While the terms bolts and screws often get used interchangeably, in reality, there are a number of major differences between these two mechanical fasteners. Both bolts and screws are threaded fasteners with a head, but the similarities mostly end there.
The primary difference between bolts and screws is that bolts are designed to assemble unthreaded objects, while screws are used to fasten threaded objects (with many screws creating their own threads as they are installed). Bolts are also designed to be used with a nut on the opposite end, while screws require no nut and don’t typically travel through the opposite end of the workpiece.
Let’s take a closer look at the differences between bolts vs screws, and examine how they are used in real-life applications.
What is a Bolt?
A bolt is an externally threaded fastener that’s used in conjunction with an internally threaded nut to fasten various parts together. A washer is also frequently used to evenly distribute the clamping force and extend the lifespan of the clamped material.
Bolts, unlike screws, are fastened by torquing the nut while the bolt is held stationary using a wrench or pliers. They must be longer than the overall length of the material being fastened, as the nut needs something to grip onto.
Bolts are often partially threaded, with the unthreaded portion providing additional strength and resistance to shearing forces. They are highly reliable and are typically used in applications where heavy clamping force is required.
What is a Screw?
A screw is an externally threaded fastener used to hold two or more objects together. They are either fastened into pre-tapped holes, or they can be designed to tap their own hole as they are driven into a given material.
Unlike bolts, they aren’t designed to be used with nuts, so they won’t typically protrude out the opposite side.
Screws are typically used in situations where light to medium clamping force is required. They aren’t as reliable as bolts and will fail long before a bolt will.
They aren’t typically used with washers, although certain screws such as SEM screws come with washers pre-installed.
Differences between Bolts and Screws
You can usually tell the difference between bolts and screws simply by giving them a quick glance. However, there are some fasteners that blur the line between screw and bolt – like lag bolts.
In addition to the differences mentioned above, there are a few other ways you can differentiate bolts vs screws.
|Fastening||Tightened by applying torque to nut||Tightened with screwdriver rotating head|
|Construction||Partially threaded||Partially or fully threaded|
|Reliability||High||Medium to low|
One thing you’ll notice about bolts is that they are nearly always partially threaded. This is because the threads are only used for securing the nut at the opposite end, and don’t need to work to clamp the bolt inside the hole.
Screws on the other hand can either be fully or partially threaded. Partially threaded screws add additional strength and shear resistance at the expense of increased grip.
Another obvious difference between bots and screws is their drive type.
Screws can have a wide variety of drive types including Phillips, TORX, square, and many more. Bolts on the other hand are either set through a pre-drilled hole by hand or by using a socket wrench to rotate the bolt head.
Bolts have a much higher level of clamping force than comparable screws. Due to their design, the nut and bolt assembly creates a powerful clamping force – holding the material in between under powerful compression. In effect, the nut and bot work to sandwich the material in between them.Screws have a much lower level of clamping force than bolts due to their design. The only thing holding the screw in place are the threads and friction, so they will fail long before bolts will.
Load Bearing Strength
Load bearing strength refers to the maximum load a unit can bear before it fails.
Bolts provide significantly more load-bearing strength than screws. This is due to their larger gauge, as well as the washer which distributes the load evenly.
Screws have a much lower load-bearing strength. Many screws have a maximum load-bearing capacity of just 80 – 100 lbs. Compare that to 600 pounds for an eye bolt with a quarter-inch shank.
As you might expect due to their superior load-bearing capacity, clamping force, and overall strength, bolts are generally more expensive than screws. This cost difference factors into screws being much more commonly used in most construction applications.
Unless bolts are needed for their heavy-duty load-bearing strength, you generally want to use screws. This means you’ll save significant money, and have more leftover for the rest of the job.
When it comes to bolts vs screws there are a few major things to keep in mind. They differ significantly in terms of cost, load-bearing capacity, reliability, clamping force, and fastening method.
While you can certainly get deep into the weeds on these topics, the main thing to keep in mind is that bolts are secured by applying torque to the nut on the opposite end, while screws are fastened by driving the head using a drill or screwdriver.