What are screws heads, drives and threads?
Discover the different types of screw drives, heads, and threads in our comprehensive guide. We'll cover:
- Parts of a screw
- What are the different types of screw heads?
- What are the different types of screw drives?
- Advantages and disavantages of different screw drives
- What are the different types of screw thread?
- Parts of a screw thread
- How to measure screw thread size
- What are the different types of screws?
Parts of a screw
First, let’s get an overview of a screw, as seen here. Not all screws look like this, but it helps to know what we mean by these screw parts.
The best place to start is with the head, which is designed according to the screw’s function. Essentially, heads fall into two categories: countersunk and non-countersunk.
A countersunk head screw sits flush with the surface where it’s installed, or slightly below it. Countersunk screws are designed to taper so that they can self tap into the wood, which is the material these type of screws are typically used on. An example of this type is a flat screw head.
Non-countersunk screws are the opposite. They have protruding heads and don’t taper. Countersunk screws are more prone to cause damage, as the tapered part is wider than their shank, which is the screw’s body. Of course, non-countersunk screws don’t have this problem. Non-countersunk screws are also far more numerous, ranging from round to mushroom heads.
View our range of screws
What are the different types of screw heads?
Screws come in different head types, which means there is a head type to suit your application.
Binder head screw
Also known as binding head screws. With a thick head and deep slot, these are available as binder machine screws. They’re ideal for binding material swatches and large manuals. The head’s undercut also make it perfect for tucking in wire connections for electrical applications.
Pan head screw
Available in different drives, such as slotted or Philips. Pan heads have low profiles and larger diameters than most screws. They also provide superior torque for securing and removing, due to the rim of the head, which is slightly raised.
Fillister head screw
Similar to a pan head screw, but with a circular top and slightly convex surface. Its diameter allows for a deeper slot. Fillister heads are also known as raised cheese head screws. These are rarely installed in new designs but are instead used as replacement screws.
Flat – slotted head screw
Presents a flush, seamless surface, or slightly below the surface. Often used on hand rails, furniture and lighting fixtures to prevent snagging of clothes and skin, and for aesthetic reasons. You’ll also find flat head screws commonly used in construction for mounting hinges and dry wall.
Hex head screw
Often confused with a hex bolt, but they’re not the same. A hex bolt uses a nut, while a hex head cap screw is installed in a tapped hole. Simple turn the head to tighten. They’re available with or without a slotted drive. The six sides of a hex head screw allow greater torque than other screws. Unlike screws with a circular head that are internally driven, these screws are installed with the force working against the outside of the head.
Oval head screw
This is basically a flat head with a rounded surface – both are countersunk. The screw head of choice for switch coverings, the oval head is a decorative alternative to other screws.
Mushroom head screw
Also called truss heads. These screws have a wide head to create a larger bearing surface while providing an aesthetic finish. Commonly used to join sheet metal together, the low profile helps resist tampering.
Round head screw
No longer as popular as it once was, most people prefer pan heads these days. However, the round head does have a deeper slot than the pan. It’s still used in some machinery, but mostly its purpose is to provide a particular aesthetic look.
Socket head cap screw
The strongest of all screw heads, with higher torque and clamping force. The circular head has a hex drive, so it needs a hex or Allen key to tighten and loosen. Ideal when space is limited and high strength is critical. Typically used in assembly lines.
Socket head button screw
The domed head houses an internal hex drive, making this a type of socket head, but lacking its high-strength capabilities. The button head is wider and has a lower profile. It’s ideal when a wider bearing surface is needed. Used for lighter fastening needs, such as automotive, electronics and manufacturing.
What are the different types of screw drives?
The screw drive is the part of the component that enables it to be rotated into place. It’s located on the screw head. Usually, screw drives refer to the type of tool used to install or remove the screw.
Understand more about different screw drives in our guide, What is the best screw drive?
Slotted
This is the most basic drive, as it’s a single slot – often referred to as slot drive screws. It works best when applied by hand, as a power tool can easily slip and damage the surrounding area.
Phillips
Two crossed slots prevent the fastener from being over-tightened. Once a certain velocity has been achieved, the screwdriver will slip from the drive. This is known as “cam out.” More secure than the slotted drive and can be used with electric screwdrivers.
Torx
Six-pointed star design. Torx allows higher torque to be applied to the screw compared to other fasteners. Unlike the Phillips design, the Torx design does not have a maximum torque level that would cause the screwdriver to cam out. Requires less user exertion.
Hex
The 6 points of contact means reduces the wear on the screw drive. As with Torx drives, the design means that less force is required to insert the fastener, so that user fatigue is reduced in comparison to other drive types. These are also handy in tight spaces, as they can be tightened without tools.
One Way
Also known as a tamper-free drive and can only be turned in one direction. Inserted using a slotted screwdriver yet can only be removed with special tools.
Advantages and disavantages of different screw drives
Let’s take a look at the pros and cons of these screw drives and where they are best applied:
|
Drive type |
Use |
Pros |
Cons |
|---|---|---|---|
|
Slotted (or Flathead) |
Commonly used in cabinetry and furniture making Inserted into place using a flat-bladed screwdriver Works best applied by hand |
Simple and inexpensive Designed to discourage overtightening of the screw |
Screwdriver is prone to slipping Slippage can damage surrounding material Most prone to stripping |
|
Phillips |
Used in a wide range of applications, from electronics to the motor industry. |
Come in a wide range of shapes, styles and materials Self-centring for ease of use Designed to allow camming out – the screwdriver slips out of the head of the screw if excess torque is applied Designed to prevent the screwdriver from slipping out sideways, therefore protecting the product and the user |
Camming out can destroy the recess of Phillips screws Design makes it difficult to apply enough torque to bore into hardwoods Widely available, so not commonly used for tamper-proof designs |
|
Torx |
Used in a range of applications from automobile to electronics |
Performs well under conditions of high torque and useful for applications where vibrations can quickly loosen poorly tightened screws Heads can be engaged in six positions – useful in restricted space, such as engines and computer equipment Self-centering Less wear and tear on the driver head A good tamper-proof option Rusted and corroded screws are easy to remove |
Worn or deformed screws are ineffective so always need replacing Needs exactly the right size driver to engage the screw Torx drivers are more expensive than other driver types Range of styles and sizes is not as extensive as some more popular screw heads |
|
Hex (or Allen) |
Commonly used in machinery and furniture making industries. |
Widely used, so readily available Design allows high torque without camming out Inexpensive to manufacture Unlikely to be damaged by the Allen wrench when installing |
Not as specialised as other screw head systems Fastener and bit sizes must match to be engaged |
|
One Way (or Security) |
Commonly used in applications where theft or vandalism is a concern, such as safes, public bathroom fixtures and security gates |
Tamper-resistant design Can easily be installed with a standard slotted screwdriver |
Difficult to remove – needs a special one-way tool extractor to unscrew it (although other methods include drilling, cutting and the use of locking pliers) |
|
Robertson
|
Used in a range of applications from woodwork to aviation. |
The tapered recess provides superb grip, reducing cam out and allowing the user to tighten with one hand Self-centring Square drive design has four handy positions of engagement – useful in tight corners Designed to cope with high torque without causing damage Faster and more efficient than Phillips heads on production lines |
Can be more difficult to source Needs exactly the right size driver to |
What are the ridges on a screw called?
That’s the thread, the spirals that wrap around the internal or external diametrical surface. External threads are called male while internal threads, such as what we see with nuts, are female.
What are the different types of screw thread?
Just as there are different types of screws, there are different threads on screws and other fasteners. Each serves a purpose. The four main types include:
Standard (unified or metric)
Unified screw threads meet standard tolerances and specifications across different industries. They’re economical, providing excellent value and are easy to apply. Both unified and metric have the same 60˚ profile.
Square
With no thread angle, there is no bursting pressure on the nut. Among power screws, square threads offer the lowest friction and highest efficiency. These are difficult to manufacture, so the cost is higher than most other types. Used for power transmission applications, square threads are often found in mechanical processes.
Acme
Used in high-load applications, acme threads are stronger than square threads in shear. With a 29˚ thread angle, acme threads are relatively easy to manufacture. Applications vary, but the most common uses include machine tools and conveyers.
Buttress
Buttress threads handle extremely high axial thrust in one direction, which translates into high thread strength. With low friction and high-shear strength, these threads maintain their integrity over time when used with split nuts. Buttress threads are often used for hydraulic sealing in applications such as automotive.
Parts of a screw thread
Using a standard thread as our illustration, let’s look at the anatomy of screw threads. They all use the same screw terminology.
The pitch:
The distance from a point on the screw thread to the point on the next thread.
How to identify thread pitch: use a pitch gauge. Check each form size on the gauge against the thread you’re identifying. When a form matches, the gauge tells you the pitch.
Thread angle:
Also called the flank, the thread angle is the distance between the sides of the thread. It tells us that both sides of the thread are angled to the same degree. Unified threads all have a 60° angle, as mentioned above.
Depth:
The distance from the thread’s crest to the root. This is measured perpendicular to the screw’s axis.
Major and minor diameters:
As the names imply, the major diameter is the largest diameter of the screw. The minor diameter is the lower extreme diameter of the thread.
Pitch diameter:
This is half the distance between the major and minor diameters.
Root:
The groove of the thread, which corresponds to the minor diameter.
Crest:
The top surface of the thread, which corresponds to the screw’s major diameter.
Helix angle:
Also known as the thread flank, this is the angle made by the thread’s helix and its relation to the thread axis. On a tapered thread, the helix angle is the angle made by the conical spiral of the thread with the axis of the thread. An easy way to think of the helix angle is that it’s the shape formed by the threads of the screw.
How to measure screw thread size
Some screws require matched mating parts, which means you’ll need the size of the screw thread. Examples include machine screws, which can be secured into a tapping hole or with a nut, and captive screws. How you measure screws depends on whether or not you’re dealing with metric threads or unified screw threads.
Metric screw thread
This is designated by pitch in millimeters (mm). You measure the distance between two adjacent threads at their peaks. The challenge here is accuracy, due to the minute spacing involved, so use a caliper. Take the major diameter of the screw. Let’s say it’s 6mm, which is represented as M6. Now let’s say the pitch is 1mm. Your screw is M6 x 1mm.
Unified screw threads
This is designated in threads per inch (TPI), which is exactly what it sounds like. It’s the number of complete threads that appear in one inch of threaded length. Let’s say you have a pan head machine screw. Your TPI is affixed to the major diameter. For instance, if your diameter is a ¼ inch and your TPI is 20, then you have a ¼ – 20 pan head machine screw.
Learn more about screw thread sizing in our guide, What are screw thread sizes?
What are the different types of screws?
Now that you have a better understanding of the parts of a screw, let’s look at different types of screws. We’ve chosen these types to give you an idea of how much they can vary, along with what screws to use for different applications.
- Available in different heads and drives
- Tend to be smaller than other screws
- Drill or tap into pre-drilled hole or nut
- Excellent for electrical applications
Learn more in our guide to machine screws.
- Available in different drive types
- Used to hold non-parallel surfaces against another
- Variety of applications, from repair work to new product designs
Find out more in our guide to set screws.
- Combines shear strength of heat-treated steel with dielectric insulation and corrosion and vibration resistance of nylon
- Nylon shell provides a cost-saving alternative to more expensive metal fasteners
- Used when accurate rotary movement is essential
- The shoulder shaft acts as bearings or bushings, or as a guide for sliding parts
- Hex socket drive for increased torque
Nylon knurled head thumb screws
- Can be tightened or loosened manually
- Large head enables easy, firm grip
- Speed of installation or removal saves time and costs
- Used for applications that require regular maintenance (not suitable for structural projects)
Our guide to thumb screws provides more information on these screws.
- Type of machine screw containing hex drive – used when clearance to install is restricted
- For demanding mechanical and industrial production applications
- Easy to install and loosen
- Superior clamping strength
Learn more about socket head cap screws in our expert guide.
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