Why an impact wrench can turn a rusted bolt but not a spring

Science · Published 1 November 2026 Swipe more like this →

In short: Imagine a tool capable of loosening massive, seized industrial bolts while being completely unable to tighten a small screw on a spring. This counter-intuitive behavior stems from the physics of angular momentum. By decoupling force from reaction, these tools allow humans to wield massive power without feeling the kickback.

The core mechanism of an impact wrench relies on storing and suddenly delivering angular momentum. Unlike a standard drill that applies continuous torque (where the user feels every ounce of resistance), an impact wrench uses a rotating mass accelerated by a motor. A rapid burst of heavy torque occurs when this weight engages with the anvil, which is the output shaft. Because the hammer mechanism is designed to spin freely between impacts, the operator feels very little reaction force; they only feel the motor accelerating the internal weight, much like a person swinging a hammer. There are three primary mechanical designs used to achieve this. The 'ball and cam' mechanism uses a steel ball on a ramp and a spring to slide a hammer assembly back and forth, often striking twice per revolution. It is common in electric tools because it prevents motor stalling but is less efficient than pneumatic designs.

A second design uses sliding pins as clutches, allowing the hammer to accelerate for a full revolution before impact, delivering stronger blows at the cost of faster wear. A third involves a rocking weight that strikes once per revolution; while simple, it can cause significant vibration. Precision is the primary challenge with these tools. Because they deliver fixed pulses of momentum rather than constant torque, the actual output varies based on how quickly the fastener reacts. If an object is very springy, the wrench may actually turn backward as energy is reflected back to the anvil while it is disconnected from the hammer. To manage this, professionals use 'torque sticks'—special socket extensions made of spring steel that flex at a specific rating to prevent over-tightening.

The tool was patented in 1935 by Robert H. Pott of Evansville, Indiana. While compressed air remains the standard for its power-to-weight ratio, cordless electric versions have surged since the mid-2000s. Notably, 'pulse tools' are distinct from impact wrenches; they use oil as a medium to transfer momentum, providing smoother impulses and noise reduction suitable for assembly rather than heavy-duty repair.

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