The Principles of Shaft Alignment

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The term shaft alignment can refer to engineering the alignment of shafts in a coupling. During shaft aligning two or more machines or machine parts are positioned so that the rotational axes of the associated shafts are as accurate as possible in alignment. An absolutely perfect alignment is impossible in practice. The inevitable effects of angular and parallel misalignments can be partially absorbed by couplings, but this increases their wear. The best possible shaft alignment reduces load change reactions, and thus wear of couplings, bearings, shaft seals and other machine elements.

With the quality of the alignment, the efficiency is also increased. The measured variables are divided both in the orientation of their plane (horizontal or vertical) as well as to their angular position (parallel or oblique), thus there are four possible variations that exist to describe an alignment. If the shafts are oblique to each other, we speak of angular displacement or gape. In most cases, all these kinds occur together, that is parallel and angular misalignment in both the horizontal and the vertical.

The parallel offset is the distance between the two axes of rotation on the horizontal and vertical planes. Because the parallel displacement occurs mostly in combination with an angular offset, the distance in the middle clutch is usually noticeable. If the machine is moving higher or further right than the static machine, there is a positive sign. If the machine is moving deeper or more to the left of the stationary machine, it gives a negative sign. Here, the stationary machine is viewed from the front. The offset angle is the angle between the axes of rotation. This will in practice be specified as the lead angle, which one can easily calculate. To the difference between the lower and upper respectively, the left and right gap dimension is divided by the coupling diameter. At a gape which is offset in the vertical and horizontal offset top and open at right, the result is a positive sign, otherwise a negative. The stationary machine is considered here from the beginning.

For many years, Machinery shafts were aligned with a simple straight edge and a feeler gauge aligned with the coupling shaft surfaces. In the mid-20th Century, gauges were used to provide more accurate results. For highly precise alignments, laser-optical measuring instruments are now used, which for the first time allow the measurement of the rotation axis instead of the coupling surfaces. For this purpose, one end of a coupling shaft is attached to a laser device, and the other a matching receiver or screen. For measurement, the coupling with the attached transmitter and receiver must be rotated approximately 60 degrees. The device uses the deviations of the laser beam to detect parallel and angular misalignment. Depending on the device it can further correct the parameters included in the calculation, and other values are calculated and displayed.

For the correct alignment of the machine shafts there are often specified requirements for alignment, taking into account the expected position of the shaft during operation

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