February 19, 2018
The Soft Foot Primer (Part 1) Are You Measuring Soft Foot, or Just Its Effect?
By Stan Riddle
Most laser alignment tools include a soft foot function. However, in truth these tools haven’t measured ACTUAL soft foot, instead they have measured the EFFECT of soft foot at the shaft or coupling as that is where the laser heads or sensors are mounted. While the EFFECT of soft foot is important, correcting the ACTUAL soft foot will also correct the EFFECT of soft foot.
The Fixturlaser Run Out Probe measures linear travel, much like a dial indicator, but does so digitally. It measures run out, or eccentricity of the coupling, due to an eccentrically bored coupling, a bent shaft, or a combination of both.
In addition, the Run Out Probe can also be used on the machine feet to measure the ACTUAL amount of soft foot or lift at the foot. It can be used in several places on each foot, to determine if the soft foot is parallel, or angular.
We recently performed a test on one of our training pumps, which we knew to have both a slightly bent foot, and a slightly bent motor riser. Using the Soft Foot app in the Fixturlaser NXA laser alignment tool, the soft foot EFFECT as measured by the NXA was 5 mils. The NXA also indicated which foot was soft.
We then used the Fixturlaser Run Out Probe to measure the ACTUAL amount of soft foot-at the foot. But we did it as most maintenance mechanics do-in just one place. The Run Out Probe measured the soft foot on one corner at 2.2 mils.
Then we checked all four corners of the foot and found that the amount of soft foot varied at each location, from 2.2 mils to 7.3 mils. This gives a much better visual representation of the amount of deformation of the foot, and the base underneath.
When possible, checking four corners of the foot allows the mechanic to “see” the problem use a step shim or soft shim to correct the problem, and document the final soft foot values.
The NXA helped me find the location of the soft foot, but the Run Out Probe helped me determine how to fix it, and document the results.