Stress Under Belts and Radial Tires with Various Wt Distributions

In a series of field experiments we measured the stresses in sand beneath the rubber belt of a prototype belted tractor and beneath single front and dual rear tyres of a radial ply type equipped tractor, using earth stress cells installed beneath the belt or tyres. The stress in the sand increased with increasing axle load for both belt and tyres. The peak vertical stress beneath the belt was about 6 times the average stress calculated as total weight divided by total area of belt. Depending on axle load, the stresses near the soil surface were similar beneath belts and tyres, whereas at 35 to 45 cm depth the stresses beneath tyres exceeded those beneath belts. Stresses beneath individual wheels on the track bogey could be distinguished near the soil surface, but at 35 to 45 cm depth only the average stress could be identified.

Analysis of the experiments using a three-dimensional finite element model confirmed that at depth the stresses beneath a belt would be smoothed out such that only groups of wheels could be identified.

To avoid compaction, axle loads should be reduced and low pressure tyres used, wherever posssible. However, the most important consideration is the moisture content of the soil.