Myths and Truths about Shaft Alignment

February 7, 2011

Tolerances For Shaft AlignmentMYTH: “Coupling alignment tolerances provided by the coupling manufacturer are good enough for shaft alignment.” Or “There’s no need to align your machines tighter than the tolerances allowed by your flexible coupling” 

TRUTH: This is wrong because a good quality flexible couplings may be built to withstand much more misalignment than what is good for your connected machines, in terms of the vibration and other forces created. Bearings and seals will wear out and fail faster than a highly misalignment-tolerant coupling. The reason for this “extra misalignment capacity” in flex couplings is that they may need to withstand significant positional changes resulting from thermal growth or dynamic load shifts. This lets you misalign machines to “cold alignment” targets. 

MYTH: “Two lasers are better than one” 

TRUTH: This is patently false. Two lasers simply means a greater expense and more components that can fail. A single laser captured by a 5-axis detector or reflected back into a transducer is more accurate and less backlash-prone than two lasers emulating the reverse indicator principle. In addition, a single laser system is not plagued by coupling span length limitations, or misalignment range limitations. 

MYTH: “All Soft Foot can be corrected by proper shimming.” 

TRUTH: Soft Foot is Machine Frame Distortion. This can be caused by problems easily fixable by shimming (like a short foot or missing shims); but it can also result from pipe strain, which can only be properly fixed by correcting the piping position and interface with the machine. A good laser system with a pipe strain measuring feature or positional change monitoring capability is the best way to detect and measure the effects of pipe strain.

MYTH: “Pipe Stress makes the alignment difficult.”

TRUTH: Pipe stress does not have any influence on the alignment unless you make corrections on the machine with the piping attached. In most alignment the corrections to eliminate misalignment are performed on one machine only,  typically  the one that is easier to move. In a pump-motor set the corrections are done on the motor. If there is sufficient room to shim and move, excellent alignment can be achieved regardless  of  how much pipe stress there is on the pump. Of course, pipe stress is undesirable and should be eliminated prior to the alignment.

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