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Bwyanfwanigan t1_j4617yr wrote

What is missing here is the practical part. You use a torque wrench to accomplish this. They have a part that let's you set the required torque, and then you turn the bolt with the torque wrench until the wrench clicks. Google torque wrench for an image. Older torque wrenches had just a pointer and scale and depended on the elasticity of the wrench handle to get the right torque. They were fairly inaccurate.

Theoretically undoing a torque bolt requires the same torque. Real world that depends on a lot of things...

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julie78787 t1_j469nx0 wrote

It actually doesn’t because the coefficients of static and sliding friction are different.

There are practical implications of that, but unless you assume frictionless parts, the torque immediate prior to motion will be greater than the torque immediately after.

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fighter_pil0t t1_j46hn81 wrote

I feel like this would be a factor for automatic torque wrenches but not manual ones. As you try to hit the torque correctly you usually slow to a complete stop.

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julie78787 t1_j46nf6e wrote

So long as you don't stop, then start, then stop, hoping to hit the correct torque.

Once you stop, the coefficient of static friction will increase the apparently torque, potentially leaving the fastener under-torqued.

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minkey-on-the-loose t1_j46fqa0 wrote

Those old pointer torque wrenches were good enough for grandpa, they are good enough for me. But those setting torque wrenches sound awesome.

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davidm2232 t1_j46seqy wrote

Both have their purpose. The click ones are great to quickly get through a torque sequence. The beam or pointer style give a nice visual representation of the torque. Snap On makes a digital one that will give an instant torque readout and beep when you reach the setpoint. Best of both worlds

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aspheric_cow t1_j4735jb wrote

I'd argue the old beam type torque wrenches were more accurate and reliable. They are so simple there's not much that could go wrong with it. They just aren't very convenient to use.

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