A. C. Bailey

SOIL STRESSES UNDER A TRACTOR TIRE AT VARIOUS LOADS AND INFLATION PRESSURES

Abstract Soil stresses were measured under a 18.4R38 R-1 radial-ply tractor tire, operated at two levels each of dynamic load and inflation pressure. Stress state transducers were placed at two depths beneath the centerline of the path of the tractor tire in two different compaction profiles in each of two soils. Peak soil stresses and soil bulk density increased with increases in both dynamic load and inflation pressure.

Hydrostatic Compaction of Agricultural Soils

ABSTRACT A three-parameter multiplicative model for soil compaction was developed and evaluated using triaxial tests on two agricultural soils. The model satisfied the boundary conditions at low and high stress levels. The effects of moisture content on the three coefficients were investigated.

The effects of reduced inflation pressure on soil-tire interface stresses and soil strength

Abstract Inflation pressures as low as 41 kPa have been recommended by agricultural tire manufacturers for minimizing an oscillatory vibration problem, commonly called “power hop”. Other benefits of these lower inflation pressures might include decreased soil-tire interface pressures, increased tire performance, and decreased soil compaction. Measurements of soil-tire interface stresses were made at four positions on the lugs and a three positions between lugs on an 18.4-R38 R-1 radial factor tire operated at four combinations of dynamic load and inflation pressure. These measurements showed that as inflation pressure increased, the soil-tire interface stresses near the center of the tire increased, while the stresses near the edge of the tire did not change. The increased stresses near the center of the tire were also transferred to the soil as a compaction increase sensed with the cone penetrometer. “Correctly” inflated tires (i.e. lower inflation pressures) also improved net traction and tractive efficiency.

Inflation Pressure and Dynamic Load Effects on Soil Deformation and Soil-tire Interface Stresses

An 18.4 R38 R-1 radial tractor tire at inflation pressures of 41 and 124 kPa and at dynamic loads of 13.1 and 25.3 kN was evaluated to determine the effects of the new load-inflation pressure tables on soil deformation and contact stresses. Measurements of rut width and deformed rut area were conducted with a profile meter. Soil-tire interface stress measurements were also made to determine stresses occurring between the tire and the soil and to determine the tire footprint length. Inflation pressure and dynamic load effects were found on rut width, contact length, and contact area. Dynamic load effects were also found on deformed rut area. Increased levels of soil-tire interface stress was found near the center of the tire when inflation pressure or dynamic load was increased.

Some Comparisons of Average to Peak Soil-Tire Contact Pressures

Comparisons are made among several methods of determining the contact pressure between a tire and the soil. Results show that on compacted soils the peak pressures measured at the soil-tire interface are much greater than mean pressures determined from measure-ments and much greater than pressures calculated by dividing the dynamic load by contact area. On uncompacted soil, peak pressures are almost equal to the inflation pressure.

Tire lug height effect on soil stresses and bulk density

Soil stresses and increases in soil bulk density were measured beneath the centerline of one new 18.4R38 radial-ply R-1 tractor tire and two similar tires with lug heights of 55% and 31% of the new tire lug height. Each tire was operated with an inflation pressure of 110 kPa, a dynamic load of 25.0 kN and 10% slip. Soil stress state transducers (SSTs) measured the stresses at three depths in both a hardpan soil profile and a uniform soil profile, each in a sandy loam and a clay loam soil. The initial depths of the SSTs ranged from 164 to 288 mm. Analysis of the original soil stress data showed that lug height did not significantly affect the peak octahedral normal stress or its corresponding octahedral shear stress. When outliers were removed from the peak stress data, however, lug height significantly affected the octahedral normal stress in the sandy loam soil. In the uniform profile of the sandy loam and in the hardpan profile of the clay loam, the new tire generated the greatest bulk density increase, which was significantly greater than the bulk density increase caused by the 55% tire. In the sandy loam with the hardpan profile, the 55% lug height tire generated a significantly greater bulk density increase than either the new or 31% tire.

Soil stress state orientation beneath a tire at various loads and inflation pressures

Abstract Stress state transducers (SSTs) were used to determine the orientation of the major principal stress, σ 1 , in soil beneath the centeline of an 18.4R38 radial-ply R-1 drive tire operated at 10% slip. Two soils, a sandy loam and a clay loam, were each prepared twice to obtain two density profiles. One profile of each soil had a hardpan and the soil above the hardpan was loose. The soil in the second profile was loosely tilled. The stress state was determined at a depth of 358 mm in the sandy loam and 241 mm in the clay loam soil. The tire was operated at two dynamic loads (13.2 and 25.3 kN), each at two levels of inflation pressure (41 and 124 kPa). When the orientation of σ 1 was determined directly beneath the axle, the mean angles of tilt in the direction of travel ranged from 6 to 23 degrees from vertical. Inflation pressure did not significantly affect the angle when the dynamic load was 13.2 kN in the sandy loam soil, and neither inflation pressure nor dynamic load significantly affected the angle in the clay loam soil. When the dynamic load was 25.3 kN in the sandy loam soil, the orientation of the major principal stress determined directly beneath the axle was tilted significantly more in the direction of travel when the tire was at 41 kPa inflation pressure than when at 124 kPa. These changes in stress orientation demonstrate the importance of measuring the complete stress state in soil, rather than stresses along only one line of action. The changing orientation of σ 1 as the tire passes over the soil indicates the soil undergoes kneading and supports future investigation of the contribution of changes in stress orientation to soil compaction.

Effects of soil and operational parameters on soil-tire interface stress vectors☆

Abstract Normal and tangential stress vectors were measured at the soil-tire interface of a pneumatic tractor tire on firm and soft soils. Stress magnitudes were determined with a transducer which was designed to measure both normal and tangential stresses. The orientation of the transducer was determined using a 3-dimensional, sonic digitizing system which was mounted inside the air cavity of the tire. Data are presented from tests conducted at zero input torque, zero net traction, and 0.15 net traction ratio which show the effects of inflation pressure, dynamic load, and soil conditions on the stress vectors.

TRACTOR TIRE ASPECT RATIO EFFECTS TRACTOR TIRE ASPECT RATIO EFFECTS ON SOIL STRESSES AND RUT DEPTHSON SOIL STRESSES AND RUT DEPTHS

A 580/70R38 tractor drive tire with an aspect ratio of 0.756 and a 650/75R32 tire with an aspect ratio of 0.804 were operated at two dynamic loads and two inflation pressures on a sandy loam and a clay loam with loose soil above a hardpan. Soil stresses were determined just above the hardpan beneath the centerlines and edges of the tires. Rut depths were measured at the centerline and edge of each tire track. The octahedral shear stress and rut depth were not significantly different for the tires. The peak octahedral normal stress was not significantly different for the two tires when the dynamic load was 17.2 kN, but was significantly greater for the 650/75R32 tire when the dynamic load was 30.9 kN. Soil stresses and rut depths increased with increasing dynamic load at constant inflation pressure, and with increasing inflation pressure at constant dynamic load. Net traction and tractive efficiency decreased with increasing inflation pressure at constant dynamic load. At constant inflation pressure, tractive efficiency increased with increasing dynamic load. In comparisons of the centerline and edge locations, soil stresses were significantly less beneath the edges than the centerlines of the tires. Ratios of the mean stress beneath the centerline to the mean beneath the edge for four combinations of dynamic load and inflation pressure ranged from 2.18 to 3.77 for the peak octahedral normal stress and 1.76 to 3.18 for the corresponding octahedral shear stress. Ratios of the rut depth at the centerline to the edge ranged from 1.04 to 1.49. In summary, for these two tires with their slightly different aspect ratios, no fundamental differences were found that would clearly indicate that one tire was better than the other.

A rationale for modeling soil compaction behavior: an engineering mechanics approach

ABSTRACT Modeling agricultural soil compaction is important as one input to a system of effective management of soil physical condition to improve crop production. The desired degree of compaction depends on the intended purpose; for example, the requirements for traction and mobility are quite different from those for infiltration and root propagation. Our goal is to develop a compaction model and related soil and soil-machine behavior models which can be used to design systems for effective management of soil physical condition. In this article we discuss our rationale in modeling soil compaction and related soil-machine systems. The status of the various modeling efforts is discussed, as are plans and needs for the future.