Lesson 4

Mechanical Fasteners

1. Reading

Some method of joining parts together is used throughout industry, to form either a complete product or an assembly. The method ise depends on the application of the finished product and whether the parts have to be dismantled for maintenance or replacement during service.

There are five methods by which parts may be joined :

a) mechanical fasteners - screw, bolts, nuts, rivets

b) soldering

c) brazing

d) welding

e) adhesive bonding

Mechanical fasteners are most widely used in applications where the parts may need to be dismantled for repair or replacement. This type of joint is known as nonpermanent. The exception would be the use of rivets, which have to bo destroyed to dismantle the parts and so form a permanent joint. Welding and adhesives are used for permanent joints which do not need to be dismantle - any attempt to do so would result in damage or destruction of the joints and parts.

Although soldered and brazed joints are considered permanent, they can be dismantled by heating for repair and replacement.

Depending on the style of the machine screws, thread dia-meters are generally available up to 10mm, with lengths up to 50mm, manufactured in high -grade alloy steel with rolled threads, the socket screw is used for higher-strength applications than machine screws. Three head shapes are available, all of which contain a hexagon socket for tightening and loosening using a hexagon key, Fig. 4.1

Both are used in conjunction with a nut for heavier applications than screws. unlike screws, bolts are threaded for only part of their length, usually twice the thread diameter.

Standard hexagon nuts are used with bolts to fasten part together, Where parts require to bo removed frequently and hand tightness is sufficient, wing nuts are used, Fig. 4.2(a). If a decorative appearance is required, a dome or acorn nut can

be fitted, Fig. 4.2(b)

Washers distribute the tightening load over a wider area than does a bolt head, screw head, or nut. They also keep the surface of the work from being damaged by fastener.

Riveting as a means of fastening is use because of its speed, simplicity, dependability, and low cost. Light riveting, used for general assembly work ip to about 6mm diameter, is carried out in industry using high-speed rivet-setting machines having cycle tims as short as 1/3 second. rivets are used on assemblies where parts do not normally hove to be dismantled, I.e. permanent joints.

The cost of riveting is lower than that of most other methods of fastening, due to the absence of plain washers, locking washers, nuts, or split pins. The more standard head types used are shown in Fig. 4.3

3. Exercise

1. What does the method of joining parts used depends on?

2. What is the permanent joint of mechanical fasteners?

3. How can the soldered and brazed joints be dismantled for repair and replacement?

4. Which screw is used for higher strength applications, machine screw or socket screw?

5. Why is the cost of riveting lower than that of most other methods of fastening?

Lesson 5

Joining

1. Reading

A weld is defined as a localized coalescence of metal wherein coalescence is produced by heating ti suitable temperatures, with or without the application of pressure, and with of without the use of filler metal. The filler metal may have a melting point approximately the same as the base metals as in arc or gas weding. This definition distinguishes welding form mechanical joining and adhesive bonding. The Definition includes some 37 different processes falling under six general categories : arc welding, gas welding, processes. Of these, the first four are of wide industrial significance.

Basic types of arc and gas welds in Fig. 5.1 are ciews of joints commonly used in plate and sheet fabrication.

Arc welding is defined as a group of welding processes wherein coalescence is produced by heating with an electric arc or arcs, with or without the application of preessure and with or without the use of filler metal. The most significant-arc welding, gas tungstem-arc welding, gas metal-arcwelding, carbon-arc welding, and arc-spot welding. In all these cases, an arc is maintained between an electrode and the work (or between two electrodes), which form the terminals of an electric circuit

The heat for gas welding is supplied by burning a mixture of oxygen and a suitable combustible gas. The Gases are mixed in a torch which gives control of the welding flame.

Acetylene is almost universally used as the combustible gas because of its high flame temperature. This temperature, estimated to be about 6000 F, is so far above the melting point of all commercial metals that it provides a means for the rapid localized melting essential in welding.

In resistance welding, coalescence is produced by the heat obtained from the resistance offered by the work to the flow of electric current in a circuit of which the work is a part, and by the application of pressure. The specific processes include resistance spot welding, resistance seam welding, projection welding, upset welding, and flash welding, Fig 5.2 shows resistance welding symbols. The resistance of the welding circuit is maximum at the interface of the parts to be joined, and the heat generated ther must reach a value high enough to cause a localized fusion under pressure.

3. Exercise

1. Explain the definition that welding is distinguished frim mechanical joining and adhesive bonding.

2. Tell the six processes of welding.

3. Why is acetylene used for the combustible gas?

4. Specify the arc welding process.

5. specif the resistance welfing process.