So; foot may sound like human foot disorder, but it is one of the most signi;cant and most overlooked machinery alignment problems. Many experts esti- mate two-thirds of all rotating equipment in ourproduction plants have this issue, yet it o;en goes undetected.
For those new to the problem, so; foot is machine-framedistortion. It happens when rotating machinery is positioned on its base, soleplate, or frame, and one or more ofthe “feet” fails to meet with the “foot points” on the frameadequately. A simple way of demonstrating the condition isto imagine a four-legged wooden chair and consider whatwould happen if one of the chair legs was shorter than theothers. ;e chair would rock.
Same with machines; they’re set o;-kilter by this distortion and will “rock” if each of the foot points on the basedoes not sit evenly. ;is ailment impacts their condition,performance, and long-term health. To correct the situation and achieve precise alignment, you must adjust all ofthe foot points and the entire machine frame.
Accurately identifying the reasons for so; foot in a machine and correcting it before attempting an alignmentwill save you time, money, energy, and potential disruption of production.
STEP 1: Learn to identify the causes, types, andeffects of soft foot.
;e ability to recognize the various causes, types, and effects of so; foot is a critical ;rst step. With this knowledge,the technician is better equipped for the second step, whichis to diagnose and correct the condition using speci;c tools.
What causes so; foot?
• Badly machined base or feet• Warped base plate or frame• Debris, dirt or paint under feet• A cracked, damaged foundation• Too many shims• Damaged or bent feet• Pipe strain• Grout decay
Types of so; foot
Several categories of so; foot have been identi;ed over theyears: squishy foot, parallel so; foot, angular so; foot, andexternal force so; foot. Here’s a description of each type.
Squishy foot occurs when an overabundance of shims isapplied to correct a so; foot problem. It also happens whena large amount of corrosion or debris builds up under themachine foot during an initial misalignment check. As thebolts are loosened during the repair, sediment and debrisare released. ;is usually happens if the machine’s bolts haven’t been tightened for a long time—or if the machineryhas never or rarely had the bolts loosened.
External force is a so; foot condition caused by electri-
cal connections or severe misalignment in conjunction
with sti; coupling stress-induced external forces such as
pipe strain. ;is condition is sometimes hard to diagnose
and can happen at any time during machine alignment.
Parallel so; foot is the most common type of so; footand is usually the easiest to spot. It occurs when a foot doesnot meet the base and produces a gap between the foot andthe frame. When tightening the bolts, as the gap begins toclose, the machine frame will start to warp to ;t the baseplate, causing misalignment. Usually, the problem can becorrected by using shims unless you do not use the rightthickness, or you use too many.
Angular so; foot occurs when a machine foot touchesthe base on either the side or outside of the foot. ;e otherside of the foot is bent away, creating an angle between thebase and the bottom of the foot. Angular so; foot is harderto diagnose because it takes place at an angle.
What are the e;ects of so; foot?
• Machine misalignment
• Bent sha;
• Increased load on bearings
• Misalignment of bearings
• Seal failure
• High vibration levels
• Increase in power consumption
Any of these situations can destroy machinery if not
STEP 2: How to diagnose and correct soft footand the tools to use.
Undoubtedly, correcting a so; foot problem can sometimes be exasperating, partly because the process o;entakes longer than the subsequent machinery alignment.However, if you can stay relaxed and think strategically,there is always a sizable reward—preventing machine destruction. Measuring so; foot can be done in several ways.Con;rming you have the right tools available will make diagnosis more straightforward and quicker. It’s vital to applythe procedures below during all machine alignments andinstallations to determine if a so; foot condition exists.
Here is a list of devices to help measure for so; foot, andtools to use to remove any debris found underneath andaround the feet.
• Feeler gauge: Use a feeler gauge to measure gap widths orthe space between two parts. ;e device contains numeroussteel blades with diverse thicknesses that can measure up toa quarter (¼) inch to as little as 1/1000 of an inch.
• Micrometer or digital caliper: A micrometer can alsomeasure gap thickness as well as con;rm shim thickness.Before inserting shims under the feet, use a caliper to measure shim stacks to verify that the shims are the thickness aslabeled and not defective• Wire brush: Used to remove the debris or corrosionfound when checking machinery feet.
• Cleaner/degreaser: Use a heavy-duty cleaner combined with the wire brush to degrease any current shimsand contact surfaces while correcting so; foot. Brakecleaner usually works, but other popular commercialcleaners might also do the job.
• Laser sha; alignment system: A laser system is easy touse and can be very useful when measuring for so; foot.;e advanced tool guides users through the measurementprocedure using a sequence of screens leading to an autodiagnosis. A dual or single laser sha; alignment systemcan help correct the condition. However, a single-lasersystem is simpler to use and more intuitive.
Correcting so; foot with shims
An effective way to correct machine foot issues is byusing shims. However, if the frame or mount is cracked,twisted, or bent, or another overall machinery destruction has emerged, correcting the condition can be morecomplicated.
The machine will most likely already have shims. It’s agood idea to clean any current ones and also clean thebase plates and feet to eliminate any dirt and debris. It’sessential to remove any corrosion, paint, or grime thatmight have collected elsewhere to avoid accidentally creating a new condition, squishy foot, during alignment.
How to avoid squishy foot
Here are ;ve rules to apply to ensure better that squishyfoot does not form while trying to eliminate an existingso; foot condition.
• Shims come in many sizes, and large shims may be created on-site. When choosing the shim size, be sure to cover no less than 80% of the footprint.
• Do not add any more than four shims under each foot.Too many shims can cause a spring-like effect, i.e.,squishy foot.
• Layer thin shims in between thick shims. It eliminates thepotential for shim creasing and makes placement easier.
• Errors or defects sometimes occur during the shim manufacturing process. To make sure everything adds up, measure stacks with a micrometer to verify shim thickness.
• Best practices recommend measuring shims thickerthan 0.030 to verify thickness. Other resources might suggest higher or lower depths, but all in all, it is up to theperson doing the work.
By following these so; foot mitigation best practices,including applying the right methods and tools to diagnose and correct the condition accurately, your chances ofresolving the problem and restoring machine health aremuch higher. Always remember to check for so; foot before aligning sha;s.
Ryan Best is an Application Engineer at PRUFTECHNIK, Inc.,based in Philadelphia.
Up to two-thirds of all rotating machinery have this problem, but it frequentlygoes unchecked.
By Ryan Best
Best Practices forDiagnosing andCorrecting So; Foot