Torque shaft design

Figure 16-16 shows the performance characteristic of a split-shaft turbine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, aud/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to pnivide the required torque balance. Compressors, pumps, and various mechanical tinvc systems make very good applications for split-shaft designs. 

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. 

Each shaft is designed for mechanical loads and critical shaft speed. Motor size and shaft design are related. A larger shaft to take the torque will re[Pg.329]

Results for the quasi-static loads and lateral and rotational responses are shown in Table 3. Dynamic simulations for these shafts design have been run using the simulation technique described by Greenway et al (2000). The peak responses to lateral motion due to Coriolis and aerodynamic effects acting alone are given in the Table 3 as well as the peak rotations due to the head rope torque. 

Shaft design must accommodate hydraulic and mechanical loads and must avoid vibration near the natural frequency. A typical overhung shaft arrangement with dimensional nomenclature is shown in Figure 21-32. Hydraulic loads on the shaft result from the torque required to turn the impeller(s) and random or systematic lateral hydraulic loads on the impeUer(s). Other sections of the book describe methods for determining impeller power. Shaft design will use impeller power to calculate torque and hydraulic forces and thus size a shaft within allowable stress limits. 

The problem could be maintenance, operation, or design, or a combination of any or all these factors. In all honesty, you should never. see this set of evidence marks because it indicates a lack of control. Now because the mechanic cannot control operational problems or design problems, the first phase to correct this situation is to control the mechanical maintenance factors, like alignment, proper bolting and torque. sequences, be sure shafts are straight and round, and dynamically balance all rotary components. Reinstall the pump and wait for the next failure. Once the maintenance factors are under control, there should appear a clear vision and path to resolve any operational and/or design weaknesses. 

Induction motor mechanical considerations. If the motor is plaeed between the eompressor and the expander its shaft ends must be eapable of full torque transmission. Unit torque requirements are generally well above those required by the motor rating. The result is speeially designed motors. 

The modified Brayton cycle is used for both gas turbines and jet engines. The turbine is designed to produce a usable torque at the output shaft, while the jet engine allows most of the hot gases to expand into the atmosphere, producing usable thrust. Emissions from both turbines and jets are similar, as are their control methods. The emissions are primarily unbumed hydrocarbons, unbumed carbon which results in the visible exhaust, and oxides of nitrogen. Control of the unbumed hydrocarbons and the unburned... 

Water injection, or steam injection systems, are being used extensively to augment power. Corrosion problems in the compressor diffuser and combustor have not been found to be major problems. The increase in work and efficiency with a reduction in NO makes the process very attractive. Split-shaft cycles are attractive for use in variable-speed mechanical drives. The off-design characteristics of such an engine are high efficiency and high torque at low speeds. 

Most materials scientists at an early stage in their university courses learn some elementary aspects of what is still miscalled strength of materials . This field incorporates elementary treatments of problems such as the elastic response of beams to continuous or localised loading, the distribution of torque across a shaft under torsion, or the elastic stresses in the components of a simple girder. Materials come into it only insofar as the specific elastic properties of a particular metal or timber determine the numerical values for some of the symbols in the algebraic treatment. This kind of simple theory is an example of continuum mechanics, and its derivation does not require any knowledge of the crystal structure or crystal properties of simple materials or of the microstructure of more complex materials. The specific aim is to design simple structures that will not exceed their elastic limit under load. 

The differential can introduce a problem when driving on a slippeiy road because the torques on the two differential pinions are always equal, thus delivering equal torques to the drive wheels. In the extreme, if the tire of one drive wheel rests on ice and therefore lacks traction, the differential allows it to spin freely while the opposite drive wheel, and the vehicle itself, remain at rest. Special differential designs have been devised to overcome this problem. Traction is also improved in some vehicles through the application of four-wheel drive, whereby additional shafts and gears are employed to distribute engine power to all four wheels of the vehicle. 

A 61-in. outside diameter positive displacement motor of a 1 2 lobe profile design (where performance data are given in Table 4-114) has rotor eccentricity of 0.60 in., a reference diameter (rotor shaft diameter) of 2.48 in. and a rotor pitch of 38.0 in. If the pressure drop across the motor is determined to be 500 psi at a circulation flowrate of 350 gal/min with 12.0 Ib/gal, find the torque, rotational speed and the horsepower of the motor. 

Both the jackshaft and spindle are designed to absorb transient increases or decreases in torsional power caused by twisting. In effect, the shaft or tube used in these designs winds, much like a spring, as the torsional power increases. Normally, this torque and the resultant twist of the spindle are maintained until the torsional load is... 

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. 

One-way A one-way or flywheel clutch is a device that transmits torque in one direction, but disengages in the opposite direction. The typical design of flywheel clutches incorporates a tooth profile that engages in one direction only. If rotation of the drive shaft is reversed, the drive hub will freewheel and cannot engage the driven hub. 

Although not a design that guarantees positive power transmission, a satisfactorily performing friction clutch will transmit the maximum torque output of the driver to the driven shaft without slip when fully engaged. The automobile clutch is the most common example, transmitting the maximum motor torque to the wheels. 

A shaft subject to torque is generally considered to have failed when the strength of the material in shear is exceeded. For a torsional load the shear strength used in design should be the published value or one half the tensile strength, whichever is less. The maximum shear stress on a shaft in torsion is given by the following equation ... 

Example 10.2 compares data of Table 10.4 with calculations based on Figures 10.6 and 10.7 for all-liquid mixing. Power and rpm requirements at a given superficial liquid velocity are seen to be very sensitive to impeller diameter. When alternate combinations of HP/rpm are shown in the table for a particular performance, the design of the agitator shaft may be a discriminant between them. The shaft must allow for the torque and bending moment caused by the hydraulic forces acting on the impeller and shaft. Also, the... 

Options for connecting the motor drive to the shaft depend on the shaft orientation. A vertical-shaft cantilever design would prefer a belt drive to reduce the cost of manufacture of the support structure and to facilitate maintenance. A horizontal shaft has the additional option of direct coupling. Variable speed can be accomplished through a gearbox or preferably through variable frequency control on the motor. In addition to the power requirements discussed previously, the startup power to overcome the torque of the rotor must be considered. This startup power is related to the time required to reach the desired rotor speed. 

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