What Are Threaded Fasteners?
Threaded fasteners (e.g., bolts, nuts, screws) are so common that it is hard to imagine a world without them. Our society depends on them. Consider the lug nuts that keep the wheels attached to our vehicles while driving or the fasteners that hold together the structural steel members of the buildings we enter and the bridges we drive over. They’re all around us, often serving critical functions. But how do threaded fasteners work?
At their most basic level, threaded fasteners function like a stretched spring. Although you would be hard-pressed to see this stretch, it is the elastic properties of the fastener material – often carbon or stainless steel – that allows the fastener to stretch. When a fastener stretches, the resulting tension produces a force that clamps the items together.
Fastener Tensioning Methods
Some of the most common methods used to tension fasteners are: (1) torque method, (2) turn-of-nut method, (3) elongation method, and (4) direct tension method.
The torque method is one of the most common and familiar methods of tensioning fasteners. A torque value is often specified for a given application, and with the use of calibrated tooling – such as a torque wrench – the desired torque value is applied to the fastener. Torque (i.e., rotational force), which is often specified in foot-pounds, produces tension (i.e., clamping force) in threaded joints.
The amount of tension produced using the torque method depends on factors such as:
- joint dimensions
- surface finishes
- material types
- hardness of the parts
- thread types and tolerances
- thread manufacturing process (e.g., cut vs. rolled threads)
- plating thickness and types
- number of times the fastener will be reused
- washers (i.e., present or not)
- speed of torquing
- component being torqued (i.e., turning the nut vs. the bolt)
- lubricants (if any)
Given the number of factors involved, it should be noted that the torque method produces a fastener tension that varies by approximately +/- 25 percent of the intended tension.
This method involves running down the nut to the bearing surface, tightening the fastener to a low initial snug-tight condition, and then applying a prescribed number of turns of the nut – often one-third to three-quarters of a full rotation – to develop the required tension. According to Shigley’s Mechanical Engineering Design, "snug-tight" is attained by a few impacts of an impact wrench or the full effort of a person using an ordinary wrench. Once the nut is snug-tight, any additional turning develops useful tension in the bolt.
The turn-of-nut method is a popular and reasonably reliable method for tensioning structural bolts without the need for torque wrenches or specialized tension measuring devices. This method controls the fastener tension to within approximately +/- 15 percent of the intended tension.
The elongation method can be used when the overall length of the bolt can be measured with a micrometer after assembly. The amount of tension in the bolt can be calculated using Hooke’s Law and the measured elongation.
While this may appear to be straightforward, a couple of factors can complicate this method. For example, using the spring analogy, the bolt can be thought of as a multi-piece spring. That is, the unthreaded shank of the bolt is a stiff spring, while the threaded portion of the bolt is a less stiff spring (due to its smaller cross-sectional area). When the bolt is loaded, the threaded portion of the bolt will stretch more than the unthreaded shank of the bolt.
To use the elongation method, you must first determine the amount of stretch each portion of the bolt contributes to the overall stretch. The elongation method often achieves a fastener tension that is accurate within approximately +/- 3 to 5 percent of the intended tension.
Direct Tension Method
The previously mentioned methods use indirect means to achieve the necessary clamp load. Although some of the methods are more precise than others, none of them control the fastener tension in a direct manner. Therefore, several methods have been developed to directly indicate when the necessary fastener tension has been achieved, thereby ensuring the joint has an adequate clamp load. A few of these methods are as follows:
- Direct tension indicating (DTI) washers are often designed with hollow bumps on one side of the washer that flatten as the fastener is tensioned. A feeler gauge is used to measure the gap produced by the bumps. When the fastener has developed the appropriate tension, the feeler gauge will no longer fit in the gap.
- Proper tension can also be controlled by using DTI bolts and bolts that contain strain gages manufactured within the fastener. DTI bolts tend to be more expensive and are not as common as DTI washers.
- Tension control (TC) bolts, which are common in critical steel applications, feature a bolt with an end spline that disconnects when the appropriate fastener tension is reached. While TC bolts only require a single person for assembly, a specialized tool is required for installation.
- Hydraulic tensioners are sometimes used to tension large diameter bolts. A hydraulic tensioner has an upper collar that is threaded onto the exposed section of thread above the nut. Hydraulic pressure is then used to stretch the bolt, creating a gap between the nut and the joint surface. The nut is then run down freely against the surface of the joint. When the hydraulic pressure is relieved, the nut continues to hold much of the tension developed.
Most direct tensioning methods control the tension within approximately +/- 1 percent of the intended tension.
Threaded fasteners offer many advantages to our society. The simple fact that we can quickly assemble or disassemble a threaded joint with common tools is great. Further, it is nice that, in many cases, working with threaded fasteners does not require specialized training or an advanced skill level. This inherent simplicity, however, can lead to overconfidence and possibly even catastrophic damage if an inappropriate tensioning method is used.
|Tensioning Method||Tension Accuracy|
|Elongation Method||+/-3% to 5%|
|Direct Tension Method||+/-1%|
An understanding of the tensioning method employed in a specific application and the accuracy of the tension associated with the method is fundamental to ensuring the success of the threaded joint.
About the Author
Jeffrey A. Groves, P.E. is a Consulting Engineer in our Mid-Atlantic (Cherry Hill, NJ) Office. Mr. Groves provides consultation in root cause analysis, scope of damage, and value of loss consultation for industrial, commercial, and residential incidents involving equipment, machinery, and systems. You may contact him for your forensic engineering needs at firstname.lastname@example.org or (856) 662-0070.