What are screws heads, drives and threads?
Understanding the difference between a screw head and a screw drive – and the basic parts of screw thread – will help you save time in choosing the right fastener for your application. In this guide, we’ll answer your questions and walk you through everything you need to know about screw head types and drives, along with relevant topics. We’ll cover:
- What are screw heads?
- What are the different screw head types?
- What are different head screws used for?
- The advantages and disadvantages of different head screws
- What are screw drives?
- Types of screw drives
- What are different screw drives used for?
- The advantages and disadvantages of different screw drives
- What are screw threads and fasteners?
- Types of screw threads
- Parts of screw threads
What are screw heads?
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 parts of screws.
The best place to start is with the head, which is designed according to the screw’s function. You’ll come across different screw-head shapes but 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 material you’re fastening. An example of this is a flat screw head.
A non-countersunk screw is the opposite. These 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 head screw shapes are also far more numerous, ranging from round to mushroom, or truss heads.
What are the different screw-head types?
You have a range of screw-head types to choose from. The screw type you need depends on several factors, such as the screw’s purpose, which we’ll go into further along. For now, become familiar with the different heads.
Binder head screw
A dome head screw with a deep slot, these are special types of screw heads. Also called binding head screws, they typically have a 10% larger bearing surface than pan head screws.
Flat – slotted head screw
Presents a flush, seamless surface, or slightly below the surface with a slotted screw head. Flat head screw types are often used on handrails, furniture and lighting fixtures to prevent snagging of clothes and skin, and for aesthetic reasons. You’ll also find the flat head screw commonly used in construction for mounting hinges. They’re also popular drywall screws.
Fillister head screw
Similar to a pan head screw, a fillister head screw has a circular top, slightly convex surface and greater height. Its diameter allows for a deeper slot. A fillister head is very similar to a raised cheese head screw, except that it’s taller. These are often used as replacement screws.
Oval head screw
A type of machine thread 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 screw is a decorative alternative to other screws.
Pan head screw
Pan head machine screws are available in different drives, such as slotted or Phillips. These screw-head profiles are low with a larger diameter than most screws. They also provide greater torque for securing and removing, due to the rim of the head, which is slightly raised.
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. Available with or without a slotted hex. The six sides of a hex head screw allow greater torque than other screws. Unlike screws with a circular head that are internally driven, a hex head screw is installed with the force working against the outside of the head.
Round head screw
No longer as popular as it once was, most people prefer pan heads these days. However, the round head screw 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.
Mushroom head screw
Also called a truss head screw. Mushroom head screws have a wide, domed 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.
Socket head screw
The name has many variations, including cap head screw, cap screw socket head, socket head cap screw and cap head socket screw. One of the strongest of all screw heads, the socket cap head screw has high torque and excellent 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.
What are different head screws used for?
Countersunk screw uses are similar – namely when you want a flush finish. But otherwise, screw heads vary according to application, as seen in these examples below. Keep in mind that you can use a screw head cover to conceal the screw you use. Another point: if you’re looking for electrical insulation from your screw head, choose a plastic screw thread and head as your material of choice.
● Binder head screws
This screw-head type is often used for bookbinding or for materials that need to be temporarily secured. Their industrial applications are limited, but they’re excellent at fastening light-duty electrical equipment and for assembling small mechanical components.
● Flat head screws – slotted
The most effective uses for these flat-top screws are woodworking and cabinetry, such as fastening hinges, handles and other hardware. It’s popularity in woodworking is why it’s also known as a flat head wood screw.
● Fillister head screws
These are another versatile screw. Compared to a flat head, they provide a greater bearing surface and improved grip due to their raised head design. Fillisters are most often used in electrical and electronic equipment.
● Oval head screws
Common for fastening architectural hardware, such as switch covers, handles and hinges. They’re also popular screws for automotive, specifically for fastening body panels and trims when countersunk screw types are needed to provide a smooth finish.
● Oval head screws
Common for fastening architectural hardware, such as switch covers, handles and hinges. They’re also popular screws for automotive, specifically for fastening body panels and trims when countersunk screw types are needed to provide a smooth finish.
● Hex head screws
Sometimes called a hexagon screw head, these are installed with a wrench. The head protrudes with six access points, making them easy to tighten or remove, which can sometimes make them handy in tight spaces, depending on the size of the wrench needed. They’re also popular as electronic screws. Other applications include machinery and equipment, such as gears and pulleys
● Round head screws
Round-head-screw uses include surface-mounted applications where the screw head needs to be flush or slightly raised, such as in furniture. The round screw head provides a smooth, decorative finish.
● Mushroom head screws
Ideal for electrical, plumbing and HVAC applications. The curved, low-profile head discourages tampering while adding an aesthetic touch.
● Socket head screws
A socket head screw is typically used on assembly lines in manufacturing, specifically in automotive, machine tooling, metal fabrication and furniture.
The advantages and disadvantages of different head screws
Different screw-head types all have their pros and cons. Each serves a specific purpose, which underlines the fact that not all screws are appropriate for every application. Know the advantages and disadvantages of each before choosing any screw, which will you avoid surprises.
|
Screw head |
Advantages |
Disadvantages |
|
Binder |
– Non-permanent, so it’s easy to update manuals & catalogues – Undercut of head allows wire connections for electrical applications to be tucked underneath |
– Not designed for high levels of tension and stress (for light-duty applications only) – Can sometimes cost more than another type of screw head |
|
Flat (slotted) |
– Sits flush so that it doesn’t catch skin, clothes or moving objects – Easy to machine, which makes them economical |
– Slot design limits the amount of torque that can be applied, making it less secure for high-tension applications – Prone to stripping if over-tightened or if screw driver slips |
|
Fillister |
– Smaller diameter & higher profile than round or pan heads makes it effective for a deeper slot |
– Not designed for high-speed, high-torque installation |
|
Oval |
– Low profile – Provides a decorative finish
|
– Head shape creates a large surface area that requires more force to drive in the screw |
|
Pan |
– Stands up to vibration – Large mating surface area for a firm grip |
– Prone to stripping – Not as strong as other screw heads, so should not be used for heavy-duty applications |
|
Hex
|
– Highest torque screw head available – Stands up to vibration
|
– High profile – Hex shape can sometimes be helpful in tight spaces, but not always. The high profile and size of the wrench will require space |
|
Round |
– Decorative finish for surface-mounted applications |
– Shape may not transfer as much torque as other screw head types, making it less secure for high-tension applications |
|
Mushroom |
– Larger bearing surface – Decorative finish |
– Due to its configuration, should not be used where strength is required |
|
Socket |
– Resists tampering – Exceptional clamping force, so fewer screws are needed compared to other screws – High torque head screws for sure grip
|
– Usually more expensive than other screws types – Can be difficult to drive in tight spaces |
What are screw drives?
The screw’s drive is the part of the component that enables it to be rotated into place. It’s located on the screw head. Think of the shape cut into the head of the screw. Those shapes are the screw drives. The screw drive determines the type of tool needed to drive the screw into the materials you’re fastening. Usually, screw drives refer to the type of tool used to install or remove the screw.
A Phillips head screw sometimes goes by that name simply because it requires a Phillips screwdriver for installation. It’s a tool almost everyone is familiar with. Yet Phillips refers to the drive, not the head. A drive-type screw can be, for example, an oval head screw with a Phillips drive, whereas a Phillips screw is any screw head with a Phillips drive.
Types of screw drives
Get to know screw-drive types and learn the differences between them.
Slotted
The slotted head screw is the most basic drive, as it’s a single slot
often referred to as slot drive screws. The flathead screw is an example. 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. A pozi drive screw is similar, but has an added cross to reduce cam out. A Philips screw driver can fit in a
pozi drive screw head, but a “pozidriv” screwdriver can’t fit in a Phillips drive.
Torx
Six-pointed star design. Torx allows higher torque to be applied to the screw compared to other fasteners. Unlike the Phillips design, the star head screw does not have a maximum torque level that would cause the screwdriver to cam out. Star drive screw heads require less user exertion.
Hex
The six points of contact 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.
Robertson
Also known as a square drive screw, is a type of screw head that features a square socket in the centre of the head. The square drive also allows for greater torque to be applied, making it ideal for applications where high levels of tightening force are required.
What are different screw drives used for?
Some screw drives are better than others at achieving the performance you need for your specific application. Also, remember that different screwdriver heads exist for a reason. For example, a square screw head – or Robertson screw head – needs a Robertson screwdriver, but a slotted screwdriver can handle slotted and one-way screws. For best results, always use the appropriate screwdriver.
|
Screw drive: |
Best for: |
|
Slotted |
Woodworking & cabinetry |
|
Phillips |
Electronics & appliances |
|
Torx |
Applications needing a high level of security, electronics, automotive & computers |
|
Hex |
Electronics, automotive, construction, & industrial equipment, such as assembling conveyor systems or attaching parts in machinery |
|
One Way |
Discouraging tampering in applications such as gates, licence plates and safes |
|
Robertson |
Woodworking, cabinetry & light construction |
The advantages and disadvantages of different screw drives
Screw head drive types all have varying levels of holding power, resistance to cam out, and ease of use with different tools. Knowing the specific characteristics of each screw drive can help ensure that you use the right screw for the job, which can help increase the overall efficiency, accuracy and quality of your work.
|
Screw drive |
Advantages |
Disadvantages |
|
Slotted |
– Simple and inexpensive – Designed to discourage overtightening of the screw |
– Prone to slipping, which can damage surrounding material – To prevent a stripped screw head, manual screwdriver must be used carefully |
|
Phillips |
– Come in a wide range of shapes, styles and materials – Designed to allow screw cam out: screwdriver slips out of screw head if excess torque is applied – Prevents the screwdriver from slipping out sideways, protecting the application and the user |
– Camming out can destroy the screw’s recess – Design makes it difficult to apply enough torque to bore into hardwoods – Widely available: not commonly used for tamper-proof designs |
|
Torx |
– Six-pointed star screw head performs well under conditions of high torque – Resists vibrations – Heads can be engaged in six positions, making it useful in restricted spaces, such as engines and computers – Less wear and tear on the driver head – Tamper-proof – 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 screwdrivers are more expensive than other driver types – Limited range of styles and sizes
|
|
Hex |
– Widely used, so readily available – Design allows high torque without camming out – Inexpensive to manufacture – Unlikely to be damaged by the Allen key when installing |
– Not as specialised as other screw head systems – Fastener and bit sizes must match to be engaged |
|
One Way |
– 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 can also work) |
|
Robertson |
– Tapered recess provides superb grip, reducing cam out and allowing user to tighten with one hand – Square drive design has four handy positions of engagement – Useful in tight corners – Designed to cope with high torque without causing damage
|
– Can be more difficult to source – Needs exactly the right size driver tom engage screw |
What are screw threads and fasteners?
Screw ridges are the thread – those spirals that wrap around the screw’s internal or external diametrical surface. External threads are called male while internal threads, such as what we see with nuts, are female. Threads engage with the material being fastened, providing a secure mechanical connection. They’re designed to allow the screw to insert into and turn within the material. It’s this action that creates tight and secure interlocking.
Screws are one type of fastener that use threads. Other common types of fasteners include nuts and bolts. You can learn about these other fasteners, including bolt head types, in our guide, What are the different types of nuts and bolts?
Types of screw threads
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 of thread 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. In the Unified standard, the standard thread direction is right-handed, meaning that if you look at the screw from the tip, the thread spirals in a clockwise direction as you move away from the tip and towards the head of the screw.
Square
Square thread angle has no angle, of course. And that means there’s 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 for this screw threading 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 screw thread
Using a standard thread as our illustration, let’s look at the anatomy of screw threads. Screw thread terminology is the same as what’s used for screw terminology.
The pitch
What is screw pitch? It’s the distance from a point on the screw thread to the point on the next thread. How to identify the pitch of screw thread: 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 screw 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 crest to the root. This is measured perpendicular to the screw’s axis.
Major and minor diameters
Does screw diameter include thread? Yes. As the names imply, the major diameter is the largest diameter of a 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, helix angle 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. Your screw thread measurement depends on whether or not you’re dealing with metric threads or unified screw threads.
You can learn more in our guide What is the difference between metric and standard fasteners?
ISO metric screw thread
This is designated by pitch in millimetres (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. To measure screw thread, 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.
In this example, your M6 screw thread is 6mm x 1mm. Metric screw thread sizes are always determined by major diameter and screw thread pitch.
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 socket head screw cap. 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 socket head screw.
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 screws. We’ve chosen these types to give you an idea of how much screws can vary, along with their different applications. For an extensive look at different types of screws, you can check out our complete range now.
Pan head machine screws
● Available in different heads and drives
● Tend to be smaller than most screws
● Pan head self tapping screw also available – a self tapping screw has a
pointed tip to create its own threads as its driven into the material being
fastened
● Excellent for electrical applications
Pan heads and many other screw heads within this guide are types of machine screws. You can learn more in What are machine screws?
Set screws
● Available in different screw head drive types
● Not commonly used, as they serve specific screw
functions, such as holding non-parallel surfaces against
another, such as securing a shaft in a coupling or a gear to prevent slipping or rotation
Discover more about these critical screws in our guide, What is a set screw?
Shoulder screws
● 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
Learn more about these important fasteners in our Guide to shoulder screws.
Nylon knurled head thumb screws
● Can be tightened or loosened manually
● Large screw 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)
Find out more in our Guide to thumb screws.
Socket head cap screws
● Type of machine screw containing hex drive – often used when
clearance to install is restricted
● For demanding mechanical and industrial production applications
● Easy to install and loosen
● Superior clamping strength
You can learn more about one of the strongest screw head types in our Guide to socket screws.
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Questions?
Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.