Why it Takes Less Torque to Loosen a Bolt Than to Tighten It

It may not feel like it when you're fighting with a stuck or stubborn bolt, but it's true.

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As Charlie Chaplin figured out during , tightening bolts all day can be a real pain. Lucky for Charlie, and for any one of us working on a mechanical project, there's good news at the other end of the wrench: Loosening a bolt takes less torque than you originally put into tightening it.

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If that seems counterintuitive to you, you're not alone. Mechanical objects, like nuts and bolts, aren't magic. It would seem, then, that if you torqued a nut or bolt to exactly 100 lb-ft, removing it would require exactly 100 lb-ft of torque applied in the opposite direction.

But that's not the case. And has an in-depth explanation of why that's the case, complete with demonstrations and data.

AvE's full video is nearly 13 minutes long. Most of that is dedicated to him describing how he rigged up the torque-measuring device that he uses to illustrate his hands-on example. And viewer be warned, there's some foul language and Canadian humor contained in the video (a word that AvE pronounces to rhyme with "Ave0").

If you're interested in learning how to rig up an oscilloscope as an ad-hoc torque-strain gauge, . To skip the dickering around and get straight to the demonstration and explanation, click on the video window immediately below these words.

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In short, it takes less torque to loosen a threaded fastener than it does to tighten it, because the pitched threads act like an inclined plane. Tightening the fastener is like pushing "uphill;" loosening it is like pulling "downhill." In AvE's test, it takes around 10 percent less torque to loosen a bolt than what was put into tightening it.

Of course, this doesn't account for what might happen in the period between installation and removal of a nut or bolt: Rust, corrosion, threads seizing, and the like. Anyone who has strained against a badly rusted fastener knows that they're sometimes impossible to remove without the persuasion of a cheater bar or blowtorch. And as AvE demonstrates, oil on the threads has the opposite effect, reducing the breakaway torque even further.

And just because it's on our minds, here's our favorite scene from Charlie Chaplin's Modern Times:

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