What's in a name?

What is the difference between a “bolt” and a “screw”? Surprisingly, many in our industry do not know the difference, or have never really thought about it. To begin with I will give you the definitions that the IFI (Industrial Fasteners Institute) gives in their Fasteners Standards handbook.

BOLT: A bolt is a headed and externally threaded mechanical device designed for insertion through holes in assembled parts to mate with a nut and is normally intended to be tightened or released by turning a nut.

SCREW: A screw is a headed and externally threaded mechanical device possessing capabilities which permit it to be inserted into holes in assembled parts, of mating with a pre-formed internal thread or forming it’s own thread, and of being tightened or released by torquing its head.

An example of a bolt would be a hex head or pan head fastener designed to bolt together two plates with holes drilled in them. You would stack the plates on top of each other - lining up the holes - and inset the bolt through the holes. You would then spin a nut on the threads and tighten the fastener by turning the nut with a wrench while holding the bolt stationary with another wrench or a driver in the case of a recessed part. Many of the hex head and double-hex head fasteners in our industry are designed as bolts but there are some fasteners with a driver recess that are designated as bolts since they were designed to be installed in this manner.

An example of a screw would be a fastener designed to hold the sheet aluminum skin on an airframe. The underlying structural framework would be tapped with threads. You would place the sheet of aluminum with holes predrilled in the corresponding locations over the framework and tighten the screw by turning the fastener itself with a driver. Where it gets confusing is that there are many hex head screws, which look like what you would consider to be a bolt, that are designed to be installed by turning the fastener itself. This makes the part a screw. Another good example of a screw would be a self-tapping fastener that makes it own threads when installed – like a sheet metal or a wood screw.

I find it easiest to remember this by thinking of a bolt as any fastener that you hold stationary while you turn the nut and a screw is any fastener that you turn the entire fastener with a driver or a wrench.

There are certainly some exceptions to these definitions and in some cases bolts may be used as screws and screws may be used as bolts. The part descriptions (especially in the case of the NAS or MS standard parts) usually state whether the component is designed as bolt or a screw. This gives the user an idea of what the fastener was originally designed to do.

Some design features may also differ between a bolt and a screw. Many commercial grade hex head bolts do not have a stepped round bearing surface (or ‘washer pad’) under the head. Since the bolt is designed to be stationary when installed this is not really an issue. However, if this type of bolt were used as a screw, there would be no bearing surface to rotate against the installation surface as the part is tightened and the hex form could then ‘dig’ into the surface, damaging it. This is why all protruding hex head type screws have this round bearing surface. Usually a screw in this configuration is designated as a ‘hex head cap screw’.
Questions, comments?
E-mail me at: ed.spanknoble@heartlandfasteners.com