Showing papers on "Critical speed published in 1981"

Field studies on the response of floating ice sheets to moving loads

Abstract: The results of field tests on the response of floating ice sheets to moving loads are presented and discussed. To measure time-dependent deflections under moving loads, a convenient method that does not require reference to a fixed datum is developed. This method is based on integrating the output of a sensitive slope indicator. The data support the available theory in most respects. Deflection under a load moving at constant speed is maximized when the speed is equal to a critical value. The critical speed is essentially a function of ice thickness and water depth. Observed critical speeds for ice thicknesses and water depths in the ranges 0.3−1.4 and 1.8−54.3 m, respectively, are generally 10% higher than predicted values; these ranges include present data and earlier data by others. The data show further that critical deflection and strain can be up to 4.6 and 1.4 times the corresponding static loading values. Discussion of this finding points out that abrupt speed changes should be avoided.

Method of eliminating truck hunting in railway trucks

Abstract: Truck hunting is eliminated in railway vehicles by applying a low coefficient of friction material to the vehicle wheels so that a lubricating film is provided between the vehicle wheels and the rail on which they ride which serves to increase the critical speed of a rail vehicle, i.e., the speed at which truck hunting occurs, to a value above its operating speed.

Feedback control of a single mass rotor on rigid supports

Abstract: This paper considers a single mass flexible rotor on rigid supports with proportional and derivative feedback control Undamped and damped free vibrations of the rotor-control system are considered as well as unbalance response Results indicate that proportional control alone increases the rotor critical speed, but does not affect the system damping Proportional control alone also decreases the system logarithmic decrement which tends to make the system more susceptible to external vibration excitations Derivative feedback control acts to increase the system damping, to reduce the response sensitivity, and is highly desirable

The Vibration Isolating Properties of Uncentralized Squeeze-Film Damper Bearings Supporting a Flexible Rotor

Abstract: The ability of an uncentralized squeeze-film damper bearing to inhibit the effects of vibration in a flexible rotor-bearing system, has been assessed in terms of non-dimensional system parameters. This analytical approach has shown that a correctly designed squeeze-film damper bearing is a very effective means of reducing both the amplitude of motion of the rotor and the force transmitted to the bearing support structure. However, the analysis has also indicated that a poorly designed squeeze-film damper bearing can produce amplitudes and forces greater than those which would arise if the bearing support remained rigid. An experimental programme has supported the validity of the above analytical technique by showing that the measured motion orbits of the journal and disk centers as the rotor passes through the critical speed, are very similar to those predicted theoretically. Also, the response curves for specific groups of system parameters show very similar trends in practice, to those which result from the analytical approach. Some indication of the ability of a squeeze-film damper bearing to reduce the effect of much greater unbalance than normal is also reported.

Stopping method of steam turbine

Abstract: PURPOSE:To shorten a length of time for the rotor of a turbine to pass its critical speed of rotation without causing an emission of a radioactive material in a condenser, by providing injection nozzles in a part of exhaust port at the side of blade final stage in the low pressure steam turbine of a boiling water nuclear power plant. CONSTITUTION:Under a low pressure turbine 3 in a boiling water nuclear power generating plant, a condenser 4 is installed. In an exhaust port 17, even-numbered spray nozzles 21 are provided at the position horizontal and symmetrical with a rotary shaft 13. When the steam turbine is stopped, the flow of its steam is stopped from a feed port 16 to decrease its rotational speed. When the turbine reaches the range of its critical speed of rotation, a flow regulator valve 23 is opened to feed steam from a boiler 22 in the station. In this way, because of a shorter time for passing the critical speed, an emission of radioactive material in the condenser 4 can be prevented.

Variable impedance bearings for large rotating machinery

Abstract: This thesis describes an investigation which was carried out under the Interdisciplinary Higher Degres (IHD) Scheme of The University of Aston in Birmingham. The investigation, which involved joint collaboration between the IHD scheme, the Department of Mechanical Engineering, and G.E.C. Turbine Generators Limited, was concerned with hydrostatic bearing characteristics and of how hydrostatic bearings could be used to enable turbine generator rotor support impedances to be controlled to give an improved rotor dynamic response. Turbine generator rotor critical speeds are determined not only by the mass and flexibility of the rotor itself, which are relatively easily predicted, but also by the dynamic characteristics of the bearing oil film, pedestal, and foundations. It is because of the difficulty in accurately predicting the rotor support characteristics that the designer has a problem in ensuring that a rotor's normal running speed is not close to one of its critical speeds. The consequence of this situation is that some rotors do have critical speeds close to their normal running speed and the resulting high levels of vibration cause noise, high rotor stresses, and a shortening of bearing life. A combined theoretical and experimental investigation of the effects of mounting the normal rotor journal bearing in a hydrostatic bearing was carried out. The purpose of the work was to show that by changing the oil flow resistance offered by capillaries connecting accumulators to the hydrostatic bearing, the overall rotor support characteristics could be tuned to enable rotor critical speeds to be moved at will. Testing of a combined journal and hydrostatic bearing has confirmed the theory of its operation and a theoretical study of a full size machine showed that its critical speed could be moved by over 350 rpm and that its rotor vibration at running speed could be reduced by 80%.

Balancing method for elastic rotor

Abstract: PURPOSE:To obtain a properly balanced state constantly in an elastic rotor to be driven nearly in or over a critical speed by analyzing vibration values at respective points on the basis of fixed deformation modes for providing balancing weights in accordance with these results CONSTITUTION:When a rotor 11 is supported by three bearings 12, each bearing 12 is provided with a vibration displacement gauge 13 beside Meanwhile, another vibration displacement gauge 13' is equipped near the belly of a sympathetic vibration mode not beside the bearing 12, and a tacho-pulse pickup 14 is provided on the shaft end When the vibration mode is selected as phi in case bearing parts of the rotor 11 are supported by pins and deformation modes are selected as delta1-delta3 in case bearings 12 are opened respectively at one spot and given forced displacements there by certain amounts, vibrations at the rotation of the rotor 11 are analyzed by an analyzer 16 on the basis of these modes Balancing weights 17 are provided on a modifying face to reduce vibrations of these components individually for obtaining a properly balanced state

Operation of rotary machine

Abstract: PURPOSE:To safely operate the rotor in the rotary machine with low vibration value by providing a static pressure rotor floating unit at a bearing unit for supporting the rotor and floating the rotor at the speeds of the range in the vicinity of its critical speed, thereby preventing the resonance of the rotary machine. CONSTITUTION:A sliding bearing 2 for supporting the rotor 1 accommodates a high pressure oil supply hole for hydrostatically floating the rotor, which hole is connected to an oil supply tube 3. High pressure oil is supplied by an oil pump 5 which operates or stops upon reception of the revolution of the rotor as an electric signal 4. A check valve 12 is provided to prevent backflow of the oil, and wasted oil is recovered through a waste oil tube 6 to an oil tank 7. Since the static pressure rotor floating unit can be accordingly operated at the speed of the range in the vicinity of critical speed to thereby increase the thickness of an oil film in the bearing unit 2, the resonance of the rotary machine can be avoided at the critical speed, and can also be resultantly safely operated with low vibration value.

Turbine stopping method

Abstract: PURPOSE:To prevent scattering of radioactive substances in condenser due to use of vacuum pump in a boiling water reactor or the like, by lowering the cooling capacity of condenser and thereby decelerating the speed of steam turbine temporarily when the turbine speed passes through the critical revolution speed. CONSTITUTION:Before turbine speed reaches the critical speed range, the amount of water passed to cooler 19 in condenser 4 is reduced for lowering its cooling capacity. Resultantly, steam introduced into condenser 4 via by-pass line 28 is condensed at the time when turbine is stopped, which causes rises of steam pressure. Therefore, turbine speed is lowered due to wind loss of steam passing through blades near the final stage. When turbine speed has exceeded the critical speed, the amount of cooling water is increased to the previous level for restoring vacuum level in turbine by lowering steam pressure of condensate.

Speed controlling method of rotary machine by monitoring axial vibration

Abstract: PURPOSE:To prevent a rotary machine from making it to an abnormal vibration state, by forecasting in advance a vibration state in the critical speed area, and controlling a speed of the rotator in accordance with the forecast result. CONSTITUTION:A signal from the vibration detector 3 is smoothed and is inputted to the speed increase deciding device 1. The inputted signal is differentiated, and a variation rate signal can be obtained. This signal is provided to the multiplier 12, and an increment DELTAA of the oscillation amplitude is found. This increment DELTAA and a vibration signal A from the smoothing circuit 4 and operated by the adder 13 as to said sum A+DELTAA, and a threshold level AB of the true natural vibration frequency NC under this operating condition is subtracted from A+DELTAA by the adder 20, and DELTAAC is found. When this DELTAA is DELTAAC>=0, the output of the step signal generator 21 is turned on. In case of DELTAAC>0, measures such as an alarm, etc. are taken place or the operation control must be changed. The output of the generator 21 passes through the AND circuit 24 is case of N-ND (N and ND represent a signal of revolutional number and the revolutional number of forecast to be executed, respectively), and this speed controlling signal SN is provided to the speed controlling device 25.