• 대한조선학회논문집
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• 제58권2호
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• pp.84-89
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• 2021

Recently, shipyards are making many efforts to reduce the number of the mounted blocks by increasing the block size. This is to improve productivity and reduce related costs by minimizing block movement and shortening the building period. However, as the blocks become larger, the weight increases considerably. If the target block has a damage due to an unexpected accident during block lifting, it may seriously cause a problem of the reusability of the block. In this study, a large-sized block of the offshore structure weighing 480 tons was lifting with a total of seven sling belts, and one sling belt was broken while it was moving, resulting in a situation in which a part of the edge of the block collided with the ground. The aim of this paper is to verify the structural integrity of the block that directly collides with the ground in the form of free fall due to the sling breakage. Considering that the hook loads acting on several sling belts holding the block are redistributed when a sling belt is broken, the hook loads were recalculated at the angle just before the sling breakage. These loads were used to check the safety of the sling belts. In addition, FE analysis was performed by calculating the amount of impact from the free fall condition, obtaining the impact area by using Hertz's contact theory, and then applying the impact load to the area.

• Tribology and Lubricants
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• 제35권3호
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• pp.158-168
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• 2019

This paper presents the investigation of two types of rotordynamic modeling issues for 2.2 kW-class, rated speed of 1,800 rpm, squirrel-cage type induction motors. These issues include the lamination structure of rotor cores, and the radial clearance of ball bearings that support the shaft of the motor. Firstly, we focus on identifying the effects of rotor core lamination on the rotordynamic analysis via a 2D prediction model. The influence of lamination is considered as the change in the elastic modulus of the rotor core, which is determined by a modification factor ranging from 0 to 1.0. The analysis results show that the unbalanced response of the rotor-bearing system significantly varies depending on the value of the modification factor. Through modal testing of the system, the modification factor of 0.079 is proven to be appropriate to consider the effects of lamination. Next, we investigate the influence of ball bearing clearance on the rotordynamic analysis by establishing a bearing analysis model based on Hertz's contact theory. The analysis results indicate that negative clearance greatly changes the bearing static behavior. Rotordynamic analysis using predicted bearing stiffness with various clearances from -0.005 mm to 0.010 mm reveals that variations in clearance result in a slight difference in the displacement of the system up to 18.18. Thus, considering lamination in rotordynamic analysis is necessary as it can cause serious analysis errors in unbalanced response. However, considering the effect of the bearing clearance is optional because of its relatively weak impact.

• 대한의용생체공학회:의공학회지
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• 제20권1호
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• pp.99-106
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• 1999

인공관절용 재료의 내마모성 향상을 목적으로 초고분자량 폴리에틸렌(UHMWPE)을 가교시켜 기계적 성질과 내마모성을 조사하였다. 가교제로 dicumyl peroxide(DCP)를 사용하고 가교보조제로 triallyl cyanurate(TAC)를 사용하여 용융상태에서 UHMWPE를 가교시켰다. DCP와 TAC의 함량이 증가할수록 가교된 초고분자량 폴리에틸렌(XUMPE)의 젤 함량은 증가하고 녹는점, 결정화온도, 결정화도, 인장성질은 감소하였다. Pin-on-disk 마모시험과 작은 하중을 가한 ball-on-disk 마모시험에서는 기계적 성질의 감소에도 불구하고 XUMPE의 마모부피가 UHMWPE보다 감소하였다. 이 두 시험에서의 마모는 주로 변형에 의한 것으로 판단되었으며, 마모부피는 Hertz 접촉 이론으로 계산한 최대접촉응력과 XUMPE의 항복강도의 비에 비례함을 보였다. 큰 하중을 가한 ball-on-disk 마모시험에서는 기계적 성질이 낮을수록 더 큰 마모부피를 보였으며, 마모자국의 표면거칠기와 마찰계수도 증가하였다. 이는 인장강도를 넘는 응력이 가해졌을 때 항복강도와 인장강도가 더 낮은 XUMPEDml 변형과 파괴가 촉진되기 때문으로 생각되었다.