Lesson 7 Fasteners and Springs

Lesson 7 Fasteners and SpringsFasteners are devices which permit one part to be joined to a second part and, hence, they are involved in almost all designs. The acceptability of any product depends not only on the selected components, but also on the means by which they are fastened together. The principal purposes of fasteners are to provide the following design features:(1)Disassembly for inspection and repair;(2)Modular design where a product consists of a number of subassemblies. Modular designaids manufacturing as well as transportation.There are three main classifications of fasteners, which are described as follows:(1)Removable. This type permits the parts to be readily disconnected without damaging thefastener. An example is the ordinary nut-and-bolt fastener.(2)Semi permanent. For this type, the parts can be disconnected, but some damage usuassyoccurs to the fastener. One such example is a cotter pin.(3)Permanent. When this type of fastener is used, it is intended that the parts will never bedisassembled. Examples are riveted and welded joints.The following factors should be taken into account when selecting fasteners for a fiven application:(1)Primary function;(2)Appearance;(3) A large number of small size fasteners versus a small number of large size fasteners(anexample id bolts);(4)Operating conditions such as vibration, loads and temperature;(5)Frequency of disassembly;(6)Adjustability in the location of parts;(7)Types of materials to be joined;(8)Consequences of failure or loosening of the fastener.The importance of fasteners can be realized when referring to any complex product. In the case of the automobile, there are literally thousands of parts which are fastened together to produce the total product. The failure or loosening of a single fastener could result in a simple nu9isance such as a door rattle or in a serious situation such as a wheel coming off. Such possibilities must be taken into account in the selection of the type of fastener for the specific application.Springs are mechanical members which are designed to give a relatively large amount of elastic deflecting under the action of an externally applied load. Hooke’Law, which states that deflection is proportional to load, is the basis of behavior of springs. However, some springs are designed to produce a nonlinear relationship between load and deflection. The following is a list of the important purposes and applications of springs:(1)Control of motion in machines. This category represents the majority of springapplications such as operating forces in clutches and brakes. Also, springs are used to maintain contact between two members such ad a cam and its follower.(2)Reduction of transmitted forces as a result of impact or shock loading. Pplications hereinclude automotive suspension system springs and bumper springs.(3)Storage of energy. Applications in this category are found in clocks, movie cameras andlawn mowers.(4)Measurement of force. Scales used to weigh people is a very common application for thiscategory.The three major classifications of springs are compression, extension, an torsion. See Fig.7.2. Compression and extension springs are the springs most often used. Deflecting is linear in these types. Torsion springs are characterized by angular instead of linear deflection. Leaf springs are of the simple beam or cantilever type. Their deflection is linear.Most springs are made of steel, although silicon bronze, brass and beryllium copper are also used. Springs are universally made by companies which specialize in the manufacture of springs. The cylindrical helical spring is the most popular type of spring; torsion bar springs and leaf springs are also widely used. Springs are universally made by companies which specialize in the manufacture of springs. The cylindrical helical springs the most popular type of spring torsion bar springs and leaf springs are also widely used. If the wire diameter (assuming a cylindrical helical spring)is less than 3 mm, the spring will normally be cold-wound from hard-drawn or oil-tempered wire. For larger diameters, springs are formed using hot-rolled bar.It is good practice to consult with a sping manufacturing company when selecting a spring design, especially if high loads or temperatures are to be encountered, or if stress reversals occur or corrosion resistance is required. To properly select a spring, a complete study of the spring requirements, including space limitations, must be undertaken. Many different types of special springs are available to satisfy unusual requirements or applications.。