special bolt,stainless steel bolt

special bolt,stainless steel bolt

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                                     Tensile strength
Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. For example, when two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough so that the friction between the two bars can overcome the longitudinal force. If the bars slip, then the bolt may be sheared in half, or friction between the bolt and slipping bars may erode and weaken the bolt (called fretting). For this type of application, high-strength steel bolts are used and should be tightened to a specified torque.

High-strength steel bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head. The property classes most often used are 5.8, 8.8, and 10.9. The number before the point is the tensile ultimate strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.

Tensile ultimate strength is the stress at which the bolt fails (breaks in half). Tensile yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2 % offset strain. When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.

Mild steel bolts have property class 4.6. High-strength steel bolts have property class 8.8 or above. An M10, property class 8.8 bolt can very safely hold a static tensile load of about 15 kN.

There is no simple method to measure the tension of a bolt already in place other than to tighten it and identify at which point the bolt starts moving. This is known as 're-torqueing'. An electronic torque wrench is used on the bolt under test, and the torque applied is constantly measured. When the bolt starts moving (tightening) the torque briefly drops sharply - this drop-off point is considered the measure of tension.

Recent developments enable bolt tensions to be estimated by using ultrasonic testing. Another way to ensure correct bolt tension (mainly in steel erecting) involves the use of crush-washers. These are washers that have been drilled and filled with orange RTV. When the orange rubber strands appear, the tension is correct.

Large volume users such as auto makers frequently use computer controlled nut drivers. With such machines the computer in effect plots a graph of the torque exerted. Once the torque ceases to rise (the point where the bolt begins to deform) the machine stops. Such machines are often used to fit wheelnuts and will normally tighten all the wheel nuts simultaneously.

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