• Tribology and Lubricants
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• v.33 no.5
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• pp.220-227
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• 2017

In this work, carbon nanotube and nano-size alumina particle are exploited as additive for lubrication experiment. We used pin-on-disk type tribometer to investigate the tribological characteristics of lubricants with respect to additives and rotational speed. We conducted more than 15 trials of tribotests for two hours for each specimen to obtain stable and accurate frictional force and to create measurable wear track on the substrate. We conducted tests at the boundary/mixed lubrication regime to evaluate the influence of additives on the tribological characteristics. We found that the friction coefficient decreased as the rotational speed increased and as additives were added. In particular, the reduction of friction by adding additives was more significant at low rotational speed than at high rotational speed. We speculate that the additives helped to separate and protect the two contacting surfaces at low speed, while the influence of additives was not significant at high speed since sufficiently thick lubricant film was formed. The wear of the substrate was also reduced by adding additives to the lubricant. However, in contrast to friction, the amount of wear at high rotational speed was less when alumina particles were added to the lubricant than the amount of wear at low speed. We speculate that the increased wear at low rotational speed is as a result of the intermittent abrasive wear caused by alumina particles with uneven shape, while the reduced wear at high speed is as a result of sufficient film thickness which prevented the abrasive wear.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2012.10a
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• pp.388-393
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• 2012

Vibration condition monitoring at low rotational speeds is still a challenge. Acoustic emission (AE) is the most used technique when dealing with low speed bearings. At low rotational speeds, the energy induced from surface contact between raceway and rolling elements is very weak and sometimes buried by interference frequencies. This kind of issue is difficult to solve using vibration monitoring. Therefore some researchers utilize artificial damage on inner race or outer race to simplify the case. This paper presents vibration signal analysis of low speed slewing bearings running at a low rotational speed of 15 rpm. The natural damage data from industrial practice is used. The fault frequencies of bearings are difficult to identify using a power spectrum. Therefore the relatively improved method of empirical mode decomposition (EMD), ensemble EMD (EEMD) is employed. The result is can detect the fault frequencies when the FFT fail to do it.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2000.05a
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• pp.922-925
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• 2000

Generally, the machining accuracy in ball end milling directly depends on the rotational accuracy affected by the spindle speeds. The effects of spindle speeds for rotational accuracy in the high speed regions are more dominant than those in the low speed regions. This paper will investigate effects that the Increased speed affects on the rotational error according to the increase of a rotational speed and machining characteristics of the high speed ball-end milling in various rotational speeds and on various materials by using the high speed air-bearing spindle.

• International Journal of Fluid Machinery and Systems
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• v.4 no.3
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• pp.334-340
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• 2011

Turbo-pumps have weak points, such as the pumping operation is unstable on the positive slope of the head curve and/or the cavitation occurs at the low suction head. To improve simultaneously both weak points, the first author invented the unique pumping unit composed of the tandem impellers and the peculiar motor with the double rotational armatures. The front and the rear impellers are driven by the inner and the outer armatures of the motor, respectively. Both impeller speeds are automatically and smartly adjusted in response to the pumping discharge, while the rotational torques between both impellers/armatures are counter-balanced. Such speeds contribute to suppress successfully not only the unstable operation at the low discharge but also the cavitation at the high discharge, as verified with the axial flow type pumping unit in the previous paper. Continuously, this paper investigates experimentally the effects of the tandem impeller profiles on the pump performances and the rotational speeds against the discharge, using the impellers whose loads are low and/or high at the normal discharge. The worthy remarks are that (a) the unstable operation is suppressed as expected and the shut off power is scarcely large in the smart control, (b) the blade profile contributes to determine the discharge giving the maximum/minimum rotational speed where the reverse flow may incipiently appears at the front impeller inlet, (c) the tandem impeller profiles scarcely affect the rotational speeds, while the loads of the front and the rear impellers are same, but (d) the impeller with the low load must run faster and the impeller with the high load must run slower at the same discharge to take the same rotational torque, and (e) the reverse flow at the inlet and the swirling velocity component at the outlet of the front impeller with the high load require making the rotational speed of the rear impeller with low load fairly faster at the lower discharge.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.9
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• pp.3184-3189
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• 2010

To develop gasoline engine fuel pump, it is needed to measure the rotational speed of the pump. In general, because gasoline fuel pump is submerged in the fuel tank, it is difficult to measure the rotational speed directly. Currently, there are two popular methods measuring the rotational speed. One of them is using a piezoelectric accelerometer, and the other is using a current sensor. Originally, a piezoelectric accelerometer had been applied to measure the frequency of the motor vibration. A current sensor is measuring current frequency of the commutator slot. In this study, both the piezoelectric accelerometer and the current sensor have been applied on the fuel pump to calculate the rotational speed at the same time. As a result, the current sensor delivered highly accurate rotational speed information compared with that of the piezoelectric accelerometer. Especially, low rotational speed region, the current sensor shows very robust measuring characteristics. To measure the rotational speed within 1% error, the piezoelectric accelerometer needs to be set with less then 0.5Hz datum storage interval, and the current sensor needs to be set with less then 2.0Hz datum storage interval.

• The KSFM Journal of Fluid Machinery
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• v.15 no.5
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• pp.32-36
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• 2012

The positive displacement pump and the regenerative pump are widely used in the range of low specific speed, $n_s{\leq}100$[rpm, m3/min, m]. The positive displacement pump is not suitable for miniaturization and operation in high rotational speed. The regenerative pump has a problem with large leakage flow and low efficiency. While the centrifugal pump has advantages of high efficiency, miniaturization and high rotational speed, efficiency drops sharply with decrease in specific speed. Therefore the purpose of this study is to design a new type of centrifugal pump that has advantages of centrifugal pumps in operation in low specific speed. The name of this new type of pump was called 'Pipe type centrifugal pump', since the flow path through the impeller is simple circular pipe. Due to the simple shape of impeller, the manufacturing process is simple and cost is low. There is strong jet flow at the outlet of the impeller. This jet induces flow path loss, meridional dynamic pressure loss and mixing loss. Large disk friction makes the efficiency be limitted in the range of low specific speed. Even though the loss and the low efficiency, 'Pipe type centrifugal pump' represents stable performance, affordable pressure ratio and efficiency better than that of other low specific speed pumps.

• The KSFM Journal of Fluid Machinery
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• v.14 no.1
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• pp.11-17
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• 2011

The numerical study was conducted to investigate the pump performance due to impeller related parameters change in centrifugal pump of very low specific speed by using CFD code. A small centrifugal pump whose specific speed is $N_s=76.2$ was used, and the performance characteristics were discussed for different number of vanes, rotational speed, and the length and height of vane. The numerical results at a very low specific speed show that the increase of the number of vanes has little effect on improvement of output pressure but results in the reduction of pressure fluctuation, and that the head increases with the increase in the rotational speed. The decreasing the length of vane has a considerable reduction of the capacity coefficient in comparison with decreasing the height of vane.

• 한국신재생에너지학회:학술대회논문집
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• 2009.06a
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• pp.521-524
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• 2009

This study describes aerodynamic noise of high speed wind turbine system, which is invented as a new concept in order to reduce the torque of main shaft, for design parameters of the rotor blade. For parametric study of high speed rotor aerodynamic noise, Unsteady Vortex Lattice Method with Nonlinear Vortex Correction Method is used for analysis of wind turbine blade aerodynamic and Farassat1A and Semi-Empirical are used for low frequency noise and airfoil self noise. Parameters are chord length, twist and rotational speed for this parametric research. In the low frequency range, the change of noise is predicted the same level as each parameters varies. However, in case of broadband noise of blade, the change of rotational speed makes more variation of noise than other parameters. When the geometric angles of attack are fixed, as the rotational speed is increased by 5RPM, the noise level is increased by 4dB.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2001.11a
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• pp.316-321
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• 2001

The paper presents the unbalance response of a rotor-bearing system supported by an active control bearing. The proportional, derivative and integral controls are employed for the control algorithm of an active control bearing to suppress the unbalance response of a rotor-bearing system. Results of analytical investigations on the unbalance responses of a rotor supported by an active control bearing are presented for various control gains. It is found that the unbalance response of a rotor can be greatly suppressed by the proportional, derivative or integral control of the bearing. The proportional control is more effective than the derivative control at low rotational speed, and the derivative control is more effective than the proportional control at high rotational speed. In the case of the integral control of the bearing , the unbalance response of a rotor is increased as a general rule. However, the integral control of the bearing is extremely superior to proportional or derivative control at very low rotational speed.

• Tribology and Lubricants
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• v.18 no.2
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• pp.99-104
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• 2002

The paper presents the unbalance response of a rotor-bearing system supported by an active control bearing. The proportional, derivative and integral controls are investigated fur the control algorithm of an active control bearing to suppress the unbalance response of a rotor-bearing system. Results of analytical investigations on the unbalance responses of a rotor supported by an active control bearing are presented for various control gains. It is found that the unbalance response of a rotor can be greatly suppressed by the proportional, derivative or integral control of the bearing. The proportional control is more effective than the derivative control at low rotational speed, and the derivative control is more effective than the proportional control at high rotational speed. In the case of the integral control of the bearing, the unbalance response ova rotor is increased as a general rule. However, the integral control of the bearing is extremely superior to proportional or derivative control at very low rotational speed.