Classification of Screw Threads – Application Of Screw threads

Classification Of Screw Threads
Screw threads having various applications can be classified as follows

1. According to location

  • external screw thread (on bolts etc.)
  • internal screw thread (in nuts etc.)

2.According to configuration

  • straight (helical) – most common, e.g., bolts, studs etc.
  • taper (helical), e.g., in drill chuck
  • radial (scroll) as in self centering chuck

3 . According to the direction of the helix

  • right hand (common)
  • left hand (occasionally)

4. According to form

  • vee thread (600 or 550 angle) – most common
  • acme thread (290)
  • square thread (generally in power screws)
  • buttress thread (45o)
  • worm thread (290 ~ 400)
  • semicircular (groove section) thread being used in recirculating type bolts, screws.

5. According to standard

  • BSW (British Standard Whitworth); thread – size is designated by TPI (threads per inch)
  • metric thread; thread size is specified by pitch or lead (in mm)

6. According to number of start

  • single start – most common
  • multi-start (2 to 4)

7. According to spacing of threads

  • TPI (no. of threads per inch), e.g. 12 TPI
  •  pitch (or lead) – distance between two successive threads (or length of travel of the nut for one rotation of the screw), in mm

8. According to compactness or fineness of threads

  • general threads (with usually wide thread spacing), pipe threads (more densed desired)
  • fine threads (generally for leak proof)

9. According to segmentation

  • full threads (common)
  • half turns as in half nuts
  • sector thread – e.g., in the jaws of lathe chucks.

Read More: Lead Screw Mechanism -types , Advantages

Nomenclature and Terminology used In Screw Threads
Nomenclature and Terminology used In Screw Threads

General Applications Of Screw Threads

The general applications of various objects having screw threads are :
fastening : screws, nut-bolts and studs having screw threads are used for temporarily fixing one part on to another part
joining : e.g., co-axial joining of rods, tubes etc. by external and internal screw threads at their ends or separate adapters
clamping : strongly holding an object by a threaded rod, e.g., in c-clamps, vices, tailstock on lathe bed etc.
• controlled linear movement : e.g., travel of slides (tailstock barrel, compound slide, cross slide etc.) and work tables in milling machine, shaping machine, cnc machine tools and so on.
• transmission of motion and power : e.g., lead screws of machine tools
converting rotary motion to translation : rotation of the screw causing linear travel of the nut, which have wide use in machine tool kinematic systems
position control in instruments : e.g., screws enabling precision movement of the work table in microscopes etc.
precision measurement of length : e.g., the threaded spindle of micrometers and so on.
• acting as worm for obtaining slow rotation of gear or worm wheel
exerting heavy force : e.g., mechanical presses
conveying and squeezing materials : e.g., in screw conveyor, injection moulding machine, screw pump etc.
• controlled automatic feeding in mass production assembly etc.

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