bearman
Active Member
The Torque Value is supposed to determine how much pre-stretch is on a bolt or stud when cold. This pre-stretch value is a defined length. This defined length is calculated (engineered) so that when everything is at operational temperatures, the bolt or stud still has enough clamping force to hold what it is supposed to. This defined pre-stretch length is more important than the torque value that it takes to get the pre-stretch. And most torque values are the result of trial and error.
In some cases (where you can get to both ends of a bolt to measure, like a rod bolt for a Chevy) they give a length measurement instead of a torque value. On really big bolts (like 2”) that hold down large tanks and towers, they have a hydraulic stretcher that actually stretches the bolt and you just spin the huge nut down by hand and then release the hydraulic pressure.
The problem with a torque value is that there is no way to compensate for differing friction losses while torqueing a nut. These friction loses can be because of dirt in the threads, improper lubrication, or the way you hold your mouth while turning the wrench. But these friction losses are exponential, so they only matter a little at low torque, but matter a whole bunch at high torque.
In a perfect world you could just determine how many threads per inch your bolt had, and then turn it how whatever amount it would take to get your required bolt stretch. But this perfect world would require all bolts to have the same starting point and that whatever we are bolting together would not be able to flex. But since we know that we need to squish gaskets between the metal parts (because these metal parts do flex) it’s very difficult to get a “same” starting point.
I assume that the initial small torqueing is used to get everything snugged up to the same starting point, and then using another 90 degrees is an attempt to get the most accurate required pre-stretch.
Which way is the best way? I don’t know. I personally use the torque value method with a click type wrench, but I clean everything the same and then lube everything the same (and use the same torque wrench) so my friction losses should be the same.
One thing I am sure of is that that if you use the torque method you will not over stretch any bolts, they might wind up being not tight enough (and you might have to re-torque them), but they will not be over stretched.
If you have a method that works for you, why change?
But all this is just my two cents. Your mileage may vary.
And no, I didn’t stay in a Holiday Inn last night, I’m a mechanical engineer.
In some cases (where you can get to both ends of a bolt to measure, like a rod bolt for a Chevy) they give a length measurement instead of a torque value. On really big bolts (like 2”) that hold down large tanks and towers, they have a hydraulic stretcher that actually stretches the bolt and you just spin the huge nut down by hand and then release the hydraulic pressure.
The problem with a torque value is that there is no way to compensate for differing friction losses while torqueing a nut. These friction loses can be because of dirt in the threads, improper lubrication, or the way you hold your mouth while turning the wrench. But these friction losses are exponential, so they only matter a little at low torque, but matter a whole bunch at high torque.
In a perfect world you could just determine how many threads per inch your bolt had, and then turn it how whatever amount it would take to get your required bolt stretch. But this perfect world would require all bolts to have the same starting point and that whatever we are bolting together would not be able to flex. But since we know that we need to squish gaskets between the metal parts (because these metal parts do flex) it’s very difficult to get a “same” starting point.
I assume that the initial small torqueing is used to get everything snugged up to the same starting point, and then using another 90 degrees is an attempt to get the most accurate required pre-stretch.
Which way is the best way? I don’t know. I personally use the torque value method with a click type wrench, but I clean everything the same and then lube everything the same (and use the same torque wrench) so my friction losses should be the same.
One thing I am sure of is that that if you use the torque method you will not over stretch any bolts, they might wind up being not tight enough (and you might have to re-torque them), but they will not be over stretched.
If you have a method that works for you, why change?
But all this is just my two cents. Your mileage may vary.
And no, I didn’t stay in a Holiday Inn last night, I’m a mechanical engineer.
