TABLE OF CONTENTS Screw threads may be classified into two basic groups:
Parallel threads
Taper threads
A parallel screw thread is a continuous helical ridge of uniform section and uniform axial spacing formed around the exterior of a solid circular cylinder or tube or on the interior of a circular hole. The thread formed is such that the axial displacement of a point that travels along a helix connecting corresponding points of successive ridge profiles is always proportional to its angular displacement about the axis.
When formed on the exterior, the thread is termed an external or male thread; an interior thread is termed an internal or female thread.
A single helical ridge is called a single-start thread as there is only one thread running down the length of the cylinder. With this type there is only one place of engagement on the periphery of the cylinder for mating thread forms.
A single-start thread is not convenient for many applications, especially where ease and speed of engagement of mating threads is important. Many closure threads incorporate a system whereby engagement is effected at more than one point on the cylinder.
This is achieved by forming a number of parallel helical ridges or threads running down the cylinder. This enables thread engagement at more than one point around the periphery of the cylinder. The number of separate threads reflects the number of starts...
