Fastener 101 - The Basics of the Threaded Fastener Grade 5, Grade 8, Stainless Steel

What Is A Threaded Fastener

• Threaded fasteners are commonly thought of as any threaded part that is used in joint assembly.

• Threaded fasteners may be removed without damage to the fastener or to the members being joined.

• Many threaded fasteners can be used more than 1 time.

How To Measure A Bolt

• To measure the size of a bolt, you must find the diameter, length and thread pitch.

• Diameter is the O.D. of the threaded portion of the fastener.

• Length is measured from underneath the head to the end of the screw on most head styles.  Flat Head screws however, are measured by their overall length.

Thread Pitch

• Thread series cover designations of diameter/pitch combinations that are measured by the number of             threads per inch (TPI) applied to a single diameter.

• Coarse Thread Series (UNC/UNRC) is the most common designation for general application bolts and nuts. Coarse thread is beneficial, because they are less likely to cross thread, more tolerant     in adverse conditions and facilitate quick assembly.

• Fine Thread Series (UNF/UNRF) is commonly used in precision applications. Because of the larger tensile stress areas, they have high tension strength. However, a longer engagement is required for fine thread applications than for coarse series threads to prevent stripping.

Differences Between Grade 5 & Grade 8

• Grade 5 fasteners are made of carbon steel, Grade 8’s are made of alloy steel

• Grade 8 fasteners have a higher tensile strength but are more brittle

• Grade 5 and 8 fasteners are available in plain and plated finishes

• Grade 5 fasteners generally have a clear zinc (silver) plating, Grade 8’s plating are normally yellow

• Grade 5’s have 3 dash markings on their head, Grade 8’s have 6

What Is Bolt Torque?

Torque specifications frequent shop manuals. That’s a given. Today, you’ll sometimes find stretch measurements for certain fasteners along with torque angle figures but, generally speaking, “bolt torque” is still king. In many cases, the use of a torque wrench is the only way possible to measure the tension that is brought to bear on a fastener.

What is Torque?

Let’s rewind for a second. What is torque? Simple. Torque is the twist or the resistance to rotation. When used in reference to a fastener, torque is the resistance to turning of the bolt or nut. Torque is based upon the fundamental law of the lever: Force x distance = the torque or twist around a point. Torque is most commonly measured in foot-pounds or inch-pounds. In practice, if one pound of force is applied one foot from the center of the fastener, the resulting torque developed would be referred to as one foot-pound of torque. If the resistance measurement is in inches, the resulting torque would be referred to as twelve inch-pounds of torque.

Simple enough. Yet there’s more: Applying the exact amount of torque to a nut or bolt induces the correct amount of tension or elongation in the bolt that is necessary to hold the parts together. By applying the correct amount of specified torque to a bolt, the danger of distortion to the final part or adjoining parts is eliminated.

Stretch and Spring

It is also very important to understand that for a fastener to properly function, it must be “stretched.”  The ability of a fastener material to rebound (like a spring) is what really provides the clamping force. Different materials tend to react differently to these conditions. As a result, different fasteners are designed for different tasks.

What if you over-tighten a bolt?

If a fastener is over-torqued, it stretches too much. Because of this, the yield will have been exceeded, and for all intents and purposes, the fastener is ruined.

Heat and Tension

Another big factor in the torque picture is heat, primarily in aluminum. The thermal expansion rate of aluminum is greater than steel. Because of this, it is possible to stretch a fastener beyond yield as the aluminum expands when heated. The solution? Produce a more flexible bolt.

There’s a definite limit on how much a specific type and grade of metal can be stressed or stretched safely. In essence, that is the elastic limit of the material.  After the correct fastener and material have been chosen, the design engineer establishes the exact amount of torque to be applied. This torque specification will induce a stress or elongation of the bolt of approximately 60-70 percent of its ultimate elastic limit.

Another factor is tension. What is it? Tension is straight pull and is measured in pounds. Torque wrenches are at times referred to as “tension wrenches.”

Common Mistakes

So what if you don’t tighten a fastener sufficiently. That resolves the over-tightening scenario, but if a nut or bolt is not tightened enough, it will eventually work loose and drop off. On the other hand, getting too rigorous with a bolt in the tightening department will likely snap it off. Neither the “too loose” or the “too tight” situations work. That’s why you should follow specific torque recommendations, and that’s also why you should use an accurate, quality torque wrench.

Another big mistake is to test the accuracy of one torque wrench against another wrench. Example: “If a wrench is used to tighten a bolt to 90-foot-pounds, and another torque wrench is used to loosen the bolt as an accuracy test for either wrench, then you will likely obtain different readings. This is a test that is frequently performed and it inevitably results in the assumption that one of the two torque wrenches is not calibrated correctly.

“What is not generally known or understood is that the break-away or break-loose torque is considerably less than the applied torque. This means that the torque required to loosen a bolt previously tightened to 90-foot-pounds would be considerably less than the 90-foot-pounds of applied torque. In view of the above, it is easy to see why one of the torque wrenches could be considered inaccurate. A torque wrench should be tested on a torque wrench-testing machine to determine its true accuracy.”

Domestic vs. Import

• Domestic and Import bolts sold in the U.S. are made to ISO (International Standards Organization) and ASME (American Society of Mechanical Engineers) requirements.

• Domestic and Import have traceability and material test reports, however import traceability and certifications can be more difficult to obtain

• Industrial Parts House is a distributor of both domestic and import bolts but recommends domestic bolts in critical applications. If a fastener failure were to happen,  contacting domestic manufacturers is much easier and quicker than connecting with an importer.

Stainless Steel

Inch

• 18-8 , which can be 302, 303 or 304 stainless, refers to 18% chromium and 8% nickel content, this is the most common type of stainless steel.

• 316 is more corrosion resistant than 18-8 because it contains molybdenum, but in inch fasteners both have the same tensile strength.

Metric

• A2 refers to metric 18-8.
• A4 refers to metric 316.
• Class 50, 70 and 80 designate increasingly higher metric strength levels.
• A manufacturer's mark is required on metric M5 or larger diameters.