나사 골 반경에 따른 박용 유압 어큐뮬레이터의 구조 안전성 평가

Abstract

A hydraulic accumulator used for marine diesel engine consist of two hemispherical threaded shells, threaded lower shell and threaded upper shell in which liquid and gas are separated by a flexible diaphragm. A hydraulic accumulator can be applied in marine diesel engine with heavy vibration and pressure pulsation so it can damp the sudden pressure shock, pulsation and cushion the pressure spike. During the operation of a hydraulic accumulator, the thread of the lower shell is subjected to concentrated stress. The purpose of this study is to investigate numerically the structural safety of a hydraulic accumulator according to the thread root radius and the friction coefficient using three different models of the lower shells with the thread root radius 0.1 mm, 0.2 mm and 0.4 mm respectively.

Charging pressure of a hydraulic accumulator is 300 bar and operating pressure range is from 280 to 300 bar. The primary stress was analyzed by calculating the membrane and bending stress of the lower shell, the upper shell and the thread where stress is concentrated under the pressure condition of 300 bar using finite element method. The mesh test is carried out to ensure reliable analysis result by applying very small element size and the gradient mesh in the thread where stress is concentrated.

Even if the thread is not subject to the major deformation, the fatigue life should be evaluated because the peak stress of the thread can cause the fatigue failure. There are three different cases of operating pressure range should be evaluated for the fatigue life according to the service condition of a hydraulic accumulator. First, operating pressure range is from 280 to 300 bar in the normal service condition. Second, operating pressure range is from 250 to 300 bar under the severe service condition. Lastly, operating pressure range is defined as the maximum pressure range of 200 to 300 bar under the abnormal service condition.

Alternating stress is calculated with the total stress and the fatigue penalty factor according to the yield strength and the tensile strength of the applied material. The fatigue life is evaluated by applying alternating

stress to the fatigue life curve of the applied material. The static analysis result of a hydraulic accumulator shows that the general primary membrane stress and primary bending stress do not exceed the

allowable stress in the lower shell, the upper shell and the thread where stress is concentrated under the pressure condition of 300 bar. However the result of evaluation of the fatigue life shows the fatigue life increased as the thread root radius and the friction coefficient increased under the severe service condition and abnormal service condition of a hydraulic accumulator used for marine diesel engine.