If you have ever installed a window, door or kitchen cabinet, you know the importance of shims—thin wooden wedges that help keep things square and level.
Engineers may not want to admit it, but large mechanical assemblies—airplanes, tractors, industrial machinery—are no different. Sometimes, mating parts just don’t meet the way they were designed, due to tolerance stack-ups. Custom-made precision shims solve the problem, but, instead of cedar or pine, shims for mechanical assemblies are made from metal.
Precision shims compensate for accumulated tolerances between mating components. They significantly reduce manufacturing costs by eliminating the need for each component to be precision machined to achieve the proper fit and function of the total assembly. Shims significantly reduce the need for re-machining. Additionally, shims are commonly used to preserve the faces between mating components, cutting down on machining time during rebuilds and retrofits.
Precision shims are used in virtually every industry. In the aerospace industry, for example, shims are used in engines, landing gear assemblies, pilot control assemblies, cockpit pedestal assemblies, fluid and hydraulic systems, and auxiliary power units.
In tractors and agricultural equipment, shims are used for power couplings, power train assemblies (to aid alignment and increase wear
resistance), sensors (to aid alignment), pivot points (to provide wear resistance), and bearings (for preloading). In industrial machinery, shims are used to boost wear resistance in motors and to aid shaft alignment in power transmission assemblies.
Shims tend to be used in lower volume applications, though there are high-volume applications in the automotive industry.
Peeling Back the Layers
Precision shims are available as solid pieces of metal in various thicknesses, or they can be made as laminations. The latter offer many advantages.
Laminated shims consist of peelable metal layers that are removed until the shim is the correct thickness. The shims are built up from layers of precision-gauge metal foil. Layers are bonded into a rigid structure that appears and functions as a solid sheet or plate. The thickness of the shim can then be adjusted simply by peeling off laminations with a knife, or in the case of some materials, using no tool at all.
Rather than going through iterations of stacking multiple solid shims of varying thicknesses, engineers need only adjust a single shim to the desired thickness and slip it into position. In this way, variations in tolerance stack-ups can be accommodated, saving considerable time. It also saves money by reducing assembly time and minimizing line-side SKUs and inventory storage space.
Laminated shims are available in either fully or partially laminated versions. The latter type of shim can be half-solid or three-quarter solid. Semi-solid shims add rigidity to a design. Like a solid shim, they provide a bearing surface on one side, while still allowing engineers to adjust their overall thickness adjustment by peeling off unwanted layers from the other side.
Laminated shims are also available in surface-bonded or edge-bonded versions.
With surface-bonded laminated shims, adhesive is applied to the entire surface between each layer, and the layers are pressed together to cure, much like how plywood is manufactured. After separation, the unwanted layers must be discarded due to deformation during peeling.
In contrast, the layers of an edge-bonded shim are pressed together first. Then, adhesive is applied only to the edges of the layers. This allows for a much easier separation of the layers in the field. Because edge-bonded shim sets are easy to peel, the layers that are not needed are preserved and can be used in other applications.
Edge-bonded shims have all the performance and cost advantages of solid shims, but have the added advantage of being easily adjustable. Another advantage of edge-bonded shims is that multiple lamination thicknesses can be combined in the same stack, giving engineers more flexibility for adjustment. This is not an option for traditional laminated shims. Each layer is typically 0.002 or 0.003 inch thick.
Either way—edge-bonded or surface-bonded—laminated shims can be produced with any number of layers.
Another advantage of edge-bonded shims is that they are less expensive than surface-bonded shims. Edge-bonded shims can be adjusted in seconds and are much easier to peel than surface-bonded laminates. Safety is improved because no knife is needed to separate the edge-bonded layers.
Shims and Design
Shims are typically custom-made for each application, because they mirror the profile of whatever component they are mating against. (Generic shims are available. These are simply strips of metal of known thicknesses that can be used to fill a void.)
Shims can be made a variety of ways. Their shapes can be produced by lasers, water jets, stamping or milling.
Shims can be manufactured from any material. Standard materials are carbon steel (both high- and low-carbon steel, as well as alloy steel), stainless steel (austenitic and martensitic stainless steels), aluminum, brass, copper, Inconel, Hastelloy and Monel. Finished laminated shims withstand reasonable handling, including shearing and machining.
The material should be selected carefully. The wrong material for the application could end up tearing, separating or deteriorating. Similarly, the bonding agent, which is a bit like rubber cement, should also be chosen carefully. The wrong bonding agent can peel off and become debris—a definite no-no in aerospace assemblies.
When designing an edge-bonded shim, it’s important to important to provide enough real estate for the adhesive to grip—just as you would with any bonded joint. If the bonding area is too small, it can be difficult to get the layers to remain adhered to each other. Also, if engineers are trying to bond on a “controlled dimension,” they should account for the space required by the bonding agent itself.
Shims should not be mistaken for gaskets. Because of the edge-bonding process and the multiple layers, gas or liquids can escape from between the layers.
Both types of laminated shims—surface-bonded and edge-bonded—offer the advantages of reducing assembly time, taking up less inventory space, and reducing the number of line-side SKUs. However, when speed of adjustment, operator safety and minimizing the overall cost of assembly are the primary objectives, edge-bonded shims provide the best solution.