MECHANICAL FASTENERS 1 PROFESSIONAL ENGLISH IN USE FOR ENGINEERING

 

PROFESSIONAL ENGLISH IN USE FOR ENGINEERING UNIT 26. MECHANICAL FASTENERS 1

 

A Bolts The photo below shows a common fastener (or fixing): a bolt, with a nut. Bolts have threads – grooves on the outside of the bolt’s shaft (or shank), which run around the shaft in a helical pattern. The threaded shaft allows a nut, which has thread inside it, to be screwed onto the bolt (by a turning motion). Nuts usually have a right-hand thread – they are screwed on by turning them clockwise (in the direction of clock hands) and unscrewed by turning them anticlockwise. However, in some situations, left-hand threads are used. B Preload in bolted joints Turning force, called torque, is applied to bolts to tighten them – that is, to make them grip tightly. As a bolt is tightened, the tension (stretching force) in its shaft increases, and the components being bolted together are pressed tightly together. This pressing force is called preload (or clamp load). If enough preload is applied, friction between the components will prevent them sliding in different directions. This is an advantage, as sliding subjects the sides of bolts to shear force (scissoring force), which can cause them to break. In steel structures, bolts that apply high preloads to prevent sliding are called high strength friction grip (HSFG) bolts. C Washers Washers are metal discs which fit between the head of a bolt or a nut and the components being bolted together. Ordinary washers – called flat washers or plain washers – have a larger outside diameter than the bolt head or nut. Their wider area is intended to spread the load (distribute pressure) over the surface of the component as the bolt is tightened. EXERCISES 26.1 Change one word in each of the sentences below to make them correct. Look at A opposite to help you. 1. Wrenches are well-known examples of fasteners. 2. In most cases, nuts are screwed onto bolts by turning them anticlockwise. 26.2 Use the words in the box to complete the text about bolted joints, taken from a bolt supplier’s website. Look at B opposite to help you. loosen shear force tighten over-tighten preload tension torque work loose In structural applications, it is essential to (1)______________ bolts correctly – firstly to make sure that joints are held together with the right amount of (2)______________, and secondly to help ensure that the bolts do not (3)______________ over time. However, that does not mean to say, ‘the tighter the better’. 26.3 Decide whether the sentences below are true or false, and correct the false sentences. Look at C opposite to help you. 1. The purpose of all washers is to spread the load from the bolt across the surfaces of the components being bolted together. 2. Plain washers are flat

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