Torque specs

[Doktor Schnell]

I'm wondering why sometimes torque specs require one to "torque to x pounds, plus 90 degrees". Why don't manufacturers just specify a slightly higher torque?

 

[Doozer]

Because they are stretching the bolt. I believe its called "torque to yield".

 

[Davi]

Torque can be affected by rust and crud on the threads. The initial torque get it to a roughally accurate place then the degrees take over and remove the uncertanty caused by the crud.
Initially developed by CAT I believe

 

[kartracer55]

Torque can be affected by rust and crud on the threads. The initial torque get it to a roughally accurate place then the degrees take over and remove the uncertanty caused by the crud.
Initially developed by CAT I believe

Not even rust and crud, it is just friction in general, even with new hardware.

As the bolt is tightened, the tensile load increases, and frictional load increases. In other words, as you continue to tighten the bolt, you need to overcome more and more "drag" from friction. As Davi said, the torque is a ball park starting point, and by controlling rotation instead of "torque" you can get a more accurate preload on the bolt.

 

[Stephenw]

Rust and crud should not be a factor, because the bolt and nut or threaded hole should be cleaned before assembly.

Use the lubricant, sealant, or thread locker recommended by the service manual.

The specified torque value is supposed to bring the fastener up to a point lower than the elastic limit. In other words, the bolt will not be stretched to a point where it will not return to its original length when the tension is released.

The final angle is a calculated value to produce a uniform clamp load. It is supposed to be more accurate than a torque value and will work well with either new or used fasteners. The angle may or may not bring the fastener beyond its elastic limit.

 

[jetrep]

If designers/engineers had they option they would provide a specification detailing a bolt stretch requirement. In practice it is very rare to have the option to measure the actual bolt stretch. Torque values are used more as a matter of convenience than anything else. It is a simple calculation to determine how much the bolt will stretch based on how much the fastener is rotated and is very repeatable. The initial torque value is provided to give you a starting point.

I have seen fancy bolts available that have a gadget in the head that is free to spin until the bolt has stretched a certain amount. They are offered as a precise way of confirming that proper clamping forces have been applied by the fastener.

 

[Mr.Magoo]

Just don't use this method on rods and main caps!!!

 

[braincrater]

Just don't use this method on rods and main caps!!!

Why? Some manufactures require torque to yield on mains and rod caps.

 

[mccaer01]

Why? Some manufactures require torque to yield on mains and rod caps.

I do believe sCATerpillar does

 

[Davi]

I do believe sCATerpillar does

my pug 306 needed to be done that way aswell

 

[Falcon67]

And remember that torque-to-yield fasteners are single use. When they come out, they go in the trash.

 

[Mr.Magoo]

Just don't use this method on rods and main caps!!!

Rephrase: Just don't use this method on rods and main caps unless instructed to.
Sometimes people read articles or see a subject like this and then utilize the practice on everything. So unless it is called for never use this method or approach. Secondly, in many of these applications the method is only for a one time use as once the bolt is stretched it will not perform adequately again on another torque - yield. Since we have got this conversation to engines, you will note that most engine manufactures call out for new bolts in areas like mains and head bolts when a tear down or rebuild takes place. This because of the stretch, torque,twist, flex, expansion or stress that the part has been through since it was applied and utilized. My initial comment was for the guy who utilizes this method when not called for and squashes his bearings do to the 90 degrees and over torquing from this method.

 

[JimVonBaden]

And remember that torque-to-yield fasteners are single use. When they come out, they go in the trash.

Not always true. On the BMW R-series motorcycles the head studs are about 8" long. The process is to torque each one to 20nm, then turn the nut 180° to stretch the stud. This process can be repeated as many times as needed over the course of the life of the bike.

Jim

 

[Davi]

And remember that torque-to-yield fasteners are single use. When they come out, they go in the trash.

Nah they get ground down to punches

 

[Rickenbackerman]

Not always true. On the BMW R-series motorcycles the head studs are about 8" long. The process is to torque each one to 20nm, then turn the nut 180° to stretch the stud. This process can be repeated as many times as needed over the course of the life of the bike.

Jim

That may be stretching, but not yielding. TTY fasteners are one-time use as Falcon67 said.

 

[JimVonBaden]

That may be stretching, but not yielding. TTY fasteners are one-time use as Falcon67 said.

What is the difference? Trying to learn, not argue.

Jim

 

[Lotek]

They are mixing torque to yield fasteners in with the procedure of torque/angle. Apples and miles per hour. tty bolts are almost always tightened using a torque/angle procedure, and are single use(check the mfg spec) but regular reusable bolts(like head, main bolts) can be tightened using the same procedure...if called for by the mfg. Clear?

 

[JimVonBaden]

They are mixing torque to yield fasteners in with the procedure of torque/angle. Apples and miles per hour. tty bolts are almost always tightened using a torque/angle procedure, and are single use(check the mfg spec) but regular reusable bolts(like head, main bolts) can be tightened using the same procedure...if called for by the mfg. Clear?

What is the advantage to using throw-away bolts?

Jim

 

[MBeaty]

What is the difference? Trying to learn, not argue.

Materials like steel can flex in an elastic manner. This means that they will stretch, but the yield strength of the material has not been exceed. While stretching in the elastic region, when the load is taken off, the material will return to its original size, hence the fastener could be reused, because physically nothing has changed.

If it is put into strain with a load that exceeded the materials yield strength, the materials shape will deform. Even once the load is taken off it will not return to its original size and shape. This is called yielding. The fastener could not be reused, because the stress and strain properties have been changed because it has yielded.

In this image, the region with the straight line in the upward direction (on the left side) is the elastic region and anything above the point that marks the "yield stress" will have exceed the yield strength of the material.

 

[bgott]

My personal theory is that the manufacturers have gone to TTY bolts because it is easier to design a robot to read angle sensors than it is to keep torque sensors calibrated.

 

[Mr.Magoo]

Not always true. On the BMW R-series motorcycles the head studs are about 8" long. The process is to torque each one to 20nm, then turn the nut 180° to stretch the stud. This process can be repeated as many times as needed over the course of the life of the bike.

Jim

True it is, you are speaking of head studs vs head bolts, generally head bolts get tossed and not re used.

 

[PrecisionTools]

Do torque to yield bolts look any different to conventional bolts? Any special markings or anything?

 

[Falcon67]

Good info in here. I don't know if TTY bolts carry special marks - I never use them on my builds. Doing a quick search shows that they may not carry any special marks but will have a different shank near the head than a stud or regular bolt. True, we muddied the water between TTY and a method of torquing bolts. I use ARP items in my engine builds along with their lube and torque specs. I use a digital torque gage between the wrench and bolt. On rod bolts, I use a stretch gage. Using the mfgs recommended lube and torque specs is about as accurate as you can get in DIY land. If you do a lot of critical work, get a good wrench and have it calibrated.

I imagine the factories use TTY bolts because it makes for quicker assembly with less chance of error.

 

[PrecisionTools]

A good article here on TTY bolts & other stuff...http://www.rv8.ch/article.php/20051227112926716

 

[Doktor Schnell]

I asked this because of a transmission mount bolt that I had to remove and retighten recently.
Could the 50 ft/lbs+90 degrees have to do with the fact that the bolt was compressing the rubber mount?

 

[JimVonBaden]

I asked this because of a transmission mount bolt that I had to remove and retighten recently.
Could the 50 ft/lbs+90 degrees have to do with the fact that the bolt was compressing the rubber mount?

Is it a stud and nut, or a bolt. Seems the bolts are more often single use, and the stud/nut combination reusable.

Jim

 

[Doktor Schnell]

It is bolt. I installed a mount insert. There was no mention of replacing a stretch bolt, if that's really what it is.
Still trying to get my head around why 50lbs+90 degrees is really different than torqueing to say 80 lbs. Both ways seem to do the same thing.

 

[SGKent]

It is bolt. I installed a mount insert. There was no mention of replacing a stretch bolt, if that's really what it is.
Still trying to get my head around why 50lbs+90 degrees is really different than torqueing to say 80 lbs. Both ways seem to do the same thing.

Perhaps however when we get blueprints from the engineer and it shows 2 x size nails at a specific location, the inspector looks to see if I followed the engineer's drawing. If the engineer at the factory specs it to 50 pounds + 90 degrees then that is the best way to do it. They may have tried different methods and found that was most reliable. Likewise, they may spec a new bolt each time it is removed even though the old one might work. For example - I always replace rod nuts. I use the old ones to torque for the plasti-gauge. Rarely do I replace rod bolts unless the power that will be generated is well beyond the original specs. Another mechanic might tell you - reuse the old nuts. The arguments can go on for a very long time. Unless there is a mitigating reason we usually follow the factory's recomendation.

 

[MBeaty]

Still trying to get my head around why 50lbs+90 degrees is really different than torquing to say 80 lbs. Both ways seem to do the same thing.

Think about tightening down a clean bolt into a clean threaded object. While the bolt is being turned in, there is very little friction while it is just thread on thread contact. As the bolt head comes in contact with the surface below it, the friction increases greatly and the force required to turn it also increases. Much of the increase in torque to turn it is simply to overcome the friction rather than to "stretch" the bolt.

With the torque + angle method this problem is diminished. The torque spec required is much lower than what the final torque will be after the angle turn. This allows the torque setting to be reached before friction has a chance to throw it off. Because the threads of the bolt are a fixed pitch, the angle turn will always put the same amount of preload on the bolt regardless of what the torque was.

 

[Davi]

Thought this may be relevant and of use to some
http://www.dragzine.com/tech-stories/engine/the-role-of-fasteners-in-critical-engine-measurements/

 

[garfunkle24]

Still trying to get my head around why 50lbs+90 degrees is really different than torqueing to say 80 lbs. Both ways seem to do the same thing.

Look at this way:

Say you install some bolts of equal size. The manufacturer has figured out that the point at which the head of the bolt stops turning is around 50/ft lbs. Hence the first step of this torque + angle procedure is to tighten these bolts to 50/lbs.

You now go ahead and tighten the bolts another half a turn each. Given that the heads had stopped turning and the linear pitch of the bolt, you know they will have stretched the same amount.

If you were to check the torque of those bolts you would find they differed, even though the bolts had stretched the same amount. Of course this means the inverse is true: if the tested torque was the same, the stretch would be different.

So to conclude: You are really trying to set the stretch or "preload" of the bolt rather than the torque value, which is just a simple way we attempt to infer a calculated preload.

 

[Doktor Schnell]

The manufacturer has figured out that the point at which the head of the bolt stops turning is around 50/ft lbs.

Ok this is beginning to make sense. I'm trying to understand this part^ here. Does this mean that at 50 lbs, the bolt is at the threshold of stretching? In other works, all slack has been taken up in the threads and the bolt head is just beginning to press on the mating surface?
This is interesting stuff...thanks for the replies, guys.

 

[Rickenbackerman]

You've first got to understand the difference between "stretching" and "yielding".

All bolts stretch when you torque them. Imagine a 1/4-20 at 20 ft-lbs and another at 2 ft-lbs. Both are stretched, just one more than the other. When you loosed them they go back to their original size (assuming you haven't yielded them).

When a bolt is yielded, you stretch it past its elastic limit, or yield strength. When you remove it, it's now longer than it was before you installed it. It goes in the trash.