Along with unbalance, misalignment is another major cause of unwanted vibrations. Misalignment can be internal or external with respect to a machine. Internal alignment refers to the co-axiality of bearings with respect to each other. This is accomplished by the bearing housings, casings, supports and other components, and depends on the construction of the equipment. The primary requirement is that the shaft rotation should be as concentric as possible. A good internal alignment eliminates stresses and thus ensures a smooth, vibration-free operation.

Rotating machines that are installed in pairs or trains are either driven machines or drivers. Just like the importance of internal alignment, shaft axes of two machines in a coupled condition must also be as collinear as possible during their normal operation. Often, the colinearity of shaft axes is not ensured, whether with couplings or otherwise. This is especially the case when the shafts have considerable sags. Under these conditions, colinearity of shaft axes cannot produce good alignment. This will result in vibrations.

It is also important that the alignment is achieved for the operating and not the stationary conditions. The alignment of the shaft ends can always be influenced during normal operation due to following reasons:

         Thermal expansion of supports

         Stresses from pipes

         Deformation of structures

         Modifications in the sag of the rotor with a rise in temperature.

The movement of the shaft ends due to these causes can be calculated in some instances. These calculations should be incorporated. For instance, many machines have to be kept misaligned in cold conditions so they are aligned during normal operating conditions when they become warmer.

When the causes of misalignment are beyond control, it may not be possible to obtain perfect alignment of machines. It is then recommended that they operate at the nearest possible required condition, like the required pressure and temperature in pipes and casings of pumps, compressors, turbines, etc.

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