Screwed joints: Different terms,forms and types of screw thread, designation, advantages and disadvantages

SCREWED JOINTS


Definition:

A screwed joint is mainly composed of two elements i.e. a bolt and a nut. The screwed joints are widely used where the machine parts are required to be connected or disconnected without damage to the machine or fastening.

Different terms used in screw threads:

The following term are usually used in screw threads:

1.) Major diameter- It is the largest diameter of an external or internal screw thread. The screw is specified by this diameter. It is also known as outside or nominal diameter.

2.) Minor diameter- It is the smallest diameter of an external or internal screw thread. It is also known as core or root diameter.

3.) Pitch diameter- In a nut or bolt assembly, it is the diameter at which the ridges on the bolt are in complete touch with the ridges of the corresponding nut. It is also called an effective diameter.

4.) Pitch- It is the distance from a point on one thread to the corresponding point on the next thread.

5.) Lead- It is the distance which a screw thread advances axially in one rotation of the nut. It is equal to pitch in case of single start thread, it is twice the pitch in double start, thrice the pitch in triple start and so on.

6.) Crest- It is the top surface of the thread.

7.) Root- It is the bottom surface created by two adjacent flanks of the thread.

8.) Depth of thread- It is the perpendicular distance between the crest and root.

9.) Flank- It is the surface joining the crest and root.

10.) Angle of thread- It is the angle included by the flanks of the thread.

11.) Slope of thread- It is equal to one half the pitch of the thread.

Forms of screw threads:

1.) British standard whitworth (B.S.W.) threads– It is a symmetrical V- thread in which the angle between the flanks measured in an axial plane. It is generally 55°. These threads are found on bolts abs and screwed fastenings for special purposes.

2.) British association ( B.A.) threads- It is a B.S.W. thread with fine pitches. The included angle 47.5°. These threads are used for instruments and other precision works.

3.) American national standard thread- It is also known as U.S. or Seller’s thread and has flat crest and roots. The included angle is 60°. These threads are used for general purposes e.g. on nuts, bolts, screws and tapped holes.

4.) Unified standard thread- This thread has rounded crests and roots with the included angle of 60°.

5.) Square threadThe square threads, because of their high efficiency, are widely used for transmission of power in either direction. Such type of threads are usually found on the feed mechanism of machine tools, valves spindles, screw jacks etc. The square threads are not so strong as V- threads, but they offer less frictional resistance to motion than whitworth threads. It has an included angle of 0°.

6.) Acme thread – It is a modification of square thread. It is much stronger than square thread. These threads are frequently used on screw cutting lathes, brass valves, cocks and bench vices. It has an included angle of 29°.

7.) Knuckle thread- It is also a modification of square thread. It has rounded top and bottom. These threads are usually found on railway carriage couplings, hydrants, neck of glass bottles etc. It has an included angle of 30°.

8.) Buttress threadIt is used for transmission of power in one direction only. It has low frictional resistance characteristics of the square thread and have the same strength as that of V- thread. The spindles of bench vices are usually provided with buttress thread. It has an included angle of 45°.

9.) Metric thread- It is an Indian standard thread and is similar to B.S.W. threads. It has an included angle of 60°.

Common types of screw fastenings :

1.) Through bolt- A through bolt or simply bolt has a cylindrical bar with threads for the nut at one end and head at another end.

2.) Tap bolt- A tap bolt or screw differs from a bolt. It is screwed into a tapped hole of one of the parts to be fastened without the nut.

3.) Studs- A stud is a round bar threaded at both ends.

4.) Cap screws- The cap screws are similar to tap bolts except that they are of small size and a variety of shapes of heads are available.

5.) Machine screws- These are similar to cap screws with head slotted for the screw driver. These are generally used with a nut.

6.) Set screws- These are used to prevent relative motion between the two parts.

Locking devices:

A large number of locking devices are available, some of which are following-

1.) Jam nut or locknut

2.) Castle nut

3.) Sawn nut

4.) Penn, ring or grooved nut

5.) Locking with pin

6.) Locking with plate

7.) Spring lock washer

Designation of screw threads:

According to Indian standard, the complete designation of screw threads include;

1.) Size designation- The size of screw thread is designated by the letter ‘M’ followed by the diameter and pitch, the two being separated by the sign × .

2.) Tolerance designation- This shall include;

A.} A figure designating tolerance grade. Here 7 is for fine grade,8 is for medium grade and 9 is for coarse grade.

B.} A letter designating the tolerance position. Here H is for unit thread, d for bolt thread with allowance and h is for bolt thread without allowance.

For example, a bolt thread of 6 mm size of coarse pitch and with allowance on threads and medium tolerance grade is designated as: M6 – 8d .

Advantages and disadvantages of screwed joints:

Advantages-

1.) Screwed joints are easy to assemble and disassemble.

2.) A wide range of screwed joints are available to be used in various working conditions because of their versatility.

3.) Screwed joints are very much reliable on any circumstances and provides worthy service.

4.) Screw joints are also used for power transmission process.

5.) It is also relatively cheaper than other fastenings.

Disadvantages-

1.) Sometimes screwed joints becomes loose because of mechanical vibrations and other loading.

2.) The strength of the screwed joints are low compared to other fastenings or joints.

3.) Here stress concentration is also variable in different different conditions.

That’s a detailed information about screwed threads, their designation, different terms and types, advantages and disadvantages. Hope you all like this article.

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