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Diagnosing Coupling Misalignment
Coupling misalignment is categorized into one of four main categories:
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Axial Misalignment
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Radial Misalignment (also known as Parallel or Offset Misalignment)
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Angular Misalignment
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General Misalignment (combination of both Angular and Radial Misalignment)
Each of the four types of misalignment have very distinctive vibration responses during the mechanical run test. The major causes and indications for each type of misalignment are detailed in the following sections.
Axial Misalignment
At the mechanical run test, axial misalignment most commonly occurs when coupling hubs are assembled either too close to each other or too far apart from each other. Note that each coupling type will have a recommended gap between the coupling halves listed in the coupling installation manual.
The end result of axial misalignment is an inefficient transmission of power. When the coupling hubs are set too far apart from each other, the engagement between driving and driven components can be less than required and potentially result in excessive points of wear. If the coupling hubs are assembled to close together, the flexibility of the coupling may be restricted and similar excessive wear can occur.
Axial misalignment can also develop when the equipment is first put into extended operation, particularly in high temperature applications. Axial growth of the driven equipment shaft, that is in contact with the hot process stream, will occur, increasing the chances of axial misalignment. Motors also heat up during use, causing the motor shafts to elongate due to thermal expansion, also potentially causing axial misalignment.
Diagnosing Axial Misalignment:
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High 1x, 2x and 3x RPM vibration readings in the axial direction. The 1x RPM reading typically dominates.
Radial Misalignment
Radial misalignment is when the driven and driver shafts are parallel but do not line up as there is an offset between the axial centers of the shafts. The offset between the shafts can be in the horizontal or vertical direction (or a combination of both).
Typically, at the mechanical run test, radial misalignment is a result of either incorrect machining of equipment mounting plates or the incorrect shimming of equipment. It can normally be corrected my adjusting the shimming of the under the feet of the equipment's free end. Usually this is this motor.
Radial misalignment also commonly occurs during the shipping of equipment to an end user location. During shipping, it is common to find that the motor and driven equipment may move relative to each other while in transit.
For all installations of rotating equipment, it is imperative to recheck the coupling alignment after the equipment is mounted in its final location, both before and after the process piping is connected.
As with axial misalignment, radial misalignment can also develop when the equipment is first put into extended operation, particularly in high temperature applications on foot mounted machines. Unequal thermal expansions may cause differing relative vertical growth between the motor and the driven equipment, resulting in radial misalignment.
Diagnosing Radial Misalignment:
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Strong radial vibration in 1x RPM with 2x and 3x harmonics.
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The 2x RPM harmonic in the radial direction can reach a value equal to or even greater than 1x.
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The radial phase measurements on both sides of the coupling are 180° out of phase.
Angular Misalignment
Angular misalignment is the difference in the slope of one shaft of the machine relative to the slope of the other shaft of the machine. As with radial misalignment, angular misalignment can occur in the horizontal or vertical direction (or a combination of both).
Typically, at the mechanical run test, angular misalignment is a result of incorrect machining of equipment mounting plates. Individual mounting plates of bearings and motors should be machined parallel to each other. A common industry requirement would be mounting planes for different equipment shall be machined parallel to each other to within 0.002 inches per foot (0.15mm/m)
Angular misalignment commonly occurs during the final installation of equipment baseplates at end user sites, especially when the baseplate is bolted directly onto another rigid steel structure as this can cause the equipment baseplate to deflect if it is not sufficiently rigid. Excessive pipe stresses that can distort equipment casings can also lead to angular misalignment.
Again, for all installations of rotating equipment, it is imperative to recheck the coupling alignment after the equipment is mounted in its final location, both before and after the process piping is connected.
Diagnosing Angular Misalignment:
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Strong axial vibration at 1x RPM possibly with 2x and 3x harmonics.
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The 2x RPM harmonic in the axial direction can reach a value equal to or even higher than 1x.
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Vibration in the radial direction, probably of smaller amplitude than in the axial direction, in 1x, 2x and 3x.
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The axial phase measurements on both sides of the coupling are 180° out of phase.
General Misalignment
(Combination of Angular and Radial Misalignment)
General misalignment is a combination of both angular and radial misalignment. It is the most common form of misalignment. General misalignment can occur in the horizontal or vertical direction (or a combination of both).
The most common causes of general misalignment at the mechanical run test are as previously described in the sections for radial and angular misalignment
Diagnosing General Misalignment:
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Reoccurring peaks at multiples of running speed.
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1x and 2x running speed typically dominate.
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The radial phase measurements on both sides of the coupling are 180° out of phase.
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The axial phase measurements on both sides of the coupling are 180° out of phase.