Misalignment, types

Grid-Type Couplings. Another type of flexible couplings, similar to disk couplings, is the grid type (Fig. 14c). These are appHed for moderate loads and allow parallel misalignment up to 0.1 mm (0.005 ia.). Lubrication is also requited. 

Bearing type Load capacity Suitable direction of rotation Tolerance of changing load/speed Tolerance of misalignment Space requirement... 

There are r vo basie r pes of misalignment, angular and parallel. Within eaeh of the.se basie types of misalignment there are eombinations of both. The.se are the most eommon eombinations ... 

BEARING TYPE LOAD CAPACITY SUITABLE DIRECTION OP ROTATION TOLERANCE OP CHANGING LOAD/ SPEED TOLERANCE OF MISALIGNMENT SPACE REOUIRMENT... 

Typieally, misalignment problems will show up at two times rpm frequen-eies with axial vibrations at one and two times rpm. With diaphragm-type flexible eouplings, vibrations may be somewhat suppressed, and eonse-quently, trains using these eouplings should be monitored periodieally to ensure they are in alignment. 

Flexible element couplings tend to be somewhat heavier than the comparably rated gear coupling. On a retrofit, heed the earlier warning about lateral criticals. The coupling can handle axial misalignment but is more restrictive than the gear type. 

Jackshafts Some machine-trains use an extended or spacer shaft, called a jackshaft, to connect the driver and a driven unit. This type of shaft may use any combination of flexible coupling, universal joint, or splined coupling to provide the flexibility required making the connection. Typically, this type of intermediate drive is used either to absorb torsional variations during speed changes or to accommodate misalignment between the two machine-train components. 

Universal joints There are a variety of universal joints used to transmit torsional power. In most cases, this type of intermediate drive is used where some misalignment between the drive and driven unit is necessary. Because of the misalignment, the universal s pivot points generate a unique forcing function that influences both the dynamics and vibration profile generated by a machine-train. 

How misalignment appears in the vibration signature depends on the type of misalignment. Figure 44.39 illustrates three types of misalignment (i.e., internal, offset, and angular). These three types excite the fundamental (lx) frequency component because they create an apparent imbalance condition in the machine. 

This is a difficult question to answer because there are vast differences in machinery strength, speed of rotation, type of coupling, etc. It also is important to understand that flexible couplings do not cure misalignment problems - a common myth in industry. Although they may somewhat dampen the effects, flexible couplings are not a total solution. 

Combination or skewed misalignment occurs when the shafts are not parallel (i.e., angular), nor do they intersect at the coupling (i.e., offset). Figure 54.8 shows two shafts that are skewed, which is the most common type of misalignment problem encountered. This type of misalignment can occur in either the horizontal or vertical plane, or in both the horizontal and vertical planes. 

There are two misalignment planes to correct vertical and horizontal. Therefore, in the case where at least two machines make up a machine train, four types of misalignment can occur vertical offset, vertical angularity, horizontal offset, and horizontal angularity. These can occur in any combination and, in many cases, all four are present. 

Both offset and angular misalignment can occur in the horizontal plane. Shims are not used to correct for horizontal misalignment, which is typically illustrated in a top-view drawing as shown in Figure 54.11. This type of misalignment is corrected by physically moving the machine to be moved (MTBM). 

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. 

---