Why Do I Need a Torque Wrench?


      noun \ ˈtȯrk \

Because STAHLWILLE is known mostly for its torque wrenches and torque related products, we are often asked, “Why is using a torque wrench important?”  We will attempt to answer that question here.

Most sources will define torque as a twisting force that causes rotation.  When discussing torque as it relates to tightening fasteners (generally some type of threaded bolt) there are three primary components of the equation.  First, the axis of rotation is the point around which something rotates (the bolt).  Second, you have the distance between the axis of rotation and the force being applied (for our purposes, the wrench).  Lastly, you have the force being applied at the end of that distance, opposite the axis of rotation (how hard you push/pull).  Torque wrenches are engineered so when used correctly, the user knows within a small margin of error, how tight their work is – without guessing.

Torque is measured in force multiplied by distance.  For example, if your fastener calls for 75 foot pounds of torque it is asking for the equivalent of having a wrench 1 foot in length with a 75 pound weight at the end.  The resulting force equals a torque value of 75 ft lbs.  Using the same math, a 2 foot wrench with 10 pounds of force applied at the end puts a torque value of 20 ft lbs on the fastener.  In most cases your hand applies the force and the length of the wrench creates the distance.  That is the most basic view of torque value, force multiplied by distance.  Just FYI, European countries use Newton’s instead of Pounds for force and Meters instead of Feet for distance.   Nm (Newton/Meters) and Foot/lbs (or Inch/lbs) provide the same information.  The parts manufacturer can use either or both.

To understand the importance of proper tightening, you have to understand how the fastener works.  Again, for the purpose of constructing a basic answer to our initial question, we will use the common threaded bolt.

Bolts are like springs. The tighter they are (up to a limit) the more pressure they put on the threads of their matching parts (nut or threaded hole).  When you tighten a bolt, you stretch it and make it slightly longer. The bolt resists this flexing and acts like a spring.  A bolt that’s been permanently stretched by over-tightening will not apply the correct amount of clamping pressure and is likely to fail, either by loosening or breaking.  It will also not “spring back” to its original shape so loosening and retightening the bolt is not safe or possible.

The goal in fastening is to apply a correct amount of tension to the bolt and have that tension remain when the bolt is in use. At the perfect torque value, several conditions exist:

  1. The union of the threads (for example the bolt and nut) create a bond that is stronger than any force trying to separate them.
  2. The threads remain at optimum tightness for indefinite periods of time without loosening or failing.
  3. The fastener can be loosened and tightened repeatedly, as needed, without impacting the effectiveness of the fastener.

It is important to know the specified torque value for each job since not all fasteners are made equally (materials used, thread pattern/size, etc.) and not all applications are the same.

When a threaded fastener is too loose or under tightened, the metallurgic reaction or union between the fastener and matching nut or threaded hole is not completed and the fastener can come loose with vibration or movement.

When the threaded fastener is too tight, the threads are stretched too far making the material weak and subject to failure.  Over tightening can cause the fastener to break or the threads to strip.  Over tightening can also change the shape of the fastener permanently making retightening unsafe or impossible.  A properly tightened fastener can be loosened and tightened repeatedly without concern.

Both under tightening and over tightening a bolt can result in catastrophic failure.  It is important to tighten each bolt or nut to the manufacturers specified values.  One tool that allows you to set a specific torque value is the torque wrench.  A properly calibrated torque wrench adds safety, reliability and consistency to any critical fastening job.  A properly calibrated torque wrench, used correctly, eliminates guesswork and reduces the likelihood of failure.

I hope we answered our initial question, “Why is using a torque wrench important.”  As we discovered, there is more to tightening a bolt than just turning a wrench.  If you have additional questions regarding what we believe are superior torque products, click on CONTACT US above and let us know.

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