Screw vs. bolt, what is the difference?

[silagehauler]

I always like fun discussions on trivial matters that do not affect our lives in any meaningful way.

So with that being said, what is the difference between a bolt, and a screw?

Ready, set, discuss!

 

[alwaysFlOoReD]

A bolt uses a nut, a screw uses two.

 

[jkwilson]

A bolt uses a nut, a screw doesn't. A screw can be a bolt.

 

[FMC]

a screw is pointy like on the end

 

[Mr.Nutcase]

a bolt threads into nut or it thread to hole..
A Screw has a point

 

[jakemac]

A bolt passes though the material and uses a "clamping" force to securely hold it's load.

A screw passes into the material and uses a "grabbing" force to secure it's load.

 

[katy]

Sounds like you all forgot about machine screws.

 

[General Geoff]

The term 'bolt' predates the common availability and usage of helical threading. Historically, bolts were held by pins, clinches, or other mechanical means. A modern screw is a bolt when it's held on by a nut as opposed to a threaded work piece. The distinction from an engineering standpoint is that of use, not of physical appearance.

What people commonly call cylinder head bolts, are actually screws, because they thread into and are held fast by a tapped hole in the block.

A machine screw which uses a nut, is a bolt. If that same screw is used in a threaded work surface without a nut, it is no longer a bolt.

 

[2oolhound]

I've always categorized bolts as hex heads requiring a wrench or socket to tighten and a screw in various varieties: machine screws; wood screws; sheet metal screws etc. Screws use slot, philips, robertson, allen wrenches etc to tighten. The heads differ from bolts by the shape of their heads also ie: pan heads, allen heads, round heads, counter sunk etc.

 

[metaleltr]

Here is how they are classified on mcmaster carr, If you go under the bolts category, one of the subsections includes carriage bolts

 

[Wamsutta]

Anything bigger than 1/4'' is a bolt.

 

[ihateminimumwage]

A bolt passes though the material and uses a "clamping" force to securely hold it's load.

A screw passes into the material and uses a "grabbing" force to secure it's load.

This is it, including machine screws.
Bolt uses a nut, screw threads into the actual material. (We had a whole section of a class on it when I went back to school).

 

[beatcad]

there are many screw head(flat & Phillips) screws on car trim.
most from the mid 60s and back were flathead. dammit!
all the old cars I had I replaced 'em with SS Phillips heads.
I curse the man who made the flathead screw.
and yes the Phillips head is named for the guy that invented it, last name Phillips.

I rarely "restore" anything. if I pull out a flathead screw i'll replace it w/ a Phillips if its for looks(countersunk). if it anit cosmetic i'll put in a normal hex head bolt, or an allen head if I wanna get fancy/industrial

about the only thing a flathead screw is good for is a cover to yer plug or light switch cover or maybe a worm gear clamp.
i'll even use a socket on a worm gear clamp.

 

[beatcad]

sorry. I got away from the orig queston.
and the pics that metalguy from mcmastercar showed may or may not be universally accepted.

but I know and will stand firmly on this.
it doesn't matter the head.

a fastener that screws into something like wood or steel(sheet metal screw) and tightens when it's tight. that's a screw.
if a fastener goes through 2 pieces of material w/ a nut on the other side than it's a bolt.

 

[jnyost]

Most bolts are really screws.

SHCS - socket head cap screw (allen bolts)
HHCS - hex head cap screw

 

[Craptain]

It's a screw when she screams. It's a bolt when her old man walks in.

 

[RECox286]

Pretty much like the discussion about if you know the

difference between a screw and a staple...



I don't know either, I've never been stapled !

Uncle Bob

 

[Adam.C]

The difference is unclear.i.e. there is no clear distinction.

Here's my definition:
Bolts have hex or square heads and are driven with wrenches. Screws have recessed heads, driven by screw drivers, allen drivers, torx etc.

Machinery's Handbook defines the difference based on the usage (which I have a problem with).
That difference is basically, if you use a nut with it, it's a bolt. If you screw it into something already threaded, or something that doesn't require a receiving thread (wood, sheet metal screws) its a screw.

I think 9 times out of 10, my definition works. The 10th time, the difference is stupid e.g. by the machinery's hand book definition, head bolts would be screws because they don't have nuts and the heads are driven.

 

[bushmechanic]

All bolts are screws, but not all screws are bolts.

A screw is what's called a simple machine; in this case, an inclined plane wrapped about a central axis in order to increase it's length of engagement with a substrate, or narrow it's angle of engagement.

A bolt is, in the context of this forum, a screw that is a fastener. Many screws are not fasteners. This is why the first statement in this post is true.

 

[smiffy]

i think it also vary s country to country as ive always been taught a screw is self tapping a set screw is fully threaded a bolt has a shank with a threaded end and what the above picture calls a set screw is a grub screw over here

 

[Dave455]

A bolt passes though the material and uses a "clamping" force to securely hold it's load.

A screw passes into the material and uses a "grabbing" force to secure it's load.

This is totally correct!

Thus, a screw has threads all the way to the head (so it can hold thin stuff) whereas a bolt does not!

The type of head is irrelevant - a screw with a hex head is simply a set screw, with a slotted or Pozidriv recess a machine screw, and with no head a grub screw!

Size is also irrelevant, I have set screws of 3/4 Whitworth, but they are still scres as they are threaded to the head!

 

[Adam.C]

This is totally correct!

Thus, a screw has threads all the way to the head (so it can hold thin stuff) whereas a bolt does not!

The type of head is irrelevant - a screw with a hex head is simply a set screw, with a slotted or Pozidriv recess a machine screw, and with no head a grub screw!

Size is also irrelevant, I have set screws of 3/4 Whitworth, but they are still scres as they are threaded to the head!

Lag screws aren't fully threaded. I agree with your assessment, but not what you added.

 

[larry_g]

We went through this one time at work as we were developing a look-up system for a computerized parts system. We finally decided to use the Machinerys Handbook definition of "A bolt is torqued by a nut and a screw is torqued by turning the head of the screw." That allows a screw to be a bolt in application. However a carriage bolt cannot be a screw.

For the most part we used a lot of the McMaster Carr classifications.

lg
no neat sig line

 

[Dave455]

Lag screws aren't fully threaded. I agree with your assessment, but not what you added.

Aha! In the U.K. we call those 'coach screws'. I know they are not threaded all the way, but I always thought that was for manufacturing reasons! Definitely a screw though! We also have 'coach bolts'!

 

[bushmechanic]

Aha! In the U.K. we call those 'coach screws'. I know they are not threaded all the way, but I always thought that was for manufacturing reasons! Definitely a screw though! We also have 'coach bolts'!

As I said, any helical inclined plane is a screw, regardless of how it's meant to be used. This is a fundamental physical definition.

A bolt is a screw that is a certain type of fastener. The word describes an action more than it does an object, as that action provides the object it's secondary definition.

 

[Fretters]

Thus, a screw has threads all the way to the head (so it can hold thin stuff) whereas a bolt does not!

Not necessarily. Screws and bolts can both be either full thread or partial thread on the shank. Using bolt and screw in the general context. As Bushmechanic said though, a screw is a screw, whatever the function.

 

[DodgeMech]

What about a hex head fastener that goes through a pre made hole in something, then screws into captive threads in a frame? Would that be a scrolt?

 

[WNYflyer]

A bolt uses a nut, a screw doesn't. A screw can be a bolt.

^ This

In most basic terms, a bolt utilizes a nut for clamping. If no nut and you turn the head for tightening it is a screw.

 

[oldtractors]

Another way to look at it is if you can turn the fastener by the head, it is a screw, otherwise it is a bolt. You can use screws as bolts by putting a nut on them. A plow bolt, elevator bolt, or carriage bolt is not a screw.

 

[jes_si]

It's a screw when she screams. It's a bolt when her old man walks in.

 

[Professur]

Hmmm .... does a screw become a bolt if it's screwed into a hidden captive nut? Or one of those bent sheet metal tabs masquerading as a captive nut?