Determining Torque: The Facts About Required Torque, Tension and Clamp Loads

The function of a bolt is to apply sufficient clamp force such that when external forces are applied no separation or transverse movement will occur.

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Tightening fasteners to apply that clamp force using calibrated torque tools is certainly preferable to simply going by feel.  Or, as one senior technician calls it “The One Grunt or Two Grunts Method”.  However measuring torque alone does not insure a proper installation.

  • Torque is an indirect indicator of Tension. 
    • It is the measure of the twisting force that threads fasteners together. 
  • Tension is the force that holds an assembly together. Think of it this way:
    • A bolt will stretch, like a spring, as it is tightened once all the parts of the assembly are brought together.
    •  The more torque that is applied to the fastener the more tension is created as the bolt stretches.

If a bolt is like a spring and more torque equals more tension, why doesn’t a certain amount of torque always result in the same amount of tension?

While the relationship between torque and tension is linear, many factors can affect this relationship including surface texture, human error, rust, oil, debris, even material type and locking elements.  For this reason the results of tightening to a specific torque setting can vary in the field or on the assembly line. 

There is a method used to estimate the torque/tension relationship in an assembly.  The Torque/Tension Equation; T = (K D P)/12­­­­­­­­­­­­­­­­­­­­­ can be used to prescribe a torque value that will achieve a certain tension or clamp load

T = Torque (ft.-lbs.)

D = Nominal Diameter (inches)

P = Desired Clamp Load Tension (lbs.)

K = Co-Efficient of Friction

The K factor can vary depending on the condition of the threaded fasteners. 

Commonly used factors range

  • 0.10 = Lubricated with wax, oil or other coating
  • 0.20 = Plain, uncoated
  • 0.30 = Galvanized
  • 0.30+ = Dirty or Rusty

Most Bolt Torque Charts use this formula.  Because these are estimates and conditions can vary in real world applications this formula and torque charts should only be used as a guide. 

 

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The best way to determine correct torque is through experimentation under actual assembly conditions.
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Other methods of achieving specific Clamp Load are:

  • Turn-of-the-Nut.  Specific Tension can be accurately correlated to the amount of rotation of a fastener once the parts of an assembly have been brought together.
  • Direct Tension Indicator.  Used in some structural applications, a DTI washer has protrusions that compress as the joint is tightened.  The amount of that compression is measured by a feeler gage that correlates to a specific tension.  Another type of DTI washer is the Squirter® type that contains a small amount of silicone material that is expelled when the proper tension is reached.
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Need more info than you see here about torque? Call us at (800) 634-0406.

Kyle Domer
Do Fasteners Need to be UL Listed or NEC Approved for Use in Solar Applications?

The short answer is NO.

General commodity commercial fasteners do not fall under the umbrella of products that would require a UL listing or ICC-ES report. There are three main governing bodies that cover specifications for fasteners. ASTM (American Society for Testing and Materials) covers primarily bolts and fasteners for construction applications. SAE (Society of Automotive Engineers) covers fasteners for automotive, machinery, and OEM applications. ASME (American Society of Mechanical Engineers) standards B18.2.1 and B18.2.1 cover dimensional tolerances for square and hex head bolts, screws, and nuts. Mudge Fasteners provides fasteners to these three sets of specifications (ASTM, SAE, and ASME). A fourth specification group that is also applicable is AASHTO (American Association of State Highway & Transportation Officials). AASHTO specifications are typically only called out on highway projects and many AASHTO designations can be directly converted to an ASTM equivalent.

ICC (International Code Council) is the primary governing body responsible for writing and maintaining building, plumbing, mechanical, and fire codes adopted by local and state municipalities, and the federal government. ICC-ES (International Code Council-Evaluation Services, Inc.) is the branch of ICC responsible for performing technical evaluations of building products, components, methods, and materials to ensure that they meet the requirements of the codes. This evaluation process culminates in an ICC-ES report that provides evidence that the evaluated product or system meets code requirements. However, in the case of most anchor bolts and fasteners, the applicable sections of the codes (International Building Code, International Residential Code, International Property Maintenance Code, Etc.) direct you to the ASTM specifications covering said anchor bolts or fasteners. As the ASTM specifications already cover anchor bolts and fasteners in detail, there is no reason for ICC to write their own specifications for these items. For the same reason, ICC-ES does not need to perform evaluation testing of anchor bolts or fasteners. As long as the bolts meet the applicable ASTM specification(s), they will meet the applicable Code section(s).

UL Listings are a service provided by Underwriters Laboratories Inc (UL). Manufacturers submit products to UL for testing and safety certification on a voluntary basis. There are no laws specifying that a product must be tested and given a UL Mark. While there are many municipalities that have laws, codes or regulations which require a product to be tested by a nationally recognized testing laboratory (such as UL) before that product can be used within their jurisdiction, anchor bolts and fasteners are typically not among these products. This is for two reasons; the first being that products required to be tested and receive a UL Listing are typically potentially hazardous products, including marine products, life saving devices, fire suppression and containment products, chemicals, and industrial, mechanical and automotive equipment. The second reason is that, just as with ICC-ES, the ASTM specifications already cover bolts and fasteners in depth, and it would be redundant to re-cover the same information.

In summation, as long as a bolt or fastener is manufactured to the correct ASTM, SAE, ASME, and/or AASHTO specification, there is no need for it to be tested or evaluated by ICC-ES, UL, or other similar listing services. When you purchase from Mudge Fasteners, you can request a complete certification package, ensuring that your product meets the applicable specification(s).

Kyle Domer
What is the Difference Between a Self-Drilling Screw and a Self-Tapping Screw?

One of the most important commercial fasteners in use today is the Self-Tapping Screw. Among the earliest “engineered” fasteners, Self-Tapping Screws were made from hardened steel and its use was instrumental to the Industrial Revolution. Still in use today, there are now many types and styles of Self-Tapping Screws made to perform specific fastening jobs.

Unlike Military or Aerospace Fasteners, terminology used in the Commercial Fastener Trade is often imprecise and can sometimes cause confusion; one example is the distinction between Self-Tapping and Self-Drilling Screws.

  • A Self-Tapping Screw can be referred to as simply a Tapping Screw. However it is also often called a Sheet Metal Screw because they are primarily used in sheet metal. 
  • Whichever name is used, these names are for screws that form mating threads (“tapping” the threads) in a pre-drilled hole in the substrate into which they are driven.
  • A Self-Drilling Screw is a Self-Tapping Screw with the added feature of a drill point. The drill point looks a lot like the point of a drill. It will drill a hole and form the mating threads in one operation.
  • Here is where the confusion comes; many times the person specifying a screw will interchange the term Self-Tapping Screw (or “Self-Tapper”) with Self-Drilling Screw. If a screw will drill its own hole it is a Self-Drilling Screw.

Other screws that might be considered to be in the Self-Tapping Screw category are:

  • Thread-Cutting Screws (TCS) - These are screws that can be used to cut threads in a pre-drilled hole. There are many types of TCS and each has a different thread-form to accomplish that goal.
  • Thread Rolling Screws (TRS) - These are screws that will roll or extrude threads in a pre-drilled hole in the substrate without removing any material creating a fit with zero clearance.

While the terms Self-Tapping and Self-Drilling are not interchangeable, these screws come in numerous configurations and are widely used in the commercial, industrial and construction markets.

Have a question you want the Fastener Expert to answer? Contact us and your question and answer may be featured next!


Featured Self-Tapping Screws

Elco AllFlex Self-Tapping Screws
Thread-cutting self-tapping fasteners with Stalgard® GB corrosion resistant coating to prevent galvanic reactions.
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Elco Tap-Flex Self-Tapping Screws
These thread-forming screws provide the strength, ductility, and resistance to embrittlement failures in metal applications.
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Kyle Domer