Cardan shaft alignment

Now possible without disassembly

PRÜFTECHNIK service technician aligning a cardan shaft
Figure 1: A PRÜFTECHNIK service technician aligning a cardan shaft

It was an exciting service call at a tinplate rolling mill for the service team of PRÜFTECHNIK Alignment Systems GmbH. Using the newly developed, unique system for laser-optically aligning cardan shafts while still installed, the team successfully aligned a shaft without disassembling it first – and saved a great amount of time and effort in doing so.

But let us start from the beginning. Why do cardan shafts need to be aligned in the first place? Isn’t their very purpose to bridge offsets?

Possible arrangements of a cardan shaft
Figure 2: Possible arrangements of a cardan shaft (Z arrangement, W arrangement)
Possible arrangements of a cardan shaft

Functionality and requirements of cardan shafts

Cardan shafts are frequently used in industry to transmit power between drive inputs and outputs whose axes are displaced relative to each other. A cardan shaft is an articulated shaft with one or two universal joints that can be installed in a variety of arrangements in the drive train.

In general, there are two cardan shaft arrangements (Fig.2)
In both arrangements, the functional purpose of the cardan shaft is the same: it enables a power transmission between drive axes with a considerable offset.

If a power transmission is implemented between two offset shafts using just one joint, a non-uniform rotation occurs at the driven shaft – even if the drive shaft itself is running smoothly. During a single rotation, the driven shaft first moves faster and then slower than the drive shaft. This is referred to as the cardan error. A cardan error not only results in inconsistent rotations, but also leads to increased vibration levels in the system due to the uneven loads, eventually causing joint and bearing damage in the shafts and machines. This can be prevented by using a cardan shaft with two universal joints, where the second joint compensates the non-uniform movement of the first joint. For this to function properly, the angle of deflection ß at both universal joints must be the same – otherwise the driven shaft will again rotate unevenly.

In summary, cardan shafts are suitable for bridging vertical or horizontal offsets. The angle of deflection must be the same on each universal joint of the cardan shaft – and it is precisely for this reason that cardan shafts must be precisely aligned.

Bridging the parallel offset with a cardan shaft bracket
Figure 3: Bridging the parallel offset with a cardan shaft bracket

Aligning cardan shafts traditionally requires disassembly

Let us go back now to the service call at the steel plant. The task here was to align a roller drive connected by a cardan shaft with a Z arrangement. At this plant, the importance of aligning cardan shafts had long been recognized and alignment was already being performed using laser-optical equipment. Until now, however, it had only been possible to do so with the cardan shaft deinstalled.

To position the sensors opposite each other, the parallel offset between the shafts was bridged by mounting a cardan shaft bracket on the shaft flange on one side. Figure 3 shows the assembly on the left roller shaft.
The laser is mounted on a rotating device, and the detector is attached to the motor shaft. The deflection angles are measured with the machines uncoupled and the cardan shaft removed.

This is a time-consuming process as the cardan shaft first needs to be disassembled and the cardan shaft bracket must be carefully installed to ensure that it rests flatly against the shaft of the roller.

 

 

ROTALIGN Ultra iS - System zum Wellenausrichten
Figure 4: ROTALIGN Ultra iS

Cardan shafts in place during alignment

It would often be easier if the cardan shaft could simply be aligned while still installed, as is the case with other couplings and intermediate shafts. This is where the newly developed “Aligning cardan shafts while installed” system for the ROTALIGN Ultra iS laser-optical measurement platform from PRÜFTECHNIK comes in.

Chain-type brackets
Figure 5: Chain-type brackets

To align a cardan shaft while it is still installed, two newly designed brackets are required in addition to the ROTALIGN Ultra iS system, which consists of a computer, laser and detector. At the steel factory, a chain-type bracket was used to mount the laser on the motor shaft. Because cardan shafts frequently bridge large offsets, long rails are often necessary, which is why the new bracket has a third rail for stability.
Depending on the available space, the detector is mounted either with a chain-type bracket or using another new development – the cardan rotating arm bracket. Because the space available for rotation on the drive shaft side was limited in the present case, the new rotating arm bracket was used. How the rotating arm bracket works will be described in detail later on.

Laser-optical alignment in three steps

Just like the alignment of couplings based on conventional laser-optical methods, the laser-optical alignment of an installed cardan shaft using ROTALIGN Ultra iS is performed in three steps.

 

 

Step 1: Mounting the sensor equipment and displaying the measurement object in ROTALIGN Ultra iS

Step 1: Mounting the sensor equipment and displaying the measurement object in ROTALIGN Ultra iS

When setting up the system, the user obtains assistance from the ROTALIGN Ultra iS display. In the present case, the laser was mounted on the roller shaft using a chain-type bracket. Opposite to it on the drive shaft, the detector was mounted using the rotating arm bracket. To align the shafts, the position of the motor can be adjusted. The dimensions were determined using a tape measure.

Measurement of cardan shafts
Step 2: Measurement

Step 2: Measurement

To take the laser-optical measurements of the cardan shaft, the PRÜFTECHNIK service technicians used the multipoint measurement mode. With this method, five measurements are taken at any five rotational positions of the shaft.

Because cardan shafts generally bridge a large offset between two machines, the laser quickly strays away from the detector surface, even if the angle of rotation is only small. This is where the new rotating arm bracket comes into play – by swiveling the bracket and moving the detector up or down, the laser beam is brought back onto the detector area and a measurement can be taken.

The compact design of the brackets and the small minimum rotating angle of 60° enabled the technicians at the tinplate mill to measure the cardan shaft under restricted space conditions.

Results and interpretation screen of ROTALIGN Ultra iS
Step 3: Results and interpretation

Step 3: Results and interpretation

Immediately after measurement, ROTALIGN Ultra iS displays the results of the alignment situation of the cardan shaft in a clear and straightforward manner.
 
In this particular case, the cardan shaft at the steel factory was sufficiently aligned – the difference between the two deflection angles at the universal joints in the vertical view was only 0.06°, and in the horizontal view the angles were actually identical. Thus, both shafts were within tolerance and an alignment correction was not necessary. The foot values that specify how to adjust the cardan shaft to create an even better alignment could be ignored.

Summary:

Significant savings in time and effort during the laser-optical alignment of installed cardan shafts

The patented PRÜFTECHNIK method of aligning cardan shafts while installed saves the user the considerable time and effort involved in removing and reinstalling the cardan shaft. The demands on the user are lower as well – a cardan shaft alignment procedure while the shaft is installed is very similar to the alignment of standard couplings.

The application for aligning cardan shafts while installed and the necessary brackets are available for ROTALIGN Ultra iS.

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