Henkel has helped Ford's engineers to develop a heat shrink bonded joint for assembling a steel drive gear onto the steel crank of a straight-six engine; this offers advantages over both laser-welded and bolted joints.
Where optimal joint strength is the main requirement, heat shrink bonding can be the best technology. The heat in the assembly helps to create a rapid cure and the contraction of the outer component exerts a compressive load on the curing adhesive. As a result, the total strength is far in excess of the combined adhesive shear strength and dry interference strength.
Heat shrink bonding is most commonly used for joining steel to steel. However, it is also suitable for joining dissimilar materials for which consideration needs to be given to differential expansion during service.
Henkel has considerable experience of shrink bonding similar and dissimilar materials in cylindrical joint configurations. Its knowledge helped Ford to engineer a Loctite-bonded joint that has saved costs as well as guaranteeing the required performance from the assembly. The process is now employed in the production of the Volvo SI6 (short inline six) straight six-cylinder engine at Ford Bridgend.
The components of a shrink fit joint have an interference fit at working temperature but are assembled with a clearance. To achieve this, the outer component is heated and the other, to which the adhesive is applied, is kept cold. This was the method Henkel recommended to Ford for securing a steel drive gear onto the steel crank of the Volvo SI6 to ensure no movement.
Laser welding was an option but a clear drawback would have been the capital expenditure involved. Bolting the drive gear to the crank was also evaluated but deemed unsuitable as it would have increased the overall weight and size of the engine. Henkel was asked to assess the suitability of bonding the structure and it was a process that involved a considerable amount of testing and finite element analysis (FEA) to prove its suitability.
Henkel has many tools at its disposal to assist in finding the best process and adhesive product for the task, including extensive laboratory and testing facilities both here in the UK and in mainland Europe. The company has also developed software-based analysis tools, one of which was used for the Ford project.
This PC-based software programme is called RetCalc plus – short for retaining calculation – and it predicts the strength of metal assemblies bonded with Loctite anaerobic retaining compounds. Any manufacturer can ask for this analysis free of charge.
The procedure is simple: Henkel inputs known information supplied by the customer and RetCalc 'calculates the unknown' - so, by supplying the loads and performance required, RetCalc will recommend the most appropriate substrates and joint design. Alternatively if the substrate is known, then the load that can be applied in specific environments can be calculated.
Other calculations can include minimum and maximum taper diameter, joint length, clearance and assembly temperature. In other cases, details of the operating environment, temperature extremes, the continuous operating temperature and operating medium might be known, but calculations involving tolerances are needed.
With more design engineers specifying adhesives in their original designs, RetCalc is proving invaluable for many companies. It helps to calculate the parameters of designs and, as a consequence, saves time and cost, thereby ensuring that much of the testing work is completed before any metal is cut.
In Ford's case, RetCalc was an important element in proving the suitability of its specially developed Loctite 128467 anaerobic adhesive for the high-strength bond. This slow-curing product is optimised for the process and is now creating an instant bond between drive gear and crank that has all the performance qualities required by Ford.