Flexible shaft

With close-clearance seals it is important that the shaft be closely maintained in its rotating position flexible shaft design (operation above the first lateral critical) is usually not acceptable. Bearings that maintain the closest alignment of the shaft are obviously the best for such applications and, for this purpose, close-clearance journal-type bearings are used. 

Critical speeds of a turbine operating below its first critical should be at least 20% above the operating speed range. The term commonly used for units operating below their first critical is that the unit has a stiff shaft, while units operating above their first critical are said to have a flexible shaft. There are many exciting frequencies that need to be considered in a turbine. Some of the sources that provide excitation in a turbine system are ... 

The previous section discussed the flexible shaft with rigid bearings. In the real world, the bearings are not rigid but possess some flexibility. If the flexibility of the system is given by Ki, then each support has a stiffness of Ki,/2. In such a system, the flexibility of the entire lateral system can be calculated by the following relationship ... 

The shaft is a forging and may he designed as a stiff shaft or flexible shaft. A stiff shaft design means that the shaft will operate helow any of its critical speeds. Usual practice limits design operation to 60% of the first critical speed. This requires a heavier shaft than the flexible design that allows the shaft to pass through its first critical speed at 40-60% of normal and maximum operating speeds. 

Another way (Anadrill) is to use removable sources. Figure 4-285 shows the sources being installed in the tool at surface. The two sources, neutrons and gammas, are mounted on the same flexible shaft. They are moved from the shield to the sub without being exposed. Furthermore, if the BHA becomes stuck, they can be fished out with an overshot that connects to the fishing head on top of the neutron source. 

Shaft stiffness Most machine-trains used in industry have flexible shafts and relatively long spans between bearing-support points. As a result, these shafts tend to flex in normal operation. Three factors determine the amount of flex and mode shape that these shafts have in normal operation shaft diameter, shaft material properties, and span length. A small-diameter shaft with a long span will obviously flex more than one with a larger diameter or shorter span. 

Most turbines have relatively long bearing spans and highly flexible shafts. These factors, coupled with variations in process flow conditions, make turbine rotors highly susceptible to shaft deflection during normal operation. Typically, turbines operate in either the second or third mode and should have narrowbands at the second (2x) and third (3x) harmonics of shaft speed to monitor for mode shape. 

The material-flexing group includes laminated diskring, bellows, flexible shaft, diaphragm, and elastomeric couplings. 

Flexible shaft or spring couplings are generally used in small equipment applications that do not experience high... 

Tail Fuze M190 is an inertial type consisting of a fuze-body assembly, arming assembly, flexible shaft, and arming-vane assembly. Its cylindrical body of 1.5-inches diam and 6.89-in length is threaded externally for assembly to the fuze adapter of... 

STIRRER DRIVE FLEXIBLE SHAFT (Pulley Drive Optional)... 

Stirring Device — The lid shall be equipped with a stirring device (Fig F13), mounted in the center of the lid and carrying two two-bladed metal propellers. A stirrer shaft may be coupled to the motor by a flexible shaft, or a suitable arrangement of pulleys... 

On pressurized tanks, the seal has to be frictionless and useful over a wide range of pressures, temperatures, and corrosion conditions. Displacement detectors can be magnetically coupled, or they can use a torque tube, diaphragm and force bar, spring balance, flexible disk, or the flexible shaft design. All of them can be used to detect a liquid-vapor interface, a liquid-liquid interface, and if the level is constant, they can detect density as well. The external displacers are usually installed with level gauges (Figure 3.117) so that the operator can visually inspect their calibration and performance. 

The shim-rod primary indicators at the top plug platform consist of standard counters with five digits driven through suitable reduction gearing attached to a flexible shaft, which in turn is driven directly from the... 

The high rotational speeds in a centrifugal compressor demand attention to the critical speed of the assembly, sonic velocity in the gas being compressed, and vibration of the rotor. In the normal case of a flexible-shaft machine, operating speeds should be between the first and second critical speeds. These usually differ by a factor of at least two to five or higher, and it should he possible to operate a compressor with comfortable margins on both sides. 

Similarly, a pendulum with a soft, flexible shaft has one natural frequency of oscillation, and several definite multiples of it, if the shaft is bending. However, as shown in Fig. 13.1, if two pendula are loosely coupled by a soft spring (or two guitar strings by a piece of rubber), there will still be a most-likely natural frequency, but two "side-band frequencies" will appear. 

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