Hampson Group Ltd.

Selecting the correct laminated shim

Material Selection

Where electrolytic reaction with dissimilar metals is a possibility, consideration should be given to the use of Attewell’s unique cadmium-plated steel LS3 variant which is prepared with a coating to both sides of each foil prior to lamination. If the shim is to be used in a stressed location, it is important, especially in the aluminium range, that the shim strength is similar to the surrounding structure. Materials can also be supplied that are AMS-DTL-22499 compliant. 

* Polyester (Attewell) LM1 is Mylar® RL32 peelable one-side corona-treated polyester film

Aluminium Type 1A

Laminated sheets from Type 1A aluminium that meet the quality standards demanded by Boeing, are manufactured from individually treated layers of foil as thin as 0.05mm (0.002”). Attewell is thought to be one of only three suppliers globally to supply The Boeing Company with this product.

Weight: always a consideration

The use of polyimide film in the manufacture of laminated shims is making a significant contribution to the continual demand for weight reduction in the manufacture of aircraft. Polyimide film for use in aerospace applications has good physical, chemical and electrical insulation properties over a wide temperature range but is much lighter than traditional metal fillers and spacers. Polyimide material is laminated from 0.075mm (0.003”) and 0.13mm (0.005”) and can be backed with a solid portion made from a cotton-based phenolic resin. The material is also the obvious choice for use in composite assemblies.

Choosing the shim thickness

Having calculated the maximum variation that any mismatch might produce it is recommended that shim thickness should be specified just above that figure. A selection from the thickness table (see Table 1 of Standard Range*) could offer economical benefits since standard material is normally ex-stock, avoiding the need for special stock to be manufactured. Sheets range from 0.15 – 6.35mm thick.

Foil thickness

Foils ranging from 0.025mm (0.001”) or 0.1mm (0.004”) are used in the manufacture of standard shim stock and will impact on the final degree of accuracy achieved on the assembly fit. Foils of 0.05mm (0.002”) or 0.075mm (0.003”) normally satisfy most assembly demands.

Hybrid shims

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 whilst still providing for adjustment. It should be noted that 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.

Specified shapes

Attewell’s standard shim stock sheet size is 1220mm x 610mm (48” x 24”) and is specified in Table 2 of Standard Range*. Sheets can be profiled by punching and die pressing. As a general rule punched hole diameters must be twice the material thickness and the flange or remaining width similarly must be twice the material thickness. Light, single curvature forming can be achieved but double curvature forming invariably results in laminate failure.

Peel strength

Attewell is able to provide a peel strength according to the wishes of the customer. Shims are normally manufactured to ensure simple and easy adjustment with a knife or by hand.

Materials

Standard Range