Flexible shaft design

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. 

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. 

Note that not all these data will apply in aU instances, bnt the more background the mannfac-turer can be given, the more suitable and economical the flexible shaft design will be. 

Flexibility of service and fuels are criteria, which enhance a turbine system, but they are not necessary for every application. The energy shortage requires turbines to be operated at their maximum efficiency. This flexibility may entail a two-shaft design incorporating a power turbine, which is separate and not connected to the Gasifier unit. Multiple fuel applications are now in greater demand, especially where various fuels may be in shortage at different times of the year. 

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. 

Flexible shafting can be supplied in a large variety of design configurations and materials in response to customer needs. There are two primary types of flexible shafting power drive... 

Flexible shafting for power drive applications is designed for continuous and intermittent operation where torque must be carried in one direction of rotation only. Power drive flexible shafts are specified as either right hand for clockwise rotation, or left hand for counterclockwise rotation. (See core selection Table 5.12 and Fig. 5.64.)... 

Typically, the plant engineer is not expected to design a flexible shaft that can be left to the engineering department of the flexible shaft manufacturer. However, the engineer must know what is expected of the flexible shaft and must provide the manufacturer with certain basic data so that a suitable selection or design can be provided. Some of the critical factors are ... 

In many instances, a predesigned shaft or a flexible shaft/flexible coupling will solve the plant engineer s power transmission problem. However, if the requirements are unusual, a special shaft may have to be designed. Obviously, whenever possible, predesigned shafts should be selected because they are the less costly solution. 

Used intelligently, flexible shafts are the product designer s allies. They are more flexible than universal joints they are more versatile than gear systems because they are totally unaffected by the exact angle or offset necessary finally, flexible shafts offer an inherent shock-absorption capability, ease of installation, and maintenance unmatched by other forms of rotary-motion transmission. 

Follow these simple, basic suggestions, and the design flexibihty of a rotary-motion flexible shaft will be applied to utmost advantage. 

The number of speed reduetions is another way to classify conveyor drives. Most common of the speed-reduction methods is the two-step system, in which the motor is coupled to a speed reducer and the slow-speed shaft of the reducer is connected to the conveyor-drive shaft by a V belt or a roller chain. The second reduction not only permits the use of a simpler speed reducer but also allows a more flexible layout of the motor and reducer mounting plate. On many installations this eliminates the need for a specially designed drive mount. 

Bellows couplings consist of two shaft hubs connected to a flexible bellows. This design, which compensates for minor misalignment, is used at moderate rotational torque and shaft speed. This type of coupling provides flexibility to compensate for axial movement and misalignment caused by thermal expansion of the equipment components. Figure 59.7 illustrates a typical bellows coupling. 

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