These applications aren’t as straightforward as others. Their special considerations call for special tools and approaches.
By Brad Van Schyndel, Centerline, Inc.
As shown in the opening images on this page, a cardan shaft is, in the simplest terms, a spacer shaft with a universal joint coupling on each end. (Its name comes from a 16th-century Italian mathematician, Girolamo Cardano.) This type of arrangement allows power to be transferred between two machines that are offset from each other.
Widely used in industry, cardan shafts have proven practical on applications where space is limited—as well as in situations where an element in the machine train (e.g. paper roll) may need to be actuated (dynamically positioned) to an alternate position when the machines are not running. The universal joint allows for limited movement without uncoupling. To ensure sufficient lubrication circulation, which in turn prevents the universal joints from seizing, cardan shafts are normally installed with an angle from 4 to 6 degrees at the universal joints. Experience, though, has shown that the angle between the shafts of the driver and driven unit should be kept to a minimum, preferably less than 4.36 mrads (0.25 degrees). Ideally, the angles between the driver and driven shafts and the cardan shaft, shown as β1 and β2 in Fig. 1, would be equal. Geometrically, this would equate to zero angularity existing between the driver and driven unit: In other words, the shafts of the driver and driven machine would be parallel to each other.
If excessive angularity does in fact exist between these components, the result will be a rapid fluctuation of the driven shaft RPM during operation. This, in turn, generates damaging vibration, as well as an adverse load through the machine train, leading to premature wear of critical machine components. Precise alignment minimizes these rotational irregularities, so that uneven bearing loading during cardan shaft rotation is also minimized. Thus, the service life of the components is extended and the chance of unexpected machine failure is reduced.
Using laser shaft alignment to detect and correct problems
Accurate laser shaft alignment of the driver to the driven machine requires that the cardan shaft and its couplings be removed. Then, through the use of a laser-alignment system and a specially constructed cardan offset bracket, the angle between the machines can be easily determined and corrected. After removal of the cardan shaft, the cardan bracket (Fig. 2) is mounted to the shaft face of the stationary machine. In the case of a motor connected to a roll, the bracket would be attached to the roll shaft (Fig. 3). The bracket can be attached to the shaft using the coupling bolt-holes or—if available—a threaded hole in the center of the shaft.
The cardan bracket allows virtual positioning of the rotational axis of the stationary machine in line with the rotational axis of the moveable machine. The laser is mounted on the bracket and the receiver is mounted normally on the driver shaft—either with the standard compact chain brackets, compact magnetic brackets or other optionally available brackets. Measurements are taken and the alignment condition is determined. MT
Brad Van Schyndel is an applications engineer for Centerline, Inc., in Appleton, WI. He has more than 13 years of field experience with laser alignment of rotating machinery. Telephone: (920) 730-0615; email: firstname.lastname@example.org.
A Feature-Rich Laser-Alignment Tool
The LUDECA ROTALIGN® ULTRA is well-suited for laser alignment of cardan-shaft applications. This system features optional wireless communication, as well as a cardan shaft alignment mode that allows the user to focus only on the angularity that exists between the driver and driven shafts. The ROTALIGN® ULTRA system also calculates the necessary corrections required to remove the angularity and monitors alignment corrections in real-time as adjustments are made.
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