A method for determining and verifying the heat assembly temperature of an interference fit gear based on the control of the root residual stress

Abstract

This application discloses a method for determining the heat fitting temperature and verifying the process of interference fit gears based on residual stress control at the tooth root, relating to the field of gear assembly technology. This method overcomes the limitation of existing heat fitting processes that only consider assembly feasibility as a single objective. For the first time, it uses the residual stress state in the transition fillet region at the tooth root of hardened gears as the core constraint for determining the heat fitting temperature, establishing a correspondence between heat fitting temperature and changes in residual stress at the tooth root. Through bidirectional multi-gradient temperature verification, dual-criteria screening, and closed-loop process control, it effectively avoids the problem of residual compressive stress relaxation caused by excessively high temperatures in traditional empirical methods, improves the bending fatigue strength and reliability of gears, and solves the potential fatigue failure of high-speed, heavy-load gears caused by improper heat fitting processes. It forms a scientific, systematic, and replicable standardized process verification procedure.

Description

Technical Field

[0001] This application belongs to the field of gear assembly technology, and in particular relates to a method for determining the heat fitting temperature and verifying the process of interference fit gears based on residual stress control at the tooth root. Background Technology

[0002] In high-speed, heavy-duty gear transmission systems, such as wind turbine gearboxes and construction machinery gearboxes, interference fits are commonly used to achieve reliable connections between gears and shafts. Compared to keyed connections, interference fits avoid stress concentration and fretting wear problems associated with keyways, can transmit greater torque, and significantly improve the load-bearing capacity, operational stability, and service life of the transmission system. The heat-fitting method (heated enclosure) is currently the most widely used process in interference fit gear assembly due to its simplicity and high efficiency.

[0003] Currently, the conventional method in the industry for determining the heat-fitting temperature of gears mainly relies on the design interference. The theoretical heating temperature required to generate the minimum assembly clearance is calculated using the thermal expansion formula, and then an empirical margin of 30-50°C is added to this as the final heat-fitting temperature. This method focuses solely on assembly feasibility, only ensuring that the gear's inner bore expands and can be smoothly fitted onto the shaft, without fully considering the impact of heating temperature on the gear body material properties, especially on the mechanical state of the critical area at the root of hardened gear teeth.

[0004] Hardened gear teeth typically undergo strengthening treatments such as carburizing and quenching, and induction hardening. After carburizing and quenching, high-intensity shot peening is used to increase the compressive stress in the tooth root transition radius, thereby improving the gear's bending fatigue strength. The tooth root transition radius is a weak point in the gear's bending fatigue. Current hot fitting processes use crude temperature selection methods and lack quantitative data. Excessively high heating temperatures may cause unfavorable relaxation or transformation of residual compressive stress on the tooth root surface, significantly reducing the gear's bending fatigue strength.

[0005] Meanwhile, traditional processes rely on experience margins, and temperature selection is highly subjective and has poor repeatability. This makes it difficult to adapt to gears of different materials, specifications, and heat treatment states, which can easily cause batch-to-batch quality fluctuations and also makes it impossible to achieve controllable protection of residual stress at the tooth root during the heat assembly process. Summary of the Invention

[0006] To address some or all of the technical problems existing in the prior art, this application provides a method for determining the heat fitting temperature and verifying the process of interference fit gears based on residual stress control at the tooth root.

[0007] This application provides a method for determining the heat fitting temperature and verifying the process of interference fit gears based on residual stress control at the tooth root, including the following steps: Step S1, Test Design and Sample Preparation: Based on the design interference of the gear and shaft, calculate the theoretical heating temperature required for the theoretical minimum assembly clearance. Set multiple temperature gradients based on the theoretical heating temperature. At least one of the temperature gradients should include a verification temperature that is lower than the theoretical heating temperature required for the theoretical minimum assembly clearance. Prepare verification samples with the same material, batch, and heat treatment process as the product gear. Step S2, Reference Residual Stress Test: The initial surface residual stress in the tooth root transition fillet area of ​​all verification specimens is measured using non-destructive testing methods, and the distribution characteristics are recorded as reference stress data. Step S3, Simulated hot assembly-cooling cycle test: The verification samples are grouped and heated to each set temperature gradient and held at that temperature. The actual assembly environment is simulated for cooling, and the complete temperature-time curve is recorded. Step S4, Residual stress test and comparative analysis after thermal cycling: After the sample cools to room temperature, measure the residual stress at the tooth root after thermal cycling at the exact same position as the benchmark test, and calculate the change in residual stress. Step S5: Determine the optimal hot fitting temperature window: Plot the relationship curve between the hot fitting verification temperature and the change in residual stress at the tooth root. Simultaneously meet the dual criteria of assembly feasibility and residual stress stability, determine the optimal hot fitting temperature window, and compile standardized process specifications. Step S6, Process Verification and Closed-Loop Control: Assemble the actual product according to the prepared process specifications, and conduct spot checks on the residual stress at the tooth root of the assembled product to verify the process stability and form a data closed loop.

[0008] Preferably, in step S1, at least 5 temperature gradients are set by expanding upward and downward in both directions based on the theoretical heating temperature required for the theoretical minimum assembly gap; the interval between adjacent temperature gradients is set to 20°C to 80°C according to the gear material, size and heat treatment state.

[0009] Preferably, in step S1, the verification sample is a complete gear or a sector-shaped sample with at least one complete tooth; the total number of samples is not less than three times the number of temperature gradients.

[0010] Preferably, in steps S2 and S4, the residual stress is measured by X-ray diffraction, and at least three teeth are measured for each sample, with the center of the tooth root and the positions on both sides of each tooth being measured.

[0011] Preferably, in step S5, the assembly feasibility criterion is that the hot assembly temperature is not lower than the minimum safe assembly temperature that can be operated on site; the residual stress stability criterion is that the absolute value of the residual stress change is less than a set threshold, or the residual stress after thermal cycling remains within a preset residual compressive stress range.

[0012] Preferably, the absolute value threshold for the change in residual stress is set to 50 MPa; the preset residual compressive stress range is -300 MPa to -800 MPa.

[0013] Preferably, the interference fit gear is a hardened tooth surface gear that has undergone carburizing and quenching or induction hardening treatment.

[0014] The method for determining the heat fitting temperature and verifying the process of interference fit gears based on residual stress control at the tooth root, as proposed in this application, has the following advantages and positive effects: (1) This invention breaks through the limitation of existing hot fitting processes that only consider assembly feasibility as a single objective. For the first time, it takes the residual stress state in the transition fillet region of the tooth root of hardened gears as the core constraint condition for determining the hot fitting temperature, and establishes the correspondence between the hot fitting temperature and the change of residual stress at the tooth root. Through bidirectional multi-gradient temperature verification, dual criterion screening and closed-loop process control, it effectively avoids the problem of residual compressive stress relaxation caused by excessive temperature in the traditional empirical method, improves the bending fatigue strength and reliability of gears, and solves the potential fatigue failure of high-speed heavy-duty gears caused by improper hot fitting process.

[0015] (2) A scientific, systematic, and replicable standardized process verification procedure has been formed, which can be achieved by optimizing process parameters without any modification to existing production equipment. The implementation cost is low and the results are quick. This method is applicable to interference fit hardened gears of various materials, specifications and heat treatment states. It can provide personalized and precise process parameters for different products, ensuring the consistency of product quality in different batches and greatly reducing quality loss and rework costs. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for further understanding of the embodiments of this application and constitute a part of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 This is a schematic diagram of the interference fit between the shaft and gear in this application.

[0017] Explanation of reference numerals in the attached figures: 1-Shaft, 2-Gear. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0019] like Figure 1 As shown, gear 2 is interference-fitted with shaft 1.

[0020] This application presents a method for determining the hot fitting temperature and verifying the process of interference-fit gears based on residual stress control at the tooth root. The interference-fit gears are hardened gears that have undergone carburizing and quenching or induction hardening. The residual stress state in the transition fillet region at the tooth root of the hardened gear is used as the core constraint condition for determining the hot fitting temperature. The method includes the following steps: Step S1, Test Design and Sample Preparation: Based on the design interference δ between the gear and the shaft, calculate the theoretical heating temperature T0 required for the theoretical minimum assembly clearance, and set multiple temperature gradients T0 as the reference. i Temperature gradient T i The test specimens must include at least one verification temperature lower than the theoretical heating temperature T0 required for the minimum assembly clearance, in order to explore the possibility of low-temperature assembly and to prepare verification specimens made of the same material, batch, and heat treatment process as the product gears.

[0021] Specifically, based on the theoretical heating temperature T0 required for the minimum assembly gap, at least five temperature gradients T are set, extending both upwards and downwards. i For example: T0-ΔT1, T0-ΔT2, T0, T0+ΔT2, T0-ΔT1. The interval between adjacent temperature gradients (ΔT1 and ΔT2) is set from 20°C to 80°C depending on the gear material, size, and heat treatment condition.

[0022] The verification specimens are complete gears or sector specimens with at least one complete tooth; the total number of specimens is not less than three times the number of temperature gradients.

[0023] Step S2, Reference Residual Stress Test: The initial surface residual stress σ_res0 in the transition fillet region of the tooth root of all verification specimens is measured using non-destructive testing methods, and the distribution characteristics are recorded as reference stress data.

[0024] Step S3, Simulated Hot-Cooling Cycle Test: The verification samples are grouped and heated to their respective set temperature gradients and held at that temperature. Cooling is then performed to simulate the actual assembly environment (e.g., stationary in air or on a tooling with convection). The cooling rate should be recorded and kept as consistent as possible with actual assembly conditions. Complete temperature-time curves are recorded.

[0025] Step S4, Residual Stress Test and Comparative Analysis after Thermal Cycling: After the sample cools to room temperature, measure the residual stress σ_resi at the tooth root after thermal cycling at the exact same position as the benchmark test. Calculate the residual stress change Δσ=σ_resi-σ_res0 under each temperature scheme, as well as the stress relaxation rate or fluctuation range.

[0026] Step S5: Determine the optimal heat-fitting temperature window: Plot the heat-fitting verification temperature T. i The relationship curve between the change in residual stress at the tooth root Δσ (or the final stress value σ_resi) and the temperature window for hot fitting is determined by satisfying both assembly feasibility and residual stress stability criteria, and standardized process specifications are compiled.

[0027] The final hot-fitting process specification for this product clearly stipulates: the recommended hot-fitting temperature range (e.g., 180℃±10℃), the heat preservation time, the cooling method, and the key process control points.

[0028] Step S6, Process Verification and Closed-Loop Control: Assemble the actual product according to the prepared process specifications, and conduct spot checks on the residual stress at the tooth root of the assembled product to verify the process stability and form a data closed loop.

[0029] Specifically, in steps S2 and S4, residual stress is measured using X-ray diffraction (XRD). At least three teeth are measured for each sample, and the center and two sides of the tooth root are measured for each tooth.

[0030] The assembly feasibility criterion is that the hot-fitting temperature is not lower than the minimum safe assembly temperature T_min that is operable on site; the residual stress stability criterion is that the absolute value of the residual stress change is less than a set threshold, or the residual stress after thermal cycling remains within a preset residual compressive stress range. The set threshold for the absolute value of the residual stress change is 50 MPa; the preset residual compressive stress range is -300 MPa to -800 MPa. The temperature range with the least impact on residual stress (Δσ approaching 0) is preferentially selected.

[0031] The specific implementation plan for this application is as follows: The following example, using a wind turbine planetary gear assembly as an illustration, further illustrates this application, but is not limited to this embodiment. This embodiment addresses the interference fit assembly of a wind turbine gearbox planetary gear, employing the method of this application to determine the optimal heat-fitting temperature window.

[0032] Basic product parameters: This planetary gear is a hardened gear made of 18CrNiMo7-6, which has undergone carburizing and quenching treatment. The inner diameter is Φ300mm, and the design interference with the planetary gear shaft is 0.25mm.

[0033] Step S1: Test Design and Sample Preparation. Based on the design interference fit between the gear and the shaft, the theoretical heating temperature T0≈190℃ required for the theoretical minimum assembly clearance is calculated. Five verification temperature gradients are set: 130℃, 150℃, 180℃, 190℃, and 220℃. 25 sector-shaped samples from the same batch are prepared (5 samples per temperature group).

[0034] Step S2: The initial surface residual stress σ_res0 in the tooth root transition fillet region of all 25 specimens was measured using X-ray diffraction (XRD). Three teeth were measured for each specimen, with the stress distribution characteristics recorded at the tooth root center and both sides. The test results showed that the average initial residual stress of all specimens was -650 MPa (compressive stress), which was used as the benchmark stress data.

[0035] Step S3: Simulated Hot-Fitting-Cooling Cycle Test. The samples were divided into 5 groups and placed in an oven to be heated to 130℃, 150℃, 180℃, 190℃, and 220℃ respectively. Precise temperature uniformity control was performed, and the samples were held at these temperatures for 2 hours. After the holding period, the samples were removed and placed on a windless fixture to simulate the actual assembly environment and air-cooled to room temperature. The complete temperature-time curves were recorded.

[0036] Step S4: Residual Stress Testing and Comparative Analysis after Thermal Cycling. After all samples have completely cooled to room temperature, the residual stress σ_resi on the tooth root surface of each sample is measured again at the exact same location as in the benchmark test. The change in residual stress Δσ = σ_resi - σ_res0 under each temperature gradient is calculated. The test results are as follows: 130℃ group Δσ ≈ -3MPa; 150℃ group Δσ ≈ -5MPa; 180℃ group Δσ ≈ -15MPa; 190℃ group Δσ ≈ -40MPa; 220℃ group Δσ ≈ -120MPa.

[0037] Step S5: Determining the optimal hot-fitting temperature window. Plotting the relationship curve between the hot-fitting verification temperature and the change in residual stress at the tooth root shows that the higher the temperature, the more pronounced the relaxation (positive change) of the residual compressive stress at the tooth root. Considering the actual on-site operating conditions, the minimum safe assembly temperature T_min was determined to be 160℃. Simultaneously satisfying both assembly feasibility and residual stress stability criteria, the optimal hot-fitting temperature window was ultimately determined to be 170℃-185℃. This window ensures smooth assembly while controlling the change in residual stress within 20MPa.

[0038] Step S6: Process Verification and Closed-Loop Control. Based on the determined optimal temperature window, the heat assembly process specification for the planetary gear was formulated: heating temperature 180℃±5℃, holding time 2.5 hours (considering the actual heat capacity of the parts), and air cooling assembly. The first batch of products was assembled according to this specification, and the assembly process was smooth and without jamming. After assembly, the residual stress at the tooth root of the first batch of products was measured. The results showed that the residual stress matched the benchmark value well, verifying the stability and effectiveness of the process.

[0039] This application breaks through the limitations of existing hot-fitting processes that only consider assembly feasibility as a single objective. For the first time, it uses the residual stress state in the transition fillet region of the tooth root of hardened gears as the core constraint for determining the hot-fitting temperature, establishing a correspondence between the hot-fitting temperature and the change in residual stress at the tooth root. Through bidirectional multi-gradient temperature verification, dual criterion screening, and closed-loop process control, it effectively avoids the problem of residual compressive stress relaxation caused by excessively high temperatures in traditional empirical methods, improves the bending fatigue strength and reliability of gears, and solves the potential fatigue failure of high-speed, heavy-load gears caused by improper hot-fitting processes.

[0040] This application establishes a scientific, systematic, and replicable standardized process verification procedure that requires no modification to existing production equipment and can be achieved simply by optimizing process parameters. It is low-cost and yields quick results. This method is applicable to interference-fit hardened gears of various materials, specifications, and heat treatment states, providing personalized and precise process parameters for different products. This ensures consistency in quality across different batches of products, significantly reducing quality losses and rework costs.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for determining the heat fitting temperature and verifying the process of interference fit gears based on residual stress control at the tooth root, characterized in that, Includes the following steps: Step S1, Test Design and Sample Preparation: Based on the design interference of the gear and shaft, calculate the theoretical heating temperature required for the theoretical minimum assembly clearance. Set multiple temperature gradients based on the theoretical heating temperature. At least one of the temperature gradients should include a verification temperature that is lower than the theoretical heating temperature required for the theoretical minimum assembly clearance. Prepare verification samples with the same material, batch, and heat treatment process as the product gear. Step S2, Reference Residual Stress Test: The initial surface residual stress in the tooth root transition fillet area of ​​all verification specimens is measured using non-destructive testing methods, and the distribution characteristics are recorded as reference stress data. Step S3, Simulated hot assembly-cooling cycle test: The verification samples are grouped and heated to each set temperature gradient and held at that temperature. The actual assembly environment is simulated for cooling, and the complete temperature-time curve is recorded. Step S4, Residual stress test and comparative analysis after thermal cycling: After the sample cools to room temperature, measure the residual stress at the tooth root after thermal cycling at the exact same position as the benchmark test, and calculate the change in residual stress. Step S5: Determine the optimal hot fitting temperature window: Plot the relationship curve between the hot fitting verification temperature and the change in residual stress at the tooth root. Simultaneously meet the dual criteria of assembly feasibility and residual stress stability, determine the optimal hot fitting temperature window, and compile standardized process specifications. Step S6, Process Verification and Closed-Loop Control: Assemble the actual product according to the prepared process specifications, and conduct spot checks on the residual stress at the tooth root of the assembled product to verify the process stability and form a data closed loop.

2. The method for determining and verifying the heat fitting temperature and process of interference fit gears based on residual stress control at the tooth root, as described in claim 1, is characterized in that... In step S1, based on the theoretical heating temperature required for the minimum assembly clearance, at least 5 temperature gradients are set by expanding upwards and downwards in both directions; the interval between adjacent temperature gradients is set to 20°C to 80°C according to the gear material, size and heat treatment state.

3. The method for determining and verifying the heat fitting temperature and process of interference fit gears based on residual stress control at the tooth root, as described in claim 1, is characterized in that... In step S1, the verification sample is a complete gear or a sector-shaped sample with at least one complete tooth; the total number of samples is not less than three times the number of temperature gradients.

4. The method for determining and verifying the heat fitting temperature and process of interference fit gears based on residual stress control at the tooth root, as described in claim 1, is characterized in that... In steps S2 and S4, residual stress is measured using X-ray diffraction. At least three teeth are measured for each sample, and the center and two sides of the tooth root are measured for each tooth.

5. The method for determining and verifying the heat fitting temperature and process of interference fit gears based on residual stress control at the tooth root, as described in claim 1, is characterized in that... In step S5, the assembly feasibility criterion is that the hot assembly temperature is not lower than the minimum safe assembly temperature that can be operated on site; the residual stress stability criterion is that the absolute value of the residual stress change is less than a set threshold, or the residual stress after thermal cycling remains within a preset residual compressive stress range.

6. The method for determining the heat fitting temperature and verifying the process of interference fit gears based on residual stress control at the tooth root, as described in claim 5, is characterized in that... The absolute value threshold for the change in residual stress is set at 50 MPa; the preset residual compressive stress range is -300 MPa to -800 MPa.

7. The method for determining and verifying the heat fitting temperature and process of interference fit gears based on residual stress control at the tooth root, as described in claim 1, is characterized in that... Interference fit gears are hardened gears with hardened tooth surfaces that have undergone carburizing and quenching or induction hardening treatment.

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