Leaf-spring Design Advanced
Page 51
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ANEW type of semi-elliptic spring, known as the Featherlight. has been produced by Toledo Woodhead Springs, Ltd., after four years' research and development. It is claimed to be up to 35 per cent. lighter than comparable types, correspondingly cheaper, and more fully " engineered " than any other spring introduced so far.
The secret behind the Featherlight spring hinges partly on the calculations which have been put into its design. Mr. Frank Woodhead, chairman and managing director of Toledo Wood head Springs, had for many years believed that conventional semi-elliptic springs could be improved and made more efficient.
His own idea was that tapering the ends of the spring leaves would go a long way towards improving efficiency, and an experimental spring was produced accordingly.
Although only roughly constructed, this spring showed definite promise and it was then decided to elicit the help of Pfint and Partners, Ltd., consulting engineers. Thus Mr. Michael A. Plint and Prof. S. J. Davies set to work to discover what made the leaf spring " tick."
Spring Design Lax Their investigations revealed that some conventional types of multi-leaf spring were anything but efficient. Although they were simple and generally trouble-free, they were in most eases wasteful of material. There was uneven stress distribution throughout the leaves and this necessitated a far greater mass of metal than was needed in theory..
These theoretical findings were confirmed by strain-gauge tests, and subsequent calculations using rational mathematical methods led to the production of designs in which an ideal state of uniform stress throughout the spring was approached far more closely than .ever before. This brought about large savings in material and cost.
An important feature of the new design of spring is that, although the leaves have to be made in a special machine which was constructed by Toledo Woodhead
Springs and about which the makers are unwilling to disclose any information, extreme accuracy of manufacture is not essential. to obtain the results arrived at by PlMt and Partners.
Indeed, the dimensional accuracy of the springs used for tests was less than that of corresponding conventional springs.
The experiments have been conducted so far only with single-rate springs and have shown that a private-car three-leaf • spring can be replaced by a two-leaf Featherlight. spring, whilst the equivalent of a conventional six-leaf spring is a Featherlight four-leaf assembly.
In practical terms this can result in a saving of approximately If cwt. in the case of a 12-ton-gross goods vehicle. Furthermore, this weight saved is unsprung weight, improving roadholding characteristics of high-speed vehicles.
A further outcome of the co-operation between Plint and Partners and Toledo Woodhead Springs has been the development of the Toledo Woodhead-Plint pitch and roll -indicator. This is a recording device producing simultaneous records of pitching and rolling movements on squared paper.
The indicator incorporates a freely suspended gyroscope, which is positioned with its axis approximately perpendicular to the surface on which the vehicle is travelling. The gyroscope carries a pointer which marks the recording surfaces, these being fed through the machine. at right angles to each other.
The machine has the advantage that the record is available for immediate inspection after a test run has been made