By Doug Lescarbeau, Loctite Brand Anaerobic Technology Director, Henkel Corporation
With more than 300 billion fasteners used in the U.S. every year, it is critical that they never fail in service. The latest threadlocking adhesives defy the root causes of fastener failure and are designed to ensure that threaded fasteners remain secure and sealed for their entire service life.
True or False: A properly torqued bolt will not loosen.
This statement may be true if a fastener is not subject to vibration or thermal cycling. But seasoned engineers know that any assembly can fail — sometimes catastrophically — if a fastener loosens
Root cause failure analysis is often used on critical pieces of equipment to identify the mode of failure. For example, the root cause of the crash of a $73 million drone aircraft turned out to be a failed fastener. However, root cause failure analysis often does not determine the reason that the fastener failed, and this is what allows engineers to prevent future failures.
Inadequate clamp force is a significant root cause of fastener failure. Clamp force must be adequate at the time the fastener is assembled, and must be maintained throughout the useful life of the fastener. Proper assembly techniques are required to develop sufficient initial clamp load. Then clamp load must be maintained, typically by using a mechanical or chemical locking system.
The second root cause of fastener failure is corrosion. When rust develops on the nut and bolt, the two parts seize together, making disassembly impossible. Corrosion is prevented by using fasteners that are made of (or plated with) materials that withstand exposure to the operating environment moisture, in the operating environment, or by applying a chemical sealant or lubricant to the fastener
Bolted Joint Mechanics
A bolted joint is basically a wedge wrapped around a cylindrical part. The mating threads effectively wedge the two parts together. Clamp load is generated by applying torque to the fastener, stretching the bolt and compressing the assembly. The more the bolt is rotated, the greater the clamp load on the assembly.
Many believe that by applying a specified assembly torque, a bolted joint produces consistent clamp load. Engineers often take assembly torque directly from charts, assuming a completely linear relationship between tightening torque and clamp load.
When tightening a bolt with a torque wrench, only 10 to 15% of the energy goes into stretching the bolt and producing clamp load. The majority of the energy is converted into heat through friction.
Many hard-to-control variables affect the relationship between tightening torque and clamp load. They have a direct impact on long-term reliability and threaded fastener failure. These variables include, but are not limited to, tolerance, surface roughness, bolt size, surface coatings, cleanliness, and thread lubrication.
Friction occurs both on the threads and under the head of the bolt. When a bolt is assembled to a specified torque, friction can have negative implications on clamp load. The higher the friction, the more the wrench’s energy is turned into heat rather than being used to stretch the bolt. If clamp load is not measured, the assembly could have much less clamp load than expected. But measuring bolt stretch is a time consuming and delicate process and is primarily reserved for critical, rarely serviced applications such as wind towers or automotive cylinder head assemblies.
When a threaded system is assembled, lubricants can greatly change the friction co-efficient. Bolts received from a manufacturer may have surface treatments that can unintentionally lubricate the assembly, such as a permanent coating/plating, residual cutting fluids, anti-corrosion oils and so on. As these coatings are not documented, their influence on the bolted joint is often overlooked.
The surface roughness of bolts varies based on their manufacturing process. Though threaded fasteners conform to well-recognized standards, every bolt manufacturer has unique differences in surface finish and under-bolt head profile that can produce a wide variance in clamp load when bolts are torqued as recommended.
Laboratory study #1: Clamp load variation related to bolt variability
Henkel recently conducted an experiment to verify the theory that differences in surface finish and under-head bolt design produce wide variability when torquing bolts from different manufacturers to recommended levels.
The study used 5/8-in. NC Grade 5 zinc-plated steel bolts and nuts from five bolt manufacturers. At the time of the study, zinc plated steel was the most commonly supplied bolt material.
The bolts were assembled with a calibrated torque wrench to 112 ft-lb (152 Nm), the standard SAE Grade 5 recommendation for steel bolts. As the zinc coating prevents rust, no wet oil film was present.
Each bolt system was placed into a Skidmore-Wilhelm clamp load tester. When the bolted assembly was torqued, the tester determined the clamp load generated by squeezing a hydraulic reservoir that produced a pressure that could be measured and directly correlated with the diameter of the piston.
The first study tested bolts in as-received condition to illustrate the variance in clamp load. Graph 1 illustrates how this produced a standard deviation in clamp load of 21% or 4100 lb.
In the second part of the study, bolts from the same five manufacturers were assembled using a chemical threadlocker. The results shown in graph 2 demonstrate a reduction in clamp load scatter.
In absolute values the range of clamp load data from highest to lowest dropped to 1300 lb with the threadlocker, versus 4100 lb without threadlocker regardless of manufacturer.
Laboratory Study #2: Impact of lubricity (K) factor on initial clamp load
A practical rule of thumb used for approximating clamp load, “K value” combines all the difficult to quantify variables for clamp load into one number. End users commonly ask for the K value as if it is an independently measurable number rather than a collection of variables.
For critical applications, measure and calculate the K value on site. A single bolt is set up with a run-out gage to determine bolt stretch, and the amount of torque required to achieve this value is experimentally calculated. This setup accommodates all the variables of this exact bolted system and determines appropriate torque.
While suppliers of threadlockers are often asked to provide K values, it is impossible to provide accurate values as suppliers do not control factors such as bolt size, manufacturer, surface roughness, tolerance, surface coatings, cleanliness, accuracy of the assembly tool, and clamp load.
To test lubricity Henkel selected a wide range of chemical threadlockers and mechanical locking systems for use in the Skidmore-Wilhelm tester. This test again used 5/8-in. bolts. (Most standard values for K are calculated on 3/8-in. bolts, but this test was done on a larger size to focus on larger applications.)
The standard formula of K=T/DF was used to calculate the value of K.
K = Universal Constant
T = Torque in foot pounds
F= Tension of clamp load in pounds force
D= Nominal bolt diameter in feet
With a dry coating like zinc, the assembler can choose to select a dry or wet film coating torque. Testers selected to use wet film specifications to best accommodate the threadlocking material. The appropriate wet film assembly torque was applied with a calibrated torque wrench. K values vary, which is normal. Bolting clamp load is subject to much variability and these values are all close enough once all practical elements are included.
The goal of any material is to be repeatable and close to the standard value. Thus a good result is a K close to what an oiled bolt delivers. A lower K value (indicating higher lubricity) is acceptable, as long as the assembly torque is dropped to ensure bolts are not over-tightened. A higher K value (indicating lower lubricity) means the correct clamp load will not be generated.
In this study, Henkel also tested a newer mechanical locking device involving two interlocking washers with built in ramps designed to stop vibrational loosening. However, this device requires a lot of energy to overcome friction and delivers a substantial increase in K value. If the same torque specification for an oiled bolt is used to tighten the device, a significant drop in clamp force results as shown in Graph 3.
Maintaining clamp load to prevent loosening and failure
There are many methods of locking threaded assemblies and combating loosening, including both mechanical and chemical threadlocking systems.
Mechanical locking devices include spring, star and tab washers; nuts with nylon inserts; castellated and lock nuts; tooth flanged bolts; and ramp washers. Some devices are more effective than others at resisting loosening, but no mechanical locking device is capable of sealing threads. Therefore, these devices always leave assemblies vulnerable to rust and corrosion.
Chemical threadlocking systems are single-component anaerobic adhesives. These adhesives are applied to the threads as a liquid, and remain liquid until isolated from oxygen between the metal surfaces. Once the coated fastener is assembled, the anaerobic sealant cures into a tough thermoset plastic.
Threadlockers offer excellent temperature resistance, rapid fixture/cure speeds, and easy dispensing. Contrary to common belief that threadlockers are permanent, these adhesives reliably lock and seal fasteners for the life of the assembly, but can be removed when required for disassembly.
It is important to choose the correct threadlocker that meets the assembly and disassembly requirements of the application. Threadlockers are available in low strength formulations that are easy to remove, medium strength grades that can be removed using common hand tools, and high strength formulations that require tools and heat of at least 450°F to disassemble. A nut and bolt assembled with a threadlocker can be repeatedly reused by simply brushing off the cured threadlocker from the disassembled parts, applying new threadlocker, then reassembling the fastener.
Chemical threadlockers do three things well: lubricate the assembly to convert torque into high clamp load; fill voids to lock the threads and maintain consistent clamp load over time; and seal threads to prevent corrosion and seizure to ensure that disassembly is consistent and predictable. With threadlockers, disassembly torque does not increase over time because no rust develops in the joint.
Comparing chemical and mechanical threadlockers
A Junker test machine assesses how well fasteners alone, fasteners with mechanical locking devices, or fasteners with threadlocker hold up when subjected to severe levels of shock and vibration. Fasteners alone or paired with mechanical locking devices typically lose 40 to 60% of their clamp load after 60 seconds on the Junker machine. Fasteners treated with threadlockers lose less than 1% of clamp load.
Junker test data and cost data demonstrate that threadlocking adhesives are a reliable and cost effective way to fight both root causes of fastener failure. In its liquid state, the adhesive facilitates assembly to prevent galling and provide consistent clamp load. It unitizes the nut and bolt by forming a tough thermoset plastic between the threads, eliminating empty spaces and physically preventing relative motion between the parts. Because threadlockers seal the assembly, they prevent leaks and corrosion.
Threadlockers can be used on any shape or size fastener and resist temperature extremes, deliver rapid fixture/cure speeds, and are easy to dispense. You can choose from liquid, gel, stick and tape forms, and can use them on any shape or size fastener. Unlike mechanical locking devices, no complex parts inventory is needed and threadlockers usually cost less than mechanical locking devices.
Advances in threadlocking technology
The latest generation of anaerobic threadlockers are easier to apply and significantly more robust than earlier formulations. Both blue removable and red permanent threadlockers are available in oil tolerant and primerless formulations that simplify assembly processes by eliminating the need to clean and prime the fastener. New dispense equipment further improves the assembly process — manual dispensers ensure consistent application, and semi-automatic and fully automatic dispense systems are also available.
For blue removable technology, you can choose from three formats – liquid, stick and tape — for the product that best suits your assembly process. Red permanent threadlockers are available in liquid or stick format.
While first generation threadlockers were effective at maximum continuous operating temperatures of 300ºF, today’s products generally withstand 20% higher temperatures. Select formulations can withstand extreme temperatures up to 650ºF without degrading.
Other product innovations meet specialized application requirements. Threadlockers with an MSDS health rating of 1 enhance occupational health and safety by eliminating allergic skin, CMR, and Prop 65 hazards. Purity certified, low halogen threadlockers meet the requirements of nuclear power generation facilities, and food grade threadlockers comply with the Federal Food Drug and Cosmetic Act and FDA regulation 21 C.F.R. 175.300. Threadlockers are available to lock and seal plastic fasteners. Products are also available that are Mil Spec certified.