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Terminology Used For Screw Threads & Types Of Screw Threads



Screw thread is a continuous helical groove of specified cross-section produced on the external or internal surface. A screw thread formed on a cylinder is known as straight or parallel screw thread, while screw thread formed on a cone or frustum is known as the tapered screw thread.

1. Pitch:- 


The distance measured parallel to the axis from a point on a screw thread to a corresponding point on the next thread is called pitch or in other words the distance from crest to crest or root to root is called pitch of the thread.


2. Lead:- 
The distance moved by a nut or a bolt in axial direction in one complete revolutions called
lead.

3. Crest:- 
The crest is the height at which an external thread is raised, or the depth at which an internal thread is indented. For common applications, screws and bolts are measured at the crests, while nuts are measured at the roots.

4. Root:-
The inner most part of the thread is called root.


5. Flank:-
The surface between the crest and the root is known as flank of the thread.

6. Angle of thread:- 
The angle between the flanks measured on an axial plane is called angle of
thread.

7. Depth:- 
It is the distance between crest and root measured at right angle to the axis.


8. Nominal diameter:- 
The diameter of the cylindrical piece on which threads are cut is called nominal diameter.

9. Major diameter:- 
Diameter at the crest of the thread measured at right angle to the axis is called major diameter and is also known as outside diameter.

10. Minor diameter:- 
The diameter at the core or root of the thread is called minor diameter. It is also called as core diameter.


11.Axis of a thread:- 
This is an imaginary line running longitudinally through the center of the screw.

12.Helix angle:- 
On the straight thread, It is the angle made by the helix of the thread at the pitch line with the axis.

13.Lead angle:- 
On the straight thread, It is the angle made by the helix of the thread at the pitch line with a plane perpendicular to the axis. Lead angle is measured in an axial plane.


14.Flank angle:- 
Flank angle is the angle made by the flank of a thread with the perpendicular to the axis of a thread.

15.Included angle:- 
Included angle is the angle between the flanks or slope of the thread measured in an axial plane.


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