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SCREW .vs. BOLT

What is the difference between a screw and a bolt?

There are those people who will categorically tell you that they know the difference between a screw and a bolt.  In general, they have made an erroneous assumption about what the difference is and are thinking of a category of screw that is unsuitable for use as a bolt.  Screws that are unsuitable to use as bolts are wood screws, lag screws, sheet metal screws and all of the screws that fall into the category of self tapping screws.

fig: BOLT - SCREW - STUD

The definition of screw is an externally threaded headed fastener (is the recessed drive socket in a setscrew considered a head), which is tightened by applying torque to the head, causing it to be threaded into the material it will hold.  A bolt on the other hand is an externally threaded headed fastener, which is used in conjunction with a nut.  To obtain reliable and repeatable fastener torque the bolt / nut combination should always be tightened by holding the bolt head stationary and turning the nut.

The afore-mentioned screws that are unsuitable to use for bolts are unsuitable because they cannot be used with a nut.  Nearly any bolt with a common head can be used as a screw by tightening it by the head into a tapped hole.  Examples of bolts that cannot be used as screws are carriage bolts, plow bolts and track bolts.  i.e. some bolts can be used as screws; some screws can be used as bolts.  If an externally threaded fastener is tightened by the head into a captive nut (nut-plate, etc.), it is still a screw.

The words "bolt" and "screw" are ambiguous.  Bolt or screw is not a physical thing; it is a matter of how a physical thing is used.  However, there are those who have set about trying to define these words as precise engineering terms.  The very nature of the common English use of these words renders that impossible.  They further set themselves up as the ultimate authority on the meaning of these two words and chide accepted official standards for not adopting their “authoritative” definition.  Acceptable standards such as The Machinery's Handbooki, the various government and military parts standards, and ASMEii parts standards are among the typical standards we as engineers rely on.


 

i

http://new.industrialpress.com/machineryhandbook

 

ii

ASME: American Society of Mechanical Engineers http://www.asme.org/kb/standards


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