Manufacturing Trend 2010/11, Technical Diagnostics Section

"Instead of firefighting and major repairs"

In machine groups where the driving and driven machines are connected by a shaft coupling, the most common cause of vibration after imbalance is the improper alignment of the shaft centerline. However, precise alignment is a key task for reducing the load on bearings and extending their lifespan.

Shaft alignment errors (inaccurate alignments) can result in: high forces, increasing the load on the shaft coupling and overloading the bearing increased energy loss reduced lifespan increased maintenance requirements The typical errors in shaft alignment (or collinearity) are parallelism error and angular error, or their combination. Parallelism error occurs when the shafts of machines connected by a shaft coupling are misaligned parallel to each other. When there is an angular error, the shafts of machines connected by a shaft coupling form an angle with each other. In case of a combined error, the shafts can even be offset. In practice, we almost always encounter a combination of the two basic errors. However, during our measurements, it is advisable to separate and treat the two basic error cases separately. The tolerance limits for alignment are separately defined in standards.

Traditional Measurement Principles

Shaft alignment with straightedge and feeler gauge This well-known "ancient" method has the outstanding advantage of being moderately equipment-intensive. However, its drawback is its strong sensitivity to the manufacturing accuracy of the shaft coupling, the condition of the tools used, and the expertise of the personnel. Another issue is that the quality of the alignment is difficult to quantify. For important equipment, it is definitely worth switching to one of the following methods.

Mechanical dial indicators can achieve adequate accuracy, and the method can eliminate the influence of manufacturing inaccuracies of the shaft coupling on the measurement. Furthermore, the measured values can be quantified well, allowing the quality of the alignment to be assessed. The downside of the method lies in the complex setup of the devices, possible mechanical deflections, the highly demanding readings, and the manual calculations required. Two dial indicators are needed for the measurement, which can be mounted radially and axially, or radially on both sides. The measurement is done by rotating both sides of the shaft coupling together.

Dial indicator radial and axial measurement Two dial indicators fixed on a support arm measure the parallel displacement of the coupling (top indicator) and the angular error (side indicator). The readings of the dial indicators are taken in positions corresponding to 6-12-9-3 o'clock.

Dual-sided radial dial indicator measurement Dial indicators fixed on the shaft on both sides of the coupling measure the displacement of the shaft centers relative to each other and the angular error. It is advantageous that shorter support arms are used in this method, reducing the impact of their deformation on the measurement results. However, this method requires more space. The readings of the dial indicators are also taken in positions corresponding to 6-12-9-3 o'clock. One dial indicator shows the displacement between the centers, and the difference in values shown by the two dial indicators indicates the angular error.

During the recording of dial indicator data, it is advisable to perform credibility checks, as reading (or interpretation) errors can easily occur, leading to complete failure of the shaft alignment.

Before the introduction of laser systems, systems equipped with data acquisition and evaluation units were capable of recording the data from electromechanical dial indicators based on the above principles. After verifying the acceptability of the data, the shaft alignment error and the necessary corrective movements were immediately calculated. The application of these systems eliminated reading and manual calculation inaccuracies. However, the cumbersome mechanical setup, relatively large space requirements, measurement errors caused by mechanical play, and the inability to bridge large distances remain disadvantages.

Laser Shaft Alignment

Laser single-axis alignment devices are based on the dual-sided radial method. Instead of dial indicators, one or two combined laser emitter-detector units are used, fixed on the shafts on both sides of the coupling. These units measure the displacement between the centers of rotation of the shafts and the angular error of the shafts. Shaft alignment using these systems is based on trigonometric calculation principles. The measurement values are recorded by the evaluation unit, which immediately calculates the extent of the shaft alignment error and the necessary corrections. Laser systems have rapidly evolved in recent years. The characteristics of each generation of devices are presented below.

Single-axis alignment with invisible laser Compared to dial indicator systems, installation is simple, there is no mechanical play, and it can bridge larger distances. The inaccuracies caused by mechanical play in dial indicator methods and, of course, reading and manual calculation errors cannot occur with this method. In most cases, these systems work with a combined laser emitter-detector unit and a reflective target.

Single-axis alignment with visible laser – 1st generation Devices equipped with visible lasers are easier to handle than systems using invisible laser beams. The maximum measurement distance can reach up to 10 m, and these systems almost without exception use two combined laser emitter-detector units. A new service is the automatic check of "soft foot" (or three-leggedness) and continuous live display during correction movements.

Single-axis alignment with visible laser – 2nd generation This category includes the best tools currently available: their measuring heads contain inclinometer angle position sensors and are suitable for cable or even wireless communication.

The 2nd generation systems offer several new services to facilitate alignment work:

Alignment error and correction value calculation based on less than 180 degrees of shaft rotation; the best instruments can calculate from just 35–40 degrees of shaft rotation consideration of the effects of thermal expansion in the correction (some systems' measuring heads also have temperature sensors, such as the Damalini EasyLaser series) alignment of uncoupled shafts, cardan shafts, and flanged arrangements continuous measurement (so there is no need to stop at each angular position to perform the measurement, then rotate the shaft further). Main advantages The practical importance of calculating from less than 180 degrees of rotation does not need much explanation: everyone has probably been annoyed that installation conditions did not allow for traditional measurement (in the 9–12–3 o'clock positions). With the new systems, this is not a problem: even from a small rotation angle – starting from any position – the shaft alignment error and necessary corrections can be determined. However, the direction of rotation should not be changed between the three measurements. Backward rotation would measure the natural mechanical play of the shaft coupling, leading to erroneous calculations. Continuous measurement provides incredible assistance when aligning heavy rotating parts that are difficult to accelerate and decelerate. From just one slow, continuous rotation (minimally 45 degrees), the shaft alignment error and necessary corrections can be calculated. Moreover, this method - based on a full revolution - can even detect other machine faults.

Practical advice for shaft alignment work

The use of modern instruments alone is not a guarantee that the alignments will be perfect, as they do not eliminate the misalignment of the shaft coupling (its skew, runout), the runout of the shaft ends (bending), and the errors in the machine base (three-leggedness). Therefore, before starting shaft alignment, it is advisable to check the following: Runout of shaft coupling halves Geometric errors due to manufacturing or assembly of the shaft coupling can cause significant vibrations even after the most precise shaft alignment. Flatness of machine feet and base frame If the mounting points of the base frame and/or the machine feet do not lie in the same plane, correction is necessary (shimming) to avoid surprises when tightening the bolts. This error can be detected using a dial indicator or the "soft foot" feature of laser measuring systems. One more important advice: no matter how accurately we measure the required correction values with modern instruments down to micrometer precision, if we do not have suitable shimming washers for executing the correction. The shim must have uniform thickness, should not rust, break, or flatten over time. It is most practical to use pre-manufactured high-quality stainless steel shimming washers (such as EasyLaser washers) because alignment can be done quickly and accurately: simply assemble the shimming washer corresponding to the correction value indicated by the instrument under the movable machine feet, and the correct vertical alignment is achieved.

Rahne Eric (PIM Kft.) pim-kft.hu, gepszakerto.hu

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