When we think about what causes misalignment, cardan shafts are often overlooked. When they are present in a mechanical arrangement, maintenance can be quite difficult due to their size, design, and accessibility. Because of this, alignment checks are often skipped, and alignments postponed, causing continuous wear of the cardan shafts, which results in continued and worsening misalignment.
To best understand cardan shaft alignment, we must understand what a cardan shaft is and what it does. Rather than a shaft, a cardan shaft is more of a coupling whose purpose is to connect two machines that offset – one being a stationary machine, often referred to as a motor, and the other machine being the “driven” one. Cardan shafts are specifically designed to allow for parallel misalignment caused by these two machines being offset.
In a yacht, the arrangement is called a drive train and is placed between the engine and gear, transmitting torque and rotation. The cardan shaft serves as a universal or U-joint because the two machines are not able to be directly connected to each other.
Cardan shafts usually have two joints, one at each end, and is referred to as a single cardan. A double cardan will have three or more of these joints. The joints of the cardan shafts allow for necessary movement without becoming uncoupled.
The construction of the U-joint is a key component to how the cardan shaft functions and is able to transmit forces. Each U-joint consists of two yokes and four pivot pins. The hole in the yoke carries the pivot pins, which oscillates as the joint rotates. All of the bearing occurs in the small space between the pins and the yoke holes, which means that proper lubrication is crucial at all times.
The lubricant must be constantly circulated to prevent the joints from sealing, which is why cardan shafts are usually installed with three to six degrees of angle present at each joint. The difference between these angles should always be less than 0.25 degrees and, in a perfect world when precision alignments are always possible, the difference would be 0.
Even though cardan shafts are constructed for flexibility, they are not able to absorb angular misalignment between the shafts. While offset does not affect alignment, angularity does. It can be vertical or horizontal, and can cause excessive vibration and speed fluctuations.
Angular misalignment can be present in the same plane or in separate ones, depending on the layout of the machinery. When angular misalignment is present, there’s a huge fluctuation in RPMs of the driven shaft.
Traditionally speaking, cardan shafts are designed for tight spaces, and historically alignment procedures have required that the cardan shaft be completely removed for alignment to be achieved. An “offset” fixture or bracket is mounted on the coupling flange of the driven machine, with a laser sensor mounted on the rotating part. A second laser sensor would need to be mounted on the driver. The bracket would allow for virtual positioning of the rotational axis that connects the two machines. Data would then be collected on both the vertical and horizontal planes.
With technology constantly outdoing itself, cardan shaft alignments are now becoming faster, safer and more cost effective. Current laser systems allow for these alignments to be performed with the shaft in place. This is made possible by using two specially designed brackets with a handheld computer, laser transmitter and receiver. One bracket serves as a sort of chain and has a “third arm” that can be directly mounted to the shaft. The other bracket features a rotating arm, where a sensor is mounted. When the shafts rotate, the bracket arm turns, allowing for the sensor to move while picking up the laser being transmitted.
Cardan shaft alignments can be intimidating and complex, to say the least. However, with the new equipment available to perform these services, there is absolutely no need to put off this procedure to prevent cardan shaft joint wear. With minimal effect on your maintenance budget, you can prevent a lot of headache.
Rich Merhige is the owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale, which specializes in rotating and reciprocating machinery. This column is co-written by Teresa Drugatz, marketing manager at AME. Contact them through info@AMEsolutions.com or +1 954-764-2678. Comments on this column are welcome at firstname.lastname@example.org.