9.2 Prealignment and Shim Fabrication

Why Prealignment and Shim Fabrication Matters

Module 15313, Prealignment and Shim Fabrication, teaches the rough-alignment work that must happen before any precision dial-indicator or laser method is used. Skipping this step is one of the most common real-world millwright mistakes: a technician jumps straight to precision alignment on a machine that has soft foot, pipe strain, or a bolt-bound baseplate, and no amount of careful indicator reading will produce good numbers because the underlying installation is still fighting itself. On the exam, this module is tested through soft-foot identification, shim material/fabrication questions, and leveling procedure sequencing — all foundational to the Alignment domain's 20.8% weight.

Rough Alignment: The Step Before Precision Alignment

Prealignment (rough alignment) gets the two machines close enough — typically within a few thousandths using a straightedge and feeler gauge, or a caliper across the coupling rim and face — that the fine dial-indicator or laser work in Module 15314 doesn't have to correct for gross errors. Rough alignment also gets equipment level and properly supported using:

• Jackscrews — threaded fasteners built into or added to the baseplate that let a technician fine-tune horizontal and vertical position with small, controlled adjustments before final bolt-down.
• Wedges — tapered shims driven under a foot to make coarse height adjustments quickly, later refined or replaced with precision shims.
• Shims — thin, precisely-sized spacers placed under each foot to set final vertical height and correct angular tilt.

Soft Foot: Definition and Types

Soft foot is a condition where one or more of a machine's mounting feet do not sit flush on the baseplate — bolting the machine down twists the frame, which distorts the internal bore alignment of the bearings even though the shaft centerline looked fine before bolting. Soft foot must be found and corrected during prealignment, because it will silently ruin an otherwise-perfect precision alignment.

A machine can pass a static soft-foot check and still fail once running hot — this is why the Alignment domain also tests thermal growth and cold-alignment targets as a related concept.

Correcting Coupling Stresses During Rough Alignment

Beyond soft foot, prealignment must clear other stresses that would otherwise corrupt the final alignment reading:

• Pipe strain — piping bolted to a pump or compressor nozzle before the machine is aligned can pull the casing out of position. The fix is to support and align piping independently so it meets the nozzle without force, not to "align out" the strain with shims.
• Bolt-bound conditions — foundation bolts sized too tightly in their holes prevent the small horizontal moves alignment requires; slotted or oversized bolt holes must be used at each foot.
• Coupling gap — the axial spacing between the two coupling halves must match the coupling manufacturer's specification before final alignment; too tight a gap can cause thrust loading, too wide a gap prevents proper engagement.

Shim Fabrication: Materials and Best Practices

The exam-relevant fabrication rules for shims are specific and testable:

• Approved material: stainless steel is the standard for precision shims because it resists corrosion, holds a flat, repeatable thickness, and is cleaner to install than cut sheet stock.
• Never substitute: wood, cardboard, washers, or wedges left in place as permanent shims — none of these hold a controlled, verified thickness and all compress or degrade under load.
• Tolerance bands: thinner pre-cut shims (roughly 0.020 in. and under) are ground stock held to about ±0.0005 in.; thicker shims (roughly 0.025 in. and up) are rolled stock held to a wider tolerance, around ±0.005 in. — always verify actual thickness with a micrometer or caliper rather than trusting a printed or etched marking.
• Stack rules: use no more than about four shims per foot whenever possible. Favor fewer, thicker shims over many thin ones — a thick stack of thin shims compresses unevenly over time and reintroduces soft foot. When a large gap must be filled, use one thick block plus a small number of thin shims to fine-tune, rather than stacking many thin shims together.

Exam Scenario

A pump fails a soft-foot check: loosening one foot's bolt causes the frame to spring noticeably before the technician even touches a wrench to the shims. This points to induced (springing) soft foot — likely pipe strain pulling the casing — and the correct fix is to re-support and re-align the piping, not to keep adding shims under that foot.

Key Takeaways Recap

Rough alignment and soft-foot correction must happen before any precision method is attempted; soft foot has four distinct forms (parallel, angular, induced, dynamic) with different causes and fixes; and shims must be stainless steel, individually verified, and limited to about four per foot with thicker shims preferred over stacks of thin ones.