Shaft couplings and power transmission go together with shaft alignment. Shaft alignment is seldom performed without the opportunity at least to look at the flexible shaft coupling that connects the machines being aligned. During shaft alignment, machines with lubricated couplings are generally inspected or preventive maintenance is performed. Dry-type couplings also require inspection for damage such as fatigue or cracking.
This guide does not attempt to discuss all the details of shaft couplings and how they are
designed, applied, and used. Instead, they are briefly discussed in this section in the areas where they play an important role in the process of aligning shafts and in the behavior of machines due to shaft and coupling misalignment.
Flexible Couplings
A flexible coupling transfers or transmits power from one machine to another and makes accommodation for some shaft misalignment. There are two types of flexible couplings: one allows for misalignment by sliding, the other by flexing.
Typical couplings that allow for misalignment through sliding are gear-type couplings and flexible grid-type couplings. The misalignment of two shafts is accommodated during rotation of the shafts by the sliding of gear meshes (see Figure 6-1) or, in the case of the grid coupling, the grid to the grooves in the hubs. The grid coupling also has some bending involved, but this is used for torsional loading.
Shaft Couplings and Power Transmission
Figure 6-1 Gear Coupling
Courtesy of Falk Corp.
Shaft Couplings and Power Transmission
Figure 6-2 Grid Coupling
Courtesy of Falk Corp
Couplings that allow for misalignment due to bending are flexible diaphragm couplings and flexible disk couplings. The diaphragm coupling can use a single steel diaphragm or a
convoluted diaphragm made up of several layers of thin flexible steel plates. Figure 6-3 shows a diaphragm coupling.
Shaft Couplings and Power Transmission
Figure 6-3
Diaphragm Coupling
Shaft Couplings and Power Transmission
Figure 6-4
Flexible Disk Coupling
Restoring forces are very important because all couplings resist being misaligned. The coupling tends to try to run in a straight direction, and this imposes preloads on the shaft, trying to force the shaft into a particular sector of the bearing.
Restoring Forces and Moments
You should be aware of coupling behavior in misalignment. All couplings resist being
misaligned and try to operate in a non-misaligned condition (hence the term “restoring forces”). These forces act on the shafts in the form of a moment arm trying to bend the shaft and, in doing so, adding stresses to the shaft. Misalignment under these conditions can fatigue a shaft (and/or coupling) and eventually result in failure (see Figure 6-5).
Resistance to being misaligned occurs only under conditions where torque is transmitted and not in a standstill condition. When machines are borderline aligned and shafts move into a region or area of misalignment while torque is being transmitted, coupling lockup can occur on gear-type couplings.
Shaft Couplings and Power Transmission
Figure 6-5
Stub Shaft Replacement
Misalignment
Coupling misalignment differs from shaft misalignment. Coupling alignment or misalignment is the angle in degrees from the axis of one shaft to the axis of another shaft. The coupling
manufacturer usually provides allowable coupling misalignment in terms of degrees of
misalignment. If the manufacturer gives a number in thousandths of an inch (0.001=25 µm) for offset, it is the measurement of the distance between flex planes of the coupling times the tangent of the allowable angle of misalignment.
Taking into consideration the restoring forces of the coupling and the bending moments acting on the shafts, a rule of thumb can be proposed. If the misaligned shafts are graphed on paper with the proper scaling and a line is extended from the centerline of one shaft to a point of intersection on the opposing shaft, this is the point where the moment occurs (see Figure 6-6).
Shaft Couplings and Power Transmission
Figure 6-6 Point of Moment
Figures 6-7, 6-8, and 6-9 illustrate three types of misalignment. These illustrations can be in the form of vertical or horizontal misalignment. For the purposes of illustration, the angles of shaft misalignment are constant in all examples. Because couplings are the concern in this section, the angles will be given in degrees and mils per inch (25 µm) (mrad). A shaft coupling spacer of 12 inches (30.4 cm) is assumed.
The first example (see Figure 6-7) is of a machine to be moved with only angular misalignment between shafts or across the coupling.
Shaft Couplings and Power Transmission
In Figure 6-7, the angle of the shaft (MTBM) is 15 mils per foot (375 µm per 30 cm). The angle at the coupling flex plane at P1 is 0. The coupling misalignment at P2 is .072 degrees.
Figure 6-8
Angular Misalignment and Offset at P2
Figure 6-8 illustrates an offset at P2 of 15 mils (375 µm) and the same angle (15 mils per foot) (375 µm per 30 cm) or .072 degrees. The misalignment of the coupling will be .072 degrees at P1 and 0 degrees at P2.
Note: The actual offset of the shaft (MTBM) at the appropriate measuring point (P1) will be 0.
Figure 6-9
P1 P2
Shaft Couplings and Power Transmission
Figure 6-9 shows the same offset and angle to the other side of the shaft. Note how the shaft (MTBM) crosses the centerline of the stationary machine shaft. The misalignment of the coupling at P1 is the same .072 degrees. The coupling misalignment at P2 is .143 degrees or twice the coupling misalignment at P1.
Note: The actual shaft misalignment as measured at P1 would be 30 mils (750 µm). With the above referenced misalignment, a bending moment would be introduced into the shaft (MTBM) at approximately 25 inches (62.5 cm) from P1.
Advantages and Disadvantages of Coupling Types
The advantages and disadvantages of various couplings are shown in Table 6-1. Table 6-1
Coupling Advantages and Disadvantages
Coupling Type Advantages Disadvantages
Gear-type couplings • Transmit more power – have a greater power to weight ratio than other coupling types.
• Accommodate for axial shaft movements due to rotor movement by design, or rotor movement due to thermal growths.
• Require lubrication - Must be stopped to lubricated. The exception to this is a continuous lube coupling that provides lubrication with the use of a
pressurized oil system. If the oil is kept clean, this can add to the life of a gear coupling.
• Coupling lock up – This can occur under certain operating conditions. This phenomenon can limit the travel or movements of shafts in the axial direction or limit the movement of the coupling to allow for misalignment. Disk-type-couplings • Disk pack couplings can
transmit more power per given size or weight than other types of non-lubricated couplings.
• Inspection can be performed while running.
• Failed disks can be replaced relatively easily.
• Failure of disks and life is proportional to misalignment
• Corrosion and fretting
Diaphragm-type couplings
• Simple design
• No lubrication required
• Will tolerate greater angular misalignment
• Limited axial travel
• Larger diameter
• Heat generation due to windage Convoluted diaphragm
couplings
• Smaller than other diaphragm- type couplings
• Can accommodate more axial
• Complicated in design
• Heavier than other diaphragm-type couplings
Shaft Couplings and Power Transmission
Table 6-1 (continued)
Coupling Advantages and Disadvantages
Coupling Type Advantages Disadvantages
Elastomeric-type couplings
• Tire couplings • Impose small radial forces on bearings due to offset misalignment
• Centrifugal force
• Thrust loads
• Geared rubber • Cost is low
• Requires no bolting
• One hub must be moved for installation.