Mechanical fastening use

Nailing is a satisfactory way of fastening Exel Composites FRP shapes to wood and to other materials that provide enough grip to hold the nail. Common nails can be driven through 1/16″ thick FRP without re-drilling holes – tempered nails will go through 5/16″ thick material.

FRP heavier that 5/16″ requires pre-drilled holes, slightly oversized, to admit the nail and to allow for expansion and contraction between the FRP and the material to which it is nailed. It also advisable to pre-drill slightly oversized holes before nailing long lengths of lighter FRP sections. Never nail FRP to FRP.

Self-tapping screws have been used successfully in many applications involving mechanical connections when high strength fasteners are not required A better use of self­tapping screws is in combination with adhesives. In this application the screws can serve to hold the adhesive bonded surface of the two parts together while the adhesive cures in addition to contributing limited mechanical strength to the connection. Appropriately sized pilot holes should be provided in the Exel Composites shape for the screws. In corrosive environments, stainless steel or monel screws should be used – unless a suitable coating of polyester or epoxy can be applied to the exposed screw heads to prevent corrosive attack of the fasteners.

Lag screws are not recommended because they do not bite well into the FRP.

A very satisfactory connection can be made with Exel Composites FRP components by using standard bolts, nuts, and washers. Since FRP materials can fail under high localised stress conditions, such as those encountered around a bolt, the tighter the bolt is in the hole, the more effective it will be. Always use flat washers on both sides of bolt connections.

The strongest joint between pieces of Exel Composites shapes is obtained by using a combination of properly fitted bolts with adhesives applied to the properly prepared mating surfaces.

Mechanical fastening can be done by using bolts or screws into tapped holes. However, the properties of tapped holes are not good nor will the connection be strong.

For removable cover plates, sheet metal screws can be used. The strength of the connection can be improved by use of the threaded inserts bonded into place with suitable adhesives.

When removable bolts are required, threaded metal inserts or fasteners should be installed in the FRP and preferably bonded in place with a suitable adhesive. FRP threads will wear out quickly and may not give sufficient holding strength. Many types of metal inserts and fasteners are commercially available, for example: “Molly” nuts, “Tee” nuts, “Dzus” fasteners, BF Goodrich “Rivnuts” and “Helicoils”. Some types must be bonded in place while others can be mechanically fastened.

Another way of installing removable bolts is by tapping the FRP, applying epoxy or polyester adhesive in the hole and inserting the bolt after covering the threads and shank with grease or some other releasing agent. The bolt can be withdrawn after the adhesive has formed and hardened around the threads. This method is not recommended when an exceptionally strong connection is required.

When bolts are to be installed permanently, a tight connection is easily made by tapping the FRP and applying epoxy or polyester adhesive to the hole just before inserting the bolt.

Emhart Fastening Systems Group “POP” Rivets are very effective in joining Exel Composites FRP sections. These rivets are available in various sizes and head styles in aluminium, steel, Monel, copper and stainless steel. Other types of rivets, such as Drive Rivets, those formed by a rivet gun or the conventional rivet formed with a ball peen hammer, can produce an effective mechanical connection. The strength of the connection can also be improved with suitable adhesives. Back-up washers are recommended for distributing load stresses. As in drilling operations, it is necessary to use a slightly larger drill than the exact diameter of the rivet. For a ¼” rivet, use a NQ 30 drill rather than a ¼” drill.