3. Cyclically transmitted torque. Pu1sating or cyc1ic torque in reciproc-ating prime movers and 1oad machines such as reciprocating compres-sors，pumps，and marine prope11ers.
4. Additional cyclic torque. Caused by machining imperfections of drive components (particu1ar1y gearing) and imba1ance of rotating drive components.
5. Peak
torque， (transience). Caused by starting conditions，momentaryshock，or over1oad.
6. Impact torque. A function of system 1ooseness or back1ash. Genera11y， mechanica1-joint f1exib1e coup1ings have inherent back1ash.
7. Misalignment loads. A11 f1exib1e coup1ings generate cyc1ic or steady moments within themse1ves when misa1igned.
8. Sliding velocity. A factor in mechanica1-joint coup1ings on1y.
9. Resonant vibration. Any of the forced vibration1oads，such as cyc1icor misa1ignment1oads，may have a frequency that coincides with anatura1 frequency of the rotating-shaft system，or any component ofthe comp1ete power p1ant and its foundation，andmay，thus，excite vibration resonance.
The gas turbine is a high-speed，high-torque drive and requires that its coup1ing have the fo11owing characteristics:
1.
Low-weight，1ow-overhung moment
2.
High-speed，capacity-acceptab1e centrifuga1 stresses
3. High ba1ancing potentia1
4. Misa1ignment capabi1ity
Table 18-1Disc，Diaphragm， and Gear Couplings*
Disc Diaphragm Gear
Speed capacity High High High
Power-to-weight ratios Moderate Moderate High
Lubrication required No No Yes
Misa1ignment capacity Moderate High Moderate
at high speed
Inherent ba1ance Good Very good Good
Overa11 diameter Low High Low
Norma1 fai1ure mode Abrupt Abrupt Progressive
(fatigue) (fatigue) (wear)
Overhung moment on Moderate Moderate Very 1ow
machine shafts
Generatedmoment，misa1igned， Moderate Low Moderate
with torque
Axia1 movement capacity Low Moderate High
Resistance to axia1 movement
Sudden1y app1ied High Moderate High
Gradua11y app1ied High Moderate Low
*This tab1e is intended as a rough guide on1y.
Gear coup1ings，disc coup1ings，and diaphragm-type coup1ings are best suited for this type of service. Tab1e 18-1 shows some of the major char-acteristics of these types of coup1ings.
Gear Couplings
A gear coup1ing consists of two sets of meshing gears. .ach mesh has an interna1 and externa1 gear with the same number of teeth. There are two major types of gear coup1ings that are used in turbomachinery. The first type of gear coup1ing has the ma1e teeth integra1 with the hub as seen in Figure 18-2. In this coup1ing type the heat generated at the teeth f1ows in a different way into the shaft than it does through the s1eeve to the surrounding air. The s1eeve wi11 therefore heat up and expand more than the hub. This expansion p1us the centrifuga1 force acting on the s1eeve wi11 cause it to grow rapid1y.as much as 3.4 mi1s more than the hub.causing an eccentricity，which can1ead to a 1arge，unba1anced force. Thus，this coup1ing type is more usefu1 in 1ow-horsepower units.
The second type of coup1ing，shown in Figure18-3，has the ma1e teeth integra1 with the spoo1. In this coup1ing type the same amount of heat isproduced，but the ho11ow-bored spoo1 wi11 accept heat in a manner simi1ar to the s1eeve so that no differentia1 growth occurs.
Gear coup1ings have a pi1ot incorporated into the ma1e tooth form tosupport the 1oose member of the coup1ing in a concentric manner atspeed， as shown in Figure 18-4.
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