Power take-off fatigue durability evaluation method based on bench test and whole vehicle load test

Abstract

The application discloses a method for evaluating fatigue durability of a power take-off based on bench test and vehicle load test, and belongs to the technical field of automobile component fatigue durability, and comprises the following steps: obtaining a failure cycle number, performing statistical analysis, obtaining a power take-off strength distribution and a 50%-S-N curve slope; assembling a drive shaft torque testing device, and performing strain-torque relationship calibration; testing drive shaft torque and rotating speed signals in a test field, and performing torque-rotating cycle spectrum compilation; converting bench durability test results and test field test results to corresponding rotating cycles under a reference torque; calculating the power take-off strength distribution and the test field load distribution, and evaluating the failure risk of the power take-off by using a stress-strength interference model. The method obtains the strength distribution of the power take-off through the bench test, obtains the load input distribution of the power take-off based on the load test, converts the load input and the strength distribution of the power take-off to the same reference which can be compared, and quantitatively evaluates the durability risk by using the stress-strength interference model.

Description

Technical Field

[0001] This invention belongs to the field of automotive component fatigue durability technology, specifically relating to a method for evaluating the fatigue durability of power take-offs based on bench tests and vehicle load tests. Background Technology

[0002] Four-wheel drive better meets the vehicle's power requirements. The power take-off (PTO) in a four-wheel drive vehicle typically transfers power from the transmission to the rear axle; it's usually found in four-wheel drive vehicles with a front-mounted engine. Under normal circumstances, it operates in two-wheel drive mode, distributing drive torque to the other two wheels only as needed. After the PTO performs its torque distribution function, its durability must be ensured to complete the task; that is, under certain operating conditions, the PTO must not experience internal gear breakage, bearing damage, or housing cracks. During vehicle powertrain calibration, calibration engineers set the torque value and duration distributed to the rear wheels. After locking the calibration parameters, a drivetrain durability test is conducted. If the drivetrain durability test passes, the durability performance meets the requirements; if it fails, the PTO structure needs to be improved or the torque distribution recalibrated. In the development process, time is precious; therefore, assessing the PTO's durability performance before the vehicle drivetrain durability test is essential and requires an efficient and accurate technical method. Currently, there is no way to evaluate the durability performance of a power take-off (PTO) based on bench durability test results and actual vehicle load measurements, so as to determine whether the PTO's durability performance meets the requirements before the vehicle test results.

[0003] For example, patent document CN112178067A, published on January 5, 2021, describes "a power take-off assembly and its application". Using this invention, the load on the engine can be reduced when starting at low temperatures, avoiding the problem of difficult engine starting. This patent is more inclined to a new type of power take-off structure to greatly improve starting performance, but it does not yet involve durability performance.

[0004] For example, patent document CN113418720B, published on July 19, 2022, describes "a power take-off (PTO) bench test device and its test method." This invention patent can achieve special functions under specific conditions and is increasingly widely used in special-purpose vehicles and agricultural vehicles; it can meet the requirements of PTO gear engagement and disengagement tests and life tests in special-purpose vehicle PTOs. This patent is a bench durability test method, but it does not specify whether the durability performance of the PTO meets the requirements for different power performances.

[0005] For example, patent document CN113203876A, published on August 3, 2021, describes "a method, apparatus, electronic device, and storage medium for confirming a power take-off (PTO) fault." This patent disclosure relates to the field of fault handling technology and discloses a method, apparatus, electronic device, and storage medium for determining a PTO fault. The PTO fault determination method in this embodiment includes: after determining that the PTO is unloaded based on its current output power and that the clutch is engaged based on its current pressure, comparing the rotational speed collected by the speed sensor corresponding to the output shaft of the PTO with the target rotational speed of the output shaft; wherein, the target rotational speed is the rotational speed corresponding to the current gear of the clutch; and determining whether the speed sensor is faulty based on the comparison result. This embodiment compares the rotational speed collected by the speed sensor corresponding to the output shaft of the PTO with the corresponding target rotational speed when the PTO is unloaded and the clutch is engaged, and can accurately determine whether the speed sensor is faulty based on the comparison result, reducing misjudgments of clutch faults. Summary of the Invention

[0006] To address the current situation where existing technologies lack data on bench durability tests and actual vehicle load measurements to evaluate the durability performance of power take-offs (PTOs), this invention provides a fatigue durability assessment method for PTOs based on bench tests and vehicle load tests. This method obtains the strength distribution of the PTO through bench tests, obtains the load input distribution of the PTO based on load tests, and transforms the load input and strength distribution of the PTO to the same comparable benchmark. A stress-strength interference model is then used for quantitative assessment of durability risk.

[0007] This invention is achieved through the following technical solution:

[0008] The fatigue durability assessment method for power take-off devices based on bench tests and vehicle load tests includes the following steps:

[0009] S1: Obtain the failure cycle number through the power take-off bench durability test;

[0010] S2: Statistical analysis of failure cycle data yields the strength distribution of the power take-off and the slope of the 50%-SN curve;

[0011] S3: Assemble a drive shaft torque testing device, including installing strain gauges and telemetry equipment on the drive shaft, and calibrating the strain-torque relationship;

[0012] S4: According to the transmission system durability specifications, the torque and speed signals of the transmission shaft are tested at the test site, and the torque-rotation cycle spectrum is compiled by using the rotating component counting method to analyze the torque and speed signals.

[0013] S5: Based on the principle of damage equivalence, the results of bench durability tests and test field tests are converted to the number of rotations corresponding to the reference torque;

[0014] S6: Calculate the strength distribution of the power take-off and the load distribution of the test field, and use the stress-strength interference model to assess the failure risk of the power take-off.

[0015] Furthermore, step S1 specifically includes the following:

[0016] The power take-off (PTO) bench durability test includes high-level load loading and low-level load loading. The torque value corresponding to the high-level load loading is the maximum input torque designed for the PTO, and the torque value corresponding to the low-level load loading is two-thirds of the maximum input torque of the PTO. The test speed is set to 450 rpm, and the sample size should not be less than 3.

[0017] The failure determination criterion is to record the number of failure cycles when the change in the monitoring signal of the acceleration sensor installed on the power take-off caused by the failure of internal parts such as gears and bearings reaches a certain specified vibration level.

[0018] Furthermore, step S2 specifically includes the following:

[0019] Statistical analysis was performed on the failure cycle data to calculate the cumulative failure probability of the power take-off (PTO) under high-level and low-level loads. The cycle number corresponding to a 50% cumulative failure probability under both high-level and low-level loads was taken. Simultaneously, the slope b of the 50%-SN curve was calculated, which was obtained using the following formula:

[0020]

[0021] In the formula, T H The torque corresponding to high-level loads; T L The torque corresponding to the low-level load; N H N represents the number of cycles corresponding to a 50% failure probability under high-level loading. L This represents the number of cycles corresponding to a 50% failure probability under low-level load.

[0022] Furthermore, step S3 specifically includes the following:

[0023] A drive shaft torque telemetry device was constructed, and a full-bridge strain gauge was installed on the drive shaft. The strain signal characterizing the torque was converted into a voltage signal using the telemetry device. Torque-voltage calibration was performed on a torque test bench to test the drive shaft torque.

[0024] Furthermore, step S4 specifically includes the following:

[0025] On the high-speed ring track of the test track, the torque and speed of the drive shaft were tested according to the durability specifications of the whole vehicle transmission system. The torque signal was obtained by the torque testing device, and the drive shaft speed was calculated from the rear wheel speed in the vehicle CAN information. Drive shaft speed = (left rear wheel speed + right rear wheel speed) ÷ 2 × main reduction ratio;

[0026] The rotating component counting method was applied to statistically analyze the number of rotations of the drive shaft in different torque ranges during testing. The calculation formula is as follows:

[0027]

[0028] In the formula, c j This represents the total number of rotations of the drive shaft within the same torque range.

[0029] j represents a certain torque range;

[0030] n is the number of speed ranges corresponding to the same torque range;

[0031] t imin This is the lower limit time point within a certain speed range;

[0032] t imax This refers to the upper limit time point of a certain speed range;

[0033] The total number of torque division intervals J = (measured maximum torque - 0) ÷ 10.

[0034] Furthermore, in step S4, the durability specification of the vehicle transmission system includes five test conditions: high-speed shifting, high-speed driving, reverse gear test, parking and hill start test; each test includes five conditions, and a total of three tests are conducted.

[0035] Furthermore, step S5 specifically includes the following:

[0036] For a given torque T1 and its corresponding number of rotations C1, there will be a corresponding torque T2 and its corresponding number of rotations C2, satisfying equation (3):

[0037]

[0038] In the formula, b is the slope of the 50%-SN curve of the power take-off device. Based on formula (3), the bench test results in step S1 are converted to the same reference torque T. nom The corresponding number of rotations will convert the test data from the test site to the same reference torque T. nom The corresponding number of rotations is selected, and the middle value of the torque range is chosen to represent the torque value of that range.

[0039] Furthermore, step S6 specifically includes the following:

[0040] First, the distribution of rotational revolutions in the test data converted to the reference torque is analyzed and calculated. The data follows a normal distribution with a mean u. s =1157 Nm, standard deviation σ s =86, the probability density function of the normal distribution is shown in equation (4):

[0041]

[0042] Secondly, the distribution of the number of rotations under the reference torque was analyzed and calculated based on the durability test results of different power take-off sample benches. The data followed a two-parameter Weibull distribution with shape parameter β = 4.58 and characteristic life η = 8462. The probability density function of the two-parameter Weibull distribution is shown in equation (5):

[0043]

[0044] The stress-strength interference model was used to analyze the probability of failure of the power take-off (PTO) under the durability test conditions of the whole vehicle transmission system. Under the reference torque, the intersection area of ​​the stress distribution in the test field and the strength distribution of the PTO is approximately the probability of failure. The calculation expression is shown in Equation (6):

[0045]

[0046] The cumulative distribution function of intensity δ can be expressed as: Equation (6) can then be derived as

[0047]

[0048] make Then the integral of the first term in equation (7) is the integral under the standard normal density curve from The area up to +∞ can be used express;

[0049] For the integral of the second term in equation (7), let but

[0050]

[0051] s=tη

[0052]

[0053] Therefore, equation (7) can be rewritten as equation (8):

[0054]

[0055] make Equation (8) can then be rewritten as equation (9):

[0056]

[0057] Based on the distribution parameters of the test field data and the distribution parameters of the bench durability test, F is solved using MATLAB programming, which is the probability of the power take-off unit failing due to durability.

[0058] Compared with the prior art, the advantages of the present invention are as follows:

[0059] 1. The fatigue durability assessment method for power take-off based on bench test and vehicle load test of the present invention achieves power take-off failure through bench durability test, obtains power take-off strength distribution, and obtains the slope of 50%-SN curve;

[0060] 2. This method involves constructing a power take-off torque testing device, testing the torque and speed of the power take-off on an actual vehicle, and obtaining the load spectrum of the power take-off under the durability test specifications of the whole vehicle transmission system, that is, the number of rotations of the transmission shaft corresponding to different transmission shaft torque ranges;

[0061] 3. By applying the fatigue cumulative damage theory and the stress-strength interference model to evaluate the relationship between the load input distribution and the strength distribution of the power take-off (PTO) in the test field, the durability risk of the PTO can be assessed before the transmission system durability test, providing decision support for the PTO design. Attached Figure Description

[0062] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0063] Figure 1 This is a flowchart illustrating the fatigue durability assessment method for power take-off devices based on bench tests and vehicle load tests according to the present invention.

[0064] Figure 2 This is a cumulative failure probability curve of the power take-off bench test results;

[0065] Where a is the cumulative failure probability of the power take-off under high load, and b is the cumulative failure probability of the power take-off under low load.

[0066] Figure 3 A schematic diagram of the slope of the 50%-SN curve;

[0067] Figure 4 This is a schematic diagram of a drive shaft torque testing device;

[0068] In the diagram: 1. Power take-off (PTO), 2. Full-bridge strain gauge, 3. Telemetry equipment rotor, 4. Coupler, 5. PTO torque input shaft, 6. Drive shaft, 7. Welding terminal, 8. Connecting wire;

[0069] Figure 5 A statistical diagram showing the number of rotations corresponding to different torque ranges;

[0070] Figure 6 This is a schematic diagram of the stress-intensity interference model. Detailed Implementation

[0071] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0072] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0073] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0074] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0075] Example 1

[0076] like Figure 1The diagram shown is a flowchart illustrating the fatigue durability assessment method for a power take-off (PTO) based on bench testing and vehicle load testing in this embodiment. The assessment method specifically includes the following steps:

[0077] S1: Obtain the failure cycle number through the power take-off bench durability test;

[0078] The power take-off (PTO) bench durability test includes high-level load loading and low-level load loading. The torque value corresponding to the high-level load loading is the maximum input torque designed for the PTO, and the torque value corresponding to the low-level load loading is two-thirds of the maximum input torque of the PTO. The test speed is set to 450 rpm, and the sample size should not be less than 3.

[0079] The failure determination criterion is that when the change in the monitoring signal of the acceleration sensor installed on the power take-off caused by the failure of internal parts such as gears and bearings reaches a certain specified vibration level, the failure cycle number is recorded.

[0080] S2: Statistical analysis of failure cycle data yields the strength distribution of the power take-off and the slope of the 50%-SN curve;

[0081] Statistical analysis was performed on the failure cycle data to calculate the cumulative failure probability of the power take-off (PTO) under high-level and low-level loads. The cycle number corresponding to a 50% cumulative failure probability under both high-level and low-level loads was taken. Simultaneously, the slope b of the 50%-SN curve was calculated, which was obtained using the following formula:

[0082]

[0083] In the formula, T H The torque corresponding to high-level loads; T L The torque corresponding to the low-level load; N H N represents the number of cycles corresponding to a 50% failure probability under high-level loading. L This represents the number of cycles corresponding to a 50% failure probability under low-level load.

[0084] S3: Assemble a drive shaft torque testing device, including installing strain gauges and telemetry equipment on the drive shaft, and calibrating the strain-torque relationship;

[0085] A drive shaft torque telemetry device was constructed, and a full-bridge strain gauge was installed on the drive shaft. The telemetry device was used to convert the strain signal characterizing the torque into a voltage signal. Torque-voltage calibration was performed on a torque test bench to test the drive shaft torque.

[0086] S4: According to the transmission system durability specifications, the torque and speed signals of the transmission shaft are tested at the test site, and the torque-rotation cycle spectrum is compiled by using the rotating component counting method to analyze the torque and speed signals.

[0087] On the high-speed ring track of the test track, the torque and speed of the driveshaft were tested according to the durability specifications of the vehicle's powertrain. The torque signal was obtained by a torque testing device, and the driveshaft speed was calculated from the rear wheel speed in the vehicle's CAN information. The driveshaft speed = (left rear wheel speed + right rear wheel speed) ÷ 2 × final drive ratio. The durability specifications of the vehicle's powertrain include five test conditions: high-speed shifting, high-speed driving, reverse gear test, parking, and hill start test. Each test includes five conditions, and a total of three tests are conducted.

[0088] The rotating component counting method was applied to statistically analyze the number of rotations of the drive shaft in different torque ranges during testing. The calculation formula is as follows:

[0089]

[0090] In the formula, c j This represents the total number of rotations of the drive shaft within the same torque range.

[0091] j represents a certain torque range;

[0092] n is the number of speed ranges corresponding to the same torque range;

[0093] t imin This is the lower limit time point within a certain speed range;

[0094] t imax This refers to the upper limit time point of a certain speed range;

[0095] The total number of torque intervals J = (measured maximum torque - 0) ÷ 10;

[0096] S5: Based on the principle of damage equivalence, the results of bench durability tests and test field tests are converted to the number of rotations corresponding to the reference torque;

[0097] For a given torque T1 and its corresponding number of rotations C1, there will be a corresponding torque T2 and its corresponding number of rotations C2, satisfying equation (3):

[0098]

[0099] In the formula, b is the slope of the 50%-SN curve of the power take-off device. Based on formula (3), the bench test results in step S1 are converted to the same reference torque T. nom The corresponding number of rotations will convert the test data from the test site to the same reference torque T. nom The corresponding number of rotations is selected, and the middle value of the torque range is chosen to represent the torque value of that range;

[0100] S6: Calculate the strength distribution of the power take-off (PTO) and the load distribution at the test site, and use a stress-strength interference model to assess the failure risk of the PTO. This includes the following:

[0101] First, the distribution of rotational revolutions in the test data converted to the reference torque is analyzed and calculated. The data follows a normal distribution with a mean u. s =1157 Nm, standard deviation σ s =86, the probability density function of the normal distribution is shown in equation (4):

[0102]

[0103] Secondly, the distribution of the number of rotations under the reference torque was analyzed and calculated based on the durability test results of different power take-off sample benches. The data followed a two-parameter Weibull distribution with shape parameter β = 4.58 and characteristic life η = 8462. The probability density function of the two-parameter Weibull distribution is shown in equation (5):

[0104]

[0105] The stress-strength interference model was used to analyze the probability of failure of the power take-off (PTO) under the durability test conditions of the whole vehicle transmission system. Under the reference torque, the intersection area of ​​the stress distribution in the test field and the strength distribution of the PTO is approximately the probability of failure. The calculation expression is shown in Equation (6):

[0106]

[0107] The cumulative distribution function of intensity δ can be expressed as: Equation (6) can then be derived as

[0108]

[0109] make Then the integral of the first term in equation (7) is the integral under the standard normal density curve from The area up to +∞ can be used express;

[0110] For the integral of the second term in equation (7), let but

[0111]

[0112] s=tη

[0113]

[0114] Therefore, equation (7) can be rewritten as equation (8):

[0115]

[0116] make Equation (8) can then be rewritten as equation (9):

[0117]

[0118] Based on the distribution parameters of the test field data and the distribution parameters of the bench durability test, F is solved using MATLAB programming, which is the probability of the power take-off unit failing due to durability.

[0119] Example 2

[0120] This embodiment provides a method for evaluating the fatigue durability of a power take-off (PTO) based on bench tests and vehicle load tests, specifically including the following steps:

[0121] Step 1: Conduct a bench durability test on the power take-off (PTO). The bench test consists of two loading levels. The torque value corresponding to the high-level load is the maximum designed input torque of the PTO, and the torque value corresponding to the low-level load is two-thirds of the maximum torque of the PTO. The rotational speed is 450 rpm, and the sample size is 3, but not less than 3. The failure criterion is that the change in the monitoring signal of the acceleration sensor installed on the PTO caused by the failure of internal parts such as gears and bearings reaches a certain specified vibration level. An example of the bench durability test specification is shown in Table 1.

[0122] Table 1 is an example table of bench test specifications.

[0123]

[0124] The failure cycle count was recorded. An example of the power take-off durability test results is shown in Table 2.

[0125] Table 2 shows an example of the results of the power take-off (PTO) bench durability test.

[0126]

[0127] Step 2: Perform statistical analysis on the failure data. The data follows a two-parameter Weibull distribution. Calculate the cumulative failure probability of the power take-off (PTO) under high-level and low-level loads. Example results are shown below. Figure 2 As shown; take the cycle number corresponding to a 50% cumulative failure probability under high-level and low-level loads, and calculate the slope b of the 50%-SN curve, as illustrated in the diagram. Figure 3 As shown, the calculation formula is as shown in equation (1), and the calculation result is b = -0.11.

[0128]

[0129] In the formula, T H The torque corresponding to the high-load condition is 900 N·m.

[0130] T LThis is the torque corresponding to the low-level load, with a value of 600 N·m;

[0131] N H This represents the number of cycles corresponding to a 50% failure probability under high load conditions, with a value of 20156.

[0132] N L The number of cycles corresponding to a 50% failure probability under low load is 788315.

[0133] Step 3: Assemble a driveshaft torque telemetry device. Install a full-bridge strain gauge on the driveshaft. Use the telemetry equipment to convert the strain signal characterizing the torque into a voltage signal. Perform torque-voltage calibration on a torque test bench. This device is used to test the driveshaft torque. The testing device is as follows: Figure 3 As shown in Table 3, examples of torque-voltage relationship calibration results are presented.

[0134] Table 3 is an example table of torque and strain gauge voltage calibration data records.

[0135] Serial Number Torque (N·m) Voltage (V) 1 200 1.26 2 400 2.54 3 600 3.83 4 800 5.12 5 1000 6.42 6 1200 7.73 7 800 5.21 8 400 2.66

[0136] Step 4: On the high-speed test track, according to the vehicle drivetrain durability specifications, the driveshaft torque and speed are tested. The torque signal is obtained from the torque testing device in Step 3. The driveshaft speed is calculated from the rear wheel speed in the vehicle's CAN information: Driveshaft speed = (left rear wheel speed + right rear wheel speed) ÷ 2 × final drive ratio. This specification includes 5 operating conditions, as shown in Table 4.

[0137] Table 4 is a schematic diagram of the working conditions and test sequence for the transmission system durability specifications.

[0138]

[0139] One test includes cases 1 to 5, and three tests are performed, resulting in three load samples.

[0140] The rotating component counting method was applied to count the number of rotations of the drive shaft in different torque ranges during testing. A schematic diagram of the calculation process is shown below. Figure 4 As shown, the calculation formula is as shown in Equation 2.

[0141]

[0142] In the formula, c j The total number of rotations of the drive shaft corresponding to the same torque range

[0143] j represents a certain torque range

[0144] n is the number of speed ranges corresponding to the same torque range.

[0145] t iminThe lower limit time point of a certain speed range

[0146] t imax The upper limit time point of a certain speed range

[0147] Table 5 shows the results of the number of rotations corresponding to different torque ranges in the three tests. Since the power take-off unit mainly operates when the vehicle is moving forward, and the proportion of reversing is relatively small compared to forward, and the maximum torque corresponding to the reversing state is less than the maximum torque corresponding to the forward state, this invention only considers the positive torque in the forward state. The number of torque ranges and the range size are the same in the three tests. The total number of torque ranges J = (measured maximum torque - 0) ÷ 10. The maximum measured torque of this invention is 900 Nm, which is divided into 90 torque ranges, each with a torque range of 10 Nm. The number of cycles corresponding to each torque range will be different.

[0148] Table 5 shows examples of the number of drive shaft rotations corresponding to different torque ranges in the three tests.

[0149]

[0150] Step 5: Based on the principle of damage equivalence, convert the bench durability test results and the test field test results to the number of rotations corresponding to a certain reference torque. For a given torque T1 and its corresponding number of rotations C1, there will be a corresponding torque T2 and its corresponding number of rotations C2, satisfying equation (3):

[0151]

[0152] In the formula, b is the slope of the 50%-SN curve of the power take-off, which has been calculated in step 2. Based on formula (3), the bench test results in step 1 are converted to the same reference torque T. nom The corresponding number of rotations is shown in Table 6 (example). The test data from the test site is converted to the same reference torque T. nom The corresponding number of rotations is used to select the middle value of the torque range to represent the torque value of that range. The results (examples) are shown in Table 7. The reference torque of this invention is 1000 N·m.

[0153] Table 6 shows the number of rotations corresponding to the conversion of the PTO bench durability test results to the reference torque.

[0154]

[0155] Table 7 shows the number of rotations corresponding to the reference torque after converting the test data from the three test sites.

[0156]

[0157] Step 6: Calculate the strength distribution of the power take-off (PTO) and the load input distribution at the test site, and apply the stress-strength interference model to assess the durability failure risk of the PTO. This includes the following:

[0158] First, the distribution of rotational revolutions converted from the three test field data to the reference torque was analyzed and calculated. The example data of this invention follows a normal distribution with a mean u. s =1157 Nm, standard deviation σ s =86, the probability density function of the normal distribution is shown in equation (4);

[0159]

[0160] Secondly, the distribution of the number of rotations under the reference torque was analyzed and calculated based on the durability test results of different power take-off sample benches. The example data of this invention follows a two-parameter Weibull distribution with shape parameter β = 4.58 and characteristic life η = 8462. The probability density function of the two-parameter Weibull distribution is shown in equation (5):

[0161]

[0162] The stress-strength interference model was used to analyze the probability of power take-off (PTO) failure under the durability test conditions of the whole vehicle powertrain. A schematic diagram of the stress-strength interference model is shown below. Figure 5 As shown, under the reference torque, the area where the stress distribution in the test field intersects with the strength distribution of the power take-off is approximately the probability of failure, and the calculation expression is as shown in equation (6).

[0163]

[0164] The cumulative distribution function of intensity δ can be expressed as: Equation (6) can then be derived as

[0165]

[0166] make Then the integral of the first term in equation (7) is the integral under the standard normal density curve from The area up to +∞ can be used express.

[0167] For the integral of the second term in equation (7), let but

[0168]

[0169] s=tη

[0170]

[0171] Therefore, equation (7) can be rewritten as equation (8).

[0172]

[0173] make Equation (8) can then be rewritten as equation (9).

[0174]

[0175] Based on the distribution parameters of the test field data and the distribution parameters of the bench durability test, the probability of F = 0.000115 was solved using MATLAB programming. This means that the probability of the power take-off (PTO) failing due to durability is 0.0115%, which is extremely low. This indicates that the PTO can simultaneously meet the requirements of durability and power performance.

[0176] The calculation procedure for this embodiment is as follows:

[0177] Clear;

[0178] syms y; % Defines the variable y;

[0179] beta = 4.58263; % Shape parameter β of the power take-off strength distribution;

[0180] yita = 8462.13; % Characteristic life η of power take-off strength distribution;

[0181] Us = 1157; % Average stress input of the power take-off test field load μ s ;

[0182] sgmaS = 86; % Stress standard deviation σ of load input at the power take-off test site s ;

[0183] B = (yita) / sgmaS;

[0184] A = (-Us) / sgmaS;

[0185] ft=exp((-1 / 2)*(B*y+A)^2-y^beta);

[0186] k1 = normcdf(A);

[0187] k2 = vpa(int((ft), 0, inf));

[0188] k3 = k2 * B / sqrt(2 * pi);

[0189] F = 1 - k1 - k3; % Calculation result of power take-off durability risk;

[0190] R = 1 - F; % Calculation result of power take-off durability and reliability.

[0191] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0192] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0193] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for evaluating the fatigue durability of a power take-off (PTO) based on bench tests and vehicle load tests, characterized in that, Includes the following steps: S1: Obtain the failure cycle number through the power take-off bench durability test; S2: Statistical analysis of failure cycle data yields the strength distribution of the power take-off and the slope of the 50%-SN curve; Calculate the cumulative failure probability of the PTO under high-level load and low-level load respectively. Take the cycle number corresponding to a 50% cumulative failure probability under high-level load and low-level load. At the same time, calculate the slope b of the 50%-SN curve. The slope of the 50%-SN curve is obtained by the following formula: （1） In the formula, The torque corresponding to high-level loads; This is the torque corresponding to the low-level load; This represents the number of cycles corresponding to a 50% failure probability under high-load conditions. The number of cycles corresponds to a 50% failure probability under low-level load; wherein, the power take-off bench durability test includes high-level load loading and low-level load loading, the torque value corresponding to the high-level load loading is the maximum input torque designed for the power take-off, and the torque value corresponding to the low-level load loading is two-thirds of the maximum input torque of the power take-off; S3: Assemble a drive shaft torque testing device, including installing strain gauges and telemetry equipment on the drive shaft, and calibrating the strain-torque relationship; S4: According to the transmission system durability specifications, the torque and speed signals of the transmission shaft are tested at the test site, and the torque-rotation cycle spectrum is compiled by using the rotating component counting method to analyze the torque and speed signals. S5: Based on the principle of damage equivalence, the results of bench durability tests and test field tests are converted to the number of rotations corresponding to the reference torque; S6: Calculate the strength distribution of the power take-off and the load distribution of the test field, and use the stress-strength interference model to assess the failure risk of the power take-off; Step S6 includes the following: First, the distribution of rotational revolutions in the test data converted to the reference torque was analyzed and calculated. The data follows a normal distribution with a mean of... =1157 Nm, standard deviation =86, the probability density function of the normal distribution is shown in equation (4): （4） Secondly, the distribution of the number of rotations under the reference torque was analyzed and calculated based on the durability test results of different power take-off sample benches. The data followed a two-parameter Weibull distribution with shape parameter β=4.58 and characteristic life η=8462. The probability density function of the two-parameter Weibull distribution is shown in equation (5): （5） The stress-strength interference model was used to analyze the probability of failure of the power take-off (PTO) under the durability test conditions of the whole vehicle transmission system. Under the reference torque, the intersection area of ​​the stress distribution in the test field and the strength distribution of the PTO is approximately the probability of failure. The calculation expression is shown in Equation (6): （6） strength The cumulative distribution function is expressed as Then equation (6) is derived as follows: （7） make Then the integral of the first term in equation (7) is the integral from the standard normal density curve. arrive The area, using express; For the integral of the second term in equation (7), let ,but ； ； ； Therefore, equation (7) is rewritten as equation (8): （8） make , Equation (8) can then be rewritten as equation (9): （9） Based on the distribution parameters of the test field data and the distribution parameters of the bench durability test, F is solved using MATLAB programming, which is the probability of the power take-off unit failing due to durability.

2. The fatigue durability assessment method for power take-off devices based on bench tests and vehicle load tests as described in claim 1, characterized in that, Step S1 includes the following: The test speed is set to 450 rpm, and the number of samples should not be less than 3. The failure determination criterion is to record the number of failure cycles when the change in the monitoring signal of the acceleration sensor installed on the power take-off caused by a failure of the gear or bearing parts inside the power take-off reaches a certain specified vibration level.

3. The fatigue durability assessment method for power take-off devices based on bench tests and vehicle load tests as described in claim 1, characterized in that, Step S3 includes the following: A drive shaft torque telemetry device was constructed, and a full-bridge strain gauge was installed on the drive shaft. The strain signal characterizing the torque was converted into a voltage signal using the telemetry device. Torque-voltage calibration was performed on a torque test bench to test the drive shaft torque.

4. The fatigue durability assessment method for power take-off devices based on bench tests and vehicle load tests as described in claim 1, characterized in that, Step S4 includes the following: On the high-speed ring track of the test track, the torque and speed of the drive shaft were tested according to the durability specifications of the whole vehicle transmission system. The torque signal was obtained by the torque testing device, and the drive shaft speed was calculated from the rear wheel speed in the vehicle CAN information. Drive shaft speed = (left rear wheel speed + right rear wheel speed) ÷ 2 × main reduction ratio; The rotating component counting method was applied to statistically analyze the number of rotations of the drive shaft in different torque ranges during testing. The calculation formula is as follows: （2） In the formula, This represents the total number of rotations of the drive shaft within the same torque range. rpm(t) represents the number of revolutions per minute (rpm). j represents a certain torque range; n is the number of speed ranges corresponding to the same torque range; This is the lower limit time point within a certain speed range; This refers to the upper limit time point of a certain speed range; The total number of torque division intervals J = the measured maximum torque ÷ 10.

5. The fatigue durability assessment method for a power take-off unit based on bench testing and vehicle load testing as described in claim 4, characterized in that, In step S4, the durability specification of the vehicle transmission system includes five test conditions: high-speed shifting, high-speed driving, reverse gear test, parking and hill start test; each test includes five conditions, and a total of three tests are conducted.

6. The fatigue durability assessment method for a power take-off unit based on bench testing and vehicle load testing as described in claim 1, characterized in that, Step S5 includes the following: For a given torque and the corresponding number of rotations Then there will be a corresponding torque. and the corresponding number of rotations Satisfying equation (3): （3） In the formula, b is the slope of the 50%-SN curve of the power take-off device. Based on formula (3), the bench test results in step S1 are converted to the same reference torque. The corresponding number of rotations will convert the test data from the test site to the same reference torque. The corresponding number of rotations is selected, and the middle value of the torque range is chosen to represent the torque value of that range.

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