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  • Torque wrench working principle

    Discussion in 'The main mechanical design forum' started by Ashish Menkudale, Apr 30, 2015.

    1. Ashish Menkudale

      Ashish Menkudale New Member

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      I would like to know the working principle of torque wrench. For analyzing a design of bolted joint (specifically to calculate the thread strip margin for a stud)a bolt which is torqued using a torque gun, when does the wrench clicks ?


      Does the wrench click when

      1. the stud/bolt will encounter the end of threads into threaded insert (female part) and the further motion is opposed (opposing torque exceeds the calibrated wrench torque after the end of threads) ?

      OR

      2. after the specific number of threads produces enough amount of counter torque to oppose the applied torque by wrench ?
       
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    3. Dana

      Dana Well-Known Member

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      The torque wrench doesn't know what (friction or end of threads or whatever) is causing the torque; it just clicks when its preset torque is exceeded.

      However, in general, unless the threads bottom out in the hole (which usually should be avoided), it's the friction on the threads, as opposed to the angled component of the force on the helical inclined plane. That's why the torque-tension relationship is approximately the same for fine or coarse threads.
       
    4. Ashish Menkudale

      Ashish Menkudale New Member

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      Thanks Dana.

      Could you please be more specific about relation between friction produced by contact area provided by threads and torque by wrench ? Can you help me with formula or a source for these details ?

      For my problem, I have a torque value as an input and I need to find out how much will be the engagement length corresponding to that torque.
       
    5. Dana

      Dana Well-Known Member

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      Friction is independent of contact area for reasonably rigid materials, so the length of engagement of a threaded fastener should not significantly affect the torque.

      There is a linear relationship between torque and tension for fasteners. I don't have a formula offhand, but it's been extensively studied, look it up... bolted joint design is usually done from emperical tables rather than analysis.

      In general, the allowable tension on a threaded fastener is a function of the material tensile strength and the thread root cross sectional area. The required thread engagement length is then a matter of the thread tooth shear strength matching the root tensile strength. Torque isn't directly affected by the engagement length, but if the engagement length is insufficient, then the threads may strip before you reach the torque corresponding to the desired bolt tension. A rule of thumb is to use a thread engagement length equal to at least the bolt diameter in hard materials (e.g. steel) and double that in soft materials (e.g. aluminum).
       

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