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How engineering adhesives can help achieve better assemblies at reduced cost

Retaining bearings, shafts, bushes and other cylindrical assemblies with engineering adhesives
 When it comes to the use of bearings, one option involves the incorporation of engineering adhesives for both original design and maintenance applications.
Adhesives used in cylindrical assemblies bond one part that is inserted into another, are referred to as ‘retaining adhesives’ or ‘retainers’. The technology behind adhesives that are employed for such bonded applications is known as anaerobic. As such, they cure in the absence of air when in contact with metal surfaces. These adhesives are applied as a liquid that not only covers the area of the components to be joined, but also fills all the surface irregularities. As the surfaces are brought together and the air is excluded, the adhesive begins to harden to form a tough plastic. This action not only bonds securely the parts but, because the adhesive provides 100 per cent contact over the entire surface, it also prevents any moisture from entering the joint.  Beyond that, retaining bearings through the use of anaerobic adhesives means that fretting can be eliminated, joint strength can be increased, designs can be simplified and, significantly, worn parts can be reclaimed.
Increasing competition and customer expectations have forced machinery and equipment manufacturers worldwide to look for newer ways to reduce this process costs and improve the operational reliability of their products. Augmentation of cylindrical assembly strength by using engineering modern adhesives helps the machinery manufacturers in achieving lower assembly costs while also increases the reliability of the most critical assemblies in the machines – thereby improve the overall dependability of the product.It is obvious that though the interference fits offer a better alternative to complex assembly methods such as keyways and splines; requiring fewer parts, simpler and generally less expensive design and resulting in improved performance; even this method has serious drawbacks.
Even with finer finish, surface irregularities reduce actual surface contact to around 20-40 and of the available surface area. This considerably reduces the actual frictional pressure between the mating parts that keeps them together and reduces the retaining effectiveness. The micro gaps between the parts allow the entrapped air to fret and corrode the parts – which develop into cracks that are difficult to identify (refer Table 1).
Anaerobic adhesivesLoctite anaerobic adhesives were invented in 1950s and today, these anaerobic adhesives are the preferred method of retaining cylindrical assemblies for machine manufacturer’s world over.
The curing of Loctite anaerobic products is a process in which a liquid cures and hardens into a thermoset plastic material. This hardening process will only occur in the absence of air when confined within the metal surfaces (non-metal parts can be used). The cured material has a very high cohesive strength and joins the mating parts to their ultimate assembly strength.
Anaerobic adhesives for retaining applicationLoctite anaerobic retaining compounds help prevent the onsets of loose bearings, worn keyways and housing and fretting corrosions.
The retaining compound is normally spread on one of the mating surfaces either as a bead or coating) directly from its original container – a plastic bottle – or by automatic or semi-automatic application equipment. The parts then assembled and within a few minutes the adhesive begins to cure to allow final positioning of the components. Handling strength is usually achieved within 30 minutes while complete strength can be realised within 4 –12 hours depending on the grade of the adhesive. This curing time can be improved with the help of an activator. The controlled strength of Loctite retaining compounds enables dismounting of the assembled parts using the usual hand tools.
The anaerobics being liquid in nature, flow and fill all the spaces and surface irregularities between mating parts and provide a 100 and surface contact. It is possible to assemble even mating parts of the dissimilar materials.
The benefits of using Loctite retaining adhesives in place of the conventional methods of assembly can be summarised as under:
Reduced manufacturing cost•  Due to the high cured strength over a wide range of gaps, Loctite retaining compounds allow usage of clearance or light press fits – thereby eliminating the high costs of precision surface finishes and close tolerances between mating parts•  Eliminate the processing costs involved in producing the press or shrink fits by avoiding using expensive machines such as presses, grinding / honing machines and heaters.•  Eliminate precision machining and additional material costs involved with keyways and multiple splines.•  Reduced rejections of mating parts. Significantly lower assembly times. Improved production efficiencies.•  Reduced inventories of multiple locking accessories / fits.
Improved assembly reliability•  By filling the surface roughness and clearances with a rigid thermoset plastic, the area in surface contact is greatly increased – improving the stress distribution.•  The effect of pressure between the surfaces are reduced or eliminated.•  Improved fatigue resistance.•  Consistency of load carrying performance is improved even over uncertain tolerances.•  Eliminate the fretting corrosion.Where can retaining adhesives be used?Slip FitsUsing an adhesive to bond parts enables the male and female components to be slip-fitted into place instead of being forced together. Yet despite the ease of assembly, the bonded slip-fitted component can transmit three or four times more torque than an unbonded press-fitted unit. Not only is it stronger but has no assembly stresses and eliminates remedial work such as reaming and shaft straightening.
Press and Shrink FitsThe use of retaining adhesives does not necessarily entail re-design. Used together with existing interference fits, they can uprate an assembly to withstand higher loads without the need for modifications. Used on heat shrink fits they will provide high resistance to complex dynamic loading and differential thermal expansion particularly where dissimilar materials are used. Bonded interference fits are also an ideal solution for situations where interference is limited by yield stress and component thickness.
How Loctite helps in doing this?An increasing number of manufacturers use retaining compounds for improving the bond strength of their press fit or shrink fit assemblies; the usage of retaining compound allows them to relax the tolerance specifications – which reduce the manufacturing costs – while improving the reliability of the assembly. The following test results prove a point in the case:
It is apparent that the press fit joint bonded with retaining compound offers much higher retaining strength compared to one without it. In fact, at an interference of 0.05 mm the push out force required to disassemble the bonded joint is almost 3.8 times the push out force required to disassemble an unbonded press fit joint.  What also should be noted that this ratio is as high as 3.3 even when the interference levels are relaxed by 90 and to 0.005 mm.
 
Authored by:
 
Pradhyumna Ingle, Business Director – General Industry, Henkel Adhesive Technologies India Pvt. Ltd.
For more information, contact at marketing.ag@henkel.com or visit www.henkel.in / www.loctite.in

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