By Doug Lescarbeau, Anaerobic Technology Manager, and Tina Adams, OEM Market Manager, Henkel Corporation
Retaining compounds offer a number of benefits in fastening applications, including reducing weight, lowering cost and assembly time, and even reducing some dimensional precision requirements. Here’s a closer look at when and how you can use retaining compounds.
Ancillary benefits of retaining compounds include their ability to increase the number of viable substrate materials for an application and their ability to seal an assembly to prevent corrosion.
Competitive snowmobilers, snow-cross racers and XGames athletes have come to rely on Polaris snowmobiles to help them dominate in extreme events like racing, long-distance jumping, and freestyle competitions that involve dangerous back flips and extensions. The sleds are designed for terrain domination, enduring the punishment of full-out driving maneuvers, high loads and even crashes. The engineers who build the machines must be sure that the sleds will function reliably under severe conditions and hold tight under stress and at high temperatures.
To ensure that Polaris snowmobiles endure wide temperature fluctuations, hard landings, vibration and extreme impacts while continuing to perform reliably, engineers used Loctite anaerobic adhesives – specifically threadlockers and retaining compounds – to ensure the safety of the sleds and eliminate any risk of assembly failure. Threadlockers are used on all threaded fasteners throughout the machine, eliminating vibrational loosening. At critical interfaces such as engine mounts, these adhesives reliably secure the threaded fasteners that attach the engine to the chassis. Failure here could mean disaster.
Then there are the interference fits found throughout the machine. Interference fits join cylindrical parts such as shafts, bearings or bushings to smaller gears or housings, and rely upon high friction to lock parts together so that even large amounts of torque cannot separate the bond. To achieve maximum joint strength and optimal performance, these joints must be precise. The inner part of the interference fit is machined to be imperceptibly bigger than the mating hole of the outer part. The larger component is then forced into the smaller part, both deform slightly to fit together, and the two parts “unitize” and operate as one.
But even when the appropriate allowances for interference fitted parts are calculated to achieve maximum friction, failure can occur. And in order to calculate and achieve such exacting levels of dimensional precision, both the cost and the time required for component production increase.
Retaining compounds enable engineers to design robust, lightweight interference fitted joints at reduced cost and in less time than traditional interference fits by reducing the required dimensional precision. Retaining compounds provide strength and reliability and are increasingly used in general industry to secure interference fitted parts.
Ancillary benefits of retaining compounds include their ability to increase the number of viable substrate materials for an application and their ability to seal an assembly to prevent corrosion. Without a retaining adhesive, substrate selection for a reliable press or shrink fit may be limited because of the high levels of stress on the joined components. Only certain materials can withstand the stresses and forces required to merge the two parts. For example, powdered metal or aluminum may fail while steel will easily achieve the required joint strength requirements. A retaining adhesive strengthens the overall assembly, making substrate selection less critical to parts performance.
A recent Design World/Henkel study of design engineers found that almost 42% of the 400 surveyed recognized that retaining compounds can be used to increase the reliability of assemblies. The respondents noted that the combination of retaining adhesives and interference fits improved reliability over a mechanical fit alone.
In addition, the respondents noted that retaining adhesives help lower overall manufacturing and assembly costs by reducing the precision of dimensional tolerance requirements. The majority of survey participants reported experience problems with fretting corrosion and stated that they could use retaining adhesives to solve this problem.
While less than half of the respondents are currently using retaining compounds, the majority of respondents who do use adhesives select them because of the confidence they have with them from previous design experiences. The survey results indicate that many engineers would benefit from further education about the use and advantages of this technology.
Retaining compound basics
Retaining compounds are liquid anaerobic structural adhesives that cure or polymerize when confined without air between close fitting metal surfaces. These adhesives are made from engineered plastics. Related to interference and slip fits, engineered plastic adhesives let you create joints that are high strength, lower weight and lower cost than traditional interference fits.
Retaining compounds are specifically formulated to join cylindrical, unthreaded metal parts together. These adhesives fill the empty space between mating interference fitted parts and cure to chemically unitize the assembly. Since the combination of the interference fit and the retaining compound is much stronger than the interference fit alone, you need not go to extremes to calculate dimensional tolerances.
The cured retaining adhesive fills the inner space between components and provides a physical and chemical barrier that eliminates fretting, oxidation and galvanic corrosion. The adhesive also seals the interference fit joint against leakage.
Retaining compounds are used in three distinct ways for interference fitted joints:
• added to existing interference fit designs to increase joint strength and reliability;
• engineered into new interference fit designs to deliver smaller, lighter weight assemblies that operate reliably with relaxed tolerances requiring less precise machining;
• used to structurally bond a slip fit that completely eliminates the need for an interference fit.
Bonded slip fits require no machining to tight tolerances and are as reliable and effective as interference fits.
Traditional interference fits generate strength solely from the metal-to-metal contact of surface peaks, an area that represents only 15% of the joint surface area. Liquid retaining compounds fill the surface irregularities and clearance gaps between metal parts, then cure to create a very dense and high strength adhesive bond that increases joint strength and achieves maximum load transmission. The cured resin increases the area of surface contact to 100% so the distribution of stress and joint reliability are improved and part life is increased.
Components bonded with retaining compounds do not require a fine surface finish and allow relaxed tolerances for joining dissimilar metals. Adhesive bonded assemblies experience less stress in the joint and are more robust under differential thermal expansion. This means that softer substrates like aluminum will not be damaged by force or thermal expansion and lose their interference.
Retaining compounds prevent fretting corrosion in interference fits. Fretting corrosion results when micro-movement occurs between a bearing and a shaft or housing, causing small sections of the assembly to weld together. When the assembly moves the next time, these small welded sections rip off and result in a cratered surface. By filling the inner space between interference fitted parts, retaining compounds prevent micro-movement that is the root cause of fretting corrosion.
Retaining compounds can replace brazing and welding with a quick and simple process that does not require specialized equipment, skilled operators, consumable components or equipment maintenance. Unlike brazing and welding, retaining adhesives will not create inherent burn and fume hazards and can be quickly applied and cured.
Application and removal
In their liquid form, retaining compounds may be applied using precision manual, semi-automatic and fully automatic dispense equipment.
An example of a fully automated high precision dispenser is the Rotospray applicator, which applies a metered amount of adhesive in a 360° bead or band onto a pre-defined location in the bore of a part. Rotospray application is precise and repeatable and the applicator can be positioned on any axis and operated in either semi or fully automated mode.
Hand pumps are precision applicators for manual dispense that control the volume and location of adhesive. The heart of the hand pump is an adjustable stroke peristaltic pump that dispenses a metered amount of adhesive each time it is manually actuated.
Parts assembled with retaining compounds come apart with the standard disassembly methods for press and shrink fit components. These methods range from gear pullers to hydraulic oil injection to hydraulic presses. No special processing steps are required to break the adhesive bond. You can select from either medium or high disassembly strength formulations. Medium strength retaining compounds may be preferred for joints designed for routine disassembly or service.
Advances in chemical retaining
Recent advances in chemical retaining have made these materials more robust. Formulated for simpler processing, the latest retaining compounds do not require cleaners, primers or activators to enhance cure speed or strengthen bonds. Once assembled, they resist higher temperatures than earlier formulations.
While clean bonding surfaces deliver the best results for any assembly, the newest oil tolerant retaining compounds are formulated to cut through oils and surface contaminants. The result is consistent bond strength despite variations in the cleaning process.
Historically, a chemical activator or primer had to be applied to non-ferrous parts to cure anaerobic retaining compounds. The newest primerless retaining compounds cure on these surfaces as well as they cure on steel. The primer is no longer required to achieve polymerization, making the process robust and eliminating an assembly step.
Size, weight and efficiency are key parameters for most new interference or slip fit designs, targets that drive assemblies to higher operating temperatures. The latest temperature resistant retaining compounds extend the continuous operating temperature ranges from -5 4°C to 230°C (-65°F to 450°F). Even when high temperature capability is not required, these formulations increase the safety and robustness of the assembly’s design.
Gold Medal Retaining
Pro snow cross racer and freestyle rider Levi LaVallee, along with his gold medal winning snow cross race team, relies exclusively on Polaris as their racing machine of choice. LaVallee holds nine Winter XGames medals, including six golds, and holds the world record for the 412.5-ft distance jump achieved on his snowmobile in 2010. Levi attributes much of his success to the reliability and performance of his Polaris sleds.
“The one thing that makes you feel good about doing crazy things like distance jumps and double back flips is knowing that, when Polaris is putting their sleds together, they’re using Loctite,” said LaVallee. “The racers that we’re running now are the Polaris IQR600. We’ve had that chassis for the last few years and we’ve absolutely had phenomenal results with it, from XGames gold medals to the world distance jump to winning snow-cross championships. It’s just a great machine and it says a lot about Polaris and how strong their equipment is, how well they build their machines.”
Polaris engineering specifies Loctite products throughout their manufacturing process. In addition to the threadlocking adhesives found on every threaded fastener assembly on the machine, Polaris has a number of retaining applications on their sleds.
These adhesives are critical on the main bearing interfaces. For bonding the snow drive clutch cover bearings and the moveable sheave bearings, the clutch has to operate reliably in an environment with temperatures ranging from -40F to +200F. In this case, Polaris engineers specify Loctite 648 to ensure that the bearings are fully seated and that they retain their position throughout the life of the machine. Loctite 648 high strength bearing retainer provides excellent performance for dynamic, axial and radial loads. The adhesive cures on metals without an activator and cures through contaminants including oils, cutting and corrosion protection fluids, minimizing time spent cleaning parts.
On the snow clutch drive, Loctite 648 is hand applied to the inner diameter of the moveable sheave and the bearing is pressed in place. The retaining adhesive increases the component’s reliability and ensured the bearing will not fail prematurely. In a second clutch application, Loctite 620 secures the spider assembly to the stationary shaft, possibly the most important part of the clutch assembly since the weights react off the spider assembly and allow the clutch to function as expected.
For applications in the engine that reach operating temperatures up to 450F, Loctite 620 high temperature, high strength retaining compound endures temperature extremes. Specifically for engine applications like retaining the crown shaft bearings, Loctite 620 is actually formulated to survive higher temperatures than the engine is likely to achieve at its hottest. To accelerate cure time during assembly, Loctite 7088, a non-aerosol primer stick, is applied to the fitting before adhesive is applied.
When LaVallee attempted his world record distance jump on New Years Eve 2012, he blasted off the takeoff ramp at 100 mph, launched through the air across part of San Diego Bay and touched down forcefully more than 400 feet later. The sled was fine, despite having come down hard at 111 mph from a maximum altitude of approximately 70 ft in the air onto a landing platform made of metal and wood. After some celebration, Levi jumped back onto the sled, which started right up and ran in top form. LaVallee credits Loctite threadlockers and retaining compounds for the performance of his sled.
LaVallee also shared a very personal application for Loctite retaining compounds that he considers critical to his safety during freestyle competition. He defies gravity, flipping his 500 lb snowmobile high in the air while performing unbelievable jump flips. In 2013’s gold medal winning performance, LaVallee completed nine jumps in 75 sec and nailed huge extensions and perfect landings on a no-hands back flip, a no-feet backflip and a backflip with his hands on the handlebars but feet off and fully extended to his back bumper. To control his sled and launch solid extensions during flips, LaVallee maintains a death grip on the handlebars.
“If your handlebars rotate forward or back, even just a little after you’ve clamped them as tight as you can, you can be in major trouble,” said LaVallee. “Before the competition, I asked Henkel if they could help me stop my handlebars from moving. They suggested using a retaining compound to lock the bars in place. I haven’t had an issue with movement since. Loctite is as good as gold.”