From a designer’s perspective, the combination of interference fits with retaining adhesives considerably opens up the potential for innovation. In particular, bonded shrink-fit assemblies offer solutions that are certainly greater than the sum of their parts, paving the way for new design concepts and cost savings. As well as enhancing the reliability and service life of an assembly, this combination opens up a wealth of design opportunities. Higher load transmission and performance can be obtained from existing design and geometry solutions as well as the potential for equal performance with relaxed tolerances and the reduction in size and weight of the assembly.
Retaining compounds are liquid anaerobic structural adhesives that cure or polymerise when confined without air between close-fitting metal surfaces. They are commonly used in cylindrical assemblies to bond one load-bearing part into another – such as a bearing into a housing or onto shafts, or fixing gears and pulleys onto shafts. They are also ideal for applications such as sealing engine and boiler core plugs, fixing oil filler tubes in castings and restoring the accuracy to worn machine tools.
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 that abrade and reduce the contact surface even more. This leads to corrosion, accelerated wear and ultimately, failure.
Liquid retaining compounds fill the surface irregularities and clearance gaps between the 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 increased.
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 costly remedial machining.
Welding or soldering are limited to compatible metals and the parts can be distorted by the high temperatures required. Heating of the material can lead to residual stresses and structural degradation and additionally make disassembly for subsequent maintenance difficult or even impossible.
The bigger picture
Used on their own or in combination with traditional assembly methods, retaining adhesives can enhance product design way beyond the commonly appreciated benefits. Firstly, 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. To 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 are painstakingly calculated to achieve maximum friction, failure can occur. And 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.
The potential to create assemblies that are more compact and lighter in weight by using friction joints and engineering adhesives is also considerable and good examples of these qualities can be found at Ford, Bridgend, the manufacturing plant for the Volvo S16 short, six-cylinder engine. A design upgrade required the drive gear to be securely fitted on the crank to ensure no movement. Laser welding and bolting the drive gear were options but that 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 as it avoided the high capital cost involved in laser welding.
Another cost saving on the Volvo S16 involved the potential re-design of a mid-shaft bearing. To achieve the strength needed to prevent bearing spin and resultant wear, an increase in wall thickness was originally proposed that would have increased the weight and cost of the component. However, by introducing engineering adhesives into the assembly, Ford was able to structurally bond the slip fit bearing into the lightweight housing, completely eliminating the need for a friction fit.
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. For example, powdered metal or aluminium may fail while steel will easily achieve the required joint strength. A retaining adhesive strengthens the overall assembly, making substrate selection less critical to parts performance.
Recent advances in retaining compounds have made these adhesives even more robust. Formulated for simpler processing, the latest adhesives do not require cleaners, primers or activators to enhance cure speed or strengthen bonds. And once assembled, they resist higher temperatures than earlier formulations.
Size, weight and efficiency are key parameters for most new interference or slip fit designs, targets that drive assemblies to high operating temperatures. The latest temperature resistant retaining compounds extend continuous operating temperature ranges considerably. And even when high temperature capability is not critical, these formulations increase the safety and robustness of the assembly’s design.
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