Shaft alignment
The term shaft alignment (engl. Shaft alignment ) referred Mechanical alignment of shafts at a coupling . When aligning shafts, two or more machines or machine parts are positioned so that the axes of rotation of all connected shafts are as precisely as possible in alignment.
Measured quantities of misalignment (radial-axial measuring method)
A perfect alignment is impossible in practice. The effects of the unavoidable angular and parallel misalignments can be partially absorbed by clutches , which, however, increases their wear. The best possible shaft alignment reduces load change reactions and thus wear on couplings, bearings, shaft seals and other machine elements. The efficiency increases with the quality of the alignment.
The measured variables for alignment are subdivided according to their level (horizontal or vertical) as well as their angular position (parallel or oblique); there are thus four possible deviations for describing an alignment. If the waves are at an angle to one another, one speaks of an angular offset or gap . In most cases in practice, all sizes occur together, i.e. parallel and angular misalignment both in the horizontal and in the vertical.
Parallel offset
The parallel offset describes the distance between the two axes of rotation on the horizontal and vertical plane. Since the parallel misalignment usually occurs in combination with an angular misalignment, it is usual to keep the distance in the center of the coupling. If the moving machine is higher or further to the right than the stationary machine, the sign is positive . If the moving machine is lower or further to the left of the stationary machine, the sign is negative. The stationary machine is viewed from the front.
Angular misalignment
The angular offset indicates the angle between the axes of rotation. In practice, this is specified as the angle of inclination , as it is easy to calculate. For this purpose, the difference between the lower and the upper or the left and right gap dimension is divided by the coupling diameter. In the case of a gap that is open at the top in the vertical offset and on the right in the horizontal offset, this results in a positive sign, otherwise a negative one. The stationary machine is viewed from the front.
Alignment procedure
For many years, machine shafts have been aligned with a simple straightedge and feeler gauge on the coupling surfaces. The dial gauges used since the middle of the 20th century enable much more accurate results. Laser-optical measuring devices are used today for high-precision alignments , which for the first time enable the measurement of the axes of rotation instead of the coupling surfaces. For this purpose, a laser device is attached to one coupling half and a suitable receiver or mirror to the other. For the measurement, the coupling with attached transmitter and receiver must be rotated by approx. 60 °. The device calculates the parallel and angular offset from the deviations in the laser beam. Depending on the device used, further correction parameters can be included in the calculation and further values can be calculated and displayed.
Requirements
For the correct alignment of the machine shafts, specifications are often given for the alignment that take into account the expected position of the shafts in the operating state. This ensures that the position of the shafts in relation to one another lies in the middle of the working range of the coupling during operation, which then ideally only has to absorb a marginal deviation from the ideal shaft alignment. This minimizes clutch and bearing wear. In operation, a lower machine vibration then indicates the successful use of specifications.
Coupling specifications
Often, specifications are already given by the manufacturer, ideally for the entire unit. Specifications are often given in gap and offset or as dial gauge specifications.
Thermal growth
If there are no specifications from the manufacturer, the relatively different thermal growth of the machine can be estimated or calculated based on the different operating temperatures (e.g. motor-pump). It is also possible to use laser-optical sensors to measure the displacement of the machines in relation to one another. B. mounted at a standstill and then measured the change in shaft alignment until normal operating conditions are reached. The specifications for the shaft alignment resulting from this measurement can then be taken into account again for the alignment.