Torque Tolerance

[thstone]

In summary:

Threaded fasteners can clamp materials together only when they are holding with the proper amount of tension. For this to happen they must be properly tightened. To this day a simple, inexpensive, and effective way to consistently and accurately tighten a fastener does not exist. There are a number of tensioning methods that that do this better than a simple torque wrench but they are both complicated and expensive. In most situations, the less-than-perfect traditional method of a torque wrench is sufficient.

Why is a torque wrench with widely-varying known errors sufficient? Because engineers compensate for the inability to consistently and accurately determine bolt tension by massively over-designing joints. This accommodates inaccurate tightening and avoids catastrophic failure. Designers will specify more or larger bolts than needed in order to ensure that the joints are sufficiently clamped together. It is true that fewer or smaller fasteners could be used if bolt preload was more accurately and consistently controlled - but that is not practical nor realistic. And historically, the over-design of the fastener has been far cheaper than controlling the tightening process in a factory (and which is nearly uncontrolled in a DIY/backyard mechanic scenario).

Thus, the fairly large inaccuracies of measuring tension using a torque wrench are taken into account during the design phase such that large errors in applied torque will not result in failures.

What does this mean to you? Simply use the best tool that you have and torque to the specified value.

And now you know why manufacturers use 12 bolts to hold two parts together when it sure seems like 4-5 could have done the job!

[BoomerRoadie]

Originally Posted by thstone

In summary:

What does this mean to you? Simply use the best tool that you have and torque to the specified value.[/B]

And now you know why manufacturers use 12 bolts to hold two parts together when it sure seems like 4-5 could have done the job!

HAHA!! Yes...in summary...

Ok, I'm good now. I have been taking in a ton of information from people like you. This has been helpful. And I'm sure I was over complicating things, but it has more to do with understanding what goes into the actual torque, not the torque itself.

I also do not see tools as investments. Tool are assests. Investments have, by definition a quantifiable return, whereas assests do not. So the real qualifier for an assest is, will it fulfill its duty over its period of expected performance? There is much more to that definition, but for these purposes, your final summary says everything that's needed.

Thank you!

[JFP in PA]

Originally Posted by BoomerRoadie

I also do not see tools as investments. Tool are assests. Investments have, by definition a quantifiable return, whereas assests do not. So the real qualifier for an assest is, will it fulfill its duty over its period of expected performance? There is much more to that definition, but for these purposes, your final summary says everything that's needed.

Thank you!

An investment is something that gives you some type of return on what you paid for it. It could be an asset, or some other type of vehicle. Quality tools that give you the ability to properly maintain your vehicle without having to pay someone to do it because you lack the tools, those savings is where your quantifiable return comes from. And when you decide to stop working on the cars, you can sell them (now an asset) for whatever the market will bear, recover some, if not all, of your original capital investment.

[lkchris]

Originally Posted by thstone

. To this day a simple, inexpensive, and effective way to consistently and accurately tighten a fastener does not exist.

Actually, lock washers are indeed simple and inexpensive and effective. And you don't have to calculate or measure anything--just flatten them!

Not the whole story, obviously ...

PS: don't reuse crush washers

PPS: just remember ... threaded fasteners are springs.

[lkchris]

Originally Posted by thstone

Designers will specify more or larger bolts than needed in order to ensure that the joints are sufficiently clamped together. It is true that fewer or smaller fasteners could be used if bolt preload was more accurately and consistently controlled - but that is not practical nor realistic. And historically, the over-design of the fastener has been far cheaper than controlling the tightening process in a factory (and which is nearly uncontrolled in a DIY/backyard mechanic scenario).

Let's not undersell engineers or suggest sloppiness is ok for mechanics ...

There are plenty of instances I suggest where things are not overbuilt ... race cars, for example. (See Colin Chapman). Aircraft ... weight gets pretty critical.

Weight is pretty critical in a world where fuel economy is a big deal, too, as our politicians mandate. I owned several Mercedes before I purchased my Boxster and my first impression was the Boxster was pretty chintzy. Then I thought a little more about it having an aluminum jack.

You want the mechanics that maintain the airplanes you fly in to follow the rules. I worked in nuclear weapons and I know you want rules followed there, too. Let's encourage following the rules in Porsche maintenance, too. I've big doubts whether cheap tools are following the rules.