Robert Dunkel PEng, Director of Technical Service at Henkel Corporation, explains how the use of threadlocking adhesives can dramatically improve the reliability of machines assembled using threaded fasteners, partly due to the way threadlockers prevent fasteners from loosening, and partly due to threadlockers acting as a lubricant during assembly and removing the variability that occurs when tightening as-received fasteners.
Root cause failure analysis is often employed when critical pieces of equipment fail. True root cause failure analysis forces you to ask 'why' five times. The fifth question sometimes identifies the mode of failure (a fastener loosened) instead of the root cause (insufficient clamp load).
Case studies and laboratory data provide new insights into the root cause of typical mechanical loosening and sealing failures. They explore the reasons why some of these root causes are most effectively addressed with non-traditional chemical solutions.
Bolted joints exhibit a very complex relationship between clamp load and the torque-tension relationship of threaded fasteners. A common perception is that a bolted joint produces consistent clamp load given a specified torque. However, load cell tests of identical bolts sourced from multiple manufacturers demonstrate that the actual clamp load, when torqued to a specification, can vary by over 20 per cent if used under as-received conditions. The use of a chemical thread treatment can dramatically reduce deviation and provide additional reliability benefits.
Gas and fluid leakage are common maintenance reliability issues. Root cause analysis has been applied to typical flange and threaded fittings configurations. The results identify several failure modes that can be countered with specific types of chemical sealants.
Machinery equipment reliability can be influenced by many factors. There were observed a number of factors that can influence this. Many are considered 'non-traditional' even though they have been around for many years.
True root cause of failure
Root cause failure analysis aims to break down a problem into the most basic elements and find the cause.
In a review of case studies, it was observed that the fifth question, or ultimate root cause, will often identify a loosened fastener or a leaky gasket. This fails to bring us down to the level of the assembly process to get to why this occurs and to prevent it from happening again. In examining either a loosened bolt or leaky gasket, a fastening system that has failed to provide adequate clamp force is often the true root cause – one that is avoidable with proper assembly techniques.
Mechanics of bolted joints
A bolted joint is, in its most basic form, a wedge wrapped around a cylindrical part. As the bolt is turned, the threads effectively wedge the two parts together. The more the bolt is rotated, the more clamp load is achieved.
In a practical system, both the nut and the bolt have tolerances to ensure that they will not bind when assembled. By default, this means that a bolted joint will have variable gaps, depending on the combination of tolerances in the parts (see image, right).
The surface finish of bolts is related to how they are manufactured, the dies used to produce them, and the technology used to form the part. Though threaded fasteners conform to widely recognised standards, each manufacturer has slightly different processes that will lead to small, but ultimately significant differences in surface finish and under-head profile, with a resultant effect on the bolted system.
When a threaded system is assembled, the presence or absence of a lubricant will greatly change the lubrication co-efficient. Bolts procured from a vendor may have permanent coating or plating, residual cutting fluids, anti-corrosion oils or other substances. The challenge is that these are not documented and are often overlooked as to their influence on the bolted joint.
Most fasteners are used in dynamic systems subject to vibration. Though high vibration captures attention, even low-force vibration over time can cause issues. In the electrical industry, it is common for electrical panels to require re-torqueing of set screws due to the effect of the AC current over time.
A bolted system has variables that are difficult to model. As discussed, tolerances, surface roughness and surface friction all impact assembly and can lead to potential issues.
This effect is tied to tolerance. Under vibration, a machine moves back and forth. Due to required manufacturing tolerances, the bolted assembly has empty space. In a simplistic manner, it is often assumed that if you tighten a bolted assembly properly, the friction of the surfaces will stop the assembly from loosening.
Under the constant vibration that machinery produces, the bolted assembly will eventually move relative to its components. The rate at which this occurs will vary, but disassembly can occur very rapidly if a harmonic is reached.
Returning to the description of the joint as a wedge, any movement back and forth is like standing on a ski slope and sliding back and forth. Under these conditions, you would tend to move downhill (the path of least resistance). On a bolted joint system, the tendency of a part is to come apart.
Many methods of combating this are available on the market. One of the more effective means is to place a self-curing chemical into the joint to fill the machining tolerances and produce a solid filler. This addresses the root cause of the failure by eliminating the empty space in the threaded assembly (see image, right).
Mechanical systems for stopping vibration cover a wide spectrum of designs and effectiveness. Some mechanical systems actually speed up the rate at which fasteners come apart. The best of these work, but are very costly in comparison to chemical threadlocking systems.
Chemical threadlocking, despite being proven in over 50 years of service, is still considered a relatively new and unproven technology. Nevertheless, it provides a cost-to-benefit ratio that delivers high performance at a low cost.
Friction effect on clamp load
When a bolted system is assembled, the clamp load is generated by putting energy into the system, which stretches the bolt. The bolt stretch compresses the assembly to create the clamp load. The most commonly used method of achieving this is to apply a known torque that 'equates' to a certain clamp load. It is a common perception that with a specified assembly torque, a bolted joint produces consistent clamp load.
All nuts and bolts have a surface roughness that produces friction. This has implications when fastener systems are assembled using torque or energy input to correlate the clamp load generated to the amount of energy going into generating clamp load through bolt stretch.
Other methods of determining the correct bolt stretch exist, but have limitations in general use. One example is torque-to-yield bolts, which are commonly used in automotive cylinder head assembly. Another is the direct measurement of bolt stretch using a run-out gauge, which is commonly used in wind-tower base securing.
The following test was conducted to quantify this.
Bolt variability from different manufacturers
Knowledge within Henkel had long held that the surface finish, along with the variances in under-head bolt design, would produce a wide scatter when each bolt was torqued to the recommended amount.
As an experiment to verify this hypothesis, zinc-plated M16 bolts and nuts from five different manufacturers were used. Bolts were assembled with a 152Nm calibrated torque wrench.
Each bolt system was placed into a Skidmore-Wilhelm clamp load tester. When the bolted system was torqued, it compressed a hydraulic reservoir, producing a pressure that could be measured and directly correlated, thanks to the knowledge of the diameter of the piston, to the clamp load.
The first test utilised bolts in as-received condition to illustrate the variance in clamp load. Using bolts in the as-received condition is common. The table below illustrates how this produced a standard deviation in clamp load of 18 kN (21 per cent).
In an era of cost avoidance, it is not uncommon for manufacturers to shop for interchangeable items based on cost. As a result, the clamp load generated can vary significantly, even using a properly calibrated torque wrench.
In the second part of the experiment, bolts were assembled from the same five manufacturers with a chemical threadlocker to observe the effect. The results shown in the graph below demonstrate a significant reduction in clamp-load scatter.
In absolute values, the range of data dropped to 5.7kN of clamp-load difference, from highest to lowest values. Utilising a liquid threadlocker reduced the scatter of clamp loads from different bolt manufactures.
One of the most significant benefits of chemical threadlockers is their ability to reduce the variability of bolted assemblies. The consistent lubrication that results from specifying the material to be used as the assembly fluid is a major benefit. Chemical threadlockers not only stop vibration loosening, but also allow more consistent clamp load by removing variability from the system.
Non-traditional chemical threadlockers – alternative forms
One of the barriers to chemical threadlocking is that maintenance staff have found carrying liquid materials an issue due to the potential for spillage. One industrial company has been actively involved in trying to resolve this concern by introducing patented technology that allows for solid format, or 'stick', threadlockers.
This innovation was targeted at applications performed in the field or at the machine's location. The goal was to develop products that could easily go into a tool box, or even a pocket, for easy access. This stay-in-place format enables users to apply chemical threadlockers to all fasteners ahead of time, thereby accelerating re-assembly and providing an additional side benefit.
Non-traditional chemical threadlockers – primerless on inactive materials
A concern voiced by end users was the requirement for flammable primers on chemically inactive materials such as aluminium. Many plants and mines have banned their use for safety reasons.
Chemical threadlockers normally require free metal ions to cure. These are found on the surface of metals that corrode, such as steel. One industrial company has been active in developing materials that cure without a primer, which eliminates both a step and a flammable solvent from the workplace. These primerless materials are a large step forward in making this technology more accepted in the workplace.
More about anaerobic adhesives for threaded fasteners
Follow the link for more information about the Loctite Success Secured campaign; the microsite has customer case studies, videos, an adhesive selection 'wizard', webinars and a wealth of other useful information.