Method for preventing shattered-rim fracture in railway wheels

The invention is in the field of railway wheels and, more particularly, relates to preventing shattered-rim fracture in railway wheels.

Recently, railroad transport volume, which is sometimes expressed in terms of railroad freight revenue ton-miles, has been increasing to meet an increasing rail transport demand. As a result, railway cars are hauling ever-heavier average loads. For instance, lightweight aluminum coal cars, which can have maximum gross loads up to 286,000 lbs., have become increasingly common. The wheel loads in such “heavy haul” coal cars are approaching permissible limits, a circumstance which results in lower load safety margins for the wheels of such rail cars.

One serious problem with railway wheels is known as shattered-rim fracture.

Shattered-rim fracture is a phenomenon whereby a rolling contact fatigue crack initiates at an internal defect in the wheel rim subsurface and propagates to ultimately cause substantial damage to the wheel. Left unchecked, the shattered-rim fracture can cause catastrophic failure of the railway wheel and derailment of the rail car. This problem can be exacerbated with increasing wheel speed, unbalanced loads, heavy braking, and other circumstances common in the railway industry.

Given the general increase in freight revenue ton-miles in the rail industry, there is an increasing need to prevent shattered-rim fracture in railway wheels.

It is a general object of the invention to provide a method for preventing catastrophic failure of railway wheels.

Another object of the invention is to provide a method for more effectively preventing shattered-rim fracture in railway wheels.

A further object is to provide a method for testing railway wheels for load suitability during intended usage.

Still another object is to provide a method for grading railway wheels as to load suitability.

Yet another object is to provide methods as characterized above that can be easily and effectively carried out.

Another object is to provide methods of the foregoing types that can be selectively carried out at different safety level rankings.

It is believed that a major cause of the shattered-rim fracture phenomenon is Mode II (in-plane shearing) crack propagation from defects in the rim of the railway wheel. It has now been discovered that shattered-rim fracture in railway wheels can be substantially avoided if the rim of the wheel does not contain defects of a size such as to lead to a Mode II stress intensity factor range (ΔK II ) at the maximum intended load that is greater than the threshold Mode II stress intensity factor range (ΔK IITH ) required for cracks to propagate from the defect. The invention makes use of this discovery to prevent shattered-rim fracture in railway wheels. In general, the invention contemplates an analysis of the likelihood of Mode II crack propagation from defects in the rim of a railway wheel. Such analysis may be used to determine the suitability of a railway wheel for carrying a maximum intended load, or alternatively, may be used to determine the maximum suitable load for a railway wheel.

In accordance with a highly preferred embodiment of the invention, a method is provided for more efficiently and effectively preventing shattered-rim fracture in a railway wheel. Generally, the method comprises providing a railway wheel that is expected to be subjected to an intended load, measuring the size of the largest defect within the rim of the wheel, and determining whether the defect size of this defect is below a pre-determined maximum permissible defect size for the intended load. The predetermined maximum defect size is ascertained by evaluating the likelihood of crack propagation from the defect upon subjecting the wheel to the intended load. In accordance with another highly preferred embodiment of the invention, the method for preventing shattered-rim fracture generally comprises the steps of measuring the maximum size of defects in the railway wheel rim, and determining the maximum usage load suitable for said wheel based on a predetermined correspondence between permissible load and defect sizes.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a fragmentary perspective view of a railway wheelset that includes a typical railway wheel.

FIG. 2 is a side elevational view of the railway wheel illustrated in FIG. 1 .

FIG. 3 is a section taken in the plane of line 3 — 3 in FIG. 2 .

FIG. 4 is a flowchart representing a general Mode II crack propagation analysis for railway wheels.

While the invention is susceptible of various modifications and alternative constructions, a certain illustrated embodiment thereof has been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed but, on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention.

The invention is particularly applicable to the analysis of railway wheels of a conventional type. Referring now particularly to FIGS. 1-3, there is shown a portion of an illustrative wheelset 10 that includes a typical railway wheel 11 mounted on an axle 12 . The wheel 11 in this instance comprises a hub 13 , a rim 14 , and a connecting plate 15 , with the rim 14 comprising a tread 16 and a flange 17 . Shattered-rim fracture in the illustrated wheel would occur when a crack develops in the rim 14 .

The likelihood of crack propagation from a defect, such as an internal inclusion, pore, void, vacancy, or the like, may be determined using an analysis of the likelihood of Mode II (in-plane shearing) crack propagation from defects in the rim of the railway wheel. Generally, the Mode II stress intensity factor range (ΔK II ) for a defect of a given size can be estimated for a railway wheel for a given wheel diameter and intended wheel load. If the Mode II stress intensity factor range (ΔK IITH ) estimated for a given defect in the rim of the railway wheel is greater than the threshold Mode II stress intensity factor range (ΔK IITH ) for the wheel rim, then one would expect that cracks would propagate from the defect. If, on the other hand, the Mode II stress intensity factor range (ΔK II ) is estimated to be less than the threshold Mode II stress intensity factor range (ΔK IITH ), than one would expect that cracks will not propagate from the defect. The maximum permissible defect size thus is found when

for a given defect size and intended load.

As illustrated in FIG. 4, one may generally conduct a Mode II crack propagation analysis by calculating the contact stress between the railway wheel and a rail, taking into account the wheel load and wheel diameter. The Mode II stress intensity factor range (ΔK II ) then is estimated for the defect in the railway wheel rim. Generally, one may calculate the Mode II stress intensity factor range for a threshold crack having the shape of a penny with a diameter equal to the size of the defect, i.e., diameter of the equivalent flat bottom hole detected ultrasonically. The calculation of the Mode II stress intensity factor range is a standard calculation in the art. Likewise, experimental evaluation of the threshold Mode II stress intensity factor range (ΔK IITH ) is a standard procedure in the art. This threshold range may be estimated via crack growth tests, as described, for example, in Hamada et al., “Measurement of Mode II threshold stress intensity factor range (ΔK IITH ),” I.C. M & M '97 (1997) pp. 311-12. Once the Mode II stress intensity factor range (ΔK II ) and the threshold Mode II stress intensity factor range (ΔK II ) have been determined, the relative magnitude of the stress intensity factor range (ΔK II ) can be compared to the magnitude of the threshold Mode II stress intensity factor range (ΔK IITH ) to determine the likelihood that a crack will propagate from the defect.

Determination of the foregoing stress intensity factor ranges may be inexact and subject to estimation and, in conducting a Mode II crack propagation analysis, estimates of both the Mode II stress intensity factor range (ΔK II ) and the threshold Mode II stress intensity factor range (ΔK IITH ) may vary. In particular, empirical estimates of the threshold Mode II stress intensity factor range (ΔK IITH ) may vary depending on the level of safety desired, with lower estimates of the threshold Mode II stress intensity factor range corresponding to higher margins of safety. For instance, if plural crack propagation tests are conducted, the minimum, maximum, mean, or other test value may be taken as the threshold Mode II stress intensity factor range estimate. Likewise, estimates of the Mode II stress intensity factor ranges (ΔK II ) for defect sizes may vary depending on the level of safety desired, with higher estimates of the Mode II stress intensity factor range corresponding to higher margins of safety.

In practice, the evaluation of load suitability or assignment of a load rating to a railway wheel does not necessarily involve calculation of the stress intensity factor ranges, but more likely is conducted with reference to pre-calculated values. Most preferably, different Mode II stress intensity factor ranges (ΔK II ) for different defect sizes may be estimated and a record prepared that correlates defect sizes with permissible loads. One may prepare such a correlation record by determining a plurality of wheel loads that, for a corresponding plurality of defect sizes, would correspond to a Mode II stress intensity factor range that is not greater than the threshold Mode II stress intensity factor range for the wheel rim. In making such determination, either the defect size may be calculated from a predetermined wheel load, or the wheel load may be calculated from a predetermined defect size, and an additional margin of safety may be imposed on the calculated values, if desired. The correlation record thus prepared may be used in conjunction with actual measurements of defects in railway wheels to determine load suitability or to assign load ratings to the wheel.

In light of the foregoing, the invention encompasses the preparation of a table that correlates permissible defect sizes to permissible loads for a given wheel. Such a correlation table may be prepared by estimating defect sizes for wheel loads that will result in Mode II stress intensity factor ranges that are not greater than the threshold Mode II stress intensity factor range, and tabulating the defect sizes and wheel loads. In keeping with this aspect of the invention, two possible correlation records between defect sizes and permissible loads for conventional steel railway wheels of various diameters have been prepared and are set forth hereinbelow.

The “Rank I” Table represents a safety level for railway wheels in which the threshold Mode II intensity factor range was estimated as having a value of 18 MPam, and wherein this estimated value for ΔK IITH was used to derive maximum permissible defect sizes. The Rank II Table represents an acceptable safety level in which an estimated threshold Mode II stress intensity factor range (ΔK IITH ) of 20 MPam was used to derive maximum permissible defect sizes. These values were estimated based on empirical crack propagation tests, with 18 MPam being the mean test result and 20 MPam being the maximum. In keeping with this aspect of the invention, the user may select the safety level or the acceptable safety level.

In carrying out the invention, the foregoing Tables may be used in determining the maximum permissible defect size in railway wheel rims. The size of an actual defect in a given railway wheel rim can be compared to the maximum permissible defect size expressed in one of the foregoing Tables for a given intended load. Generally, the largest defect in the rim should be measured, with the size of the defect preferably being measured ultrasonically in accordance with known techniques. If the maximum defect size is below the maximum permissible defect size for the intended load, then the railway wheel will be deemed suitable for the intended load; otherwise, the railway wheel will be deemed unsuitable for the intended load. For instance, using the Rank II Table, if the intended wheel load for a 30 inch wheel is 185 kN, and if the maximum defect size in the wheel is found to be 0.800 mm, the wheel may be deemed unsuitable for use with the intended load. On the other hand, if the maximum intended load for the wheel is 100 kN, the wheel may be deemed suitable for use with the intended load.

In carrying out a further aspect of the invention, a method is provided for the assignment of load ratings to railway wheels. To this end, the defect size measured in the rim of the railway wheel may be compared to the maximum permissible defect size in the Rank I or Rank II Table to determine the maximum load suitability for the wheel. For instance, using the Rank II Table, if the maximum defect size in a given 30 inch wheel is found to be 0.800 mm, then the maximum wheel load should be no more than 170 kN and the wheel may be assigned this load rating. The wheel should be deemed unsuitable for carrying greater loads, inasmuch as the maximum permissible defect size for greater loads is 0.774 mm, which is exceeded by the actual size of at least one defect in the wheel. Again, one may choose values from the Rank II Table if a lower safety level is deemed acceptable, or instead from the Rank I Table if a higher safety level is deemed necessary.

The utility of the Rank I and Rank II Tables in predicting shattered-rim fracture may be confirmed by referring to real railway wheels. For example, an actual railway wheel that failed due to shattered-rim fracture is reported in Marais, “Wheel Fractures in Heavy Haul Freight Wheels Due to Substance Defects,” Twelfth International Wheelset Congress (1998) pp. 306-13. The fracture in this wheel was found to have originated from an inclusion in the rim of the wheel. The wheel load was 130 kN at the time of fracture, the diameter of the wheel was 34.24 inches (870 mm), and the inclusion size at the origin of the fracture was approximately 1.0 mm. Referring to the Rank I and Rank II Tables, it is seen that the maximum permissible defect size for this wheel under this load is 0.969 mm (referring to the Rank I Table) and 1.196 mm (referring to the Rank II Table). It is thus seen that the defect in the rim was close to or exceeded the estimated maximum defect size permissible for the wheel rim. The defect was greater than that deemed permissible according to the Rank I Table. While the defect was within the permissible range according to the Rank II Table, the Rank II Table may be deemed useful as an aid in estimating permissible defect sizes.

Of course, those skilled in the art may make other estimates of the threshold Mode II stress intensity factor range for the particular railway wheel being evaluated. Those skilled in the art also may evaluate or empirically determine other permissible loads or permissible defect sizes for a given load. Accordingly, the specific Rank I and Rank II Tables should be regarded as exemplary of broader aspects of the invention.

From the foregoing, it can be seen that the invention provides a method for effectively preventing catastrophic failure of railway wheels due to shattered-rim fracture. With empirical tables developed for desired safety levels, the method can be quickly and efficiently carried out. The methods of the invention further may be employed to easily test and assign load ratings to railway wheels.