1. Technical Field
The invention relates to reinforced resilient pneumatic tires and more particularly to a vehicle tire reinforced by a thin annular composite band which is stabilized by a plurality of radial elements in the tire sidewalls to enable the tire to run in an unpressurized condition. More particularly, the invention relates to a pneumatic tire in which the band element is formed by a plurality of helically wound layers of material strips, in which at least the outer strips are prestressed in tension to improve the endurance of the band and minimize band stresses placed thereon during both pressurized and unpressurized conditions.
2. Background Information
Various tire constructions have been devised over the years which enable a tire to run in an under-inflated or non-inflated condition such as after receiving a puncture and loss of pressurized air for extended periods of time and at relatively high speeds. This enables the vehicle operator to safely drive the vehicle to an appropriate location for repair or replacement of the punctured tire. Certain of these safety tires, referred to as "run flat tires", have been successful for certain applications and certain types of tire constructions. Most of these run flat tires achieve their run flat capability, by the placement of reinforcing layers or members of relatively stiff elastomeric material in the side walls of the tire which enable the tire to support the vehicle weight even with the complete loss of internal air pressure. Examples of such prior art run flat tire constructions which use such sidewall inserts are shown in U.S. Pat. Nos. 3,911,987; 3,949,798; 3,954,131; 4,067,372; 4,202,393; 4,203,481; 4,261,405; 4,265,288; 4,287,924; 4,365,659; 4,917,164; and 4,929,684.
In addition to these prior art run flat tire patents, various run flat tire constructions have been developed which utilize a thin annular band which extends circumferentially throughout the tire beneath the tread area. Examples of such banded run flat tires are shown in the following patents.
U.S. Pat. No. 4,428,411 describes a method to make a particular band for use in a run flat tire which uses a series of side-by-side elements in the form a helix. The band has hoop compression as against a conventional breaker belt that has no significant compressive strength but is used only to resist tension loads endured by the tire when pressurized. However, the band has no residual stresses after manufacture which reduces the critical operational stresses after integration into a tire as is the band of the present invention.
U.S. Pat. Nos. 4,673,014 and 4,794,966 teach a method to acquire desirable prestressing in a fabricated band made of helical elements. Physically bending the larger diameter helix element around a smaller mandrel and securing it with a resin impregnated tape does acquire a desirable level of prestressing. However, the band of the present invention is achieved by inherent thermal shrinkage as the sequentially overlapped material layers eliminate any physical bending of a structural element as is required in the method of these two patents.
U.S. Pat. No. 4,456,048 teaches a method of acquiring a change in band stiffness as a function of deflection. The band has a lower stiffness for normal pressurized operation and has a higher stiffness to support load when the tire is uninflated and experiences larger deflection. The band of the present invention does not have a variable stiffness but is prestressed and does not change the band stiffness. The band does favorably affect the range of operating stresses. Japanese Patent application No. JP 63141809 discloses a run flat tire having a banded element which is formed of layered strips of materials, such as an aramide filament which is impregnated with a high elasticity epoxy resin, which after hardening provides a stiffened band but provides no suggestion for prestressing a band in tension which is achieved during the manufacture of the band of the present invention. Thus the tire of this disclosure requires that elastomeric side wall inserts be utilized in combination with the band in order to achieve the desired run flat characteristics.
Other run flat banded pneumatic tires are shown in U.S. Pat. Nos. 4,111,249; 4,318,434; 4,428,411; 4,459,167; and 4,734,144.
Banded tires have been fabricated with band materials made of steel, aluminum, titanium, and epoxy and thermoplastic composites with glass, KEVLAR (aromatic polyamide) and graphite fiber reinforcement. The common failure mode with the light, economical laminate band construction is interlaminar shear within the band's primary bending neutral axis. This is a fatigue failure and is directly related to the spectrum of cyclic operating stress. As in all fatigue failures, the lower the stress, the longer the operating life.
Prestressing techniques lower the critical operating stress. This critical operating stress occurs in the footprint of the tire and is induced by road surface irregularities, aggravated by ride dynamics. This feature was identified by operating loaded tires on dynometer wheels and then compared with actual road tests. Dynometer tests showed an increase in cyclic life by a factor of 10-20 times. Thus, anomalies in the road surface can greatly shorten tire life when compared to dynometer testing.
There are three recognized approaches to improve the interlaminar fatigue life of the bands: lower the critical operating stress, improve the toughness features of the matrix and the introduction of reinforcing fibers thru the failure plane. This latter approach uses a weaving process and is applicable to wet or preimpregnated fiber tows or thermoplastic tape.
The proposed approach of the present invention uses a prestressing technique to reduce the critical operating stresses. The unique feature identified in this approach is that of beneficial residual stresses are introduced in the band by the fabrication process. The sequential application of reinforced thermoplastic tapes employing heat and high pressure to the contacting surfaces results in a gradual buildup of residual stress. The geometric features of the invention retain the desired strength of the reinforcing fiber in the hoop direction and provide adequate lateral strength, with the level of beneficial residual stress being achieved in the preferred embodiment with a winding angle preferably of .+-.15.degree.-20.degree. off circumferential.