Retaining bearings, shafts, bushes and other cylindrical assemblies. How modern engineering adhesives can help in achieving better assemblies at reduced cost?
Increasing competition and customer expectations have forced pump manufacturers worldwide to look for newel ways to reduce the process costs and improve the operational reliability of their products. Increase of cylindrical assembly strength by employing modem adhesives helps the pump manufacturers in achieving lower assembly costs while also increases the reliability of the most critical assemblies in the pumps, thereby improving the overall dependability of the product.
The most common examples of cylindrical assemblies used in a pump can be: assembly of sleeve to bore, assembly of piston shaft to bearing and gears shaft to seal assembly, and assembly of shaft to impeller. These assemblies are subjected to various loads and stresses - axial pull, torque, shock, vibration, expansion or chemical attack, etc, - and their life expectancy depends, to a large extent, on the accuracy of Fitting. There are, however, other adverse conditions such as corrosion, fatigue of parts due to elastic strain, pressure between parts, work hardening, fretting, etc, which adversely affect the reliable life of these assemblies.
Based on the performance required, any of the following conventional methods of assembly have traditionally been used:
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. Clearly, using modern adhesives to retain cylindrical assemblies in the pumps could be the answer to both, the customers requirement of an improved product offering and the finance managers expectation of a better bottom line.
Selecting the Right Engineering Adhesive
It is very important to select the right adhesive for any assembly to extract the maximum benefits that the engineering adhesives offer. The selection depends on a variety of criteria that include the operating conditions, size of the assembling and functional requirements. Loctite offers the widest range of engineering adhesives to meet the specific assembly and operational requirements. Further, in order to enable the selects and application of the right engineering adhesive, it offers assistance right from design to implementation. This has enabled Loctite to partner the worlds leading pump manufacturers in improving the performance reliability of the design and economise the production costs.
Even with finer finish, surface irregularities reduce actual surface contact to around 20-40% 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 in cracks that are difficult to identify.
Henkel Loctite invented the anaerobic adhesives in 1950s. However, these one-part anaerobic adhesives were 1st used for "retaining" the cylindrical assemblies only in 1963. Through the sixties and into the seventies technical advances were made and both manufacturing and maintenance engineers discovered and adopted anaerobic retaining technology. Today, these are the preferred method of retaining cylindrical assemblies for pump manufacturer 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.
Loctite anaerobic retaining compounds help prevent the onset 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 are then assembled and within a few minutes the adhesive be- gins 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% 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 listed as under:
- 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 cost 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.
All the above benefits aid in reduction in manufacturing costs.
-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.
The above factors significantly improve the reliability of the assemblies.
In fact, 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 reduces the manufacturing costs, while improving the reliability of the assembly. Table I shows the test results to prove a point in the case.
It is obvious 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.
|Posted : 8/12/2005|