In a recent article (Aug. 28, 2020), we focused on the importance of correcting soft foot on the machines in our plants and facilities. Here, we focus on the proper ways to check for and eliminate it.
As we discussed previously, soft foot is the condition that exists when the four feet of a machine are not equally supported by the base. This can be caused by bases or machine feet being in poor condition. Too many shims under a machine’s feet is a common problem (should be a maximum of four). Dirt or foreign material can collect under the feet depending on how well a based is cleaned, and errors in the shimming process will often occur if we don’t physically measure the shims before we insert them under the feet.
There are two types of soft foot we must consider, short leg and anglular soft foot
SHORT LEG
It’s easy to see and correct Short Leg. As the name implies, Short Leg occurs when one or more of a machine’s feet are shorter than the rest.
To identify Short Leg, we should break the coupling or remove belt tension, then loosen all four hold-down bolts. Measure the existing shim packs and consolidate shims to no more than three or four per pack. Then measure the gap under each foot. The gap should be no more than 0.002” (0.05mm). Quick tip: If there are shim packs in place, wiggle them. If they are loose, add shims until the shim pack is snug. Re-consolidate shims after correcting Short Leg
ANGULAR SOFT FOOT
Angular Soft Foot is harder to identify and correct than Short Leg. Angular Soft Foot is the condition when all four corners of a machine foot are not supporting the load evenly.
Machine feet can be bent or damaged during shipping and handling. Machine bases can warp or twist over time. In either case, the result is the same. To check for Angular Soft Foot, the machine needs to be close to its final position. Thus, this check is performed only after a rough-in alignment has been completed. At this point, it is particularly important to ensure that the vertical angularity has been corrected, as this type of correction can induce Angular Soft Foot.
Next we should break the coupling or remove belt tension. This time, we loosen one bolt at a time, and with feeler gauges measure and document the gap under all four corners of the foot. Once we have documented the foot, we can re-torque the bolt and go the next bolt in the sequence (usually a criss-cross pattern). It is important that we do not correct any feet until all feet have been checked.
While re-machining of the feet or base are good solutions for Angular Soft Foot, they’re not easy to do with installed machines. This leaves us with step-shimming as a solution. Step-shimming is acceptable, provided the following rules are applied:
♦ For “A” Shims: one extra shim only, with no shim being cut smaller than one half of the shims original dimension
♦ For “B” Shims: ideally one extra shim only, up to two shims maximum with no shim being cut smaller than one half of the shims original dimension
♦ For “C” Shims: ideally two extra shims only, up to three shims maximum with no shim being cut smaller than one half of the shims original dimension
♦ For “D” Shims: ideally three or less extra shims, up to four shims maximum, with no shim being cut smaller than one half of the shims original dimension
Good base/foundation specifications will help minimize the occurrence of soft foot on new installations. However, it will inevitably occur. In fact, experience shows us that Angular Soft Foot is so prevalent in industry today that seeing a machine without step shims should raise the question as to whether this condition had even been checked.
CHALLENGE
A quick check to see how well you are doing in the area of soft foot is to walk through your operations and observe the shimming practices under machine feet. Look for feet with more than three or four shims, homemade shims made out of improper material, and the presence of step shims.
LASER-SYSTEM PRECAUTIONS
Most laser systems have a built-in soft foot function. It’s great that technology draws our attention to this condition. It is also important to recognize that such systems DO NOT measure soft foot directly.
Laser systems measure shaft deflection at the coupling and then calculate soft foot from that. While they do a reasonable job of Identifying which feet have problems, we should exercise caution when determining actual corrections.
The best way to determine the corrections required is with feeler gauges. This will always ensure that we make the right correction the first time, thereby minimizing the time needed to complete an alignment task.TRR
ABOUT THE AUTHOR
Tim Dunton is Director of Product Development and thought leader for Reliability Solutions, a UT-RMC training partner based in Northwest Florida. He has over 40 years of experience in vibration analysis and technical training related to the reliability of rotating machinery, including extensive work and qualifications in machinery troubleshooting, problem-solving, and condition-monitoring program design, implementation and operation. His background also includes significant experience in workforce development, curriculum development, instructional design, and Reliable Manufacturing. Dunton holds DTI Class 1, CMRP, and CMRT Certifications. Email: timdunton@reliabilitysolutions.net.
Tags: reliability, maintenance, availability, RAM, soft foot, machine alignment, shims, equipment baseplates, maintenance management, skills development, training and qualification
