What is Soft Foot?
Soft foot refers to a condition where one or more feet of a motor or pump do not make solid contact with the baseplate. This can result in a rocking motion, negatively impacting alignment measurements and overall machinery performance. Soft foot can be caused by an uneven foundation or baseplate, twisted or damaged feet, or improperly stacked shims under the feet to correct vertical alignment.
Soft foot significantly affects shaft alignment measurements, introducing noise and vibration that can compromise the machinery’s operation. It is crucial to address soft foot issues before proceeding with shaft alignment, as unresolved soft foot can lead to inaccuracies in alignment and long-term damage to the machinery.
How Shafts Are Connected
Mechanical couplings are commonly used to connect motor and pump shafts, with designs ranging from rigid to flexible. Rigid couplings are typically used for vertical pumps, whereas horizontal pumps often employ flexible couplings, which can be either metallic or elastomeric. Each type has its advantages and limitations: elastomeric couplings may degrade in high-temperature or chemically aggressive environments, while metallic couplings might be unsuitable for corrosive conditions.
Flexible couplings can accommodate some degree of shaft misalignment, defined by maximum permissible angular and parallel offsets. However, this flexibility should not justify neglecting precise shaft alignment. Misalignment can cause significant damage to the pump, motor, and coupling. Some flexible couplings rely on lubricated, sliding components to manage misalignment and may function less efficiently with perfectly aligned shafts, as slight movement is necessary to maintain lubrication.
Detecting Misalignment Problems
Noise and vibrations often signal issues within a running pumping system. Vibration analysis, a critical component of preventive maintenance, can identify shaft misalignment. High vibration levels at one or two times the shaft rotational rate frequency typically indicate misalignment. Prompt detection through vibration analysis can prevent extensive damage to the pumping system.
Impact of Soft Foot on Shaft Alignment
Soft foot can severely distort alignment readings, leading to improper corrections that exacerbate the problem. Misalignment due to soft foot can cause premature wear on seals, packing, shafts, and bearings, leading to excessive leakage, vibration, noise, and reduced efficiency. It also increases power consumption and maintenance costs. Therefore, achieving accurate shaft alignment requires first addressing any soft foot issues.
Correcting Soft Foot
Correcting soft foot involves several steps:
1. Visual Inspection: Begin with a thorough visual inspection to identify any visible gaps or unevenness between the feet and the baseplate.
2. Measurement: Use precision tools like feeler gauges to measure the gap between each foot and the baseplate. This helps determine the extent of the soft foot condition.
3. Shim Adjustment: Place shims under the feet to fill the gaps and ensure that all feet are in solid contact with the baseplate. Use as few shims as possible and ensure they are clean and flat to avoid introducing additional variables.
4. Recheck Alignment: After correcting soft foot, recheck the shaft alignment using alignment tools such as dial indicators or laser alignment systems. Make necessary adjustments to achieve proper alignment.
5. Torque Verification: After shimming and aligning, verify the torque on the mounting bolts to ensure they are evenly tightened, preventing distortion of the baseplate or motor frame.
Summary
Addressing soft foot is essential for accurate pump and motor alignment. It involves careful inspection, measurement, and adjustment to ensure all feet are in solid contact with the baseplate. Proper alignment reduces the risk of noise, vibration, and premature wear, enhancing the efficiency and
longevity of the pumping system. Regular maintenance and monitoring, using tools like vibration analysis and alignment devices, are crucial for detecting and correcting soft foot and alignment issues promptly. Ensuring a stable foundation and correct alignment practices can significantly reduce operational disruptions and maintenance costs, leading to more reliable and efficient machinery performance.
We’re Here to Help
A-Line Alignment Tools:
Our tools are designed to provide accurate measurements of motor and pump shaft positions at multiple points, ensuring precise alignment. From feeler gauges to dial indicators, we offer a comprehensive range of alignment devices to suit your needs.
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