shimming

Shimming is the practice of inserting thin metal or composite wedges, called shims, between two components to adjust their relative position, alignment, or spacing. The shim fills a gap that cannot be closed by design alone, compensating for tolerances that stack up during manufacturing or assembly. A shim might be 0.005 inches thick steel, or a laminated pack of three 0.010 inch pieces, depending on how much adjustment is needed.

The work appears simple but demands precision. A bearing housing that is 0.015 inches too high relative to its mounting surface will cause misalignment and premature wear if left uncorrected. Rather than remake the part, a machinist inserts a shim under the housing feet. The shim moves the bearing to the correct height. Laser alignment tools or dial indicators confirm that runout or deflection falls within acceptable limits. Shims are especially common in machinery where two shafts must run parallel and coaxial, such as gearboxes and pump assemblies.

Shim materials and installation

Steel shims dominate in structural and mechanical work because they resist deformation under load and are inexpensive. Stainless steel is chosen when corrosion is a concern. Composite shims, made from phenolic resin or fiberglass, appear in electrical applications and where thermal expansion mismatches matter. Thickness ranges from 0.002 inches for fine tuning to 0.125 inches or thicker for coarse adjustment. Shims are cut to fit the support points, often with notches or slots to clear fasteners or features. They are installed dry, glued with threadlocker or epoxy, or left loose depending on whether they need to be removable for future adjustment.

Shimming enters the process when measured clearance or runout exceeds tolerance and a fix is urgent. It is not the preferred design solution; a good design minimizes the need to shim by tightening tolerances or using adjustable mounting feet. However, in field service and maintenance, shimming becomes essential. A pump that has vibrated or settled over time often needs shims to restore alignment. Thermal growth or foundation creep may necessitate reshimming months or years after installation.

The term itself comes from the noun shim, which has obscure etymology but likely relates to Old English words for wedge. In woodworking, a shim is a wood wedge used to tighten a door frame. Industrial shimming uses the same principle: a thin wedge closes a gap and holds position. The plural is shims, and the verb form is shim or shimmed. A technician who specializes in precision alignment is sometimes called a shim man, though the title is less common now.