A bolt in the world of fasteners is usually made of metal has a head on one end that is often hex shaped (giving the term hex head bolt) and machine threads on the other end. Bolts are used with nuts that have a matching thread and are rotated on the bolt creating compression or fastening situation. Washers are placed between the nut and bolt for extra strength (torque area) and a lock washer is applied under the nut so the tension applied will not release.
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Bolts can be made of many different metals that allow different strength levels so they can have varying applications.
Bolts come in different grades. What does that mean?
Bolts have markings on the head. The most common markings indicate that it is a grade 2, 5, or 8. It can indicate metric also. Typically a grade 2 bolt will have a smooth head and the least tensile strength. A type 2 is made of a softer metal (low or medium carbon steel) and by design will stretch or wear before it breaks. Plow bolts are typically grade 2 so they may stretch and not break if the plow hits a rock. Grade 5 is the next most common graded bolt. 3 marks or bars on the head of the bolt indicate it is a grade 5.
Grade 5 bolts are made of medium carbon steel and are usually quenched and tempered so they are capable of higher tensile strengths. Grade 8 bolts are made of medium carbon alloy steel that is quenched and tempered and have a higher tensile strength than the grade 5. Grade 8 has 6 lines or marks on the head. There are other grades of bolts but Grade 2, 5 and 8 will cover most applications for USS or SAE bolts.
What is a USS or SAE bolt?
USS stands for United States Standards and is commonly referred to a coarse threaded bolt. SAE is Society of Automotive Engineers and is commonly referred to as a fine thread bolt. T.P.I. or threads per inch measure bolt threads. Both styles can be used with nuts but if the bolt is in a high vibration area (like an engine) it will most often be a fine thread. One of the main benefits of fine thread in a vibration situation is simply more threads. Where a coarse threaded nut might vibrate, loosen and fall off a fine thread has more threads and would only loosen.
What other styles of bolts are available?
Metric bolts are another common bolt used in the market today. They are also marked on the head indicating tensile strength. The three most common metric markings in order of tensile strength are 8.8, 10.9 and 12.9.
Stainless bolts are referred to as A-2 bolts. They may or may not be stamped A-2 but are usually non-magnetic. They do not have the tensile strength other bolts have but can resist rust or corrosion better.
As bolts can actually be made of many materials and head configuration as well as thread length the styles of bolts is endless. Bolt fabrication companies can build bolts with specified sleeves, shoulders, lengths, and materials etc.
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