1. Field of the Invention
The present invention relates to a mounting system and in particular but not exclusively to a counting system for motor vehicle engines/gearboxes.
2. Description of the Prior Art
Currently, vehicle engines/gearboxes are mounted to motor vehicles by means of compliant mounts which isolate engine/gearbox vibrations from the vehicle body. In a simplified form, the engine/gearbox mount may comprise a single coil spring. Such mounts suffer however from lack of inherent damping and poor lateral control. Rubber mounts provide an inexpensive form of mount which can be designed to provide appropriate damping and to give good lateral support. However, the effectiveness of the rubber mount is limited by the need to compromise between low frequency stiffness which improves vibration isolation while the engine is idling, and higher frequency stiffness which improves vibration isolation at high engine and vehicle speeds, and which improves the constraint of the engine/gearbox.
Low frequency vibration isolation of the mount may be improved by means of harmonic damping effects produced by the use of two spring elements with a mass located therebetween. With such a system, the low frequency vibration of the engine/gearbox is damped by the vibrations of the mass acting between the spring elements. In order for this to work properly, the natural frequency of the mass must be outside the natural body frequency of the vehicle body and the engine order frequencies. If this is not achieved, the system will amplify those inputs which coincide with the natural frequency of the mount. In order to ensure that this is avoided, the natural frequency of the mount needs to be 30 Hz or lower. This frequency can only be achieved by using springs with a low spring rate and by using a large mass. This is not desirable since low spring rates reduce low frequency vibration control and also the large mass is undesirable since minimising the mass of the motor vehicle improves fuel economy and reduces costs. Furthermore, harmonic damping mounts of this form are of inherently complex design and expensive to produce.
Low frequency isolation of the engine/gearbox can be improved by the use of a hydraulic mount known as a hydramount in which, fluid is pushed through a long tube between a pair of chambers. With this system, if the vibration is of high frequency, the fluid has sufficient inertia to resist movement, resulting in the vibration being isolated solely by the flexibility of a rubber diaphragm by which the engine/gearbox is connected to the hydraulic circuit. During low frequency vibration fluid passes more freely between the chambers resulting in the rubber diaphragm being effectively more flexible. Hydromounts of this form are of complex design and are expensive components. Furthermore, due to the hydraulic nature of its operation, a degree of damping is unavoidable as fluid is forced from the chambers into the smaller diameter connecting tube. Moreover, hydraulic mounts do not generally resist large fore-aft movements as they have little inherent strength in these directions.