Mechanical fasteners provide a clamping force between two pieces of material.There is a wide variety of types of fasteners and a variety of materials used to make them. There are threaded fasteners, non-threaded fasteners and special purpose fasteners. Steel fasteners are the most common material used, although aluminum alloys, brass and nickel are amongst other metals used. Aluminum alloy fasteners have the advantage over steel of being much lighter, non-magnetic and more corrosion resistant fasteners. Nickel has the particular advantage of strength at high temperatures.
The choice of fastener will depend on a number of factors:
-Environmental…For example: high, normal and low temperature; corrosive, abrasive and oxidising conditions.
-Nature of the external loading on the fastener…For example: tension, compression, shear, cyclic, impact.
-Life and service requirements…For example: the need for perhaps frequent assembly and disassembly.
-Design of the components being joined and types of materials…For example: stress concentrations; the elastic, plastic and thermal properties of the materials.
Quantity of fasteners required and cost
Threaded fasteners
With a threaded fastener, the clamping force holding the two pieces of material together is produced by a torque being applied to the fastener to stretch it and this is maintained during the service life of the fastener. The fastener is in tension and the two pieces of material in compression. Bolts mated with nuts and screws with threads in the material are examples of this type of fastener. Low- or medium-carbon steel bolts will typically have a minimum proof strength of about 225 to 400 Pa, the proof strength being the maximum stress the bolt can withstand without acquiring a permanent set. For quenched and tempered mediumcarbon bolts the minimum proof stress is 600 to 650 MPa and quenched and tempered alloy steel about 970 MPa. Threaded fasteners are particularly useful for joining components that are likely to need to be dismounted during the life of the product.
Non-threaded fasteners
Rivets, eyelets, nails and pins are examples of non-threaded fasteners. Rivets can be used for joining dissimilar or similar materials, both metallic and non-metallic, to give permanent joints. When the force applied to the rivet is sufficiently high, plastic deformation occurs and the shank of the rivet increases in diameter as its length decreases. That part of the shank within the hole increases in diameter until it fills the hole and the unsupported part of the shank outside the hole continues to deform until a head is formed. A ductile material has to be used for the rivet material, e.g. mild steel, copper, brass. aluminium alloy. For some materials the required ductility is obtained in the cold state, in other cases the riveting takes place hot. Where the riveting force might damage or distort the materials being joined, tubular or semi-tubular rivets might be used instead of solid rivets.
Pins, either in the solid or tubular forms, are widely used for fastening. For example, taper pins are used to join wheels onto the ends of shafts, the pin just being driven through holes in the two parts until it is fully home and giving a tight fit. Taper pins are usually supplied in mild steel but can be alloy steel, stainless steel or brass. Another form of pin is the split cotter pin. This is used where movement in the joint is required or as a locking device : for slotted nuts on bolts.
There is a wide variety of forms of spring-retaining clips. A simple form is a C-clip which is used to lock components on shafts, the clip generally fitting into a groove on the shaft . Most C-clips are made from hardened and tempered carbon steel.
Fatigue properties of fastened joints
The use, for example, of bolts or rivets as fasteners for joints can introduce fretting damage. Fretting is the wear process that occurs at the areas of contact of two metals undergoing small cyclic slip. The damage is referred to as galling or scuffing. Such damage can lower the fatigue strength by factors as high as three for aluminium alloys.