Last Updated on April 19, 2022 by Marsh Fasteners
Bolts are available in a wide variety of different grades, which usually indicate a bolt’s tensile strength. Typically, bolts are steel fasteners that are engineered with external male threads. Marsh Fasteners explains the tensile strength and grades of bolts for various applications. Read on to find out more.
Tensile Strength Of Bolts
So, what is the tensile strength of a bolt? In simple terms, the tensile strength is the maximum amount of stress or pressure a bolt can withstand. Therefore, the higher the tensile strength, the greater the impact the bolt can bear, making it suitable for high-stress applications such as industrial machinery or securing heavy objects to each other. The most common high tensile bolts are:
- Grade 8.8 bolts: Also known as structural grade bolts, they are typically made of steel and plated with several coatings.
- Grade 10.9 bolts: As these bolts are most often used in the automotive industry, they are also referred to as ‘car bolts’. They are made of boron or carbon steel and have high tensile strength (1040 MPa).
- Grade 12.9 bolts: Known for their strength, these bolts are most often used in the manufacturing and construction industries to join medium or heavy industrial parts such as an automotive engine.
- Grade 14.9 bolts: It's simple: these are the most durable bolts you can use in your application. Perfect to join large structural elements to each other, the grade 14.9 bolt has a whopping maximum tensile strength of 1400 MPa.
Now that you know how tensile strength is useful in choosing the right bolt for your application, Marsh Fasteners explains the various grades of bolts and the applications they are used for. Here is a general breakdown:
SAE American Grades
These grades start at grade 2 and go up to grade 8, which is the strongest SAE American Grade bolt available.
A grade 2 bolt is distinguished through its complete lack of markings and will usually have a tensile strength of between 60,000 and 74,000 psi. Depending on the application they are used for, the Grade 2 bolts can be partially or completely threaded. These bolts are generally used for non-critical joints and applications in design, OEM and maintenance, especially as it is a cost-effective option. The nominal size range of the grade 2 bolt is ¼ inch to 4 inches.
Manufactured from medium carbon steel and hardened for greater strength and durability, a grade 5 bolt is distinguished by 3 radial lines and promises tensile strengths between 105,000 and 120,000 psi. They are most commonly found in automotive applications or those that require medium strength. The most common sizes of Grade 5 bolts range from ¼ inch to 1½ inches.
The high-strength, sought-after grade 8 bolt can be distinguished by six raised dashes. Its medium-carbon alloy steel has been quenched and tempered, ultimately making it possible to achieve an impressive tensile strength of 150,000 psi, compared to a grade 2 bolt’s 64,000 psi. In other words, a grade 8 bolt is more than twice as strong as a grade 2 SAE American Grade bolt. You will often find grade 8 bolts in demanding applications, such as in automotive suspensions. Other applications that require the strength and tolerance of a grade 8 bolt are industrial machinery, motors, pumps, valves and tanks!
The heavy-duty grade S bolts are built for strength and durability. They are engineered to withstand stresses ranging from 45 to 49.9 MT per 6.45 cm3.
Ultimately, the bolt that you select will depend on how strong it needs to be. The more demanding the application, the higher the grade that will be required.