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

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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