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How to test the service life of compression springs | compressione spring

How to test the service life of compression springs | compressione spring

Dec 06, 2021

One sample selection
   When preparing samples, care should be taken to eliminate some abnormal factors that affect fatigue strength. There are two types of factors that affect the spring fatigue strength test: one is internal factors, such as chemical composition, metallographic structure, etc.; the other is external factors, such as surface condition, shape and size, temperature, surrounding medium, etc. Generally, it is based on the inherent fatigue strength of the material (internal factors), and the external factors are considered for correction. Therefore, in the test process, the test piece should try to eliminate the influence of external factors. When conducting the first type of test, samples can be prepared according to the conditions of use, or samples can be taken directly from the product. The second and third types of tests should be sampled according to specific requirements.
  A. Specimen size The spring size is always different, so even under the same amount of deformation, the stress and fatigue strength between them are not exactly the same. In order to accurately reflect the fatigue strength of the sample, the measurement accuracy of the size should be improved on the basis of reducing the size error.
   1) The measurement accuracy of the sample size should be greater than 0.5%. When the material size is less than 2 mm, its accuracy should be 0.01 mm. If 0.5% of the sample size is less than 0.01mm, the accuracy is 0.01mm.
  2) When measuring the size of a circular cross-section (such as outer diameter D2, spring wire diameter d), two diameters perpendicular to each other on the same cross-section should be measured, and the average value should be used as the diameter.
  3) According to the spring shear stress calculation formula:
   It can be seen that in order to reflect the maximum shear stress, the maximum value of spring D/D should be measured.
  B. Sample shape
  1) In order to ensure that the spring does not produce eccentric load after being stressed, the parallelism of the two ends of the spring and the perpendicularity of the whole spring should be strictly checked.
   2) In order to ensure uniform deformation of the spring after loading, the spring pitch should be uniform and the spring should not have excessive ellipticity.
  3) In order to facilitate the comparison of test results, the number of end turns and structure of the same batch of springs should be as consistent as possible.
  C. Different processing and heat treatment states have different effects on fatigue strength. Therefore, the same batch of samples should be processed and heat treated at the same time.
  1) The same batch of samples should be processed by automatic spring coiling machine as much as possible, and manual operation should be minimized.
  2) When the free height of the spring is adjusted by standing or strong pressure treatment, the fatigue strength may be reduced due to excessive load, so the spring specimen should not be adjusted by standing or strong pressure treatment.
  3) The same batch of spring test pieces should be made of the same coil of steel wire, and the surface conditions should be consistent. There should be no defects that cause stress concentration, such as rust, nicks and scratches.
  4) The heat treatment process also has a great influence on the fatigue strength. Even if the low-temperature tempering hardness after complete quenching and incomplete quenching is exactly the same, there is a significant difference in fatigue strength. Too large grain size and surface decarburization will reduce fatigue strength. Therefore, the same batch of samples should be heat treated in the same furnace.
  2. Spring life test
   (1) In order to avoid the eccentricity of the spring force, the spring force should be placed on the spring bracket during installation to ensure that the two ends of the spring are in flat contact.
   (2) The adjustment test piece has the same installation deformation and the same maximum deformation.
  (3) When determining the S-N curve, the load stress can be reduced from the maximum fatigue limit (refer to the existing spring fatigue data), and the spacing can be carried out at a ratio of 1.02 to 1.5.
   (4) During the test, in order to ensure that the load is as stable as possible, tests and adjustments should be carried out, but the time should be short. Can't be overloaded.
   (5) If the stress unit is MPa, the significant figure shall be rounded to one decimal place.
   fatigue life
1) When determining the fatigue life of the spring, the number of cycles N that can be used during the test is 1×10, 2×10, 5×10, 1×10, 2×10, 5×10, 1×10, 2 ×l07 is used as the reference fatigue life.
  2) Unless it is a special case, the test generally stops when the number of actions reaches 1×10, and it shall not be interrupted.
  3) Generally, the number of breaks of the test spring is regarded as the fatigue life. However, in some cases, the number of sample cracks is regarded as the fatigue life.
  4) The record of the number of actions is generally based on l0n, and the effective number is 3 times or more, such as 2.75×l06.
  Testing machine operation
   1) The test machine should start smoothly and without impact.
  2) Except for resonance fatigue testing machines, the resonance point may sometimes be passed during the process of reaching the operating speed or stopping the operation. In order to prevent excessive stress caused by resonance, a device to prevent resonance is installed.
  3) The same batch of samples should be tested at the same speed.
  4) The test shall be carried out continuously from beginning to end. When the operation needs to be stopped due to malfunction or other reasons, it shall be recorded in detail in the record.
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