The importance of correcting soft foot errors demands effecitve laser soft foot measurement
Soft foot errors in machine mounting faces are a source of great frustration to any alignment technician, even when you may not realise it.
Excessive soft foot errors create two main problems:
Firstly, the ability for the affected machine to be aligned accurately is compromised. Both vertical and horizontal position at the coupling may become highly reactive to the sequence and torque applied to each of the foot mounting bolts. With uncorrected soft foot errors, the time required to reach an in-tolerance alignment may easily be doubled or worse, and often there may be no choice but to leave a job "just" in an acceptable alignment condition due to time constraints. In addition, the position of the soft foot affected machine is not stable - minor adjustment of the tightness of even a single mounting bolt may push the alignment out, and it is likely that with time, vibration and thermal stresses, the original alignment will not remain good over a long period of operation.
The second major impact of not addressing soft foot errors is machine distortion. Experienced alignment technicians and vibration engineers regularly observe machinery that is negatively affected by being operated with frame distortion due to soft foot. The most commonly noticeable effect is in electric motors, where a relatively small distortion of the motor frame directly impacts the evenness of the critical air gap between the rotor and stator. Resulting variation in the electromagnetic forces as the rotor turns creates vibration, which may reach critical levels.
In addition, housing distortion of any rotating machinery will create misalignment between bearings, gears, seals, rotors and other moving parts that are mounted in precision in-line machined bores for very good reason.
From field experience we know that in many installations there is likely to be a soft foot problem to be corrected, because so often the mounting structure used for a pump/motor system or other machinery is neither sufficiently rigid or flat to allow the machines to sit flat and firmly without shimming corrections.
The outcome is that good alignment practice is to take the time to accurately measure and correct soft foot errors as part of every shaft alignment, as it has been clearly established that time invested to correct soft foot will pay back directly in reducing the overall alignment time, improve the alignment results and avoid vibration and machine damage from housing distortion.
High quality laser alignment systems offer some means of measuring soft foot errors, which is possible by recording the change in laser beam position as each machine foot is tightened or loosened.
Hamar Laser Instruments have developed what they believe to be the most effective soft foot measurement AND correction cycle, which is built into every Stealth shaft alignment laser. An algorithm based on machine research and incorporating the actual machine dimensions, guides the operator to measure coupling alignment variation as each machine foot is loosened. From this data the softfoot "gap" at each foot is accurately calculated, and a recommended correction value is displayed. Without feeler gauges, calculations or trial-and-error, normal soft foot errors are quickly identified and corrected, usually with a single shim insertion.
The Stealth soft foot correction cycle has been demonstrated to be very effective for correcting classic soft foot errors and for trouble-shooting more complex soft foot problems, saving Stealth users many hours in alignment time on site, and allowing them to sign off their alignments with greater confidence than ever.
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