Structural analysis of an electrical switch box has allowed TWI to come up with a cost reducing design. W Lucy & Co Ltd made about 2000 tank enclosures a year for medium voltage switch gears at the time of TWI's involvement. Because of these increasingly large production runs, Lucy approached TWI to establish whether a reduced weight stainless steel, the use of which would reduce material costs, could fulfil the functional requirements of the product and allow the company to use its robotic facilities to improve manufacturing efficiency.
Finite element analysis
The design was based on 4mm thick stainless steel sheets which were welded together to form an enclosing tank. Additional welds were made to stiffen the structure, with stainless steel plate ribs outside the tank and mild steelplate stiffeners on the inside. TWI was asked to study deformations and stress levels in the design using finite elements analysis (FEA) and then to explore the possibility of reducing the weight of the box using thinner sheet materials.
Using drawings supplied by Lucy, TWI was able to model the existing design using FEA; critical stresses and deformations under service and design conditions were noted for future comparison. Proposed modifications to the stiffeners were then incorporated into the model so the effect of these changes could be studied. Additional information was recorded for changes to the enclosure thickness.
Conclusions
Of the six models analysed, 3mm thick stainless steel sheet used for vertical side plates offered the most promising solution. This involved radical changes to the tank's stiffening arrangements, but resulted in stress levels and deformations which compare favourably with those of the existing design.
TWI recommended that Lucy should manufacture a tank incorporating the proposed design changes so that it could be tested under simulated service and design conditions. The new design reduced Lucy's material and manufacturing costs.