We’ll tighten up the bolted connection topic to help you better understand how mating fasteners work together in order to achieve a reliable connection.
You recall that there are three basic types of bolted connections: snug tight, pre-tensioned, and slip critical. The latter two of these connections require a designated pre-tension (preload) value for the bolt such that the internal stresses are just below the material yield point. Achieving correct fastener preload allows for the proper clamping force in the connection without loosening – which is necessary because tensioning either too little or too much will almost always cause some type of connection failure. Snug tight connections, though designed connections in their own right, don’t require consideration of preload as a function of capacity; all other connections do.
A convenient example for us to use is the connection between truss modules in a span of aluminum truss. Generally speaking, there are two types of connection methods used in entertainment truss systems today: pinned and bolted. Bolted truss (also referred to as plated truss) accomplishes the connection by bolting through the truss end plates, placing the connection in combinations of shear and either tension or compression, depending upon the connection location and the applied loads. The tension component of these connections is what we’ll examine in more detail. First, let’s understand the connection as an assembly.
What Happens in the Fastener Assembly?
When you first place the nut onto the bolt’s threads, before any contact with the bearing surfaces is achieved, the mating threads are somewhat loose in relation to each other. In most cases the thread pitch is identical, with a small gap between the external bolt threads and the internal nut threads. As additional torque is applied, the nut and bolt bearing surfaces come into contact and the mating threads also come into contact along the thread’s angular bearing surfaces. These forces act together within the assembly to achieve clamping because angular thread contact works against compression at the nut face to induce tension to the bolt. This tension causes the bolt to elongate. As more torque is applied, the more tension force is induced and the more the bolt stretches. With this type of bolted connection, our goal is to achieve the clamping force necessary for the connection to stay intact under its specific application conditions.
Clamping Force and Preload
If you truly need a designed connection where clamping force is critical, then you are obliged to consider fastener preload, not simply applied torque. Preload is related to the amount of tension in the connection, which translates directly to clamping force. You cannot have clamping force without inducing tension into the bolt. A correctly designed connection includes a preload value resulting in bolt stretch sufficient to resist all external forces in the connection, without failing the connection. Remember from our previous article that this also means preload can’t exceed the fastener proof load, which is the maximum amount of load the fastener can withstand without yielding (permanently deforming). For example, the proof load value for a 5/8 in -11 UNC Grade 8 bolt is 19,200 lbs. Under ideal conditions, the torque value required to achieve an approximate preload at 75% of proof is 212 ft-lbs. Does a stagehand carry a 1/2 in drive 24 in long calibrated torque wrench in their standard toolkit? How many ways can a stagehand tighten a bolt? There’s food for thought…
In the case of a moveable loft block we’re faced with less than ideal conditions where seemingly innocuous things like rust, dirt and bearing surface anomalies add friction to the process, such that more of the applied torque is wasted overcoming this friction, rather than properly translating to bolt tension.
The real issue here, especially with reuse of fasteners, is that we tend to rely too much on torque value, thinking that this translates into some magic preload value. It does not! There is a critically important difference between bolt torque as it relates to a tightening guideline and torque as it relates to preload. If you truly need a connection requiring minimum clamping force, be prepared to tackle a veritable shopping list of considerations, mostly related to identifying the sum of external forces the connection needs to resist and how to go about achieving the necessary fastener preload to accomplish that goal. Force transfer isn’t as simple as torque and tension.
Let’s examine in more detail what happens with thread engagement during the tightening process.
Thread Contact and Deformation
When both sets of mating threads begin to contact and the nut face tightens against the bolted material, the angular thread surfaces bear against each other in shear and the helical action occurring as a result of applied torque causes compression of the nut against these bearing surfaces, which also increases tension in the bolt. So what happens in the threads themselves? As torque is applied to the nut, the first set of mating threads engages. As the forces increase, shear transfer into the next set of threads occurs, and so on until all of the mating threads are engaged. Think carefully about how the material of the threads’ mating surfaces needs to behave for this to happen as we get a step closer to answers. Don’t worry; we won’t leave you hanging by a thread!
It should be obvious that when the first set of mating threads makes contact it closes the gap between the remaining mated threads. However, let’s not forget that this contact also induces tension – and stretch – in the bolt. At first, the forces in the thread contact surfaces are localized at the first thread. If the bolt did not stretch, or if the bolt stretched uniformly along its length beyond the engaged threads, then this force would theoretically be shared among all thread contact surfaces at once.
Of course, this is not the case because the bolt does not stretch along the unengaged threads; it stretches along its length between the first engaged thread and the bolt head bearing surface. If the nut and the bolt were of identical material properties and material hardness, the first thread would remain in contact and as additional torque is applied and more subsequent stretch in the bolt occurs, the shear forces would remain concentrated at the first thread until it eventually failed in shear, because a single thread cannot withstand this shear alone. In order for the threads to properly share this force, the threads must deform ever so slightly as to engage each sequential thread in the helix. As such, for this to occur, the nut material must be softer than the bolt material, and in fact it is. For nuts and bolts of the same grade and material, the nuts are, by design, slightly softer than the bolt material, for exactly this reason.
What Really Happens?
As torque is applied to the nut and its face compresses against the bolted material, the first set of threads engage. Increasing torque increases bolt tension, so clamping force also increases. The bolt stretches, but during this period pretension is not yet achieved so the connection remains loose. As increased torque continues to compress the nut, its threads also continue to deform so that shear transfer can distribute along subsequent threads of the connection. Once this increases to the point where correct preload is achieved, the nut threads have permanently deformed – by design – while the bolt remains in its elastic state. When the nut is removed, it cannot be reliably reused in this connection.
Wash, Rinse, Repeat
Connecting full-circle back to our truss example: what happens in repetitive-use production trusses where the norm is to bolt, unbolt and repeat as necessary? There’s no single, easy answer for this situation as there are multiple solutions dependent upon a very few variables. We can correctly infer from what we know that bolted truss connections in a repetitive use application are not intended to achieve preload, because when properly preloaded the nuts permanently deform and cannot be reused. In reality, our typical truss connection is a form of snug-tight connection, requiring only the turn-of-nut method for reliability. If you follow this practice, then theoretically the nut threads are never permanently deformed, will never cross-thread, and will last indefinitely. However, most people have a slight issue with that philosophy.
What’s the answer?
Those nuts are cross-threading because they are being torqued, or perhaps over-torqued, to the point of permanent deformation. Throw them away. If clamping force is critical, then the fasteners must be properly preloaded. This will permanently deform the nuts and they should not be reused.
Identify where a snug-tight connection is appropriate. Understand that torque is not a pure measure of connection strength. If you insist on using a torque value, understand that the truss manufacturer’s recommended torque value isn’t intended to achieve fastener preload in a designed bolted connection. If you insist on using that 3 ft cheater bar for easy torque, then expect to replace your fasteners on a regular basis. If you do follow a recommended torque value, understand its importance and never be afraid to question. Inspections are always good. Thread the nut on the bolt; turn it all the way down through the bolt’s entire thread length. If it jams or cross-threads even the slightest, throw the nut away. If there is visible damage to the bolt or its threads, throw the bolt away.
For more on fasteners, read Fasteners and How to Use Them.