Author Archives: Peak innovations Engineering

Use of Washers and Flange Heads

It is likely that most common answer to the question “What is the main benefit of flat washers or flange head nuts and bolts?” would be that the increased bearing area they provide distributes the bolt load over a greater area thus decreasing the potential for compressive yield. While true, during testing we were able to get visual evidence of two important conditions that go against many people’s assumptions.

In support of a paper on the calculation of pressure cone angle originally written for presentation at the annual meeting of ASTM F16.96 Bolting Technology Subcommittee, we took scans of under head pressure for various fastener styles. While the entire study was rewritten in an article posted to the In the News section of our website, two findings related to under head pressure are extracted here because they likely have the widest reader interest.

First, the standard flat washer, whether to SAE, ASTM or DIN standards, does not reduce peak stress anywhere rear what would be expected if it were assumed that stress reduction simply correlated to change in contact area compared to the bolt alone. Figure 1 shows the under head pressure profile for the common hex cap screw and the same fastener with a hardened flat washer placed beneath it. The profile was taken by scanning and digitizing Fuji Prescale film which had been placed under the head and nut. The fasteners were 1/2” -13 and directly tensioned in a load frame. The 5,000 and 10,000 lb loads represent about 40% and 80% the proof load of a Grade 5 fastener. While the precise reduction in peak pressure attained thru use the washer can’t be determined because portions of the film were saturated by loading it beyond it’s capacity, it likely about 30% at 5,000 lb and 50% at 10,000 lb. Based strictly on relative area, the stress reduction should be nearly 80%. While the fact that the standard flat washer is not thick enough to take full advantage of its diameter is likely not news to some readers, some likely haven’t really given the subject much thought and would be interested in seeing it. It also begs the question why consideration isn’t given to revising flat washer standards to decrease the relative dimensions of diameter and thickness.

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The other finding we thought would be of wide interest was the performance of flange head fasteners. While their OD is similar to that of a flat washer, the tapering flange thickness will in theory provide greater load distribution out to the OD. Figure 2 shows the results of testing flange fasteners in the same manner as the flat washers shown in Figure 1. In this case the pressure distribution under both the head and nut is shown because the results were marked different. One can see that under head contact was at the edge of the clearance hole, while the nut contact was at the flange OD. This is not the intent of governing standards, but rather the individual geometry of the fasteners chosen for test. A survey of six different flange bolts and nuts in our lab at the time revealed that two contacted on the OD as in Figure 2, two contacted from the OD in to about half the distance to the clearance hole, and the last two contact over essentially the entire face.

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So these test results shown that contact areas can be smaller than expected when using flanged fasteners, again resulting in higher contact stress and greater potential for compressive yield. However in this case the size and shape of that contact area is more unpredictable. This condition influences more than contact pressure. One of the resisting moments that determine how much bolt tension is generated for a given input torque during tightening increases as the radius of the contact area increases. That is, the further the contact area is from the centerline of the rotated fastener, the more resistance is created during tightening, and the less bolt tension will be created for a given installation torque. The difference between the ID and OD contact shown in Figure 2 could reduce tension by about 10-15%.

Written by Dave Archer
Principal Engineer for Peak innovations Engineering

Torque Control vs. Angle Control

In previous columns. I’ve explained why tightening fasteners to a particular torque does not mean the resulting bolt tension will be either accurate or repeatable. That’s one of the biggest challenges of using bolted joints that depend on clamp load. However, another means of tightening that does not rely on the torque tension relationship is available to manufacturing engineers. This method is often called “angle control” in assembly and “turn-of-the-nut’ in construction applications. It is a more fundamentally sound method of tension control. There is always a direct correlation between bolt elongation and bolt tension until the bolt yields, which is not the case with the torque-tension relationship. Differences between torque and angle control are more easily understood visually. Figure la could be the result of a bench test used to calculate friction coefficient or nut factor, where the resulting tension can vary for a given torque input. The amount of variation depends on geometry, surface finish and coatings, but the extent in Figure I is not exaggerated for some conditions. Unfortunately, electroplated zinc, the most common fastener coating, has some of the worst torque-tension variation.

Assuming the trace occurs in the elastic range, angle can be substituted for tension on the X axis (Figure lb) to show how this condition impacts traditional torque-controlled tightening. Because the friction effect of each bolt varies, the angle the bolt rotates before the installation torque is reached is not constant. Higher friction will cause torque to rise more quickly. Because the angle of rotation is smaller, the bolt tension generated is also proportionally less. This is the theory behind angle control. Since torque only indirectly creates tension while the bolt elongation that results from rotation is directly proportional to it, let’s rotate to a specified angle rather than a specified torque and we’ll always get the desired tension. Of course, it’s not that simple in actual use.

First, we need to determine the angle that will achieve our desired tension. If we were elongating the bolt as we would a steel rod in a tensile tester, Hooke’s Law would make it easy to estimate the elongation required for a given tension. However, for every unit of bolt elongation in a joint, there is a corresponding (but unequal) amount of compression in the clamped components. For simple hard joints, such as non gasketed steel plates, this compression can be calculated by the same method used for bolt elongation. For other less ideal joints, testing is required in fact, testing is needed to utilize angle control for non ideal joints for another reason. Joint stiffness, represented by the slope of a tension-angle trace, is not constant while the joint is being drawn together into complete contact. This is represented in Figure 2, which also illustrates a common solution to the problem. Testing determines the torque at which the transition phase ends and the linear port ion of the trace begins. Allowing for panlo-part variation, this “snug” torque is determined. The specified angle of rotation, which must be measured with sufficient accuracy, starts from this point. As bolt length-to-diameter ratios get smaller, more tension is generated per degree of rotation, so greater accuracy is required.

Less common, but more troublesome, is when the tension-angle trace is not linear, due to joint stiffness variation. While testing is required to establish angle control on all but the simplest joints, this is also true of torque control where in-joint tests or verified friction factors are needed. So, the bottom line comparison between angle control and torque control is that joints with consistent stiffness and variable friction favor angle control, while consistent friction and variable stiffness favors torque control. Joints with neither consistent stiffness nor friction require a return to the drawing board. Since the first joint type is the most common, and torque control is definitely more common in assembly applications, there are opportunities for many manufacturers to successfully employ angle control, particularly those using tightening tools with angle encoders.

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Written by Dave Archer
Principal Engineer for Peak innovations Engineering

Problems Solved

Bolted joints are the vast majority of our consulting business. Our work is divided evenly between validating joints under development and troubleshooting problems with existing joints. Our troubleshooting experience led us to develop the matrix below to help engineers solve common problems with bolted joints. Distilling even the most common problems and solutions into the confines of this column requires simplification that may invite critical judgment from veteran hands. Our experience is that many engineers could benefit from reviewing the relationship between a problem and its likely underlying causes. One assumption inherent in the matrix is that the joints are fastened with a power tool capable of accurate and repeatable shut-off, such that operator influence is unlikely. Note that angle of rotation is listed as a problem, though only a subset of controlled tools can report angle.

Thread strip, bolt failure and high prevailing torque are easily detected with any type of tool. However, they represent extreme problems. Knowing the angle required to rotate the fastener from a known and repeatable starting point to the target torque provides insight into joint conditions between those extremes. Angle of rotation governs the clamp load, and that, in turn, determines the reliability of most joints. While not as convenient or accurate as an integral angle encoder, rotation angle can be determined manually by match-marking. Such manual measurements are not done on every joint, so problems caused by clamp load being outside the expected range are not realized until product is in the field. This is one of the main drivers in the expanded use of sensored tools.

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Written by Dave Archer
Principal Engineer for Peak innovations Engineering

Nine Common Bolting Boo-Boos

As with most endeavors where success can’t be measured by keeping score, a good job of engineering is often defined by not doing a bad job. In other words, the most direct measurement of successful joint design and assembly is a lack of joint problems. As an independent test lab, we evaluate joint capabilities, failure modes and root causes of operational joint failure. This provides me an opportunity to survey what joint characteristics and development decisions are likely to reduce the capacity or reliability of bolted joints across a wide range of industries and applications. Here’s a brief look at the most common reasons why problems occur during joint design, joint assembly and troubleshooting.

JOINT DESIGN

  • Installation torque is established without specific testing of the torque-tension relationship (how much bolt tension is generated for a given torque input) or joint behavior (whether unexpected events occur during tightening). Without this knowledge, the joint’s suitability is an educated guess. Whether this is a cause for concern is largely dependent on the cost of failure
  • The compressive strength of a joint member is exceeded before the installation torque is reached. In most structural joints, the fastener should be much less stiff than the clamped members and be the first element to yield under increasing clamp load. When this is not the case, unexpected joint behavior often results
  • Inadequate consideration is given to the effect of the grip length to fastener diameter ratio on sensitivity to relaxation and resistance to loosening. Use of a larger diameter fastener is often incorrectly seen as providing an added safety factor-in fact, it can actually increase the tendency for vibrational loosening.

JOINT ASSEMBLY

  • Run-down sequence, driver type, driver speed, assembly tooling, and tightening strategy all influence the outcome of tightening a fastener. Often, no investigation of these factors is undertaken before a fastening operation is established. The lack of two-dimensional visualization makes investigation difficult
  • When fastening problems occur in a production environment, personnel with joint engineering experience or people with the authority to make design changes are often unavailable. The pressure to maintain production then leads to inadequate solutions that don’t address the root cause, resulting in a lingering problem.
  • Even when it’s performed, the benefit of joint development and validation testing is not always optimized due to disconnects between the testing organization and the plant. Test parameters can either be unrealistically lab-oriented or production can override the parameters under which test results were generated to increase production rates.

TROUBLESHOOTING

  • Attempting to assign cause to a problem without diagnostic tools is time-consuming and often results in a lot of scrap. Understanding increases exponentially when joint behavior can be viewed in two dimensions by creating an X-Y graph of two parameters. The most widely used graph is the torque-angle plot, which reveals changes in apparent joint stiffness. A wide variety of causes can be uncovered with this simple tool.
  • When bolt fracture or thread strip occurs, engineers typically call their fastener supplier first. However, it is more likely that the person who should be informed can be reached on an internal extension rather than an outside line. Unless the specified fastener was substituted for, it is far more likely the issue is based on joint design or assembly parameters than a fastener not meeting spec
  • As with any form of problem-solving in a stressful production environment, the inclination to start trying things in the hopes it will work is very strong. However, it’s better to rein in this desire and determine if this assembly problem is a process that was never really in control. For instance, did some small incremental variation make the problem visible? Or, did a distinct change in product or process occur?

Without question, the greatest barrier to assembling efficient and reliable bolted joints is the belief that, because threaded fasteners are seen as a ubiquitous commodity, the engineering required to implement them shouldn’t be more involved than picking a torque value from a table.

Written by Dave Archer
Principal Engineer for Peak innovations Engineering

More Data, Less Knowledge?

Several times before. I have discussed the need for physical testing in the development and validation of bolted joints. However, just as both the quality of the joint and the quality of the joint designer’s knowledge base will be limited by reliance on numerical analysis, the same limitations occur when experience and applied experimentation is all one has to draw from. After all, the behavior of bolted joints is based on the same mechanical engineering principles whose understanding enhance the chances that planes will fly and bridges won’t collapse.

Too many catastrophes and too little advancement would be the result of reliance solely on past experience or experimentation as the basis of engineering application. Book smarts and skinned knuckles are equally useful when the goal is a product that is sophisticated, economical, safe or cost-effective. Seat-of- the-pants, or the increasingly popular Internet engineering. should be reserved for all other projects. The proliferation of inexpensive computing power can easily result in a hollowing out of knowledge.

Just as I feel much less pressure to improve my dodgy spelling capability since the advent of the spell-checker. many engineers are more adept at outputting data from their laptops than understanding why the results of scenario A differ from those of scenario B. What if you were asked. ‘All else being equal, how would the initial bolt tension change if the thickness of clamped parts were doubled?’ Someone with a lot of practical experience
might have run across this situation previously. and feel they know the answer. Someone else with a bolted joint analysis program can input both variations of the joint und quickly give you an answer; a quantifiable one at that. Instead, let me suggest a more palatable alternative.

Use a simple analysis program to perform a series of “what if ‘studies that selectively vary common joint variables. The results will provide you with a sensitivity analysis of the impact of these changes, and hopefully, give you a better intuitive feel for joint behavior. To give that knowledge a better foothold, take time to look at the intermediate calculations and their associated help screens. For the truly ambitious. create your own spreadsheet from hand books or the VDI2230 standard. (Send me an e-mai1, and I’ll forward you contact information on analysis programs that I’m aware of.) While many inputs determine joint behavior, these variables can be grouped into the following three fundamental categories: r Strength.

Material strength properties determine the relative capability of the joint to perform as desired under the loads applied to it. The ability of the bolted joint to develop high clamp loads (so it can act like a non-separable joint) in a small, cost-effective package is without doubt its greatest competitive advantage. r Stiffness-Bolt tension is created by elongation during tightening, with the resulting clamp load causing compression in the parts between the nut member and the head The relative stiffness, or spring rate, of the bolt and clamped members is a principal factor in determining the behavior of bolted joints under service loads r Friction-Friction at the interface of fasteners and clamped members has an isolated, but critical role in bolted joint assembly and behavior Friction is both the most influential and the most elusive variable determining the torque-tension relationship that converts input torque to resulting bolt tension.

Friction is equally important in establishing the load that will cause clamped plates to slide against one another when loaded. So, what is the answer to the question of how grip length impacts initial bolt tension? It depends on the basis of how the bolt is tensioned. If, as in most cases, the bolt tension is established by torque control, increasing grip length will not change tension. as the torque-tension relationship (T = KDF or the expanded ISO 16047/VDI 2230 relationship) is independent of grip length. This is a convenient fact, because if grip length did influence torque-tension, all torque tables would need to include it. More basic, the bench top torque-tension tests on which those tables were based, and fastener finishes are classified, would be valid only for the grip length at which the tests were conducted.

However. if angle control were employed to tension the bolt in question, since the relative stiffness between the bolt and the joint members has been reduced, rotating the bolt or nut through the same angle in the thicker joint would result in lower initial bolt tension.

Written by Dave Archer
Principal Engineer for Peak innovations Engineering