shaft, select the maximum lift load Fz' and torque load Mz' in each state of the load spectrum. Taking into account the actual situation during helicopter flight, use the maximum lift load Fz' in each state of the load spectrum as the static lift load in the fatigue test load, and select the maximum torque load Mz' in each state of the load spectrum as the static torque load in the fatigue test load.

4. The method for designing the tightening torque of the hub compression screw according to claim 3, characterized in that: Step S221 further includes the following steps: S2211: When fatigue failure of the main reducer rotor shaft requires multiple fatigue test loads, the lift load Fz' and torque load Mz' of each fatigue test load level remain unchanged, and only the load levels of the rotational shear load Fxy' and rotational bending moment load Mxy' are increased. The load increase ratio for each load level is 10%-20%. S2212. According to the fatigue life index of the main reducer rotor shaft, complete the first-stage fatigue test, pre-assess the fatigue life of the main reducer rotor shaft to be 500 hours, complete the second-stage fatigue test, pre-assess the fatigue life of the main reducer rotor shaft to be 2000 hours, complete all multi-stage fatigue tests, and the main reducer rotor shaft reaches the fatigue life index. Combined with the SN shape curve of the main reducer rotor shaft material, calculate the fatigue limit of the main reducer rotor shaft after completing the multi-stage fatigue test and the life cycle number corresponding to the bending moment load Mxy'. Apply Miner's cumulative damage law, and based on the ratio of the test cycle number and the life cycle number of the multi-stage fatigue test of the main reducer rotor shaft, calculate the fatigue damage caused by the multi-stage fatigue test of the main reducer rotor shaft.

5. The method for designing the tightening torque of a hub compression screw according to claim 3 or 4, characterized in that: When conducting fatigue testing on the main reducer rotor shaft, the test load loading scheme includes the following steps: S231, lift load Fz' of fatigue test, the static load in lift load Fz' is applied evenly to four vertical loading channels with a fillet phase of 90°, and the lift load Fz' applied by the four vertical loading channels remains unchanged during the test; S232. Fatigue test shear load Fxy': A dynamic load in the shear load Fxy' is applied as a fluctuating load that varies according to a cosine law to two mutually perpendicular first horizontal loading channels. During the test, the combined shear load F0' of the two mutually perpendicular first horizontal loading channels remains constant, and the direction of the combined shear load F0' rotates 360° about the axis of the main reducer rotor shaft. S233. Fatigue test bending moment load Mxy': The dynamic load in the bending moment load Mxy' is applied as a fluctuating load that varies according to the cosine law to four vertical loading channels with a 90° fillet phase. During the test, the combined bending moment load M0' of the four vertical loading channels remains unchanged, and the direction of the combined bending moment load M0' rotates 360° around the axis of the main reducer rotor shaft. S234. The torque load Mz' of the fatigue test has opposite load directions on the two second horizontal loading channels. The static load in the torque load Mz' is first applied to the two second horizontal loading channels arranged parallel to each other, and then the fluctuating load that changes according to the cosine law is applied to the two second horizontal loading channels. The torque load value generated by the load fluctuation is equal to the dynamic load of the torque load Mz'. The period of the static load fluctuation and the dynamic load fluctuation of the torque load Mz' is the same as the period of the phase change of the shear load Fz' and the bending moment load Mxy'.

6. The method for designing the tightening torque of the hub compression screw according to claim 5, characterized in that: In step S3, calculating the friction force Fμ of the hub pressing structure specifically includes the following steps: The nonlinear finite element method is used to calculate the torque M generated by the friction force Fμ of the hub pressing structure. Fμ A finite element model was established in the finite element software ANSYS. The geometric model includes all parts of the main reducer rotor shaft hub. According to the actual assembly situation, the contact surface between the upper conical ring, lower conical ring, anti-loosening plate, hub center piece and the main reducer rotor shaft was established as a "Fiction" contact unit. The friction coefficient was set to 0.

2. Then, the clamping load Qp of the hub clamping screw and the limit load and maximum fatigue test load were applied. The torque M generated by the friction force Fμ under the limit load and maximum fatigue test load was obtained through the support and reaction load solver of the post-processing module of the finite element software ANSYS. Fμ .

7. The method for designing the tightening torque of a hub compression screw according to claim 6, characterized in that: The number of test cycles for the first level fatigue test load is 200,000 to 500,000 times.

8. The method for designing the tightening torque of a hub compression screw according to claim 6, characterized in that: The torque fluctuation coefficient is set to 10%, and 10%×Mz' is taken as the dynamic torque load in the fatigue test load.

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