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ENGINEER'S NOTEBOOK PLASTIC SPRINGS
Page 52
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spigots mounted in surface plates which are bonded to the spring.
A sales coup just announced by GKN involves the first production order for hybrid springs and the first GKN spring contract with Japan. They are to be fitted front and rear on the new Mitsubishi eight-tonne GVW FK416 truck chassis, The Japanese have expressed interest in manufacturing composite springs themselves under licence from GKN. The company welcomes this in principal, if it can keep its manufacturing secrets.
Hybrid leaf springs represent a cost as well as a weight compromise. They are less expensive than all-GRP springs, but costlier than two-leaf steel parabolics. When making comparisons, however, it should be remembered that price differential between the two materials is constantly changing.
Thanks to slicker maufacturing techniques, prompted in part by greater production volumes, composite spring prices have come down in real terms, but at the same time steel springs have become available from a wider range of sources. A premium British-made spring for a 10tonne axle could cost 20% more than a GKN composite. but a steel spring of questionable quality sourced in the Far East might cost 40% less.
The doubts about all-GRP springs shown by heavy truck makers are not shared by their semi-trailer counterparts. The fact that virtually all trailers with metal suspension use double slipper-ended springs works in GKN's favour. So too does the spring shape needed; axle spacing on a typical tandem, of around 1.35m, limits spring length. A spring for a 10 or 11-tonne trailer axle is much shorter and chunkier than its driving-axle equivalent on a tractor or a 17-tonne four-wheeler.
Wearlite end-caps are used on GKN's slipper-ended all-composite trailer springs. The Wearlite material makes contact with the cam profiles and rebound stops inside the pivoting rockers and end shoes. No smoothing or other preparation is necessary on the cam surfaces when they are new, says GKN. If direct rolling/scuffing contact was permitted between the spring laminate and new rough-surfaced shoe cams, damage could result.
An 800km endurance test on a laden semi-trailer tipper over Belgium presented no problems for GKN's latest Wearlite end-capped composite springs. Running the same combination on the same route with standard monoleaf steel springs produced sparks and a red glow from the spring ends and shoes, says GKN.
Air suspension is rapidly becoming a standard fitment on tri-axle semi-trailers, a move encouraged by the UK weight concession allowing eight rather than 7,5 tonnes to be imposed on each axle, if air sprung. Could this squash GKN's ambi lions? Morely says that among the heaviest items in most air-suspension systems are the fore-and-aft radius arms in the form of quarter-elliptic springs. These can be made in weight-saving GRP composite, and GKN has produced several examples for evaluation.
To date OKN's spring designers have been constrained by the need for their products to be direct replacements for existing steel springs. They would welcome the opportunity to be involved in an allnew truck development project where the springs could be designed from scratch.
Composite semi-elliptic leaf springs would ideally be wider but thinner than they are now, to improve ride and handling characteristics.
On passenger cars, GKN has prodduced numerous prototype springs for fitting transversely on vehicles which have independent (double wishbone or MacPherson strut) suspension.
Transverse leaf springs were last used on the front axles of sit-up-and-beg Ford Populars of the fifties, where low cost and simplicity overruled performance. But tin ability, using GRP, to shape the spring precisely in all three planes has transformed the potential of transverse units.
A transverse spring clamped to the chassis at two points (typically on either side of the engine) and cantilevered out t support wishbone-located stub axles serves as its own anti-roll bar. The bend ing loads in the spring induced during cornering react with each other through the two pivots.
A GRP spring does not necessarily have to be a leaf spring. An alternative i; a "sulcated" compression spring, moulde, in a regular wave-form, which extends ai closes like one wall of a bellows. A sulcated spring with suitable end fittings could be used as a substitute for a heavi( steel coil spring or torsion bar in an IFS installation.
Sulcated springs have been made by GKN but they are not high on the list of development priorities. The transverse leaf spring is the preferred IFS solution. Ei by Alan Bunting