FASTENERS & SEALING
Shrink bonding of auxiliary drive gear from crankshafts; Henkel high-strength anaerobic adhesive provides an instant bond
part, both deform slightly to fi t together and the two parts ‘unitise’ and operate as one. But even when the appropriate
savings. Higher load transmission can be obtained from existing designs and geometries and there’s the potential for equal performance with relaxed tolerances and the reduction in size and weight of the assembly. Traditional interference fi ts generate
ADHESIVES B
ADVANCED Boosting the scope of shrink fi ts with engineering adhesives
onded shrink fi t assemblies not only enhance the service life of an assembly but also pave the way for new design concepts and cost
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. T is leads to fretting corrosion, accelerated wear and ultimately, failure. In a bonded assembly, anaerobic retaining
adhesives fi ll 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. T e cured resin increases the area of surface contact to 100% so the distribution of stress and joint reliability are improved and part life increased. It’s easy to see why designers are
increasingly favouring retaining adhesives to replace, or complement, conventional
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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 eff ect’ 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 diffi cult or even impossible. Used on their own or in combination with traditional assembly methods, retaining adhesives can enhance product design way beyond the commonly appreciated benefi ts. Firstly, let’s consider the time and cost involved in engineering an interference fi t assembly with specifi c load capacities. Interference fi ts rely on friction alone to transmit torque. To achieve maximum joint strength and optimal performance, these joints must be precise. T e interference fi t is machined to be imperceptibly bigger than the mating hole of the outer part. T e larger component is then forced into the smaller
allowances for interference fi tted 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 fi tted joints at reduced cost in less time than traditional interference fi ts by reducing the required dimensional precision. Since the combination of the interference fi t and the retaining compound is much stronger than the interference fi t alone, you do not need to go to extremes to calculate dimensional tolerances.
CAPITAL SAVINGS T e potential to create assemblies that are more compact and lighter in weight by using friction joints and engineering adhesives is also considerable and a good example 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 fi tted 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- eff ective as it avoided the high capital cost involved in laser welding. T e ability to increase the number of viable substrate materials for an application is another major benefi t of engineering adhesives. Without them, substrate selection for a reliable press or shrink fi t 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. Recent advances in retaining compounds
have made these adhesives even more robust. T ey no longer require cleaners, primers or activators to enhance cure speed or strengthen bonds. And once assembled, they resist higher temperatures than earlier formulations.
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