• Tribology and Lubricants
• /
• 제17권4호
• /
• pp.290-296
• /
• 2001

Lift-off means that the rotating journal surface separates from the leaf surface in an air-lubricated multi-leaf foil journal bearing. Lift-off builds up minimum film thickness which will provide load capacity between leaves and journal. In this paper an analysis of air-lubricated multi-leaf foil journal bearing was performed with effects of multi-leaf foil deflection and compressible lubrication equation simultaneously. Minimum film thickness built up versus journal speed predicts the lift off characteristics and these results will be useful in designing and selecting the proper speed of the air-lubricated multi-leaf foil journal bearing.

• 한국전산구조공학회논문집
• /
• 제24권3호
• /
• pp.337-342
• /
• 2011

매우 큰 편심하중을 지지하면서 상하로 수직위치를 제어하는 의료용 리프트 칼럼은 수직위치의 정확도를 확보하기 위해 매우 낮은 구조적 변형과 진동은 물론 높은 피로강도가 요구된다. 리프트 칼럼은 일반적으로 3단 슬라이딩 붐구조로 유도전동기로 작동되며, 상하운동에 따라 변동응력을 나타낸다. 본 연구에서는 리프트 칼럼의 상하운동으로 야기되는 변동응력에 따른 피로강도를 수치해석적으로 예측하였다. 한 주기 상하운동에 따른 응력변동은 유한요소해석으로 구하였으며, 피로수명은 Palmgren-miner기법과 재료의 S-N선도를 적용하여 예측하였다. 수치해석의 신뢰성을 확보하기 위하여 리프트 칼럼의 상세형상과 체결부위을 반영한 3차원 FEM모델을 생성하였으며, 칼럼과 패드사이 계면은 접촉조건으로 처리하였다.

• 설비공학논문집
• /
• 제21권12호
• /
• pp.695-702
• /
• 2009

Dynamic behavior of Libr-water absorption system using low-temperature hot water was investigated numerically. Thermal-hydraulic model of single-effect/double-lift 100 RT chiller was developed by applying transient conservation equations of total mass, Libr mass, energy and momentum to each component. Transient variations of system properties and transport variables were analysed during start-up operation. Numerical analysis were performed to quantify the effects of bulk concentration and part-load operation on the system performance in terms of cooling capacity, coefficient of performance, and time constant of system. For an absorption chiller considered in the present study, optimum bulk concentration was found to exist, which resulted in the minimum time constant with stable cooling capacity. COP and time constant increased as the load decreased down to 40%, below which the time constant increased abruptly and COP decreased as the load decreased further.

• Tribology and Lubricants
• /
• 제39권5호
• /
• pp.203-211
• /
• 2023

This study experimentally investigates the effects of angular acceleration on the friction and wear performances of a gas foil thrust bearing (GFTB) using a typical GFTB with six pads. The outer radius of the bearing is 31.5 mm, the total bearing area is 2,041 mm² , and the bump foil and incline (ramp) height are both 500 ㎛. The newly developed GFTB test rig for measuring the friction torque and coefficient measures the axial load, drag torque, lift-off speed, and touch-down speed. The experiment is conducted for angular accelerations of 78.5, 314.2, and 328.3 rad/s² at axial loads of 5, 10, and 15 N, respectively. The test shows that the start-up friction coefficient increases with increasing axial load at the same angular acceleration, and the friction coefficient decreases with increasing angular acceleration under the same axial load. As the angular acceleration increases, the lift-off speed at the motor start-up increases, and the touch-down speed at the motor stop decreases. The wear distance of the GFTB for a single on/off cycle increases with increasing axial load at the same angular acceleration and decreases nonlinearly with increasing angular acceleration under the same axial load. The test results suggest that adjusting the rotational angular acceleration helps reduce bearing friction and wear.

• 한국공작기계학회:학술대회논문집
• /
• 한국공작기계학회 2004년도 춘계학술대회 논문집
• /
• pp.209-214
• /
• 2004

This paper deals with tipping over of hydraulic excavator's crane work. If excavator lift too heavy weight, excavator tipped up. This is 38% of whole excavator accidents. In this paper, tipping over load which is maximum load of excavator can lift with displacement of excavator links, real load and tipping over rate are calculated with Zero Moment Point. We designed the tipping-over stability criterion algorithm considering the dynamic characteristics to which ZMP theory is applied and discussed the usefulness of the proposed algorithm compared with the moment equilibrium equation through the simulation and the actual test.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2000년도 춘계학술대회논문집B
• /
• pp.491-496
• /
• 2000

The objective of this study is to increase lift and decrease drag of an airfoil by delaying flow separation with piezo-ceramic actuators. The airfoil used is NACA 0012 and the chord length is 30cm. An experiment is performed at the freestream velocity of 15m/s at which the Reynolds number is $3{\times}10^5$. Seven rectangular actuators are attached to the airfoil surface and move up and down based on the electric signal. At the attack angle of $16^{\circ}$, the separation point is delayed downstream due to momentum addition induced by the movement of the actuators. Drag and lift are measured using an in-house 2-dimensional load cell and the surface pressures are also measured. Lift is increased by 10%, drag is reduced by 50%, and the efficiency is increased to 170%. The flow fields with and without control are visualized using the smoke-wire and tuft techniques.

• Tribology and Lubricants
• /
• 제31권3호
• /
• pp.109-124
• /
• 2015

This paper presents a wear analysis procedure for the journal bearings on a stripped-down single-cylinder engine during start-up and coast-down by motoring. A journal bearing is in the mixed elastohydrodynamic (EHL) lubrication region when the shaft speed is less than the corresponding lift-off speed. Below the lift-off speed, a wear scar can form on bearing surfaces. In part 1 of this paper, we develop the appropriate formulations and the calculation procedure for the analysis. Specifically, we formulate an equation for modified film thickness in a journal bearing considering the additional wear volume. In order to obtain the modified specific wear rate induced by the modified Archard’s wear coefficient, we utilized the extended non-dimensional diagram for the specific wear rate, k, the fractional film defect coefficient, Ψ and the asperity load sharing factor, γ₂. This asperity load sharing factor is newly calculated by setting the Zhao-Maietta-Chang (ZMC) asperity contact pressure equation coupled with the central film thickness equation derived by using the ZMC asperity contact model equal to the modified central contact pressure derived by using the central (or maximum) contact pressure at the dry rough line-contact configuration. We can use the procedure introduced in this paper to determine the lifetime (or longterm) linear wear in radial journal bearings that is a result of repeated stop-start cycles.

• Tribology and Lubricants
• /
• 제31권4호
• /
• pp.141-147
• /
• 2015

Gas foil thrust bearings (GFTBs) have attractive advantages over rolling element bearings and oil film thrust bearings, such as oil-free operation, high speed stability, and high-temperature operation. However, GFTBs have lower load carrying capacity than the other two types of bearings owing to the inherent low gas viscosity. The load carrying capacity of GFTBs depends mainly on the compliance of the foil structure and the formed hydrodynamic wedge, where the gas pressure field is generated between the top foil and the thrust runner. The load carrying capacity of the GFTBs is very important for the suitable design of oil-free turbomachinery with high performance. The aim of the present study is to identify the characteristics of the load carrying performance of GFTBs. A new test rig for the experimental measurements is designed to provide static loads up to 800 N using a pneumatic cylinder. The maximum operating speed of the driving motor is 30,000 rpm. A series of experimental tests—lift-off test, static load performance test, and maximum load capacity test—estimate the performance of a six-pad GFTB, in terms of the static load, driving torque, and temperature. The maximum load capacity is determined by increasing the static load until the driving torque rises suddenly with a sharp peak. The test results show that the torque and temperature increase linearly with the static load. The estimated maximum load capacity per unit area is approximately 80.5 kPa at a rotor speed of 25,000 rpm. The test results can be used as a design guideline for GFTBs for realizing oil-free turbomachinery.

• 한국지반공학회논문집
• /
• 제38권4호
• /
• pp.21-32
• /
• 2022

강재형 말뚝기초는 안정적인 지지력 확보와 높은 시공성으로 해양구조물 기초로 널리 활용되고 있다. 일반적으로 설계수명에 도달한 해양구조물은 해체수순을 밟게 되는데 말뚝기초는 높은 인발하중과 경제적인 이유로 예외적으로 현장에 존치되는 경우가 많다. 현장에 존치된 기초는 환경오염뿐만 아니라 신규구조물을 건설하는데 장애요인이 될 수 있으므로 말뚝기초 완전해체를 위한 연구가 필요한 실정이다. 본 연구에서는 인발하중을 획기적으로 저감시킬 수 있는 신형식 이중벽 말뚝기초를 제안하고 축소모형실험을 통해 인발하중 저감성능을 실험적으로 평가하였다. 이를 위해 축소된 이중벽 말뚝기초를 제작하고 건조된 모래지반에서 말뚝 설치 및 인발 거동을 모사하였으며, 측정된 인발하중을 동등한 단면의 일반말뚝과 비교하였다. 그 결과, 조밀한 모래지반에서 이중벽 말뚝의 최대 인발력이 일반말뚝에 비해 45% 감소되는 것을 확인하였다. 본 연구를 통해 이중벽 말뚝의 인발하중 저감 성능과 메커니즘을 검증하였으며, 설계수명에 도달한 기초를 완전히 회수할 수 있는 가능성을 확인하였다.

• Tribology and Lubricants
• /
• 제40권3호
• /
• pp.103-110
• /
• 2024

Gas foil thrust bearings (GFTBs) are oil-free self-acting hydrodynamic bearings that support axial loads with a low friction during airborne operation. They need solid lubricants to reduce dry-friction between the runner and top foil and minimize local wears on their surfaces during start-up and shutdown processes. In this study, we evaluate the lift-off speeds and load capacity performance of a GFTB with Polytetrafluoroethylene (PTFE) surface coating by measuring drag torques during a series of experimental tests at increasing ambient temperatures of 25, 75 and 110℃. An electric heat gun provides hot air to the test GFTB operating in the closed booth to increase the ambient temperature. Test results show that the increasing ambient temperature delays the lift-off speed and decreases the load capacity of the test GFTB. An early developed prediction tool well predicts the measured drag torques at 60 krpm. After all tests, post inspections of the surface coating of the top foil are conducted. Scanning electron microscope (SEM) images imply that abrasive wear and oxidation wear are dominant during the tests at 25℃ and 110℃, respectively. A quantitative energy dispersive spectroscopy (EDS) microanalysis reveals that the weight percentages of carbon, oxygen, and nitrogen decrease, while that of fluorine increases significantly during the highest-temperature tests. The study demonstrates that the increasing ambient temperature noticeably deteriorates the static performances and degrades the surface coating of the test GFTB.