Fasteners may be small components, but how they’re tightened can greatly affect the safety and performance of an assembled product. In some applications, simply tightening a bolt to a set torque isn’t enough. Instead, the fastener is deliberately tightened to, and in some cases beyond, its yield point. This process is known as torque to yield.
In simple terms, torque to yield means tightening a fastener to a controlled level of elongation, taking it beyond its elastic limit so that it undergoes some permanent deformation. Engineers use this approach when a joint needs a precise preload and a high, consistent clamping force.
So, what actually happens to the bolt? And why would you intentionally stretch a fastener beyond the point where it can return to its original length? Let’s take a closer look.
Torque to yield explained
Think about what happens to a conventional bolt during tightening. As you turn it, the bolt stretches by a very small amount. While it remains within its elastic range, that stretch isn’t permanent. Release the load and the bolt can return to approximately its original length.
A torque-to-yield bolt goes further. Tightening continues to the fastener’s yield point and, depending on the specified procedure, into the plastic region. At this stage, the bolt becomes permanently elongated.
That might sound like something you’d normally want to avoid. In this case, though, it’s deliberate. The controlled elongation allows the fastener to develop the preload needed to keep the joint securely clamped and can help reduce variation in the resulting clamp load.
How does torque to yield work?
You won’t normally tighten a torque-to-yield (TTY) bolt by applying one torque value and stopping there. The process commonly uses an initial torque followed by one or more specified angular rotations.
The initial tightening seats the joint components and provides a known starting point. From there, the bolt is turned through a set number of degrees to achieve the required elongation and preload.
A typical procedure could look like this:
- Tighten the fastener to the specified initial torque
- Turn it through a defined number of degrees
- Apply another angular rotation if the procedure calls for one
- Check that the tightening operation has met the required parameters
There’s no single torque or angle that works for every application. Bolt design, joint materials, friction, target preload and other engineering requirements all play a part, which is why you should always follow the specified tightening procedure.
What do preload and clamp load have to do with it?
As you tighten and stretch a bolt, tensile force builds inside the fastener. That force is bolt preload. The bolt then pulls the components of the joint together, producing a compressive force known as clamp load. The two forces are closely related, but they aren’t the same thing. Preload is the tensile force within the bolt, while clamp load is the compressive force acting on the components being held together.
Applied torque is only part of the picture. Friction in the threads and beneath the bolt head or nut means not all the torque you apply goes into stretching the bolt. That’s one reason engineers need to look beyond a torque value alone when joint performance is critical.
Why use torque-to-yield bolts?
If permanently stretching a bolt means it may need replacing later, why use this approach at all? For the right joint, there are several benefits:
- Effective clamping: Taking the bolt to a controlled level of elongation helps generate the preload the joint requires.
- Less preload variation: The torque-plus-angle approach can reduce some of the variation caused by friction compared with relying on torque alone.
- Efficient use of the fastener: Engineers can make effective use of the bolt’s material properties without necessarily having to specify a larger fastener.
- Good sealing performance: A well-controlled clamping force helps gaskets and seals maintain the required contact pressure.
Installation needs to be closely managed and, because a torque-to-yield fastener may have permanently stretched, replacement will generally be necessary if it’s removed.
Where are torque-to-yield bolts used?
Cylinder head bolts in modern engines need to keep the cylinder head, gasket and engine block securely clamped together despite changing temperatures and loads. But this isn’t purely an automotive technique. The same approach can be useful in other engineered assemblies where maintaining the intended bolt tension is important.
It won’t suit every joint. Engineers need to consider the fastener material, joint design, expected loads and working conditions before deciding whether torque to yield is appropriate.
Is torque to yield the same as torque plus angle?
In short, no. Although the terms are often used together, they describe different things.
Torque plus angle is a tightening method. You first tighten the fastener to a specified torque, then turn it through a defined angle.
Torque to yield refers to the tightening strategy and the resulting condition of the fastener. The bolt is intentionally taken to, or beyond, its yield point so that some of its elongation becomes permanent.
The two are closely linked because torque plus angle is often used to tighten torque-to-yield bolts. Measuring the additional rotation gives engineers a way to control bolt elongation and preload after applying the initial torque.
There’s an important distinction, though. A torque-plus-angle specification doesn’t necessarily mean the bolt is being taken past its yield point. An angular tightening procedure can be specified for other reasons, including achieving more consistent preload.
Can you reuse torque-to-yield bolts?
Generally, no. Once the bolt has moved beyond its elastic range, it has permanently changed. Reusing it may make it difficult to reproduce the intended preload and could put the fastener or joint at risk.
You can’t necessarily identify one by sight either. A torque-to-yield bolt may look much like another fastener. The safest approach is to check the assembly instructions and manufacturer’s specification, which should tell you whether the bolt needs replacing.
Measure torque accurately with Crane
Crane provides torque measurement solutions for manufacturers across a wide range of industries, helping teams measure, monitor and record fastening performance.
With better visibility of the tightening process, you can identify variation, verify performance and make informed decisions about your fastening operations.
Contact Crane to discuss the right torque measurement solution for your application.