Quality Test & Inspection: Tight Not Always Right
January 1, 2007
Threaded fasteners have been around and used to assemble a variety of products for so long that it is often assumed that everything is known about them. This confident assumption often leads operators to tighten a given fastener until it “feels” tight and they are done with it. In some applications this approach may prove satisfactory, but with critical applications, which can impact a financial bottom line through costly rework or warranty repairs, the old “feels tight, must be right” method is inadequate.
Fasteners of various types can account for as much as 50% of the parts count in a typical bill of materials, so it is important that the correct fasteners are used properly to ensure product quality and efficient manufacturing. In addition, great strides have been made in developing new materials and implementing them into new products. How these components perform when the fastener is tightened must be thoroughly researched before a quality assembly can be ensured.
Modern computer software and engineering analysis methods have been developed to provide fastener design engineers with some powerful tools for performing threaded fastener analysis. The accuracy of test and measurement equipment has been improved to aid in the testing of threaded fasteners. Yet, after the product has been designed and the best fasteners are chosen, it all comes down to correct installation, and no manufacturer in these competitive times can afford to have the wheels fall off their cart.
Traditionally, torque has been used as the most practical means of determining the correct assembly of a bolted joint. It is relatively easy to measure torque either dynamically during the assembly or statically via a post-
assembly audit. For many noncritical fasteners, this method has proven sufficient. However, in critical assemblies that potentially impact safety or product warranty issues, and for products where assembly conditions are not tightly controlled, measuring torque alone may not be adequate as a means of determining joint integrity.
Torque as an Energy Transfer ProcessWhat happens when a threaded fastener is tightened? Applying torque to a threaded fastener with a hand or power tool in order to hold the assembly together is an energy transfer process. Operators apply a specified amount of force at a distance to turn the fastener to a certain point. For example, Newton-meters expresses a value of force measured in Newtons applied at a distance measured in meters; in the United States the common measurement of foot-pounds (lb-ft) is an expression of force in pounds applied at a distance measured in feet. Practice and experimentation has proven that the energy that is used to turn the fastener is transferred into clamping force that ultimately holds the parts together.
When a fastener is tightened, operators often use a torque-measuring device to determine the amount of energy that is being applied to the fastener, such as with a hand torque wrench or a rotary torque sensor used on the output end of a power tool. In fact, most modern power tools come with some torque measurement capability already built into them.
Consider the following scenario: The quality inspector asks, “How tight is that fastener?” The operator answers, “I tightened it to 50 lb-ft.” The quality inspector compares the reading to the engineering specification, which requires that the fastener be torqued to 50 lb-ft and pronounces the assembly satisfactory.
The only problem is that their torque measurement during the assembly process or through an audit after the fact told them only how much energy was used to turn the fastener, not how much of that energy was actually transferred into clamping the assembly together. The specified effort was put into tightening the fastener and the tool did its job, but is that good enough to hold the product together?
The answer lies in predicting or determining how much clamp load was produced when the fastener was tightened. This is a significant challenge because there is no direct way to measure clamp load in the actual assembly.
Clamp load is the amount of pressure that holds two or more parts together in an assembly. A simple illustration is a common C-clamp used to hold a couple of wood blocks together; as operators turn the threaded section, they apply clamp load to the blocks. When they tighten a threaded fastener to hold an assembly together, they are applying clamp load to that stack up of parts. In the truest sense, it is the correct amount of clamp load that is generated that actually holds the parts together successfully, not merely the amount of energy, measured as torque, applied to the fastener. The practical problem is how to determine the amount of energy that went into holding the assembly together.
Where Does The Torque Go?Threaded fastener analysis has shown that most of the energy applied to a fastener goes into overcoming the friction that exists under the head of the fastener and in the threads. Together, these two sources of friction can typically be as much as 80% to 90% of the applied torque energy, leaving as little as 10% of the energy to transfer into clamp load.
In a perfect world where all of the components in the assembly are identical, including the threaded portions of the fastening system, applying a specific amount of energy to the fastener produces a corresponding amount of clamp load. However, anything that changes the process, such as a shift in the friction coefficients in the threads or under the head of the fastener, may produce a huge shift in the clamp load generated in the assembly.
The state of friction in the joint is important because it holds the fastener in place and maintains the clamp load after the assembly is complete. However, a slight change in the state of friction either under the head of the fastener or in the threads can produce a significant change in the amount of clamp load generated. Either more or less friction than anticipated can produce a poor assembly.
Because as much as 90% of the torque energy is absorbed by friction in the joint, leaving only 10% of the total energy for clamp load, a 5% change in the amount of friction in the threads can increase or reduce the amount of clamp load by half-a significant shift.
For example, the presence of dirt or other contaminants on the threads can increase the amount of friction, thus reducing the clamp force and leaving a loose assembly. Conversely, an accidental drop of oil under the head of a fastener can reduce the friction and allow more energy to flow into clamp load, which can overtighten the assembly to the point of over-stretching or fracturing the fastener, which can produce a catastrophic failure.
If torque reveals only how much energy was put into the assembly, what can tell how much clamp load is holding the assembly together? The key is simple-measuring the turn of the fastener. This is because the angle of rotation of a threaded fastener is more directly proportional to the clamp load than the measurement of torque. If operators can define the relationship of fastener rotation to clamp load in a given joint, then they can use the angle of rotation during tightening to predict the amount of clamp that will be produced when the assembly is finished.
Because of the additional expense of angle measurement, torque-turn methods are usually reserved for only the most crucial fasteners in an assembled product. However, for noncritical fasteners, torque-only measurement and control is usually adequate. This can be achieved through monitoring the output of the tools on the assembly line through the use of rotary torque sensors or a torque sensor built into the tool. By closely controlling installation torque, operators can monitor the amount of energy that is being put into the assembly. This verifies the performance of the tool and ensures that a consistent amount of energy was used to fasten the assembly together.
If torque is not monitored during the assembly process, performing an audit after the assembly has been completed can approximate it. This is done by applying torque to the fastener, up to the specified torque specification, thus ensuring that the correct amount of energy was used to tighten the fastener. Post-assembly torque audits are quite common in assembly operations, and serve to verify the performance of the assembly tool.
Torque And Angle MeasurementSeveral methods have been developed to overcome the limitations of torque-only measurement. Foremost is the torque-turn method which uses the threaded design of the screw or bolt to its best advantage. Because of the helix of the threads, fastener rotation is almost directly proportional to clamp load and can be used to determine or predict the force holding the assembly together. With this method, the fastener is tightened to a preliminary torque level, predictably sufficient to bring all of the components into alignment and contact, and then the fastener is turned a specified number of degrees to produce the correct clamp load.
The specifications for the assembly can be determined through experimental testing on the fastener and verified by testing on actual assemblies. For example, the fastener was tightened to 20 lb-ft of torque and then turned 60 degrees. This produced 7,500 pounds of clamp load. This data was obtained during experimental testing using a rotary torque angle sensor and a clamp force load cell.
From this data, it is calculated that 85 pounds of clamp load will be produced with each degree of fastener rotation. After this is verified through obtaining and analyzing torque-angle signatures via testing on actual assemblies, the torque-angle data recorded during the assembly process or a post-assembly audit can estimate the clamp load and ensure a more complete assessment of product quality.
In addition to its uses in determining correct assembly methods, the examination of torque and angle data, called torque angle signature analysis, is a powerful tool for fastener engineers to analyze and troubleshoot problematic joints.
Recording torque and angle data and examining the shape and slope of the resulting curve can discover underlying problems with the assembly. Ideally, after the parts are brought into alignment, the increase of torque and angle should be a linear process producing a straight torque-angle curve. However, in the case of embedment of the head of the fastener into the surface of the part, the torque-angle signature starts out linear but begins to flatten out when a straight line is overlaid onto the plot. Careful review of a torque-angle graph often can aid in revealing the cause of a failed assembly. Assemblies that reveal this kind of behavior require closer investigation.
How Tight Is Right?Although operators have learned much about threaded fasteners over the years, recent developments in materials and assembly processes have prompted the need for additional analysis of bolted joints to ensure a high level of quality. A conscientious fastener engineer can model the assembly using specialized computer software or finite element analysis. This should be followed up with experimental testing to determine the state of friction in the joint and the relationship between torque, angle and clamp load. This is followed up with verification using torque-angle analysis on prototype assemblies.
Controlling torque is an important first step in determining the quality of a product assembled with threaded fasteners. If the tool used in the assembly is not consistent in its application of energy to the fastener, the quality of the assembled product will suffer. For those fasteners that are crucial to safety, reliability or durability of the product, torque alone may not be sufficient to ensure quality. Modern tools and diagnostic methods have been developed to determine the adequacy of the assembly operation, such as torque-angle measurement and control and torque-angle signature analysis. Remember, just because the fastener seems tight does not make the assembly right.
- Controlling torque is an important first step in determining the quality of a product assembled with threaded fasteners.
- If the tool used in the assembly is not consistent in its application of energy to the fastener, the quality of the assembled product will suffer.
- For those fasteners that are crucial to safety, reliability or durability of the product, torque alone may not be sufficient to ensure quality.