Despite the small size of nuts and washers, these pieces are essential for maintaining the strength and integrity of the equipment. They are designed to work in a way that holds things together as a mechanical fastener. The nut threads through the pieces that are being held together, while the washer helps with load distribution and protects the joint surface.
In this guide, we are looking at nuts and washers in-depth. Not only will you learn more about how these fasteners are used, but this information helps with the comparison of different types of nuts and washers. Learning about the different materials and purposes makes it easier to pick the right type of nut and washer for your application. I think it is best to just get rid of this image below with 2 charts below to avoid confusion
What Is a Nut?
The primary purpose of a nut is to create a threaded connection that secures the fastener in place. Typically, nuts are used along with bolts or threaded rods. This fastener is designed with a tapped inner diameter and can effectively secure various components.
The internal threads mate with external threads of bolts (or other types of threaded fasteners). When a nut is tightened, it creates a clamping force that helps to hold the connected parts together.
You’ll find many different sizes and varieties of nuts with different fastening purposes depending on the application. The best practice is to choose a nut and bolt combination based on grade and size. Depending on the nuts and bolts that you choose, some can be applied without tools, while others require the use of power tools for optimal performance. Additionally, certain nuts and washers have special designs that prevent the nut from loosening – ensuring strength and durability over time.
The chart below displays the diameter and height of frequently used nut sizes.
How to Pick the Right Nut
As you compare different types of nuts, you will find a variety of shapes, sizes, thread patterns, and materials. Often, the selection of the nut is constrained by your choice of bolt because the threading and size must match. Additionally, it is best practice to match the grade, class and material with the corresponding bolt that is being used. Nuts also have various types of head shapes to accommodate different types of applications.
Regardless of the size, shape, or type of nut you choose, all of these elements help hold components together through the same mechanical property. As the nut and bolt thread together, the friction creates the necessary tension to hold the components in place.
Consider the “job” for the nut and bolt, and remember that variations in sizes and shapes can impact the efficacy of the hold. If a stronger hold is necessary, then you might choose nuts with locking mechanisms to protect against movement due to vibration and dynamic loads.
Also, there are different applications. Many nuts require rotational force, but other designs are available with clip-on or push-press applications. Some also offer the benefit of adhesive thread lockers for added strength.
Production
When choosing the ideal size and threading, remember that manufacturers produce nuts in both imperial (inch) threads and metric threads. The thread pitch is an indication of the fineness or coarseness of the thread positioning. Since nuts must fit perfectly with the corresponding bolts, they can’t be mixed.
Sizing
Certain key measurements are used to communicate the nut size, including standards and strength grade marking. There are different characteristics to measure, including thread pitch, threads per inch (TPI), as well as thread diameter.
Thread & Diameter Measurements
There are two ways to measure the thread diameter:
- Outside Diameter (OD): Also known as Major Diameter, this is the distance between the thread roots. This method is the most common measurement in basic nuts.
- Inside Diameter (ID): Also known as Minor Diameter, this is the distance between the thread crests.
Also, two types of measurement systems can be used:
- Threads Per Inch (TPI): An indication of how many threads are located in one inch of the thread length, used in the imperial system.
- Thread Pitch: Measurement for metric fasteners using millimeters, indicating the distance between two thread crests.
No matter the type of measurement system being used, nut size names are always in this format:
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Outside Diameter followed by the thread pitch or TPI
So, the names are slightly different depending on which measurement system is being used.
Inch Format
With inch format, the measurement is listed as thread size – which is a number between 1 and 12, followed by “TPI.” Then, the number of threads per inch immediately follows the diameter.
Example:
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1/4"-20 Nut
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This indicates a nut with a 1/4-inch diameter and 20 threads per inch.
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3/8"-16 Nut
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This indicates a nut with a 3/8-inch diameter and 16 threads per inch.
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#2-56 Nut:
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This indicates a nut that corresponds to a #2 screw size with a 56 threads per inch pitch.
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#8-32 Nut:
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This indicates a nut that corresponds to a #8 screw size with a 32 threads per inch pitch.
Metric Format
In metric format, the letter "M" is always used with a number that indicates the number of millimeters across the diameter, followed by the thread pitch.
Example:
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M6x1 Nut
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This indicates a nut with a 6mm diameter and a thread pitch of 1mm.
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M8x1.25 Nut
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This indicates a nut with an 8mm diameter and a thread pitch of 1.25mm.
Coarse Threads
When a nut is listed with coarse threading, it means that the threads are thicker and farther apart.
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Unified National Coarse (UNC) Threads: These threads are comparable to ISO metric threads.
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Unified National Coarse Rolled (UNRC) Threads: Adding the "R" is an indication that "rolled" external threads are being used, which have a rounded root contour. These threads can be used interchangeably with UNC fasteners.
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UNJC Unified National Coarse Threads: These threads have a controlled root radius, and an augmented minor diameter exhibits enlargement, which helps to disperse strength in a bigger area. This design is ideal for high-stress applications and was created for military use. Keep in mind that these source threads are not interchangeable with other UNC fasteners.
In the metric system, the labeling and design are a bit more simplistic. ISO metric threads use the word "coarse" to indicate the type of threads being used.
When the threads are thinner in size and positioned closer together, they are known as “fine threads.”
There are three categories of fine threads in the imperial system that matches the coarse threads categories and types (listed above):
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UNF: Unified National Fine Threads.
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UNRF: Unified National Fine threads with "rolled" external threads.
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UNJF: Unified National Fine threads designed with an increased minor diameter and controlled root radius for dispersing stress over a larger area.
Similarly, fine threads in the metric system keep the labeling simple by using the words "fine" or "super fine."
Keep in mind that the methodologies for determining TPIs and thread pitches vary distinctly, so the following applies:
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Inch Threads: When a nut has a higher TPI measurement, it is an indication of finer threads. More threads are located per inch of thread length.
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Metric Threads: A nut with a lower thread pitch indicates finer threads since there is less space between each thread crest.
Standards
Different regulation standards are used for nuts, depending on their measurement system. There are two measurement systems for all fasteners: Imperial and Metric. Each individual measurement system is also broken down into Tolerance Classes. Tolerance Class rating is an indication of their precision in assembly.
Imperial
American countries follow the imperial system, known as the Unified Thread Standard, as well as the “threads per inch” (TPI), counts. This standard is regulated by The American National Standards Institute (ANSI) and The American Society of Mechanical Engineers (ASME). When parts are being used for automotive applications, they are regulated by the Society of Automotive Engineers (SAE International).
You can tell that you are working with inch fasteners if any of the following standards apply to your nuts:
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Unified Thread Standard (UTS): This is the most common system used in the United States and Canada. The regulations include Unified National Fine (UNF) and Unified National Coarse (UNC) threads. Screw thread properties are indicated based on three classes of general purpose: UN, UNR, and UNJ series fasteners.
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SAE: This standard designates the number of threads located in each inch of threading, including both coarse and fine threading for screws, bolts, ports, pipes, and flange ports.
The tolerance class ratings for Imperial measurement are as follows:
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Class 1B threads: Loose fitting nuts, most often used when easy assembly is necessary, or they are being placed in dirty arenas.
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Class 2B threads: This is the most common thread class, offering maximum strength and common precision.
Class 3B threads: This class offers the least assembly tolerance.
Metric
Since most countries around the world use the metric system, this screw thread is the most common. The thread pitches and diameters are measured in millimeters. The International Organization for Standardization (ISO) has strict specifications for coarse, fine, and superfine threads.
In Europe, the Deutches Institut für Normung (DIN) (also known as the German Institute for Standardization) standards are used.
Metric nuts offer five possible tolerances for metric nuts, ranging between 4 – 8. The most common type is a nut with a “6g” tolerance. The rating system indicates that less tolerance is available for lower numbers and more tolerance is available for higher numbers
Grades
Indentation marketings offer indications about the strength of the nuts and bolts. Different standards (imperial vs. metric) use different strength markings.
The SAE regulates inch nuts, and these nuts have lines running lengthwise and located on the top of the nut.
Instead of line indentations, metric grade markings are indicated with numbers that show the strength grade. Some metric nuts use a pattern of dots and lines located between the sides of the nuts and the circle (with indentations printed outside the circle).
Types of Nuts
It’s no surprise that nuts come in various shapes, sizes, materials, and thread patterns. Keep in mind that there are constraints in nut selection based on the bolt being used, especially when it comes to threading and size. Also, make sure you are selecting a nut material and head shape that suits your application.
Non-Locking Nuts
These nuts do not include any locking mechanisms, only the basic nut design.
Locking Nuts
These nuts are designed with locking mechanisms to keep the nut in place. Examples of locking mechanisms include pins, plastic inserts, lock wire, or misshapen threads.
Materials for Nuts
Nuts come in a variety of finishes and materials, each of which are suited for different applications and use cases.
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Aluminum: Not only are aluminum nuts lightweight, but they are also popular since aluminum is resistant to oxidation. Additionally, this material is easy to manufacture and offers thermal and electrical conductivity.
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Brass: If you need strong nuts, then brass is a great material to consider. These nuts are conductive and resistant to corrosion. They offer low magnetic permeability.
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Copper alloy: When good load capacity is a priority, then copper alloy is the way to go. This material is wear-resistant and suitable for dynamic loads.
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Fiber-reinforced plastic: Nuts are available with industrial fabric-reinforced urethanes. The benefit of using these non-metal nuts is that they are wear resistant.
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Inconel®/Incoloy®: When the nuts are in high-temperature conditions, then this type of metal is a great option. They hold strong and have oxidation/carbonization resistance in temperatures up to 1200° F.
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Monel®: This proprietary metal blend is popular with nuts because of the material’s resistance to solutions and chemicals.
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Nylon: Nylon is a tough and resistant material, which is why it is ideal for nuts. Nylon offers good pressure ratings, making it an ideal solution when the nuts are under high pressure.
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Plastic: If the price is a concern and the load is light, plastic nuts are inexpensive. Plus, they offer corrosion resistance.
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Polytetrafluoroethylene (PTFE): This insoluble compound, known as PTFE, is beneficial for nut manufacturing because it offers durability and low friction.
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Polyvinyl chloride (PVC): PVC nuts are inert, resistant to chemicals, smooth, flexible, and non-toxic.
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Polyvinylidene fluoride (PVDF): If you need less creep and stronger nuts compared to other fluoropolymers, then PVDF is an excellent option. Keep in mind that this material is susceptible to high temperatures.
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Rubber: When noise dampening and vibration management are needed in specialty applications, rubber nuts can be helpful since they can absorb the vibrations without moving out of place.
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Steel: A strong, carbonated iron is used to produce steel nuts. Remember that uncoated steel is at risk of corrosion, which is why most people choose coated steel.
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Hardened steel: Specific hardening methods are used to make the steel stronger. The drawback is that hardened steel can be more brittle than other steel types.
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Stainless steel: It’s easy to see why stainless steel nuts are popular because they offer an appealing finish as well as chemical and corrosion resistance. But this type of steel can’t be hardened like carbon steel.
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Titanium: If you need nuts that are strong, hard, lightweight, and corrosion-resistant, then titanium is a great choice. Titanium can be alloyed with other metals to increase strength and durability.
What Is a Washer?
A washer is a thin, flat disc with a circular hole in the middle. It is placed between the nut and/or bolt and the surface that is being fastened, usually made of metal or plastic. The most common washers used are flat washers. The Standards for the flat washers are set by the SAE (Society of Automotive Engineers) and USS (United States Standards). One method to distinguish between the two is by recognizing that SAE washers are thinner and smaller compared to USS washers. Similar to the nut, it is important to match the correct grade, class and materials with the bolt that is being used.
The purpose of using a washer is to distribute the pressure or load that is being exerted by the fastener, which helps reduce the risk of deformation or damage to the materials you are fastening. A washer also acts as a protective barrier to reduce the risk of corrosion or marring of the materials while also preventing wooden surfaces from cracking. Washers can be used when the fastening needs to be waterproof because sealing the assembly reduces the risk of leakage.
If the surface being fastened is uneven or irregular, a washer helps by providing a more uniform and secure clamping force. Additionally, washers help ensure ideal fastener alignment while reducing the risk of the fastener loosening because of movement or vibration. Washers are especially important when the joint is exposed to dynamic forces.
How to Pick the Right Washer
When choosing a washer, you need to know the Outer Diameter (OD), Inner Diameter (ID), and thickness of the washer. These details must align with the type of nut and bolt you are using. It is best practice to match washers and bolts with their corresponding materials (stainless steel washers with stainless steel bolts, etc.)
One of the main reasons to use a washer is because the bolt and nut assembly aren’t tight enough. Placing a washer between the nut and the surface helps by increasing the distance, which offers more leverage so you can create a tighter joint. Another method for ensuring the nut stays in place is using a lock washer.
Here are a few considerations when choosing a washer:
Sizing for Washers
Washers are measured using two systems: imperial units (in the United States and Canada), and metric design units (in most other parts of the world). These dimensions can be specified in various ways, including gaged values, standard nominal values, or other custom measurements as needed.
When Nominal ANSI is being used, it refers to the size of the fastener that the washer mates best with. For example, use a washer that matches the size of the bolt: for a ⅝-11 bolt, choose a ⅝” washer. This system specifies both the outer diameter and inner diameter of the washer, as well as the thickness of the washer.
Measurement for Washers
You need to know the washer thickness, outer diameter (OD), and inner diameter (ID) to ensure the washer matches the application in which it is being used.
Thickness is usually the first measurement to consider since it impacts the tightness of the assembly. If you need extra tightness, then go with a thicker washer. Sometimes, it’s necessary to use multiple washers. If waterproofing is a priority, then a thinner washer might be sufficient.
Also, measure the size of the washer. If it’s too small or too large, it can negatively impact the joint.
During assembly, greasing the washers can make it easier to place the fastener. Additionally, the grease helps to reduce the risk of corrosion.
Standards
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ASME B18.22M: General specifications for most flat washers with round holes, including both soft and hardened materials that will be used for general-purpose applications.
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ADS AGS970: These specifications are for aluminum alloy washers specifically.
Grade
The construction materials, treatment processes, and tensile strength determine how each washer is graded.
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Low-carbon grade 2: Washers are made with low or medium-carbon steel and are cold-forged in the manufacturing process. They offer a tensile strength of 74.
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Grade A: Washers are made with low or medium-carbon steel and offer a tensile strength of 60.
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Grade B: Washers are made with low or medium-carbon steel and offer a tensile strength of 60 – 100.
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Grade 5: Washers are made with medium carbon steel and are treated with quenching and tempering. Grade 5 offers a tensile strength of 105 – 120.
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Grade 8: Washers are made with medium carbon alloy steel and are treated with quenching and tempering. Grade 8 offers a tensile strength of 150.
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Grade C: Washers made with medium carbon steel, designed for use in a partially completed assembly to slide in and out of a position (so tensile strength is not applicable).
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2-H: Washers made with medium carbon steel.
Materials for Washers
Washer materials vary significantly to match specific application needs and environmental conditions. Review the most popular materials below.
- Stainless steel: Washers made with stainless steel are a common choice because they offer corrosion and chemical resistance while also providing an appealing finish.
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Galvanized carbon steel: Galvanized carbon steel is a type of steel coated with a protective layer of zinc to enhance corrosion resistance and durability.
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Aluminum: Aluminum is one of the most popular solutions for lightweight washers. They are easy to manufacture, resistant to oxidation, and provide the benefit of thermal and electrical conductivity.
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Brass: When strength is a priority, then brass is a common choice. Not only is brass resistant to corrosion, but it is also a conductive metal with low magnetic permeability.
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Copper base alloy or bronze: Copper alloy is an excellent choice because it offers a good load capacity while also being suitable for dynamic loads.
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Copper: Copper has high tensile strength that can withstand high temperature and pressure conditions. They are often used in large-scale manufacturing operations.
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Nickel based alloy: Not only is nickel-based alloy tough, but it is resistant to corrosion and oxidation and offers versatility in many applications. The strength of the nickel-based alloy is beneficial in extreme temperatures (both hot and cold).
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Spring steel: Spring steel is highly resilient, offering durability and weather resistance. It is known for being durable and hard, with the ability to support heavy loads without breaking.
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Hardened steel: Specific manufacturing methods are used to harden the steel so that it is stronger. One potential problem is that hardened steel can be brittle.
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Titanium: When the washer needs to be lightweight, hard, strong, and corrosion-resistant, then titanium is a popular choice.
Classes
Washer classes signify various grades, indicating tolerance, material thickness, and overall quality to ensure proper fit and performance in diverse applications.
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Type A: Type A washers are made of steel and have broad tolerances. This type of washer is ideal if precision is NOT essential for the application.
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Type B: When precision is important, then Type B washers are usually recommended. Type B offers tighter tolerances compared to Type A washers and comes with narrow, regular, and wide outside diameters to match specific bolt sizes.
What to Consider
Washers are versatile and can be used for many functions, such as protecting a surface, preventing the fastener assembly from loosening, and distributing the pressure more evenly. Here are a few considerations when using washers:
Load Distribution
Since threaded fasteners cause stress to the material when driven in, washers can help distribute the load and reduce the risk of damage. This application is common for soft materials where protection is needed against stress-related damage. For example, a washer reduces the likelihood of wood cracking when a screw is driven in
Spacing
If you are dealing with a situation where a threaded fastener has a longer length than the depth of an object, then you can’t drive it in all the way (otherwise, the fastener will stick out the back). One or more washers can be placed through the threaded fastener to create padding so you avoid driving the fastener too deep.
Vibration Absorption
Certain types of washers are designed specifically to absorb vibrations and are commonly used in machinery and other vibrating applications. Not only do they help with fastener alignment, but they prevent the fastener from loosening due to movement or vibration. These washers are made with softer materials, such as urethane, rubber, or plastic, which are more effective at absorbing vibrations. Additionally, vibration absorption can reduce the object from damage.
Liquid Protection
When washers are placed correctly, they can create a waterproof seal. These washers are made with soft materials that can press against the object’s surface completely. They are often used in pipes and pipe connections.
Surface Protection
Since washers are placed between the surface and the fastener, they can provide a protective barrier to prevent corrosion, damage, or marring of the materials.
Compensation for Uneven Surfaces
If you are trying to fasten surfaces that have irregularities or are not perfectly flat, then washers help to provide a more uniform and secure clamping force
Types of Washers
There are a few different variations of washers with very important distinctions. When choosing a washer, ensure it complements your selected bolt and matches your fastening application.
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Beveled washers: The surface is slightly angled, making it possible to join materials that are not parallel.
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Flat washers: The most common type. Flat washers are used to create better load distribution through a bigger surface area. Choose from different thicknesses to match the hold strength that is needed.
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Lock washers: Available in different shapes, such as helices, conical, toothed rings, or springs. Lock Washers help by preventing the fasteners from slipping, especially in demanding applications with high vibration levels.
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Structural washers: A popular choice for heavy-duty applications, such as construction projects. Structural Washers are often used with thick fasteners and can withstand high load pressures.
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Spring washers: Sometimes called “locking washers.” Used to prevent the fastener from coming loose in a vibrating environment. A portion of the flat part of the washer is removed, creating a twisted shape. They act as a spring by creating a preload between two surfaces. Spring washers can only be used on the nut side of the fastener (not the bolt side).
Making the Final Selection
The combination of nuts and washers can effectively ensure that the fasteners stay tight and durable. But the key to success is choosing materials, styles, and sizes that match the specific applications you are using. Sometimes, the smallest nuances can make a difference in the performance of the nut and washer combination.
The information in this guide will help you in the selection of the ideal materials, making it easier for you to apply this knowledge in practical applications.