When tightening a bolt, how tight is too tight? Or for that matter, how loose is too loose? Specifying and measuring how tightly a bolted fastener can bond separate materials together is an important technical factor in the design of bolted assemblies, with an entire subset of structural engineering dedicated just to solving these applications. In this article, we’ll review the role of ‘tightness’ – that is, clamping force and torque measurement – in selecting bolted fasteners.
Torque Explanation and Measurement
Imagine two pieces of flat steel plate, with a hole drilled through both pieces in the middle. Picture a bolt, washer, and nut set slipping through the aligned holes in the plates, and being tightened down so that the plates are now bolted together firmly. There are several forces here in play that keep the assembly tight:
- Compression – the steel plates are compressed under the nut and bolt head, which creates force pushing against the nut as it threads down the bolt
- Thread interference – the above force generated by the plates causes the nut to press against the bolt’s threads, causing friction that increasingly resists rotation of the nut
- Bolt elongation – the bolt itself is stretching here as well, as the plate’s compression attempts to push the nut and bolt head away from each other
When tightening up the assembly, all of the above forces compound to a point where the nut can no longer spin further under reasonable effort. You might swap your hand wrench out for an air wrench and attempt to tighten the nut further, and you may indeed get a little more tightening out of the assembly. Any further though, and either the nut will seize and stop rotating, or the assembly will fracture in its weakest spot – either by cracking one of the plates, popping the head off the bolt, or cracking along the bolt threads. In this scenario, the clamping force exceeded the bolt’s capacity, leading to failure.
Measuring all of the above dynamic forces, and the capacity of the fastener itself, is difficult to perform in a real production or construction environment. The objective here is to figure out a value of ‘tightness’ that aggregates all of these forces, such that you can be assured that the fastener is not too loose that it cannot perform its clamping job, but also not too tight that it will risk failure. The solution: Torque Measurement.
Torque is defined as the rotational force required to move an object about a centerline or pivot point. Imagine again our bolt and steel plate example – when applying the nut to the bolt, the nut at first spins freely, but as the two plates begin to engage, the nut takes more effort to spin. This effort to spin the nut is the torque of the fastener, and this torque increases as the clamping force of the assembly increases. Torque is measured in several units: Metric NM (newton metre), and Imperial Lb-In (inch-pound) or Lb-Ft (foot-pound).
In a bolted assembly, torque measures resistance to rotation. As the assembly tightens up, the nut takes more force to spin, and at a certain prescribed torque value, the assembly is considered ‘acceptably torqued’.
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Now that we understand torque as a concept, let’s discuss how torque values are derived in practice. All of the materials and environmental conditions involved in a bolted assembly serve to influence torque specifications. To keep this section short, we’ll look at a quick bullet list of factors that impact torque values:
- Bolt material and grade – harder or higher tensile strength bolt materials can offer higher torque ranges
- Bolt finish – an applied surface finish can change thread friction and interference, usually increasing torque values
- Bolt lubrication – using a lubricating material or finish also changes friction, usually decreasing torque values
- Thread specification – coarse threads typically have lower torque values, while finer threads have higher values
- Clamped materials – the nature of the clamped assembly can change torque values, such as using lower torques when clamping soft or highly compressible plastics
- Vibration factor – as the most common enemy of bolted assemblies, vibration tends to increase torque values in an effort to resist untightening of the nut during use
- Application factors – other factors such as if the joint requires material flexure, shock load, shear load, etc, all vary torque values
- Environmental factors – heat causes steel materials to expand, and cold to contract. These temperature swings can loose bolted assemblies over time, requiring different torque values to combat
While all of the above variables may seem daunting to consider, luckily, we have the help of multiple specifying authorities in finding a place to start in specifying torque values. Industry groups such as ASTM and SAE produce torque charts that convey standardized tightening ranges, categorized by the bolt material or grade number itself. These standardized charts are published as ‘starting values’, meaning that they only take into consideration the bolt grade and material. Because these charts have no insight into any other application or environmental variables, a qualified authority must always be consulted when torque values are critical design factors.
For a brief snapshot of how grades, materials, and finishes influence torque values, here’s a table comparing 1/2″ bolt sizes.
|Specification||Bolt Size||TPI||Proof Load (lbs)||Clamp Load (lbs)||Tightening Torque (ft lbs)|
|SAE Grade 2||1⁄2||13||7,800||5,850||24||61||49|
|SAE Grade 5 / ASTM A449||1⁄2||13||12,050||9,038||38||94||75|
|SAE Grade 8 / ASTM A354-BD||1⁄2||13||17,050||12,788||53||X||107|
A note about measurement methods: there are several ways to measure torque, not just in the tools employed, but also when to measure during assembly. In short, simple torque measurements load a torque wrench (the tool measuring the value) upon final assembly to confirm that the proper tightness was achieved. In complex assemblies, the torque profile should be measured during the entire assembly process, not just at the end, to assure that material engagement occurred correctly, and that the fastener showed no signs of abnormal loading. Knowing your torque values expected, as well as when to measure (and when to re-check over the life of the assembly), are very important.
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