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Threaded Fasteners Torque-to-Yield and Torque-to-Angle
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In the mid 1980s, we started to see a move in engine fasteners to a new process called torque-to-yield (TTY). Head bolts were the first fasteners affected, although the technology has trickled down to other critical fasteners. The theory holds that the farther we stretch a fastener toward the threshold of yield, the more load it exerts on the joint.
Now you might say, "If we want more load, we can always use a bigger diameter fastener." That’s correct. Let’s use our (hypothetical) gasket example from Victor Reinz. We need 11,900 lbs. of load on each bolt. We can get that load by stretching a 7/16" diameter bolt to the threshold of yield or by putting a very moderate load (requiring very little stretch) on a 9/16" diameter bolt. The concern is on a head bolt application is that you get lots of change in the joint. Both gasket relaxation on a new installation, as well as thermal expansion on bi-metal designs will cause changes to the joint dimension once the installation is complete. Head gasket relaxation causes loss of load from the fastener. The less stretch you have on the fastener, the more the loss of load. Let’s work our theoretical example:
- 7/16" fastener stretched .070" equals 11,900 lbs. of load;
- 9/16" fastener stretched .030" equals 11,900 lbs. of load;
- A composition gasket installed at .045" relaxes 25%, for a net loss of .011";
- 7/16" fastener loses 1/7 of the load, leaving 10,200 lbs.; and
- 9/16" fastener loses 1/3 of the load, leaving 7,933 lbs.
As you can see, we’ve got a major sealing issue with the 9/16" fastener. Obviously, it’s a big advantage to keep the fastener diameter small and use maximum stretch to seal engines. Also, keep in mind that the longer in length the fastener is, the more it stretches to get the desired load. Just look at modern engine designs today. We have a predominance of long yet relatively small diameter head bolts. You’ll also notice that on the good designs all the bolts are the same length. This makes only one engineering exercise to do rather than two or three as a tightening theory is developed.
Now, let’s look at the other side of this equation. Our head bolt will be pulled or stretched further than the installation dimension because of the thermal expansion rate of an aluminum head versus a steel bolt (Illustration 3). This can be an issue, especially with a fastener installed at the threshold of yield and a gasket that doesn’t relax (Multi-Layer Steel). On a typical cylinder head operating at 250° F, the head bolt will stretch another .005" or so as the engine reaches operating temperature.
This will often result in the fastener being moved significantly farther into the post yield zone. Repeated movement of the fastener into the post yield zone can ultimately lead to work hardening of the fastener and sudden failure (Illustration 4). You may remember the 2.5L GM engine with a head bolt near the exhaust manifold that broke during service. This was a prime example of this problem.
Torque Turn to Tighten
One thing that should be obvious by now is that if we’re going to tighten fasteners to the threshold of yield, we need a better method than measuring resistance to turn. Friction variances could easily get us into trouble.
Fortunately there is a method of tightening a fastener that is much more accurate than measuring resistance to turn. It’s called Torque Turn to Tighten (TTT), often referred to as angle turn. With this method, you use a relative low torque to run down and align the fastener (Illustration 5), then rely solely on a measured turn to tighten the fastener to the desired level. What we’ve done has not affected the friction in our fastener, it has taken it out of the equation when it comes to tightening.
For instance, 90 degrees of turn is 90 degrees of turn; old bolt, new bolt, rough threads, new threads, it doesn’t matter. The amount of stretch will be extremely uniform from bolt-to-bolt across the joint. Load scatter is kept to a minimum.
TTT is a far superior method of tightening critical fasteners regardless of whether you tighten them to yield or not. Fastener engineers use sophisticated mathematical models to calculate the amount of turn needed to get a desired load, but what has really fueled the rapid growth in this area is sophisticated electronic equipment. Sensitive electronic load sensing cells coupled with angle encoders using advanced software programs have allowed engineers to test their theories watching run down curves in real time as they tighten fasteners (Illustration 6).
Some articles I’ve read indicate that TTY fasteners are somehow "special", metallurgically speaking. If you’re comparing them to the garden variety bolt from your local hardware store, then, yes, they are. If you’re comparing them to other critical fasteners in an engine, then, no, they are not. They’re high-grade fasteners, typically grade 8 for English and class 10.9 for metric applications (Illustration 7).
One bit of confusion is that there are true TTY fasteners (Illustration 8), designed with a reduced shank area (Cummins rod bolts and Porsche rod bolts, for example), and there are standard high-grade fasteners tightened to yield. Both styles are tightened to the threshold of yield; the reduced shank style directs the elongation to the shank, where the others elongate in the threaded area. The second style is much more common in most automotive engines.
A final subject is the relative merits to re-using critical fasteners. If I had a dollar for every head bolt I’ve wire brushed and reused I could afford a pretty nice vacation next year. There are very few of us in this industry that haven’t reused critical fasteners!
However, times change, engines change, technology changes, I’ve changed. My policy is that if new critical fasteners especially head bolts are readily available, old ones are replaced. Understanding much more about fasteners and engine operating conditions today, I’m reluctant to reuse them.
A well-respected OE engineer specializing in engines tells me that critical fasteners have about six rundowns in their useful life. They use four of those at the OE manufacturing operations, leaving rebuilders just two. One rundown for checking sizes puts us on the last rundown during final assembly. My thinking is: why take the chance? Replace the fasteners! The relative cost compared to the total engine job is small and the peace of mind is high.
I’d like to thank Ralph Shoberg, President of RS Technology, Ltd. (www.rstechltd.com), and Otto Kossuth of General Fasteners, Inc. These two men gave me a fastener education and a pretty good layman’s view of a complicated subject.
Bill McKnight is Director of Training for MAHLE Clevite.
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