This invention relates to a pneumatic tire particularly suited for agricultural or logging use. The tire being designed with lateral-edge lugs having axially extending projections.
Agricultural and logging tires are exposed to a variety of working conditions requiring the tire to function well on paved surfaces, loose soil and rough and rugged terrain. Under normal working conditions the tire traction is achieved by radially extending lugs. In some severe conditions, however, the tire must be adapted to accommodate chains to assist in providing tractive movement of the vehicle. The use of chains is particularly common in logging operations and during times of the year where there may be heavy snow and ice conditions.
The tires are designed with a relatively low net-to-gross ratio in the range of 25% to 40%. The net-to-gross ratio is the ratio of the percentage of the tire contacting a hard paved surface versus the overall contact area of the tire under a normally inflated and loaded condition. In an agricultural tire the lugs are circumferentially spaced such that the tire lugs dig into the soil and provide tractive movement while an inner tread surface provides flotation and support for the vehicle. Tires for use in logging operations are very similar to agricultural tires with the exception that the tire must be substantially more durable and the lugs must be structurally stronger.
In logging operations the tires are required to run over rugged terrain and are exposed to tree trunks and other obstacles that can cut a tire. The tires are driven over the felled trees and exposed to branches and other objects that can cut and severely damage either the inner surface of the tread or the lugs.
In order to overcome the severity of working conditions the logger tire is designed with a multiple ply carcass generally a minimum of eight plies are used. Radially outward of the carcass are two steel breakers designed specifically to prevent branches from penetrating through the inner tread surface. The sidewalls are beefed up substantially thicker than agricultural tires. And the lugs are substantially thicker and heavier braced than an agriculture tire.
The design of the logger tire is such that the lugs represent a large mass of rubber that must be cured during the manufacture of the tire. During the vulcanization process the rubber must be heated in order to adequately accomplish curing of the tire. The time required is a function of the thickness of the material and the chemical composition of the material to be cured. In a logging tire the thickest area or largest area of mass is at the lateral edge of the tread lugs. It currently takes approximately in excess of three hours to cure a typical conventional logger tire. The tire cure time is dictated by the thickness of the lug and cannot be decreased under conventional curing processes without sacrificing the quality of the tire cure.
Both the agricultural and logging tires once placed into use must be durable enough to accommodate the mounting of chains. The chains are used to provide additional traction when the weather or soil conditions are such that an increase in traction is required. The use of chains is particularly common in logging operations. The chains are mounted to a tire in such a fashion that the links of the chains provide excessive forces on any area of the tire that provides a sharp corner. In particular the lugs are extremely sensitive to accommodating chains and have been known to tear or crack as a result of chain usage. The chain mounted tire being driven over logs and branches is exposed to an environment that induces chain slippage either in the axially or circumferential direction. This movement of the chain relative to the tire increases the probability of tire damage, in particular the lateral edges of logging tires are known to be susceptible to tearing. After a short period of usage it is not uncommon to see a majority of the tread lugs damaged at the lateral edge. Solutions to chain damage have historically required the addition of more rubber which further increases the amount of time required to manufacture the tire.