In the precision shaft alignment business, we encounter all types of electric motors. One type we come across are “Rolled Steel Frame” (RSF) motors. These motors have a frame housing and feet that are made from mild, rolled steel. They range from very small up to about 25hp. They are common in many industries; however, they require some special consideration when aligning.
RSF motors have a housing and feet that are stamped from mild steel and are not exceptionally ridged. Most of them don’t have 4 separate feet but instead have a plate welded onto the housing with 4 holes stamped or drilled in it. These can be the most difficult to align. RSF Motors are more prone to soft foot issues than cast iron frame motors due to their mild steel feet. The process of finding and correcting soft foot becomes even more important and must be done carefully.
Another step that must be performed carefully when aligning RSF motors is the process of torqueing down the hold down bolts during the correction phase of a precision shaft alignment. A “cross torque” pattern must be established and followed with at least 3 passes around the pattern before final torque is reached. Some RSF motors may require more passes. This will minimize flexing of the mild steel feet and creating soft foot when none may have existed. It is recommended that one person follow this tightening sequence, this will ensure that the same torque is applied to each hold down bolt/foot.
Throughout VibrAlign’s Shaft Alignment Best Practices training course, we explain the causes of these alignment issues and practice the best ways to correct soft foot and other common issues that can occur during rotating machinery installation and alignment.
