Non-destructive inspection of the state of a helical cylindrical compression spring

BACKGROUND: When springs are used, they undergo “compression-decompression” cyclic loading, resulting in a gradual decrease in elastic properties and performance, formation of microcracks, and the springs destruction. Moreover, if these are the springs of the valve mechanism of the internal combustion engine, then they are subject to heating, which intensifies the processes of loss of elasticity and destruction. In practice, spring control often comes down to its external inspection, assessment of geometric parameters and dynamic testing. The latter requires expensive equipment and a long period for the test itself.

AIM: Improvement of the non-destructive inspection method for determining the state of a coil spring using a new technical solution at the level of patentability.

METHODS: The presence of microcracks and their growth during cyclic loading leads to a change in the microshape of the cross-section of the spring coil, which affects the electrical properties, in particular, its electrical resistance. Therefore, assessing the performance of a spring comes down to determining the number and volume of microcracks. Measuring the microgeometry of the spring can be achieved using a highly sensitive microohmmeter with a division value of at least 10^-6 Ohms. In the working coils of the spring, especially on the “inner fiber of the coils” [1], the greatest tangential stresses arise during its operation. Consequently, the inner fiber of the working coils of the spring is more susceptible to the formation of microcracks.

RESULTS: To implement the proposed non-destructive inspection of the state of a helical coil spring, a device, including a microohmmeter and a mechanical type power structure for its tension and compression, is required. The tension and compression of the spring is monitored using a dynamometer. The terminals from the microohmmeter for measuring resistance are fixed on the spring coils under test. Determining the resistance value of a new spring and subsequent monitoring during operation will make it possible to identify a faulty element at an earlier stage and to determine the feasibility of restoring the spring.

CONCLUSION: The proposed technical solution simplifies monitoring the state of the entire spring without destroying it, reduces the time for monitoring its state as a whole, and reduces the energy intensity of this process.