PM—Power and Machinery: Working Space Requirement for an Agricultural Tractor Axle Suspension

Abstract

An agricultural tractor equipped with both front and rear axle suspensions offers a number of advantages when compared with a traditional tractor fitted only with seat, cab or front axle suspension. A soft suspension with low natural frequencies normally offers better vibration damping capacity than a stiffer one, but needs more travel space to avoid over-travel. The restricted space is therefore the basic constraint when designing effective suspensions.The main purpose of this work is to study the link between the need for available suspension travel space and the vibration damping capacity for agricultural tractors supplied with various types of full axle suspensions. In order to study the suspension characteristics, the dynamics of the differently designed vehicles are described with linear models, and their movements when driving over standardized tracks are simulated in the time domain.The effects of varying linear suspension characteristics on the vibration damping potential and need of travel space are studied. The possibilities of reducing extreme travel values by using non-linear damping elements are also studied and discussed as well as the effects and needs of different levelling devices to counteract variations in static load. Simulations have also been performed in order to study the vibration damping potential for vehicles with varying total mass of the rear axle and any coupling devices that may be mounted on the axle.One general conclusion of the work is that a combined use of load levellers and non-linear progressive damper characteristics makes it possible to use a soft suspension with good vibration reduction under normal driving conditions. The non-linear dampers then limit the maximum suspension strokes on rough surfaces and at high speeds, thus avoiding over-travel. Another conclusion is that, for a vehicle with both front and rear axle suspensions and with a normal seat position, the effects of the front suspension characteristics are rather limited. The use of a rather stiff front suspension may therefore be appropriate in order to avoid the need of a front load leveller.