FEATURE FASTENINGS & ADHESIVES
Boosting the scope of shrink fits with engineering adhesives
Used on their own or in combination with traditional assembly methods, retaining adhesives can enhance product design way beyond the commonly appreciated benefits, as Henkel explains
N
ot only can bonded shrink fit assemblies enhance the service life
of an assembly, but they can pave the way for new design concepts and cost savings. It is possible to obtain higher load transmission from existing designs and geometries, and there’s the potential for equal performance with relaxed tolerances, and a reduction in size and weight of the assembly. Traditional interference fits generate strength solely from metal-to-metal contact of surface peaks, an area that represents a relatively small percentage of the overall joint surface. Small micro-movements at the joint interface can produce particles which abrade and reduce the contact surface even more. This leads to fretting corrosion, accelerated wear and, ultimately, failure. In a bonded assembly, anaerobic retaining adhesives fill the surface irregularities and clearance gaps between the metal parts, then cure to create a very dense and high strength 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 increased.
RETAINING ADHESIVES It is therefore easy to see why designers
are increasingly favouring retaining adhesives to replace, or complement, conventional assembly methods. When employed without structural bonding, pins or key/keyway assemblies, for example, have uneven distribution of mass and imbalance that can lead to vibration at high speeds. Splines and serrations cause high stresses due to the ‘notch effect’ that occurs around the key that will, during the service life of the assembly, require remedial machining. Welding or soldering are limited to compatible metals and the parts can be distorted by the high temperatures required.
A ROBUST SOLUTION So, let’s consider the time and cost involved in engineering an interference fit assembly with specific load capacities. Interference fits rely on friction alone to transmit torque, and to
At Ford’s Bridgend plant, which manufactures Volvo engines, a shrink bonded solution met the dimensional and performance requirements
achieve maximum joint strength and optimal performance these joints must be precise. 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 ‘unitise’ and operate as one. But even when the appropriate
allowances for interference fitted parts BLIND RIVET NUTS: FOR A VARIETY OF APPLICATIONS
The RIVKLE range of blind rivet nuts from Böllhoff was developed specifically for applications requiring a high load-bearing (threaded) blind fastening with exceptional pull-out resistance when used in thin-walled materials. Offering good vibration and noise damping, together with electrical and thermal insulation and tolerance compensation, these are used in a wide variety of industrial and automotive applications. In one unusual application, however, the nuts have been used to help climbers maintain their skills and fitness levels. The application demanded a high strength, dependable, fastening capable of firmly attaching climbing ‘handles’ to a polyester/glassfibre climbing wall, but which
would also allow the handles to be removed and reinstalled swiftly if needed. With the need for ‘instant’ demountability, coupled with the demand for high pull-out strength and absolute fastening integrity, the M10 RIVKLE rivet nuts proved to be the safest and most practical solution for attaching the handles.
Böllhoff Fastenings T: 01482 325425
www.bollhoff.co.uk
are painstakingly 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, interference fitted joints at reduced cost in less time than traditional interference fits by reducing the required dimensional precision. Since the combination of the interference fit and the retaining compound is much stronger than the interference fit alone, you do not need to go to extremes to calculate dimensional tolerances.
FURTHER BENEFITS The potential to create assemblies that are more compact and lighter in weight by using friction joints and engineering adhesives is also considerable. An example can be found at Ford’s
Bridgend plant which manufactures the Volvo S16 short, six-cylinder, engine. Here, a design upgrade required the drive gear to be securely fitted on the crank to ensure no movement. Although laser welding and bolting the drive gear were options, these would have increased the overall weight and size of the engine. Ultimately a shrink bonded solution
proved able to meet dimensional and performance requirements. It provided an instant bond that performed as well as a laser weld, without the risk of introducing stress in the crank. It was also much more cost-effective. The ability to increase the number
of viable substrate materials for an application is another major benefit of engineering adhesives. Without them, substrate selection for a reliable press or shrink fit may be limited because of the high levels of stress on the joined components. A retaining adhesive strengthens the overall assembly, making substrate selection less critical to parts performance. Of further benefit, advances in retaining
compounds have made these adhesives even more robust. They no longer require cleaners, primers or activators to enhance cure speed or strengthen bonds and, once assembled, they resist higher temperatures than earlier formulations.
Henkel
www.henkel.com
16 JULY/AUGUST 2018 | DESIGN SOLUTIONS
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