Other Articles - April - 2017

Everything You Need to Know About Torque

Sometimes it’s hard to imagine just how much technology goes into even the smallest parts in today’s vehicles. Take fasteners: Bolts, nuts, and studs are things we seldom think about, but they can cause more problems than you may realize. You see terms such as torque, torque angle, and torque to yield, but few really understand what those terms mean.

How many times have you seen a broken bolt (figure 1), and wondered why? How many times have you seen components warp or won’t seal? How many times have you heard someone say, “These new parts are junk. We never had problems like this before, but now I can’t keep this car out of my shop?”

Turns out, there’s more to controlling leaks than just seals or gaskets. Fasteners provide preload while attaching two or more parts together. Think of bolts, studs, and nuts as springs. The bolt provides a constant surface pressure, or clamp load, to make sure the gasket seals the surfaces properly.

As you tighten bolts, they actually stretch, much like a spring. This stretching is what actually maintains the surface clamp load and keeps the bolt tight. Any force that lets the bolt lose its stretch will cause gasket or seal failure, due to a loss of clamp load.

Years ago, you simply tightened a bolt to a specific torque and your work was done. In the 1980s, engineers in Australia discovered that this might not be the best approach. Fastener engineers really started looking at bolts, nuts, and studs because of gasket failures that were occurring and to help address — believe it or not — vehicle fuel economy standards.

They discovered several interesting things: We already know that the gasket needs a certain amount of force on its surface to seal. You’re also probably aware that we’re trying to seal a surface that isn’t consistent. Heat, pressure, and vibration can all affect these surfaces. Manufacturers consider these variables when they design gaskets and fasteners.

An example of this is the typical head bolt and gasket. Victor Reinz gasket company shared how they calculate typical head bolt clamp loads. General clamp load (GA) is calculated at three times the lift off force for the surface they’re trying to seal. A typical 4.25” race engine cylinder bore develops about 1400 PSI of cylinder pressure, which is equal to 19,861 pounds of force.

So the general clamp load is 19,861 x 3, or 59,583 pounds per cylinder. With a cylinder head that has five bolts surrounding that cylinder, each bolt needs to supply 11,917 pounds of force to maintain the seal.

The engine design engineers then use this data to determine the size, tensile strength, torque, and tightening process. They apply the same type of calculations to the sealing surfaces in an automatic transmission valve body and pump.

TORQUE

Torque is simply a twisting or turning force that’s applied to a component, in this case a bolt. Let’s say a bolt calls for 100 pound-feet of torque. We generally think of that 100 pound-feet as being applied directly to the gasket and sealing surface; it isn’t. In fact, it isn’t even close.

When torquing a bolt, the amount of torque lost to friction can be as great as 90%. This means that the bolt is absorbing a lot of the torque or clamp load before it gets to the gasket surface. 45-55% of the loss is due to friction between the head of the bolt and the bolt surface; 35-45% of the loss is due to friction loss between the threads, leaving only 10% to apply the clamp load on the gasket and surface from bolt stretch (figure 2).

Think of it this way: The more torque required to overcome friction, the less torque you have available to stretch the bolt and provide clamp load on the surface. So the effects of dirty or damaged threads on clamp load can be overwhelming.

Don’t forget the washers that are under some bolt heads. The correct washer hardness and contour are critical to establishing and maintaining the correct torque.

Finally, there’s the gasket itself. The gasket can’t relax too much with use or, even with a properly torqued fastener, the gasket will fail because of lost bolt stretch and clamp load.

Due to friction variations, there can be as much as a 35% variation in the torque to the bolts clamping a component, even though all bolts are torqued to the same value. This means a lot of sealing issues have nothing to do with the gasket, but rather the bolts themselves. This is when the technology started to change and you started to hear terms such as torque angle and torque-to-yield.

When a bolt is torqued, it typically operates in one of four phases: elastic phase, plastic phase, yield point, and shear point (figure 3).

Elastic Phase — In this case, the bolt will stretch while under torque, but returns to its original condition as soon as the force releases. We’ve all looked at a torque spec for a plain old bolt and wondered, “Where did they come up with that specification?” Well, the amount of torque applied to the bolt is determined by the bolt’s diameter, material (grade 5, grade 8, etc.), thread pitch, and the desired clamp load.

Generally, torque specs are established for that plain old bolt at 75-80% of the bolt’s yield point. In other words, you’re trying to stretch the bolt enough so that it’s under tension, but not enough to permanently distort or break it.

Plastic Phase — Once a typical bolt reaches the yield point and you continue to apply torque, it’ll enter the plastic phase. We’ve all experienced this when, all of a sudden, it feels like the bolt is stretching: the “oh-no” moment. If the bolt reaches the plastic phase, it won’t return to its original length and condition after releasing the torque. Upon examination, the bolt’s shank or threads may be smaller than it was originally.

Yield Point — The point that separates the elastic phase from the plastic phase is known as the yield point. This is just before the bolt starts to distort to the point of no return.

Shear Point — Basically, the material can no longer stretch and the bolt breaks. This occurs when you exceed the plastic phase range.

TORQUE ANGLE (TA)

Torque angle was developed as a method of limiting the effects of friction loss on the bolt torque and, ultimately, on the clamp load. By using bolt rotational angle as a method to determine clamp load, friction loss isn’t as much of an issue and clamp load is applied more evenly across the clamping surface.

Torque angle consists of tightening the bolt to a specified torque, then continuing to rotate the bolt a specific number of degrees. Measuring the angle requires an angle meter (figure 4). Some meters are mechanical, while most manufacturers use electronic measuring tools to check the angle.

Torque angle has helped vehicle engineers address uneven clamp load (even though all the bolts were torqued to the same value) while improving fuel economy. It also enabled them to reduce production cost, because it allowed them to redesign torque patterns and eliminate bolts.

TORQUE-TO-YIELD (TAY)

Engineers discovered that, with the right materials, they could reduce the bolt size if they used torque-to yield technology. This method was developed by SPS and is also known as the joint control method.

A standard M16 bolt torqued to 75-80% of its yield is equal in clamp load to an M12 bolt torqued to a specific value, followed by a specific number of degrees rotation. With torque-to-yield technology, the process for tightening the bolt is usually the same as the torque-angle process.

The primary difference is the fastener: Because you’re using a smaller bolt, the amount of bolt stretch increases dramatically as compared to the standard torquing process. The tension created by stretching the bolt controls the clamp load. Typically, the longer the fastener, the more it stretches to achieve the desired clamp load.

As changes in temperature or pressure occur, the clamp load never drops below predesign values. The gasket continues to seal the surface, so the bolt doesn’t need retorquing.

Example of the effect of stretch on clamping force based on bolt size:

  • 7/16″ bolt stretched 0.070″ = 11,900 pounds of clamping force
  • 9/16″ bolt stretched 0.030″ = 11,900 pounds of clamping force

Unlike conventional bolts, you tighten a torque-to-yield bolt past its yield point into the plastic phase. This means, in the vast majority of cases, the bolts require replacement once they’ve been torqued.

We’re even seeing this technology in simple things, such as input and output speed sensor bolts on today’s transmissions. Check the repair information: It says don’t reuse the bolts in bold print.

Reusing a torque-to-yield bolt will likely lead to bolt failure at some point. The failure may not occur while you’re torquing the bolt; it’s more likely to occur as the bolt is heat-cycled during use.

These methods are standard across many industries. I just finished putting tracks on an excavator and, guess what: the pad bolts were torqued to 85 pound-feet plus 120º. Go figure!

So far, we’ve discussed how fasteners control lifting force. The shear point is also something engineers take into consideration when designing a clamping system.

Let’s say we had three pieces of metal that were clamped together with a bolt (figure 5). If those parts were subject to forces that were trying to slide the pieces apart, we’d need to take the shear point of the bolt into account, so torque isn’t the only consideration.

If the pieces of metal are exposed to forces that are trying to slide them apart, it exposes the bolt to shear forces rather than simply trying to stretch the bolt, as in a typical clamping situation.

The force that’s applied to the right axis of the fastener tests not only the ability of the clamping torque to keep the parts together, but also the ability of the bolt to prevent sliding between the clamped parts.

Let’s look at two bolts of the same grade and size to provide a clearer picture of this situation (figure 6): Both bolts are 1/2″ course thread (SAE 13 threads per inch) grade 8 fasteners. The only difference is that one bolt is fully threaded, while the other bolt has an unthreaded shank.

In our example, the threaded portion of the bolt is exposed to the shear area of the plates being bolted together, while the other example the unthreaded shank is exposed to the plate shear area. While there’s typically no specification for shear strength regarding fasteners, hardness (Rockwell/Brinell) and ductility (SHCS) are considered in the calculating which fastener the engineers use.

With this in mind, it should be clear that you need to understand the basics when replacing a fastener. Simply grabbing a bolt that fits the hole and has the correct thread type and pitch, and rotating it until it appears to be tight, may not do the job you expect.

Fastener failure can damage your work without any clues as to why you’re having problems. So the next time you see a gasket or fastener failure, take the time to look a little deeper: You may find the solution is simpler than you think.