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2.2 SCREW THREAD
In our day to day life, we come across many objects where bolts and nuts are used to join two pieces together. For example we use wooden furnitures like desks, stools, tables etc. in school, showing bolts, nuts and screws. Such machine parts which are used to connect two pieces together are called as fasteners. There are two types of fasteners, viz, temporary fasteners and permanent fasteners. Threaded fasteners like bolt and nut are temporary fasteners. The process of joining different machine parts of machine or engineering products is called as fastening.Permanent fastening such as welding, riveting etc. join two parts together permanently and they cannot be separated without breaking the fastening, but in the case of temporary fastening, the parts are joined together temporarily and can be separated easily without breaking the fastening.
(i) EXTERNAL THREAD
It is a continuous helical groove or ridge cut along the external surface of the cylinder, e.g. threads on bolts studs, screws etc. FIG 2.2(a) shows an external thread.
(ii) INTERNAL THREAD
It is a thread on the internal surface of a hollow cylinder. FIG 2.2(b) shows the internal threads, e.g. threads of a nut.
(iii) SCREW PAIR
The bolt and nut together called as screw pair. One or more such pairs are used to join two parts.
(iv) PARALLEL AND TAPER THREAD
A thread formed on the surface of a cylinder is called as parallel or straight thread. A thread formed on the surface of a cone called as taper thread. Refer FIG 2.3(b)
(B) TAPER THREAD
(A) PARALLEL THREAD
(v) RIGHT HAND AND LEFT HAND THREADS
Consider any nut and bolt. Hold the bolt firmly in left hand and rotate the nut clockwise by the right hand, the nut will screw on the bolt of the threads are right handed. It is abbreviated as RH thread. A left hand screws thread when assembled with a stationary mating bolt, screws off the bolt for clockwise rotation. It is abbreviated as LH thread.
Observe that mostly the bolts and nuts that we use in daily life have RH thread. Also we can observe that all the jewellery mating pieces have LH thread.
(vi) PITCH, P
It is "the distance between the corresponding points on the adjacent threads, measured parallel to the axis". Refer FIG2.2 (a)
It is "the distance moved by a nut or bolt in the axial direction in one complete rotation".
(viii) SINGLE START AND MULTI START THREADS
When only one helix, forming the thread runs on a cylinder, it is called as single start thread. If more then one helices run on a cylinder, it is called as multi start threads.
i.e. L=P in the case of single start
L=2P in the case of double start
L=3P for triple start and so on.
It is the edge of the thread surface farthest from the axis, in case of external thread and nearest to the axis, in case of internal thread
It is the edge of the thread surface nearest to the axis in case of external thread and farthest from the axis, in case of internal thread.
The surface connecting crest and root is called as flank.
(xii) THREAD ANGLE
It is "the angle between the flanks measured in an axial plane".
(xiii) MAJOR DIAMETER OR OUTSIDE DIAMETER
It is the diameter of an imaginary coaxial cylinder just touching the crest of external threads or roots of internal threads. It is the largest diameter of a screw thread.
(xiv) MINOR DIAMETER OR ROOT DIAMETER OR CORE DIAMETER
It is the diameter of an imaginary co-axial cylinder just touching the roots of external threads or crest of internal threads.
(xv) NOMINAL DIAMETER
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