The problem of torque wrenches

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donuk
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The problem of torque wrenches

Post by donuk »

While I was recently reassembling a set of speakers after performing worthwhile internal upgrade the problem of just how tight to set the mounting screws crossed my mind.
I do not have a torque wrench, so just used keys to get moderately tight; and did them up a bit more the next day.
If I had had a torque wrench I would have used it but would it have helped?
I began to think about it. A torque wrench measures only how tightly you are screwing a screw. It does not measure the pressure the screw in causing. It does not measure how tightly the joint is being made; it does not measure the perpendicular force. All it does is to tell you how much energy you are using to tighten the screw. (Or something similar).
What, I asked, would the reading be if the screw hole was tight (one was, and squeaked as I tightened it - so I put a bit of grease on all screws). When the screw is finally tightened, the head comes into contact with the speaker surround. The torque wrench will be measuring this friction. A shake proof washer would greatly modify the reading, as would grains of dust or a roughened surface. Or any number of things.

I concluded that torque wrenches are of limited use. I then found this site:
http://www.surebolt.com/torque_errors.htm

So gentlefolk, I think we need a re-think on some of the very precise values required. How do we measure them?

Donuk damp downtown York
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lejonklou
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Re: The problem of torque wrenches

Post by lejonklou »

Correct Don, the friction between screw head and the surface it meets is what trips the torque tool. It may appear imprecise, but if you use the tool in practice, you will quickly find the results are consistent and repeatable. It is also very obvious when excessive friction, for instance due to damaged threads, destroys all precision. A tiny bit of grease can solve this. But on many fasteners the precision does not increase by greasing the surfaces, therefore I recommend to start "dry" and lube only when necessary.
Spannko
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Re: The problem of torque wrenches

Post by Spannko »

I agree with everything you say Don.

However, I still use a torque screwdriver because it's the most "accurate" (I use the term loosely!) method we have for trying to introduce some consistency into our systems.

I can't remember the actual figures (so don't quote me), An experienced engineer can achieve a consistency of about +/- 25% when tightening by feel. A torque driver will get us to about +/- 15%. I think the use of torque nuts will get us down to about 10% and pressure sensors between the tightened surfaces will get us to about 5%.

Using a torque driver won't achieve perfection, but it's a lot better than guessing, particularly for an inexperienced bolt tightener!

Also, it's very easy to determine the "best" torque for your speakers, and it's surprisingly repeatable.
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