Page 128 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

EXERCISE 8 : Fasteners

Screw threads

Screw thread is a helical groove formed on cylindrical surfaces either external or internal. In other words Screw
thread is a ridge of uniform section in the form of helix either on a cylinder or inside a cylindrical surface. It is most
important single feature in engineering. The application of screw threads to bolts,nuts, screws, studs etc, enable
us to join two or more components together easily and also can be dismantled without damaging any components.
The first screw thread which was standardised is “Whitworth” Screw Thread (BSW) was designed by Sir Joseph
Whit Worth in 1841 in England. In 1864, William Sellers designed Sellers Thread.
External thread: It is the thread formed on the outside of a shaft e.g., bolt, screw and stud.
Internal thread: It is the thread formed inside a component say a hole or Nut.

Elements of screw thread: Fig 1 shows elements of a ‘V’ thread.
Right hand thread: is a thread that advances into the work when turned clockwise. It is the commonly used thread
unless and otherwise stated. (Fig 2a)

Left-hand thread: It is the thread that advances into engagement when turned anti clockwise. (Fig 2b)

Fig 1 Fig 2

Single start thread: it is a thread form, cut on the cylinder. Unless otherwise stated, threads are single start thread,
also called single start. (Fig 3a)

Multiple or multistart threads: a thread combination of same profile formed by two or more helices on the cyl-
inder. For single revolution, the axial movement of nut or screw is double, triple or quadruple depending on the
start of the thread. (Figs 3b, c,d & e)

Fig 3

115