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W T Henley Ltd
Design Optimisation of Plastic Holding Tool
 
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Company Profile

The original company was formed in 1837 by William Thomas Henley, who paved the way for transmission of electricity around the world. Today W T Henley manufactures an extensive range of electrical equipment used throughout the electrical supply and distribution networks, from sub-station to the end consumer. It is also a market leader for cable joints and terminations, distribution equipment and overhead line insulators. The diagram on the right depicts some composite tension insulators designed and manufactured by W T Henley. A team of experienced design engineers and a modern bulk manufacturing facility ensure that the company stays at the forefront of modern technology using the most advanced materials while maintaining the high standard and on-time delivery of their products.

 

The work reported here describes an optimisation project undertaken by CADFEM UK CAE Ltd. to perform a structural analysis and design optimisation study of a plastic holding tool design, for W T Henley Ltd. A prototype of the tool is shown on the left. The tool is used to hold a break/make connector for use in the electrical power industry. It is designed to operate at a maximum torque of 12Nm, which is applied to a bolt at the top of the tool. The bolt then drives a blade to cut through a cable within the break/make connector. The tool is currently made of aluminium. However because of cost and safety reasons it is desirable to replace the metal with some plastic materials, ideally unreinforced Nylon 66, which is one of the most inexpensive and readily available plastic materials. W T Henley has produced some prototypes made of plastic resin, based on the metal counterpart.

Failure was initially detected in the plastic resin around the threaded metal insert, following which a steel pin and a vertical web were incorporated into the top component. However the latter design was found to fail in the bottom component at the interlock section.

The picture on the right illustrates the top component reinforced with a steel pin (concealed from view) and a web, and the subsequent failure at the interlock section. A nonlinear contact finite element analysis and optimisation solution has been performed by CADFEM UK CAE Ltd. to study the large deformation and contact response of the tool and to obtain an optimum design with minimum material usage.

Simulations Details

A two-dimensional plane strain, large strain, large displacement contact analysis was considered for the tool design. The web reinforcement was assumed to add considerable in-plane stiffness to the top component. Due to symmetry only half of the model was subsequently meshed and analysed in the ANSYS program using the higher order 8-noded elements, ignoring the effect of the handle. The first part of the project was to analyse the plastic tool design with the original dimensions. The graphic below left shows the equivalent stress at the interlock region, at 55% of the design torque. The maximum stress has already exceeded the yield stress of about 95.6MPa for unreinforced Nylon 66.

The results and the critical area of the design compare favourably with the experimental results obtained by W T Henley. Based on the results a number of modifications were made. The most important factor was found to be the angle of the contacting surfaces, which was set to be 45 in the original design. The stress was found to decrease with the angle. To avoid the dislocation of the interlock section when loaded, although very unlikely, the angle was set to a smaller angle of 10 for the subsequent design optimisation. The graphic below right shows the finite element model and the boundary conditions for the modified design. The purple coloured elements depict the reinforced steel pin. The design torque of 12Nm was translated into an axial force of 3575N, given the pitch angle of the thread and the coefficient of friction between the bolt and the threaded insert.

The axial force was applied as a uniform pressure per unit thickness to the bolt region, as shown in the same figure. Contact elements were created between the top and bottom components at the interlock section, and between the inside surfaces of the two components and a rigid surface representing the break/make connector.

The fillet radius for the inner corner of the bottom component and the tip radius of the top component, and the thickness of the bottom component were chosen to be the Design Variables.

The yield stress of 95.6MPa was set as a State Variable, which represented the operational ceiling limit for the maximum stress. The Objective Function was set to be the total area of the model, which was to be minimised by ANSYS while meeting the limit of the State Variable. Using the sub-problem approximation method the optimisation solution converged to an optimised design after 6 loops. The graphic left shows the optimised dimensions of the bottom component at the interlock section. The stress result for the optimised design at 100% design torque is shown in the plot above right.

Benefits

By replacing the aluminium with unreinforced Nylon 66 the weight reduction and cost savings in both the material and manufacturing process are enormous, as opposed to an 18% increase in the total volume of the holding tool. Russ Challis, the Design Engineer at W T Henley, was very impressed with the outcome of the project. "The product will provide an extremely safe and fully insulated tool. It eliminates any risk, no matter how small, of accidents during installation of break/make connectors."

"Having prototyped the product and had it fail several times over, we were impressed by the initial studies by CADFEM UK CAE Ltd. these confirmed our own in house results. This provided us with a high degree of confidence in their final design proposals. They quickened our development time by eliminating further expensive modelling, prototyping and testing phases of the project. And once the design parameters were confirmed it gave us the platform to consider using alternative materials should, for any reason, Nylon 66 fail long term fatigue testing."

"Personally I would now be prepared to use FEA at the outset of any future project, rather than go through endless cycles of prototyping and testing."

 

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