How can impact tools deliver torques higher than the socket ratings?

Question

How can impact tools deliver torques higher than the socket ratings?

Environment

Impact tools being used with power drive (“impact”) sockets

Answer

The reason that impact tools are often capable of delivering torques that are higher than socket ratings are two-fold.

First, the torque from the tool is delivered “dynamically”—the rotating impact mechanism is brought to a sudden stop by the threaded joint.  Torque reaction from the threaded joint is provided at the threads (thread friction) and under the bolt head/nut (collar friction).  When the torque is “audited” quasi-statically with a hand torque wrench, both frictional surfaces need to move at the same time, each requiring significant torque, as a steady torque is applied to the hand wrench.  However, during the impact, as the moving metal is brought to a stop, the torque changes rapidly with time (an individual impact lasting on the order of a millisecond), and depending on the joint, both threaded surfaces do not always move at the same time, allowing the socket to see less torque dynamically than it sees with a static or quasi-static hand pull.

Second, steels are typically stronger at high strain rates.  That is, steels are stronger when loaded for very short periods with a given torque than they are when loaded at the same torque statically (for more information on material Dynamic Increase Factors, please see Werner Goldsmith, Impact, The Theory and Physical Behavior of Colliding Solids, Dover Publications Inc., Mineola, NY, 2001, p. 308-310).  The duration of each impact blow is very short, so that the resulting strain rate of the socket is very high.

These two factors help explain why impact tools are able to dynamically deliver torques that are higher than the static ratings provided by sockets.  Of course, our safety information must always be read and followed, and be sure to use only power drive (“impact”) sockets that are in good condition.