Aerospace shims

In the construction of aircraft, aerodynamic requirements have long presented the critical problem of close tolerance contours and mismatch of panel joints on the outer surface of an airframe.

Dimensional tolerances for mating these surfaces at the assembly level are affected by the method of manufacture of individual components which are subject to warpage, shrinkage or dimensional tolerance accumulation. Existing solutions to these problems require close tolerance machining of detail components, tolerance adjustment by mechanical means, or addition of aerospace metallic shims at assembly.

Hybrid shims

The wide range of temperatures within the environments in which aircraft must operate and the rapidity of drastic temperature changes experienced dictates the necessity for solutions that would not be required in less stringent uses.

Mouldable shim material exists but shrinkage of shim material after application and curing has long been a problem.

Where gaps in assemblies are likely to be larger than most, a hybrid shim, comprising a solid part and a laminated part will frequently provide the solution.

Hybrid shims offer strength while still providing for adjustment. It is possible to employ the solid part of the shim as a bearing surface. Further, a hybrid shim tends to offer better economies compared with a fully-laminated shim.

Aerospace laminated shims

Laminated shims achieve close tolerance dimensions on complex aerostructures and engineering assemblies. Using foil materials from 0.025mm  to 0.1mm, a process is used to bond multiple layers together under heat and pressure to produce a strong and rigid structure that functions as a solid, but may be reduced to precise thickness by peeling off layers.

The primary job of a shim is to provide dimensional adjustment within a mechanical assembly, precision alignment and wear compensation.

The ability to peel individual layers to achieve a shim of variable thickness provides time and process advantages when assembling components where top and bottom tolerances combine to produce a different dimension for each assembly. Laminated

shims also offer economic benefits by reducing the need for customers to maintain a large stock of solid shims.

Laminated peelable shims provide reduction in assembly time, dimensional accuracy without expensive machining.

They are used…

• Where rotating shafts, sliding surfaces or stationary surfaces must be parallel

• Where the end play is unacceptable

• Where stacked or accumulated tolerances are difficult to control

• Where forces change dimensions of an assembly or component

Standard sheet materials are metals such as titanium, steel, stainless steel, brass, aluminium alloy and also, plastics, polyimide (Kapton® and Apical®), polyester (Mylar®) film and polyamide nylon.

Components from sheet stock are easily manufactured since the material can be sheared, machined, punched, or water-jet cut.