Torque Values?

[BwanaDik]

It all comes down to bolt stretch for the clamping force, as El Toro says. At work, we have to tighten up 2 1/2 inch flange bolts and often use Hydro Torque equipment. It actually measures the stretch of the bolt to determine the clamping force. Saab Head bolts are torqued up by the amount of rotation of the wrench, not a specific torque value. A little overkill for a motorcycle obviously. The whole thing when torquing up a bolt in non-industrial situations is the "clean and dry" thing. No rust, burrs or hickies on the treads. That's the way the engineers figured the clamping force in the first place.

There's also a significant factor of safety that's built into the joint calculations. Most joints, if designed right, can stand some bad tightening, either over or under. And the bolts may or may not meet the design spec for the material. We did a check of bolt strength back in the '80's on several hunded random bolts and found that more than 30% were lower strength than specified. Think about that when buying replacement bolts from Wal Mart instead of your local fastener store

 

[El Toro]

It all comes down to bolt stretch for the clamping force, as El Toro says. At work, we have to tighten up 2 1/2 inch flange bolts and often use Hydro Torque equipment. It actually measures the stretch of the bolt to determine the clamping force. Saab Head bolts are torqued up by the amount of rotation of the wrench, not a specific torque value. A little overkill for a motorcycle obviously. The whole thing when torquing up a bolt in non-industrial situations is the "clean and dry" thing. No rust, burrs or hickies on the treads. That's the way the engineers figured the clamping force in the first place.

The goal, as you say, is to try to achieve the clamping force through proper bolt elongation. Torque specs are given because it is a relatively simple single number descriptor that someone in the field can relate to.

And the bolts may or may not meet the design spec for the material. We did a check of bolt strength back in the '80's on several hunded random bolts and found that more than 30% were lower strength than specified. Think about that when buying replacement bolts from Wal Mart instead of your local fastener store

You may remember that there was actually a scandal about counterfeit fasteners. I'm thinking it was in the 80's. It is easy to put the head markings on the bolt and pretend its a high grade and sell it for the high price. This was going on, and it took some serious effort to shut it down. It may still happen with poor suppliers. Any time someone is selling aircraft quality, SAE Grade 5 or higher, or a high Metric grade bolt for a cheap price, I would be suspicious.

Another thing that I've seen, particularly with shade tree mechanics and students, is they will buy aircraft quality or a high SAE grade that has rolled threads, and then they notice that there is an unthreaded shank, and they want threads all the way to the head, so they'll take their die and cut threads up the shank... thereby completely eliminating the consistency advantage that they paid for when they bought a rolled thread spec bolt.

If you have a bunch of suspicious fasteners, you can check their authenticity by looking at the hardness. The Brinell Hardness Number, found by a special indenting machine, can be correlated to the ultimate strength. While this isn't a fool proof check, it can sort out the pot metal counterfeits from stuff that might actually meet its label requirements.

Fasteners represent a fairly large percentage of the cost of just about any mass produced machine. Cheating on the fasteners is a way for unscrupulous companies to shave costs. Unintentionally using wrong fasteners, or cutting new threads on a high grade shank, can rob an unsuspecting end user of what he's paid for.

 

[Scoobyvroom]

Good write up El Toro... A++++

Even in my pocket reference book there is a calculator for decreasing the torque spec if you lube the threads and such.

 

[eg23232]

Tighten it till it breaks, note the torque at snap-time, and don't go so far on the new part.

That's how I thought they did it. However, I went to the Calvin & Hobbes school of engineering...

 

[Crash Cash]

If you really want to know all the technical dirt, and especially how it relates to high performance machines, read "Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook"

Carroll Smith is a racer that's been around since the '60s, and has been through the school of hard knocks. It's all cars, but high performance engines are the same, no matter what they're powering. If **** has happened, it's happened to him. And he can really write well, too.