Is it a spring, or a clamp?
Original article date: May 2000
A new all-plastic integral spring design enables objects such as cables, tubing, paper and labels to be temporarily clamped to each other or to other objects. DAVID WRIGHT reports
Annoyance doesn’t have to be unproductive. Dr Brian Wybrow used to get annoyed when he put his hand in a peg bag and pulled out one by the wrong end. It encouraged him to think there might be a better way to design a peg. The result is an innovative double-ended clamp, manufactured entirely in acetal.
When one end of the clamp is squeezed, the other opens. The jaws can clamp and release objects independently of each other. An easy way to fasten small objects together, the clamp can have many applications. Wybrow says his double-ended clamp (DEC) will be useful in tidying up cables and tubing in the automotive, aerospace and electronic industries, as well as in medical environments. But the uses could be as domestic as attaching a pair of trousers to a hanger or hanging a decoration from a Christmas tree.
At first sight, the clamp looks deceptively simple. The secret lies in the integral spring design. Two identical parts snap fit together to form two natural fulcrums. Avoiding the metal torsion spring used in pegs altogether, Wybrow chose engineering thermoplastics from the Ticona to manufacture the two components. Each have spring arms either side of the snap-fit centre. The two parts can’t slide because of staggered keyways. When the two fulcrums snap together at the centre, the restoring forces in the spring elements create clamping pressure at the jaws
Jaws with real bite
The design can be modified to suit a particular task. The jaws, for example, can be modified to have flat contours and contain more than one pair of curved ‘cable grippers’. Arrays of the clamp could be stuck together and it can be made in almost any size. Wybrow says his clamp can be designed to create any jaw clamping pressure, by snap-fitting two identical fulcrums together. The spring can also be manufactured into an all-plastic single-ended clamp.
Instead of two fulcrums, the DEC could be moulded in one piece, although this has its limitations. The clamping of one pair of jaws would still cause clamping pressure at the other pair of jaws, but when clipping thin objects, such as paper, the first clamping of jaws does not provide sufficient pressure at the other pair to clip another piece of paper.
The materials selected for manufacture depends on the application. Wybrow worked in close cooperation with his material supplier Ticona and the toolmakers and injection moulders Adreco. If flame retardancy isn’t crucial, the C13031 grade of Hastoform acetal copolymer (POM) can be used. On the other hand, for applications which require UL94V-0 flammability ratings, the 2016 grade of Celanex thermoplastic polyester (PBT) is suitable. An area where this might come in useful is in automotive manufacture, to attach cables in the car, such as in wire harnesses and cables under the bonnet.
Wybrow presented a CAD model of his design to Ticona’s Milton Keynes technical centre, which performed a series of finite element analyses with MARC software. An important factor was to get the thickness of the plastic right in particular areas, to ensure a strong clipping force but no creep. The results of the analyses allowed the design to be optimised to provide the lowest and most even levels of stress and strain possible. Because it is a complex shape, Ticona also performed flow analyses which established the ideal gate position and ensured the component would fill correctly.
To evaluate the design further, Adreco produced a prototype tool. Adreco machined a cavity and core into inserts which could then be fitted to the Ticona standard bolster system. This system allows prototype tooling to be produced much more economically and quickly than conventional methods. Inserts can be modified to meet any changes in part design dictated by the test results.
The invention was shown publicly for the first time at the Tomorrow’s World Live Event in London. It received a highly commended award. Several patents which describe a number of product variants are pending, and Wybrow would like to form a joint venture with a firm that can manufacture and market the clamps.
- Wybrow
May 2000