Screw Thread and Terms involved in Screw Thread
This post deals with screw threads and different terms involved in screw threads.
Screw Thread
The screw thread is a continuous helical groove cut made in the cylindrical surface of temporary fastening.
The screw thread is used in most of the engineering products.
Screw threads are widely used as temporary fasteners and also for power transmission in machines.
Terms used in Screw Threads
External Thread
An external thread is a continuous helical groove cut made in the external cylindrical surface.
Generally, found in bolts studs, screw etc.
Internal Thread
The internal Thread is a thread on the internal surface of a hollow cylinder.
Generally, found in nuts.
Screw Pair
A pair of bolt and nut together is called as screw pair.
These pairs are used in joining different parts.
Parallel & Taper thread
A parallel thread is those formed on a surface of a cylinder.
A taper thread is those formed on a surface of a cone.
Right Hand & Left Hand Threads
In a screw pair, if the nut is rotated clockwise to fix, then the thread on a bolt is said to be right-hand threads.
In a screw pair, if the nut is rotated anticlockwise to fix, then the thread on the bolt is said to be left-hand threads.
Pitch, P
Pitch is the distance between the corresponding points on the adjacent threads.
A Pitch is a measured parallel to the axis.
Lead L
Lead is the distance moved by a bolt or nut in an axial direction in one complete rotation.
Single start & Multi start threads
Single start threads are those formed by running only one helix on a cylinder.
Heir, L = P
Multi start threads are those formed by running only two or more helix on a cylinder.
L = nP where n is number of helices
Crest
The crest is the edge of the thread surface farthest from the axis for external threads.
In the case of an internal thread, nearest from the axis.
Root
Root is the edge of the thread surface nearest to the axis for the external thread.
In the case of internal thread farthest from the axis.
Flank
Flank is a surface connecting crest and root.
Thread angle
Thread angle is the angle between the flanks measured in an axial plane.
Outside Diameter
Outside diameter is the diameter of an imaginary coaxial cylinder touching the roots of internal threads or crest of external threads.
Root or Core Diameter
Root diameter is the diameter of an imaginary co-axial cylinder touching the crest of internal threads or roots of the internal threads.
Nominal Diameter
Nominal Diameter is the diameter of the cylinder from which external threads are cut.
Generally, the size of screw/bolt is specified by this diameter.
Form/Profile of Screw Thread
Form or profile of Screw thread is the section drawing of a thread cut by a plane containing the axis.
This post deals with screw threads and different terms involved in screw threads.
Screw Thread
The screw thread is a continuous helical groove cut made in the cylindrical surface of temporary fastening.
The screw thread is used in most of the engineering products.
Screw threads are widely used as temporary fasteners and also for power transmission in machines.
Terms used in Screw Threads
External Thread
An external thread is a continuous helical groove cut made in the external cylindrical surface.
Generally, found in bolts studs, screw etc.
Internal Thread
The internal Thread is a thread on the internal surface of a hollow cylinder.
Generally, found in nuts.
Screw Pair
A pair of bolt and nut together is called as screw pair.
These pairs are used in joining different parts.
Parallel & Taper thread
A parallel thread is those formed on a surface of a cylinder.
A taper thread is those formed on a surface of a cone.
Right Hand & Left Hand Threads
In a screw pair, if the nut is rotated clockwise to fix, then the thread on a bolt is said to be right-hand threads.
In a screw pair, if the nut is rotated anticlockwise to fix, then the thread on the bolt is said to be left-hand threads.
Pitch, P
Pitch is the distance between the corresponding points on the adjacent threads.
A Pitch is a measured parallel to the axis.
Lead L
Lead is the distance moved by a bolt or nut in an axial direction in one complete rotation.
Single start & Multi start threads
Single start threads are those formed by running only one helix on a cylinder.
Heir, L = P
Multi start threads are those formed by running only two or more helix on a cylinder.
L = nP where n is number of helices
Crest
The crest is the edge of the thread surface farthest from the axis for external threads.
In the case of an internal thread, nearest from the axis.
Root
Root is the edge of the thread surface nearest to the axis for the external thread.
In the case of internal thread farthest from the axis.
Flank
Flank is a surface connecting crest and root.
Thread angle
Thread angle is the angle between the flanks measured in an axial plane.
Outside Diameter
Outside diameter is the diameter of an imaginary coaxial cylinder touching the roots of internal threads or crest of external threads.
Root or Core Diameter
Root diameter is the diameter of an imaginary co-axial cylinder touching the crest of internal threads or roots of the internal threads.
Nominal Diameter
Nominal Diameter is the diameter of the cylinder from which external threads are cut.
Generally, the size of screw/bolt is specified by this diameter.
Form/Profile of Screw Thread
Form or profile of Screw thread is the section drawing of a thread cut by a plane containing the axis.
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