In the maintenance of industrial machinery, achieving precise alignment is a critical aspect of reliability for both new and refurbished equipment, spanning from electric motors and pumps to gearboxes and compressors.

A common problem in the pursuit of optimal alignment and seamless operation is what is termed a “soft foot” condition.

Soft foot is often a serious condition that emerges when the feet of a machine case fail to rest evenly on its supporting base. The consequence of tightening foot bolts in such a scenario is a distortion of the machine case itself. This seemingly minor disturbance not only complicates alignment efforts but also introduces unwanted load on bearings and induces internal misalignment between the stationary and rotating elements of the machine. The outcome? Poor performance and elevated levels of vibration are factors that can often lead to premature failure of the machine.

The prevalence of soft foot scenarios, though widespread, finds common ground in four-footed electric motors. Within this realm, soft foot can manifest in two key categories: non-coplanar feet and individual feet that are not flat. For the latter, corrosion damage presents a particularly challenging issue. Unlike shimming, corrosion damage necessitates a more extensive approach, demanding re-machining of the feet to rectify the effects of a soft foot condition.

Effective detection is the foundational step in addressing soft foot dilemmas. An array of methods exists, but one of the most robust entails inserting feeler gauges between each foot and the base foot pad. This technique is simple to perform and the absence of elaborate setup, rendering it an essential practice during machine installation or reinstallation.

Shaft alignment instruments can double as tools for soft foot detection. This is especially convenient given that similar equipment is employed for both situations. One technique involves selectively loosening a single foot bolt and monitoring the corresponding movement signalled by the shaft alignment indicators. Although this approach may provide insights into potential soft foot existence, it is not recommended for actual correction. A more effective alternative involves leveraging dial test indicators, which are instrumental in achieving accurate and efficient soft foot correction.

Soft foot correction, while pivotal, necessitates distinct strategies for varying scenarios. Two common manifestations are diagonal soft feet and single soft feet. Diagonal cases often stem from short feet, where shimming and removal of shims play a crucial role in correction. Conversely, single soft foot situations are often indicative of bent or angled feet, demanding a more intricate correction process involving profile measurements and fabrication of staggered shims.

A general guide and recommendations for shims are:

• Shim area should be at least 80% of the foot area.
• Shims over 0.020″ (0.51 mm) should be measured for accurate thickness.
• The total shim stack’s thickness should be measured.
• A maximum of five shims per foot (excluding angle correction) should be used, with at least one shim over 0.003″ (0.08 mm) thickness.
• The combined thickness of the three thinnest shims should be 0.010″ (0.25 mm) or more.
• No more than five stepped shims should be applied in one direction.
• Detecting and resolving soft foot challenges during machine installation offers manifold benefits. It streamlines shaft alignment procedures, curbs frame or casing distortions that erode machine efficacy, mitigates heightened vibration levels, and ultimately secures machinery’s operational longevity. In an arena where precision reigns supreme, addressing soft foot emerges as an indispensable endeavour.

For further information or advice on soft foot correction call CPM Engineering on 0161 865 6161 or email info@cpm-uk.com or find out more about our services.