Adhesives simplify the design, production and assembly of parts when compared with many traditional mechanical methods. For example, adhesives can more than double the strength of a press fitted assembly. Furthermore, experience shows that such assemblies have longer service life and can be reused after cleaning. Bonding can also allow the use of thinner wall components compared with interference-fit methods. So, what considerations are necessary to optimise the design of a bonded assembly?
In a correctly designed adhesive retained assembly, the adhesive will be subjected to a shear load during use. Relatively small changes in detail design can lead to large improvements in ultimate strength. Consider a shaft to which components are to be bonded. Conventional machining techniques create a lay in the surface that results in a coarse finish in the axial direction of the shaft and a fine finish in the circumferential direction. Note should therefore be taken of the loading direction when specifying the surface finish of a component.
Moreover, to minimise the possibility of the adhesive being scraped off the surfaces during assembly, chamfers should be specified on the leading edge of both male and female components. A 1mm chamfer of between 15 and 35 degrees to the axis of the shaft is recommended. And when two or more components are to be fitted to one end of the shaft, the construction should be designed with steps between each bond area to allow easy assembly.
The use of retaining adhesives with tapered assemblies eliminates potential fretting and increases load carrying capacity. Tapered bonded assemblies ensure a closely controlled concentricity and are an effective way of ensuring good adhesive coverage.
In practice, stresses in cylindrical assemblies will be concentrated at the edge of the joint area. This means that increasing the joint length will not necessarily lead to a proportionally stronger assembly.
Designers can create favourable geometries to deal with stress concentrations. For example, where the diameter of a shaft is 100 units, the engagement length should, ideally, not be less than 20 units.
Dissimilar materials with different coefficients of thermal expansion are often used in cylindrical assemblies, and this can result in large, undesirable tensile strains being applied to the adhesive film as the assembly reaches operating temperature. There are three techniques to ensure that adhesive bonds are able to accommodate thermal expansion.
First, an adhesive may be used to supplement an interference fit. Provided a small interference is maintained throughout the total operating temperature range, the assembly will function successfully. Secondly, where there is a clearance fit, the relatively low modulus and high coefficient of expansion of adhesives means that it is possible to eliminate or reduce tensile stress in the joint. This is achieved by specifying tolerances to ensure a substantial film thickness of adhesive is present and thus minimise the tensile strain on the product.
Thirdly, a shrink-bonded clearance fit should be specified (typically 0.05mm clearance for a 50.0mm diameter), when the outer part has a higher coefficient of thermal expansion than the inner section. The adhesive should be applied to the male component, the female component heated (to around 170ºC), then the parts assembled.
Components such as splines and keys are susceptible to fretting because of the backlash inherent in their design. Retaining adhesives fill all the voids in the assembly - eliminating movement and hence fretting between the two parts. The axial load strength of the adhesive bonded assembly means in some constructions there will no longer be a need for restraints, such as circlips, collars or pins.
Studies have shown that the maximum actual metal-to-metal contact between interference fit surfaces of a shaft and hub is only 25 to 30% - even in the most severe press or shrink fit joint. When an adhesive is added to the assembly, the contact area is raised to 100 per cent, allowing the whole joint area to transmit shear and compressive loads. This means higher loads can be transmitted by the same size of joint.
There is no doubt that many designs have been enhanced through the use of adhesives. Smaller and lighter components, simpler design, sealing against the elements are just a few of the benefits. The ideal time to consider the use of adhesives is right at the concept stage. A great deal of time and heartache (and money) can be avoided by incorporating the various bonding technologies into initial designs.
Bob Orme is senior technology specialist at Henkel, makers of Loctite brand adhesives
- Henkel has published a handy selector chart for Loctite brand products that are approved for use in the defence industry, including threadlocking, retaining, gasketing, and threadsealing adhesives, cleaners, flat face bonding adhesives, an epoxy filler, an O-ring kit, gasket removers and an anti-seize product. The chart lists both the Loctite product number and NATO stock numbers.