This paper presents the theoretical dynamic properties of an elastomeric-pneumatic isolator with orifice-type relaxation damping. The elastomeric portions of the isolator serve to contain the pressurized air, contribute elastic stiffness, and act as a cushioning and motion-limiting “snubber” in cases where severe shock transient excitations exist. The pressurized air contained within the isolator serves to support the entire static load, contribute stiffness, and provide orifice-type relaxation damping. The resulting design concept results in a low profile, low natural frequency, and very stable isolator for shock and vibration isolation.
The objective of the isolator design is to provide the maximum degree of vibration and shock attenuation of typical vehicle excitations to the vehicle operator at the frequencies where the human body is most sensitive to dynamic excitations. As a result of this fundamental objective, substantial vibration attenuation at the first mode of vibration of a seated human must be provided. This required the isolator resonant frequency to be at or below 2 cps. Examination of the dynamic environment present on typical truck and tractor cab mounting points showed considerable dynamic excitation in the proposed frequency region of the isolator resonance. Lowering the isolator resonance well below 2 cps would avoid this excitation, but at the tradeoff of very undesirably large isolator sizes, dynamic excursions, and low rocking mode frequencies. Therefore, the isolator fundamental resonance was maintained close to 2 cps and the coefficient of damping of the relaxation damping mechanism was optimized to provide minimum payload accelerations and maximum isolator damping in response to typical transient excitation time histories present on the rear axles of a typical medium-sized agricultural tractor.
The effectiveness of the system was evaluated by determining the improvement in operator comfort through the utilization of the response characteristics of simple mechanical oscillators attached to the isolated and unisolated excitation surfaces. The results show that the relaxation-damped elastomeric-pneumatic isolator concept provides large comfort effectiveness values for vehicle operators during typical vehicle operating conditions.