BUSHING

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 U.S.C. § 119(a)-(d) to GB 0720476.1 filed October 19, 2007, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to bushings and particularly, though not exclusively, to bushings for use in motor vehicle suspension systems.

2. Background Art

Typically, a bushing is located between a suspension component e.g. one end of a wishbone and the vehicle chassis or body. The bushing incorporates a resilient component for the purposes of cushioning shocks, reducing vibrations transmitted from the vehicle's wheels to the vehicle body and passengers and allowing movement of the vehicle relative to the vehicle body.

One known type of bushing consists of inner and outer metal cylindrical sleeves separated by a piece of rubber. It can be designed to have a linear or non-linear deflection versus force characteristic. Another known type of bushing is the hydraulic bushing, typically including a rubber portion having a plurality of interconnected cavities which contain hydraulic fluid. A hydraulic bushing typically has a non-linear, progressive stiffness whereby the amount it deflects tends towards a constant value with increasingly high loads.

The current automotive trend for large wheels and low profile tires results in increasing suspension loads needing to be withstood by the suspension

structure and bushings. One known solution to this problem is to make the bushing bigger, thereby increasing its load capability. However, large rubber bushes have a tendency to induce steering wheel shimmy and large hydraulic bushes tend to degrade impact performance, making the suspension feel harsh. A large bushing also introduces packaging problems in the design of the suspension.

US-A-2003/0137087 describes a series bushing comprising a first inner spring portion incorporating hydraulic fluid cavities and a second outer spring portion formed of microcellular polyurethane. The presence of the polyurethane material permits good isolation of low amplitude, high frequency vibrations but would be too weak to withstand high impact loads, such as a vehicle tire striking the edge of a pothole, for example.

SUMMARY OF THE INVENTION

According to the present invention there is provided a bushing comprising an inner resilient component connected in series with an outer resilient component whereby both components suffer a characteristic amount of deflection under any given applied force, and wherein the inner resilient component has a non-linear stiffness characteristic whereby its amount of deflection tends towards a constant value for increasingly high applied forces and wherein the outer resilient component has a substantially linear stiffness characteristic and whereby its amount of deflection under any given applied force is less than that of the inner resilient component at the same applied force.

The bushing may be advantageously utilized as a suspension mount in a motor vehicle, for example, between a suspension component and the vehicle's chassis or body. The arrangement of these two resilient components in series permits high shock loads to be reduced through the outer linear component preventing the increase in stiffness which is inherent in the inner component.

At low loads, the inner component makes the greater contribution to absorbing the shock, thus controlling the stiffness of the bushing, whereas at high loads, the outer component dominates the compliance of the bushing.

The inner and outer components may comprise any known suitable type of bushing material or device with the specified stiffness characteristic. For example, the inner may consist of a hydraulic bushing and the outer may comprise a piece of solid elastomeric material, natural or synthetic rubber. Alternatively, the inner may be composed mostly of rubber or polymer designed in such a way to achieve a progressive stiffness characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of example only with reference to the drawings of which;

Fig. 1 is a side view of a bushing according to the preferred embodiment,

Fig. 2 is a cross-sectional view along a line I-F of Fig. 1, and

Fig. 3 is a graph of applied force against deflection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The bushing of Figs. 1 and 2 comprises an inner hydraulic bushing in series with an outer rubber bushing.

The outer rubber bushing comprises a cylindrical piece of rubber 1 supported around its inner and outer perimeters by metal cylinders 2 and 3 respectively. This outer rubber bushing has a stiffness characteristic as illustrated by curve A in Fig. 3 i.e. its deflection increases in a substantially linear fashion with increasing applied force.

The inner hydraulic bushing is supported by a central metal portion 4 and the inner metal cylinder 2. This hydraulic bushing comprises an annular outer piece of rubber 5 which encloses two hydraulic fluid-filled chambers 6a, 6b, separated by two rubber portions 7a, 7b. This inner hydraulic bushing has a stiffness characteristic as illustrated by curve B in Fig. 3 i.e. it has a progressive stiffness whereby for comparatively low applied forces its deflection increases in a substantial fashion whereas for increasingly high applied forces, its deflection tends towards a constant value. For any given applied force, the amount of deflection is less for the outer bushing than it is for the inner bushing.

The two bushings are arranged in series operation and their combined effect is illustrated by curve C of Fig. 3. The combination results in a bushing which has a superior energy absorption quality compared with a single hydraulic or rubber bushing.

The example described has the further advantage of compactness, therefore relaxing packaging requirements on vehicle suspension design.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.