We’ve all heard the expressions, “…a house is as only good as it’s foundation…,” “you need solid ground…,” etc. In the world of alignment, a strong foundation is paramount for accurate results.
Brad Case, writing in the Alignment Blog, highlights a recent project where the motors soft foot played a factor:
[toggle state="closed" title="What is a Soft Foot?"] Soft Foot is a common term used for machine frame distortion. The distortion is caused when one or more feet of a machine differ in height from the others. This in turn may be due to differences when the machine was manufactured, a squishy footage with oil film etc. between foot and base, a bent foot, or it may be induced by a pipe to which the machine is attached (e.g. a pipe on top of a pump), which prevents the machine from touching all its feet to its base. [/toggle]
Previous alignment blog postings have discussed the negative effects of soft foot on rotating machinery reliability and the importance of checking and correcting soft foot before proceeding with a shaft alignment. The question is: “does soft foot need to be checked during an alignment?” Possibly.
During a recent training class an alignment check was performed on a Goulds 3196 pump, driven by a 20 HP electric motor at 1765 RPM. The pump runs intermittently–only several hours a day.
The initial set of shaft alignment measurements were taken with the following results:
Pump Shaft Alignment "Before"
As indicated by the four red coupling values and when compared to the 1800 RPM tolerances on VibrAlign’s tolerance table the alignment was out of tolerance in both the vertical and horizontal planes.
Before making a Verti-Zontal compound move, obvious and final soft foot were checked and found to be nonexistent. The Verti-Zontal compound move was completed including correcting for a bolt-bound condition in the horizontal plane.
As the mechanics where tightening the hold down bolts they noted the right rear foot now had soft foot. Where did that come from?
A 15 mil shim was required to correct the soft foot. After the hold down bolts were tightened, final soft foot was checked and found to be in tolerance (2.0 mil or less).
Pump Shaft Alignment "After"
A light “touch up” to the shaft alignment was required after taking a set of measurements. A final set of alignment measurements revealed a final alignment position of: So why did the right rear foot develop soft foot during the alignment?
The answer is in the initial alignment results. The horizontal angular misalignment was almost 12 times the allowable angular tolerance resulting in the rear feet being 172 mils out. Once the rear feet were moved over 1/8”, variations in the base or other base-related factors caused the soft foot to mysteriously appear.
When training classes to use our Verti-Zontal alignment process, we stress the importance of performing pre-alignment steps. The first step is to “rough in” the alignment to 20-30 mils vertically and horizontally for several reasons. One being the movable machine is close to the final vertical and horizontal position and the aligner is not suddenly surprised by the mysterious appearance of soft foot.
When performing an alignment, or hiring somebody to do it, how often have you had to adjust for a soft foot?
