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Threaded Fasteners Torque-to-Yield and Torque-to-Angle
Understand torque-to-yield (TTY) fasteners, you need a good fundamental understanding of threaded fasteners in general. The threaded fastener topic is a huge one.
By Bill McKnight
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All the original equipment manufacturers (OEMs) have fastener labs with lots of sophisticated equipment and well-educated people working on fastening issues. They even have their own professional association The Bolting Technology Council which holds meetings and seminars about fasteners.
I’m not a fastener engineer, and I’m not going to make you into one. I’ll keep this article thorough but fairly basic, giving you a good solid working knowledge of the business of bolted joints.
Fasteners function in an engine to hold parts together. For example, a rod bolt and nut hold the rod and cap together. Fasteners are also, in the case of head gaskets, used to load the gasket with the necessary force to seal the gasket under the forces of combustion as well as thermal expansion and contraction. Understanding some of the physics of fasteners and fastener tightening is necessary for an engine rebuilder who wants to keep fastener failures and engine failures to an absolute minimum.
Threaded fasteners in an engine can be divided into two general categories: critical and non-critical. Rod bolts, main bolts and head bolts are examples of critical fasteners. Critical fasteners can be identified because the repair procedure for the engine details exact tightening information. Pan bolts, timing cover bolts and valve cover bolts are examples of non-critical fasteners having no detailed fastening procedures. Here, we’ll focus on critical fasteners.
Bolts are elastic. When you tighten a critical bolt to specs, you’re actually stretching the bolt. As you stretch the bolt, it wants to return to its original length. Based on the quality of steel used in the fastener, the diameter of the fastener and how far you stretch it, the load or force applied to the joint (the two pieces being fastened together) changes.
Think about this for a minute. If you don’t stretch the rod bolts on the next engine you build, what would keep the rod nuts from vibrating loose and falling off as the engine runs? Yes, most of us have experienced just this sort of problem at some time in our lives!
Bolt load applied to the joint by the fasteners seals a head gasket through head lift-off during firing and changes in temperature that occur as an engine runs. To show you how important this is, I’m going to show you a sample calculation our Victor Reinz engineers used to calculate bolt load needed on an engine:
- General approximation (GA) for clamp load to seal a gasket is three times the lift-off force.
- Lift-off force for a 4.250" bore race motor with 1,400 psi firing pressure is 19,861 lbs.
- GA is 19,861 x 3 or 59,583 lbs. per cylinder. With a 5-bolt pattern, 11,917 lbs. of force is needed per bolt.
- With a 6-bolt pattern, 9,930 lbs. of force is needed per bolt.
This then becomes the initial load needed from each head bolt in order to seal the gasket. Specifying the diameter of the bolts and their tensile strength, the engineer calculates a tightening procedure that will provide the desired load to the gasket. Obviously, I’m leaving some factors out of this basic model. Hardware, cylinder head stiffness and gasket relaxation factors would also be considered and factored into the calculations. But, hopefully, you get the idea.
This is probably a good time to bring up finite elasticity in fasteners. Unfortunately, every fastener has an elastic limit, commonly referred to as its yield point, or more properly, "the threshold of yield." Up to this point, if the load on a fastener is released, the fastener will spring back to its original length. When a fastener is stretched into the yield zone, some of the elasticity is permanently lost, and the fastener will remain somewhat elongated when the load is removed. The further we stretch the fastener into the yield zone, the more elongation we get.
Many of us have observed severe elongation in fasteners as a "necking down." This occurs in the threaded area (the root diameter of a fastener is smallest in the threaded area), usually about one thread above where the fastener is engaged in the threads of the nut or the block (the threads of the nut or engine block support the fastener resisting yield). As most of you have experienced, if you stretch a fastener far enough into the yield zone, it will actually pull into two pieces.
Occasionally in automotive engine applications, the threads in the block or nut will yield before the fastener does, especially where a large number of rundowns (tightenings) have occurred. However, most of the time the bolt yields first. As you can see from the graph in Illustration 1 maximum clamp load from a fastener comes at the threshold of yield or shortly thereafter. Once a fastener is stretched farther into the yield zone, very little additional clamp load is generated and the risk of ultimate failure becomes greater. Consequently, we’d like to have some means of tightening fasteners to get the elasticity we need for load without yielding them.
Tightening critical fasteners introduces numerous additional factors into our discussion. Traditional methods have all used some means of measuring the resistance needed to turn the fastener. We’ve all used the most basic of those: "seat of the pants," "experience" or whatever you want to call it. The farther we tightened the fastener, the harder it turned, and experience (some bolts loosening and coming apart and breaking a few bolts off) taught us when to stop. Not real scientific, not very repeatable and probably not too reliable!
Torque wrenches improved this procedure immensely. We use scientific terms like Newton.meters or ft.lbs., to gain repeatability and improve reliability. We continue to rely on torque wrenches today to tighten many critical fasteners. The one thing we need to keep in mind is that we’re measuring resistance to turn.
Friction on bolted joints is the biggest factor causing resistance to turn (Illustration 2). In automotive engines, about 90% of the effort required to tighten a critical fastener is used to overcome friction. Ten percent actually stretches the fastener. This is a fairly standard number for the rigid joints we have in automotive engines. For example, a new fastener lubed both under the head and on the threads may exhibit the 90/10 relationship, while a used fastener or one with damaged threads will be 92/8.
Think about this. The more effort needed to overcome friction, the less stretch we get on the fastener and the less load on the joint. What will happen on a joint with multiple fasteners (like a cylinder head) is load scatter (variances in load from bolt-to-bolt) because of minor deviations from the 90/10 relationship. This load scatter causes uneven loads on head gaskets and may also have a negative affect on bore distortion. What we’d like to do as engine rebuilders is minimize the variances from bolt to bolt as we use conventional "resistance to turn" to tighten fasteners.
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