October 13, 2014
Uncoupled Shaft Alignment – It Doesn’t Need To Be Difficult.
By Brad Case
The night before the OJT portion of a recent Fixturlaser GO Pro Training class, at a Combined Cycle Power Plant, the shim packs of the disc type coupling between a 3300 HP electric motor and fluid drive began to fail. The machine was shut down before any damage could occur and was waiting for our class the next morning. Talk about good timing for a shaft alignment training class! (Note: The other end of the fluid drive was coupled to a boiler feed pump).
As the coupling was disassembled the shim packs fell apart.
Disc type couplings are fairly “rigid” flexible couplings that can influence the rotational centerlines of each machine so the class performed the shaft alignment uncoupled. See: http://thealignmentblog.com/blog/2013/01/10/flexible-couplins-flexible-shafts/
The motor operates at 3525 RPM and needed to be set 12 mils higher than the fluid drive to compensate for vertical growth in the drive. 3600 RPM was selected in the GO Pro tolerance table and +12 mils entered as the vertical target for the motor shaft centerline.
The initial set of results showed why the coupling failed prematurely. The vertical offset was more than 26 times the allowable offset tolerance of +/- 2.0 mils. You can see how the excessive vertical offset misalignment bent the shim packs, when viewed from on end, causing failure at these areas after thousands of cycles of excessive flexing.
The uncoupled alignment went pretty much to plan and was completed in a couple of Verti-Zontal Compound Moves.
When performing an uncoupled precision shaft alignment with any of the fully digital Fixturlaser Laser Shaft Alignment Systems there are no special programs or settings that are needed, just a few simple steps to follow when taking the alignment measurements.
First, before registering a measurement match the inclinometer values of the M & S sensor, at the top of display unit screen, within .2 to .3 degrees as shown in this example below.
During our alignment the motor shaft (with M Sensor) was the more difficult to turn of the two and was rotated first to each measuring position using a strap wrench, then the inclinometer value for the M Sensor was noted. The fluid drive shaft (with S Sensor) was then easily rotated by hand to match the S Sensor inclinometer value to that of the M Sensor and the measurement taken. Note: The shafts don’t need to be rotated together and it is ok to break the laser beams.
Second, if one of the uncoupled machines turns freely just the weight of the sensor can cause the shaft to turn when the sensors are in a horizontal plane. You can use a steady rest to rest the sensor against or counterbalance the weight of the sensor to keep the shaft from turning. In this case the mechanics simply hung the strap wrench on a bolt threaded in the coupling hub opposite the sensor.
Third, when the electric motor has sleeve bearings the magnetic center of the electric motor shaft (rotor) needs to be reset before reassembling the coupling when finished. As is shown in the following photo, the exposed motor shaft had been painted with machinist blue beforehand and scribed to indicate the correct axial shaft position for the magnetic center. Some motors will have an indexing pointer and groove machined on the motor shaft to indicate magnetic center.
Of course, as with any precision shaft alignment all “Best Alignment Practices” should be followed by taking all measurements in the same direction of rotation, checking for soft foot, replacing cruddy rusted shims with clean shims (new if needed) and by keeping the total shim count under each foot to 5 or less shims.