Rotor support system

The determination of the first bending critical speed is well established however, there is also concern with regard to the rotor support system s sensitivity to exciting forces. These come from unbalance and/or gas dynamic forces arising during operation in service. Operation with dirty corrosive gas will soon cause rotor unbalance. The rotor dynamics verification test is concerned with synchronous excitaticm, namely unbalance. The test must also verify that the separation margins are to specification. 

Rotating machines subject to imbalance caused by turbulent or unbalanced media flow include pumps, fans, and compressors. A good machine design for these units incorporates the dynamic forces of the gas or liquid in stabilizing the rotating element. The combination of these forces and the stiffness of the rotor-support system (i.e., bearing and bearing pedestals) determine the vibration level. Rotor-support stiffness is important... 

As with any other rotating machine, fans require a rigid, stable foundation. With the exception of in-line fans, they must have a concrete footing or pad that is properly sized to provide a stable footprint and prevent flexing of the rotor-support system. 

Figure 5-13. The Elliott TH expander is a single-stage hot gas expander with an axial inlet and vertical exhaust. The TH-line expanders range in horsepower from 4,000 to 50,000 and are of overhung rotor construction. The expander consists of four major components the inlet casing and its supports, the exhaust casing, the bearing housing and support system, and the rotor.

Critical speed is when the frequency of a periodic exciting frequency applied to the rotor-bearing support system corresponds to the natural frequency of the system. 

Critical speeds correspond to the natural frequencies of the gears and the rotor bearings support system. A determination of the critical speed is made by knowing the natural frequency of the system and the forcing function. Typical forcing functions are caused by rotor unbalance, oil filters, misalignment, and a synchronous whirl. 

Science of rotor-bearing-support system design and operation. Predictive maintenance technique that uses spectrographic, wear particle, ferrography, and other measurements of the lubricating oil as a diagnostic tool. 

The development of inertial guidance systems depends on finding methods to eliminate all unpredictable forces from the inertial element. In the gyro field, engineering techniques have been pushed almost to the limit in the design of mechanical rotor supports that minimize friction. A promising alternative to mechanical suspension is the use of the pressures exerted by electromagnetic fields. 

Fig. 6-10. Bell Helicopter Company engineers hold an 8-foot length of boron/epoxy shafting which will comprise one of three sections that make up the final helicopter tail rotor driveshaft system. An aluminum shaft system would require five shorter sections, with additional end fittings and bearing supports, to transmit an equal amount of power over an equal distance. The lighter weight and superior stiffness of the composite material thus permits an overall system weight reduction of 30%. Courtesy of Textron s Bell Helicopter Co.)...

For many years it has been known that a small amount of out-of-balance can improve the stability of a rotor supported in plain self acting gas journal bearings. Relatively few experiments have been cai ried out to study this phenomenon. The research reported here was undertaken to discover how a rotor responded to out-of-balance and the effect which this had on the onset of half speed whirl. If the rotor response can be predicted and a criterion established for suppression of half speed whirl, then it should be possible to assess whether the introduction of rotor out-of-balance mi t improve the performance of a beetring system. 

For the experiments, the apparatus took the form of a rigid rotor supported in a single rigidly mounted bearing. This simple symmetrical arrangement overcomes many of the difficulties associated with a two bearing system where it is necessary to have two identical bearings. 

Squeeze film dampers have long been used to combat rotor dynamic and stability problems that conventional bearings cannot solve on turbomachinery rotor systems. The use of squeeze film dampers in problem process machinery has tainted it as a treat-the-symptom solution, and many users shy away from using squeeze film dampers for this reason. Also, their limited use is explained by the difficulty in accurately predicting performance, particularly with o-ring supported dampers. 

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