Methods of testing spring fatigue performance and relaxation rate:
1. Spring fatigue performance testing requirements
All springs subjected to variable loads must undergo fatigue performance testing in the process of internal quality control, user inspection, new product type identification, certification, industry sampling and process adjustment. The testing method can be fatigue life verification test or reliability evaluation test. There are two types, but for the classification and certification of products, reliability evaluation tests must be used to assess their reliability indicators.
The required testing institutions, inspection reports and testing cycles for different testing purposes are shown in Table 1.
Testing institutions, reports and cycles required for different testing purposes
Testing purpose Testing organization testing report testing cycle life screening company or quality inspection organization recognized by the company-process adjustment at any time the company or quality inspection agency recognized by the company-synchronized with the process adjustment cycle internal quality control company or quality inspection agency recognized by the company-at least Every six months, the user submits the agreement between the two parties for inspection-synchronous with the delivery cycle, industry sampling new product type identification, government-authorized or designated ministerial (provincial) level or higher quality inspection agency, certification when necessary, national authorization, national laboratory accreditation committee When the accreditation is passed and the third-party quality inspection agency designated by the certification committee has the measurement certification mark and laboratory certification mark
For different testing purposes and product types, the minimum number of test cycles specified is shown in Table 2.
Table 2 The minimum number of test cycles (times) specified by the spring product type and testing purpose
Product category type industry sampling inspection type certification valve spring
pressure regulating spring
plunger spring 1×10
2.3×10 hydraulic spring
suspension springs are selected according to classification standards
After the fatigue test with the specified number of cycles, the relaxation rate tolerance of the spring is specified in the corresponding standard. When the standard is not specified, the mandatory testing is determined by the testing unit, and the non-mandatory testing is performed according to the requirements of the user or the tested unit. Dongguan Hardware Spring
For the reliability evaluation test, after the specified test conditions and number of cycles of the spring, its qualified crystal, first-class and high-quality products should reach the reliability level specified in Table 3. For the certified product, the reliability of its fatigue life must reach the excellent level The reliability index of the product.
Table 3 Reliability index of product grade and fatigue life
Product Grade Reliability R (%) Excellent product
Qualified product 99.99
二. Spring fatigue testing machine
A. Mechanical, electromagnetic resonance or electro-hydraulic servo testing machines are used, and supporting mainframes can also be used. When electromagnetic resonance testing machines are used, the test amplitude should not be too large.
B. The error of the testing machine should meet the relevant regulations.
C. The frequency of the testing machine should be adjustable within a certain range to meet different test requirements.
D. The functions that the testing machine should generally have: test time or times preset function; automatic timing or counting function; test amplitude display or control function; automatic shutdown function.
3. Spring fatigue test specimen
1) The sample should be manufactured in accordance with the technical documents approved by the prescribed procedures and inspected to be qualified.
2) Samples should be randomly and independently drawn from the same batch of products, and the division of inspection batches shall be carried out according to JB/T58700.
3) Number of samples:
For the fatigue life verification test, the number of samples is not less than 5 sets for engines below 4 cylinders; the number of samples is not less than 3 sets for engines with 4 cylinders or more;
For the reliability evaluation test, the number of samples is determined according to Table 6-6.
4. Spring fatigue test frequency
1) The test frequency can be determined according to the frequency range of the testing machine and the actual working frequency of the spring. Except for the random load test, the test frequency should be kept constant during the entire test.
2) The test frequency γr should be the natural natural frequency of a single test spring, satisfying (γ/γr)>10. γ is calculated according to formula (7-79).
5. Spring fatigue test amplitude The amplitude is divided into displacement amplitude and stress amplitude. For the fatigue life verification test and reliability evaluation test of coil springs, the displacement amplitude is generally used as the test amplitude.
6. Spring fatigue test procedure Refer to the spring fatigue test of Chapter 6, Section 5.
Seven. Selection of spring failure mode During the fatigue test of the spring, the failure mode is divided into two types: fracture and loss of specified function due to relaxation. According to the functional requirements of the spring, one or two of them can be selected as the failure mode of fatigue life.
8. Evaluation of spring fatigue test results
1) Determination of fatigue life: Under a given failure condition, the maximum number of cycles reached by the tested spring is the fatigue life of the spring.
2) Verification of fatigue life: After a given number of cycles, if the spring does not fail, the verification test is passed.
3) Reliability verification: According to the given reliability target value, determine the appropriate sample number n according to Table 6-6, test under the specified test conditions and requirements, and judge according to the required failure mode. If the qualified number is greater than If the minimum qualified number r required in the table is passed, the verification test is passed, otherwise it is not passed.
4) Spring relaxation rate: The spring relaxation rate ε is expressed as the percentage of load loss after the spring test:
where F-spring load measured before the test;
Fi–the spring load measured after the test.
The relaxation rate after fatigue test should be within the range specified in the drawing.