Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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Torque Setting



Torque Setting | 1 August, 2004

How do you define torque setting for screw?What does 1)Screw with Bolt size 3-48 means ? 2)Thread Size M3 means? 3)Fasterner Property Class?

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Torque Setting | 2 August, 2004

Q1: How do you define torque setting for screw? A1: Talk to people at your nut supplier. Some nut suppliers post torque tables on the web. For instance:

Q2: What does screw with bolt size 3-48 mean? A2: �3� is the thread diameter [0.099]. �48� is the threads per inch

Q3: What does thread size M3 mean? A3: �M� means it�s metric. �3� is the thread diameter [3 mm].

Q4: What does fasterner property class? A4: Property Class [clipped from Metric Fastener Tips for the Mitsubishi 3000GT and Dodge Stealth by Jeff Lucius]

Bolts and screws are designed to perform only one function: to clamp parts together. Therefore, the tensile strength and yield strength of a fastener are important. The tensile strength is the property of a fastener that resists tensional stress, or a force that causes physical lengthening of the fastener. Yield strength is a measure of how much tensional stress a fastener can withstand before it fails (breaks). Because bolts and screws are designed to clamp, compressive, shear, and bending strength and plasticity are not important. Well, at least they are not important when a bolt or screw is used as it is designed to be used, that is, not as a pivot or axle, or to locate parts (to keep them from shifting with respect to each other). The tensile and yield strengths are described for every fastener using a Property Class.

The Property Class for steel fasteners using Unified Screw Threads (the "Imperial" system) is described using SAE (Society of Automotive Engineers) Grades. The Property Class for Metric threads is described using Classes. Each metric Class symbol consists of two numbers separated by a period. The first number is 1/100 of the nominal tensile strength in N/mm2 (newtons per square millimeter; 1 N/mm2 = 145.038 psi). The second number is 10 times the ratio of nominal yield stress to nominal tensile strength. For example, a Class 8.8 bolt has a nominal tensile strength of 800 N/mm2 (or about 116,000 psi) and a nominal yield stress of 640 N/mm2 (8 times 800 divided by 10).

SAE Grade 8 (150,000 psi tensile strength) is equivalent to metric Class 10.9. SAE Grade 5 (120,000 psi tensile strength) is equivalent to metric Class 8.8. Metric Class 4.6 and 4.8 are equivalent to "mild steel".

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Torque Setting | 2 August, 2004

You might also want to check out the web site for Assembly Magazine. I have not checked it lately, but they used to have several technical articles about fasteners there. It is

Mike F

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