January 22, 2014
Understanding the Need for Offset Shaft Alignment
By Tom Shelton
While at facilities across the country I noticed a lack of understanding regarding the need for precision alignment of offset shaft driven rotating equipment. I myself have seen electricians and mechanics alike becoming frustrated trying to determine the cause of speed fluctuations and vibration in offset shaft driven equipment. Typical response is that the cause is electrical until proven otherwise. While this may be true in some instances, the casue for others may be excessive angular misalignment of the driven and driver machines.
An Offset Shaft, or Cardan Shaft as it is typically called,is a drive shaft, with u-joints, used to transfer power from a motor (driver) to a piece of equipment (driven). Offset denotes that the shafts of the driver and driven elements are not “inline” with each other. There can be any number of u-joints in a drive shaft assembly, a “single Cardan” shaft means there is a u-joint at each end of a shaft. A “double Cardan” refers to three or more u-joints in a shaft. A “single Cardan” shaft is what you will most likely encounter in the field.
There are two parts that must be understood, proper phasing and angularity of the input and output shafts.
1. Phasing refers to the position of the yokes that are part of the shaft. The two ends, or yokes, must be positioned parallel, or in line with each other. Failure to do this will cause the driven machine to speed up and slow down twice for every revolution of the driver. This will not only cause runability problems, but will cause high vibration issues.
2. The angularity of the input (driver) and output (driven) shafts in relation to the main shaft element must be equal at each end of the shaft. There are industry guidelines to establish the maximum allowable angle between the shaft and the yokes based on speed and load. The best place to get the proper information is from the machine manufacturer.
#2 is the alignment part of the job. The driven machine and the drive), when aligned correctly, will place the shaft ends in the correct position. That is, the angle of the yokes on both ends of the shaft will be equal when the driven and driver are on parallel planes in the vertical and horizontal axes. If you do not align the driver and driven machines, the speed of the driven machine will fluctuate. The worse the angular misalignment, the worse the speed fluctuation.
Offset Shaft alignments require specific shaft alignment equipment. Fortunately Vibralign/Fixturlaser has you covered. The Fixturlaser XA is capable of doing this type of alignment using a special bracket set and alignment program.
After removing the Cardan Shaft the special Offset Fixture mounts across the coupling flange of the driven machine. One laser sensor is mounted on the precision rotating turret of the Offset Fixture with the other mounted on the driver. Misalignment data is collected by rotating the sensor on the Offset Fixture and the shaft/sensor of the driver.
Angular misalignment between the driven and driver machine shafts is displayed on the results screen for both the Vertical and Horizontal planes. (Example results shown in millimeters)
Vertical and Horizontal corrections are made as needed. When the Vertical and Horiztonal angular alignment is in tolerance the driven and driver machine shafts are now parallel and the angle of the yokes on both ends of the shaft will be equal.
To see an excellent video demonstrating the effects of incorrect phasing and misalignment of offset shafts go to http://www.youtube.com/watch?v=gmV4qwLfOMY