STANDARD FORMS OF SCREW THREADS
Before discussing various standard forms of screw threads, let us learn about the elements of threads as these will be used in the discussion of standard forms of threads. The elements of threads include major diameter (D), minor diameter (d). pitch (P), pitch diameter (Pd), crest, root, depth of thread (t), thread angle (α) and flanks.
Thread forms are discussed in the following with reference to a figure given in the Appendix at the end of the book
- British Standard Whitworth (BSW) threads have been largely used for bolts, nuts, studs, etc. for general purpose such as assembling of components in machine construction by clamping together two or more parts or fabricating a structure using bolts and nuts. BSW threads have relatively coarser pitches. British Standard Fine (BSF) threads have finer pitches and are used in the construction of automobiles and aeroplanes. British Standard Pipe threads have form similar to BSW threads but pitches are fine. These are used for assembling parts in gas, water or steam work.
- British Association (BA) threads with included angle 47 are preferred to BSW threads for use in small screws (dia less than 6 mm) used in making optical instruments and clocks.
- Buttress threads have one flank normal and the other is inclined at 45° to the axis of thread while roots and crests are flat. This form is used for components carrying heavy axial loads in only one direction as in carpenter’s vice, screw processes and air plane jacks.
- Sellers’ threads or American National threads are popular in USA and find the same applications as BSW threads in UK.
- Square threads have flat crest and root and both the flanks of the tooth are normal to the axis of the threads. They give relatively less frictional resistance in working and hence are used in screw jacks, screw process, etc.
- Acme threads are the modified version of square threads. They are stronger, easy to engage and disengage and also easier to manufacture than the square threads. These are used for power transmission as in power-operated jacks and lead screw of a lathe machine, etc.
- Knuckle threads are also the modified version of square threads with both crest and root made semi-circular. They provide smooth power transmission but their production is difficult on any machine tool. These are, however, produced only by rolling or casting
- Unified threads are popular in western countries. The shapes of roots and crests vary in internal and external threads. Roots in the external threads are rounded while crests can be rounded or flat but in case of internal threads, both roots and crests are flat These threads are available in three series: unified course threads, unified fine threads and unified special threads.
- ISO threads or international threads have been adopted by the International Organization for Standards (ISO). The basic profile of thread is similar to unified threads and the threads are in two series:
- inch system and
- metric system.
The Bureau of Indian Standard (BIS) has adopted the basic profile of ISO metric threads and has developed several series of coarse and fine threads for use in India. The metric series of ISO threads is being adopted widely throughout the world. These threads may have flat or rounded roots and crests. Metric threads employ 60° angle and diameter of threads varying from 6 to 80 mm. These are extensively used in motor car industry.
Selection of a Thread Form
Selection of a thread form depends on several factors as given in the following:
Function to be performed, for example, used for clamping or transmission of power, load lifting, etc..
Size of threaded component, for example, BA threads find use only in small components.
Easy and economical production of thread forms on available machine tools, for example, it is easier to produce acme threads thansquare thread.
Measurement of thread form should be straightforward and simple to be managed with thread micrometer, etc.
Thread angle should be as sharp as is consistent with the required strength. Large thread angles cause large bursting force on the nut and consequent increase in friction. It is with this reason that square or acme threads are preferred for power transmission.
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