GB2160828A - Off-road pneumatic tire tread - Google Patents

Abstract

The pneumatic tire for use on drive wheels of general-purpose agricultural tractors performing field and transportation work has a tread comprising uniformly alternating and subdivided grousers arranged in a "herringbone" pattern at a variable angle to the equatorial plane of the tire. The tread cap includes a central zone (A) featuring the highest tread pattern saturation and intermediate (B) and shoulder (C) zones adjoining said central zone on both sides. Assuming the tread cap zones (A, B, C) to be equal in width, the ratios of coefficients (Kb, Kc) of tread pattern saturation in the intermediate (B) and shoulder (C) zones to coefficient (KA) of saturation in the central zone (A) are in the ranges of from 0.85 to 1.0 and from 0.75 to 0.85, respectively. <IMAGE>

Description

SPECIFICATION Pneumatic tire The present invention relates to tire-making industry and, more particularly, it relates to pneumatic tires.

This invention can be used most advantageously for drivewheel tires of general-purpose agricultural tractors performing field and transportation work.

The invention can also be used in the various designs of said tractors employed as prime movers, as well as in highway engineering and timber logging.

It is an object of the present invention to increase the life of pneumatic tire.

It is another object of the invention to ensure a universal use of pneumatic tire for performing field and transportation work under varying road and soil conditions.

It is one more object of the present invention to reduce the operating cost of using tractors with pneumatic tires for field and transportation work and to increase the efficiency of labor in agricultural production.

Said and other object of this invention are attained owing to the fact that, in a pneumatic tire having a cross-country tread fashioned as uniformly alternating and separated grousers arranged in a "herringbone" pattern at a variable angle to the equatorial plane of the tire, said tread including a central zone of tread cap featuring the highest pattern saturation and intermediate and shoulder zones adjoining said central zone on both sides, according to the present invention, while assuming said tread cap zones to be equal in width, the ratios of coefficients of saturation of tread pattern in the intermediate and shoulder zones of the tread cap to the coefficient of saturation in the central zone are in the respective ranges of from 0.85 to 1.0 and from 0.75 to 0.85.

A pneumatic tire featuring the herein disclosed ratios of coefficients of tread pattern saturation between the tread cap zones has an extended life owing to the provision of uniform wear over the tread cap width.

Lower values of said ratios result in nonuniform and more rapid wear of the central zone and, espe cially, of the intermediate zones of the tread cap, which affects adversely the overall operating life of the tire. An increase of those values causes a con siderable deterioration of traction-and-gripping properties and poorer self-cleaning of the tread.

In the preferred embodiment of the present invention, with a view to ensuring universal use of pneumatic tire for field and transportation work, the coefficient of tread pattern saturation in the central zone of tread cap of the tire is in the range of from 0.36 to 0.45.

A reduction of said coefficient below the recom mended limits will not provide for the desired life of pneumatic tire when used for transportation un der conditions of hard-surface roads. And, conversely, an increase of the coefficient of tread pattern saturation in the central zone of tread cap of the tire over and above the afore- specified limits will result in poorer self-cleaning of the tread and lower cross-country capacity of tractor under field and unimproved road conditions.

Other objects and advantages of the present invention will become apparent upon considering the following detailed description of an examplary embodiment thereof to be taken in conjunctio with the accompanying drawings in which: Figure 1 shows schematically the tread pattern of a pneumatic tire according to the present invention; Figure 2 shows averaged dependences of distribution of relative specific pressures, q, over the zones of tread cap under conditions of stiff and soft soil; and Figure 3 is a combined chart of the dependence of relative intensity of tread wear (tire life) E, coefficient K, of utilization of gripping weight and tire skid 3 upon the coefficient KA of tread pattern saturation in central zone A of tread cap.

The pneumatic tire of the invention has a crosscountry tread pattern in the form of uniformly alternating grousers separated into elements 1, 2, 3 (Figure 1) arranged in a 'herringbone' pattern at a variable angle a to the equatorial plane of the tire, said angle increasing towards the shoulder zone of tread cap (a, < a2 < (X3) Assuming that the tire tread cap is divided into five zones of equal width, namely, a central zone A adjoined on both sides by two intermediate zones B and two shoulder zones C, said zones respectively accommodating the central 1, intermediate 2 and the outer 3 elements of the grousers, the ratios of coefficients Ks and KC of tread pattern saturation in the intermediate and shoulder zones B and C of the tread cap to the saturation coefficient KA in the central zone A are in the respective ranges of KB/KA = 0.85 to 1.0 and KC/KA = 0.75 to 0.85.

In so doing, the coefficient KA of tread pattern saturation in the central zone A of tread cap has the maximum value and is in the range of KA = 0.36 to 0.45.

Assumed as a basis in distributing the saturation coefficient over the tread cap width (zones A, B, C) was the distribution level of average relative specific pressures in contact with the bearing surface over the tread cap width (cf., Figure 2).

Since tractors are employed in diverse soil-and climatic zones, pressure distribution was found for the conditions of hard surface (transportation work) and soft soil (general farm work).

The hard surface used in calculations was represented by concrete surface (curve 1). Soft soil was represented by stubble field (curve 2). Stubble field is the principal soil background against which the traction tests of tractors are usually conducted in accordance with standard regulations; it is the background present during the tractor operation during a sizable part of the year. High traction properties are required from tractors operating under stubble field conditions and, at the same time, such operation is accompanied by a rather intensive wear of the tire tread pattern (the generation of high traction loads is accompanied by considerable skidding under conditions of an abrasive medium such as stubble field, hence, intensive wear of the tread).

The major share of the annual amount of work done by the tractor is performed on such (or similar) bearing surfaces.

Curges 1 and 2 show the type and averaged level of relative specific pressures in contact with bearing surface over the width of the tire tread cap, the intensity of wear and its character depending directly upon the value of said pressures.

The curves 1 and 2 shown in the graph are typical of the group of all purpose tires (mainly, for traction Class 2 -wheel tractors wheeled tractors) which have similar (or close) ratios of tire profile geometry adapted in designing tires with crosscountry tread pattern used in agriculture.

Inasmuch as the character of wear and its degree (all other things being equal) depend upon the specific pressure vatues, it Figure 2 that the ratios between the saturation coefficients in the zones B and C and the saturation coefficient in the zone A should be desirably restricted within the following ranges, respectively: 0.85 to 1.0 and 0.75 to 0.85.

In the zone B, both curves 1 and 2 lie in the 1.0 0.85 range; in the zone C, it is practically only curve II (soft soil) which gets into the 0.85-0.75 range, i.e., in the case of stiff soil, the zone C is not in full contact with the bearing surface. Hence, the adopted design ratios of the coefficients of tread saturation pattern over the width of the tire tread cap.

In Figure 3, the circle, o, is used to denote a group of agricultural tires with cross-country tread (KA t 0.32); the triangle, A, is used to denote a tire with the herein disclosed tread pattern; and the square, O, is used to denote a group of agricul tural tires with cross-country tread (KA = 0.55).

Conventionally assumed to be 100% life of tire is the average life of the group of tires with cross country tread pattern of KA = 0.55, i.e., tires with saturation close to that of autotires with "highway" tread pattern.

It follows from Figure 3 that, with a decrease of the saturation coefficient KA, the tire life (.curve F) is reduced sharply whereas the curve showing the coefficient K, of utilization of gripping weight varies smoothly, the value of K, varying but slightly with an increase of K4. Consequently, a slight decrease of the coefficient of gripping weight utilization due to an increase of KA, while affecting but slightly the overall traction characteristics, may bring about a considerable increase of the tire life.

The point D of intersection of the curves K1 and is the lower limit of increase of the coefficient of tread pattern saturation KA in the central zone A.

Further increase of KA will lead to a considerable increase of tire life upon slight decrease of the coefficient of gripping weight utilization. This, however, will be accompanied by skidding of the wheel with tire (curve b), said skidding being subject to agrotechnical limitation requirements of 14% (point F on curve 3). The value of tread pattern saturation coefficient KA corresponding to said point will be the upper limit of its variation in the central zone A.

Therefore, the 0.36 -0.45 range of KA in the central zone A will be most expedient for an all-purpose agricultural tire design having a cross-country tread pattern.

Tires with the herein-disclosed tread pattern possess adequately high traction-and-gripping properties and are characterized by skid below the minimum permissible level (which is especially important in view of the prevailing tendency towards reducing the skid), while their life is longer than that of currently available (both domestic and foreign) agricultural tires with cross-country tread pattern for traction Class 2-8 tractors.

In the herein disclosed tire, the length of the separate elements 1,2,3 (Figure 1) is about 1/3 of the overall grouser length. The angles of inclination of the separate grouser elements vary within CL = 35 to 65". The spacings at which the grousers are arranged provide for an overlap zone between two adjacent grousers.

The variation of inclination angles of separate elements is effected by making bends in the grouser plane and effecting the separation therealong. The directions of grouser separations are preset in the planes normal to the planes normal to the basic faces of the central 1 and outer 3 elements.

The central 1 and outer 3 elements of the grouser are shaped as oblong pentahedrons, and the intermediate element 2 has an oblong tetrahedron. The corners of the elements 1, 2, 3 of each grouser are made rounded.

The front face of the central grouser element 1 a the center of the tread cap and the rear face of the outer element 3 at the tread cap corner are made extending in the direction of the meridian plane of the tire. Recesses provided between the grousers are made expanding towards the shoulder zones a the tread cap.

The width of tread cap approaches the width of tire profile and is within 0.9-0.98; in so doing, the radius of curvature of the tread cap is preset such that the ratio of tread camber to the tread cap width should be within 0.06 to 0.09. Referred to as the tread camber is the difference between free ra dii of the tire at the centre and corner of the tread cap.

Owing to the provision between the grousers of recesses expanding towards the shoulder zones o the tread cap and to the trapezoidal cross-section of grouser expanding towards its base, there is at tained good self-cleaning of pneumatic tire tread, as well as desired resistance of grousers to the effect of tangential forces.

The recommended ratios between the tread cap width and the tire profile width, as well as betwee the tread camber and the tread cap width, provide for a decrease in the level of specific pressures against soil and are conducive to increasing the overall working capacity of the tire.

The overlapping arrangement of adjacent grous ers provides for a smooth rolling of the wheel, fre of impact loads, and makes for uniform wear of the tractor tire tread.

In view of the foregoing, a pneumatic tire featuring structural parameters disclosed herein and their ratios over the tread cap zone offers an increase of the tire life owing to uniform wear of the tread over the width of the tread cap on hard-surface roads by 10-15% on the average, as well as high traction-and-gripping properties under conditions of soft soil while maintaining good self-cleaning ability of the tread, this making for a higher efficiency of tractor performing transportation and field work under conditions of agricultural production.

Claims (3)

1. A pneumatic tire comprising a cross-country tread fashioned as uniformly alternating and separated grousers arranged in a 'herringbone' pattern at a variable angle to the equatorial plane of the tire, said tread including a central zone of tread cap featuring the highest tread pattern saturation and intermediate and shoulder zones adjoining said central zone on both sides, wherein, assuming said tread cap zones to be equal in width, the ratios of coefficients of tread pattern saturation in the intermediate and shoulder zones of tread cap to the coefficient of saturation in the central zone are in the ranges of from 0.85 to 1.0 and from 0.75 to 0.85, respectively.

2. A pneumatic tire as claimed in claim 1, wherein the coefficient of tread pattern saturation in the central zone of tread cap is in the range of from 0.36 to 0.45.

3. A pneumatic tire as claimed in claim 1, substantially as described in the specification and shown in Figures 1 to 3 of the accompanying drawings.