Special aspects of calculation of lifting equipment reliability

Aim. When designing lifting equipment as a whole, as well as of its elements it is desirable to perform not only deterministic strength estimations, but also a probabilistic calculation of major reliability indices. Theoretical approach to the calculation of major reliability indicators of lifting equipment is described by V.I Braude. In practice the calculation of reliability of lifting equipment is usually quite difficult, because the information about values for certain indices provided in literary sources is incomplete and discordant. It causes the necessity to use average reliability indices and to introduce different assumptions to the calculation. And the calculation results turn out to be rather approximate. At the same time an approximate calculation of reliability indices allows to decide on efficient use of one or another design layout of lifting equipment and/or its structural unit.

Methods. To demonstrate the logical arguments that could be used at the calculation of reliability of lifting equipment, the article describes an example of calculation of probability of reliable operation for the lifting gear of an overhead crane, executed by a “detailed” scheme and consisting of nine elements: a three-phase induction electric motor with a short-circuit rotor; a parallel shaft double-stage gear box; a block brake with locking movement actuated by a coil spring and with breaking actuated by a shortstroke alternating electromagnet; flexible bolt coupling (with brake pulley); load drum; drum axle (or shaft); drum support; load cable and its mountings; hook assembly. Structurally, the elements of a lifting gear are connected in-series, i.e. in case of a failure of any element, the operable state of the gear is violated (a failure occurs).

Results. The known experience of operation of lifting equipment shows that the most probable failures of a lifting gear’s elements are the following failures: turn-to-turn short circuit of electric motor; wear out of bearings and gear teeth; turn-to-turn fault of a coil of a brake electromagnet; tearing up of a pulley of a flexible bolt coupling and break cheek wear out; fatigue breakdown of a drum and a bearing block, built into a drum; fatigue breakdown of a drum axle (or shaft); wear out of drum axle bearings, built into a drum; wear out (breakage) of wires and strands of a load cable; hook wear out and bearing freezing of a hook assembly. That is why the reference data used for calculation usually describe the probability of occurrence or a rate of these particular failures. Calculation was carried out with the following assumptions: Degradation (wear rout) failures were not taken into account, since they are anticipated during the phase of technical maintenance and repair; failures, caused by the violations of the rules of safe operation, were refer not to the crane failures, but to the failures of other systems. For descriptive reasons the elements of a lifting gear were chosen from the catalogue with a certain “margin” and without taking a loading mode into account.

Conclusions. The calculation results showed that neglecting various
load-bearing factors (for instance, a gear box underload by a rotation moment) may lead to excess reliability of a crane as a whole, its machinery and structure components.

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