Abstract:
A method of detecting premature wear of engine bearings in locomotive engine applications includes incorporating bismuth as a tracer element into the liner of bearing metal applied to a rigid metal backing of the bearing. A soft metal overlay free of bismuth is applied over the bearing liner and the bearing installed in the engine. The engine oil is periodically tested for the presence of bismuth. Early detection indicates premature wear of the overlay, enabling corrective action to be taken to protect the engine.

Description:
This application claims benefit of Provisional Application Serial No. 60/113,775 filed Dec. 23, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to bearings for locomotive engines, and more particularly to methods for detecting abnormal wear of locomotive engine bearings to permit corrective action to be taken. The disclosure incorporates bismuth tracer bearings disclosed in U.S. provisional patent application No. 60/113,775, filed Dec. 23, 1998, whose priority is claimed for this application. 
     2. Related Prior Art 
     Multi-layer plain engine bearings are used in locomotive engine applications to journal working components of the engine, such as the crankshaft. The bearings are made with a rigid metal backing on which a layer of functional copper-tin bearing metal is applied. The bearing layer is plated with a diffusion barrier layer of nickel and then plated with a softer overlay metal which serves as the running surface of the shaft. Under normal operating conditions, the overlay will survive for considerable time before the underlying bearing layer is exposed. However, should a bearing be improperly loaded due to, for example, misalignment between the shaft and engine block, there is the possibility of accelerated wear of the overlay, causing the bearing layer to become exposed too soon, and thus lessening the operating life of the bearing. 
     In one known attempt to monitor the wear characteristics of engine bearings in truck applications, a layer of silver was plated onto the nickel barrier layer of a sliding bearing, and the silver layer in turn covered by another layer of nickel, prior to plating the overlay. With such a bearing installed, the engine oil was periodically tested for the presence of silver. If silver was detected in the engine oil too soon (i.e., before the time in which the overlay would be expected to wear under normal operating conditions) then the conclusion was drawn that one or more bearings must be wearing prematurely, and the engine would be torn down to correct the problem in advance of damage to the engine. 
     However, in locomotive engine applications, the engine oil typically would be exposed to sources of silver apart from the silver tracer layer of such engine bearings. For example, the slipper bearings of the connecting rods of some engines contain silver which could leach into the engine oil and give a false indication of improper wear of the engine bearings. It is desirable, of course, to avoid the tremendous time and expense of tearing down a locomotive engine to replace the engine bearings and correct misalignments, etc., if such is not necessary. 
     Another drawback to the use of such a silver tracer layer is that it adds cost and complexity to the manufacture of locomotive engine bearings. Special requirements must be made for plating the silver and an added nickel layer is required to sandwich the silver between two layers of nickel. Environmental considerations must also be taken into account when plating silver. For these reasons, the use of a silver as a tracer material in locomotive engines is not satisfactory. 
     One of the objects of the present invention is to provide a method of detecting the wear characteristics of engine bearings for locomotive engine applications that minimizes or eliminates the foregoing disadvantages of silver. 
     SUMMARY OF THE INVENTION 
     According to the invention, a method is provided for detecting the wear characteristics of a multi-layer plain engine bearing in a locomotive engine having engine oil free of exposure to any sources of bismuth apart from the engine bearing. The method includes fabricating a rigid metal backing for the plain bearing and applying a functional bearing layer of copper-based bearing metal to the backing having bismuth incorporated therein. An overlay of a relatively softer metal is applied over the bearing layer and is free of bismuth. The bearing is installed in the locomotive engine and the engine oil is periodically tested for the presence of bismuth. If it is determined that bismuth is present in the engine oil too soon relative to the life expectancy of the overlay, then such would indicate premature wear of the overlay enabling corrective action to be taken to protect the engine. 
     Various locomotive engines have been analyzed and it was discovered that all were free of any source of bismuth, thus avoiding the problem of false positives associated with silver. Further, unlike silver, the bismuth can be incorporated into the traditional bearing metal alloys without having any detrimental effect on their properties, and perhaps enhancing their sliding properties. 
    
    
     THE DRAWINGS 
     These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detail description and appended drawings, wherein: 
     FIG. 1 is a perspective view of a multi-layer locomotive engine bearing according to the invention; and 
     FIG. 2 is an enlarged, fragmentary cross-sectional view taken generally along lines  2 — 2  of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a typical multi-layer plain engine bearing  10  for a locomotive engine (not shown) which, as illustrated in FIG. 2, includes a generally semicircular metallic backing shell  12 , a functional bearing layer or liner  14  applied to an inner concave surface  13  of the backing  12 , and a thin overlay  16  applied to the functional layer  14 . 
     The metal backing  12  may be fabricated of steel or other common materials according to conventional practice. The liner  14  may be the usual cast or sintered-type liners conventional to the art such as, for example, a sintered bronze copper-tin layer. The overlay  16  may comprise the usual soft electroplated layer of tin, or the like, plated onto the liner  14 . Where a bronze liner  14  is employed, a nickel barrier layer  18  is plated onto the liner  14  before plating the overlay  16  according to conventional practice. 
     According to the invention, bismuth is incorporated into the liner  14  and is not present in the nickel  18  or overlay  16  layers. The bismuth  20  is illustrated diagrammatically in FIG. 2 as distinct phases present in a matrix of the liner material  14  and serves as a tracer element, as will be explained below, to detect unusual wear of the bearing  10 . 
     It is beneficial to detect abnormally high wear rates of the liner  14  as soon as possible, since such detection may help avoid costly bearing failure and damage to an engine. In the past, silver has been used as a tracer element in engine bearings. A layer of silver is plated over the nickel barrier layer covering the liner, and an additional nickel layer is applied to the silver layer as a substrate for the overlay. By analyzing the lubricating oil to which the bearing is exposed, it is possible to detect through spectography the presence of silver, which would indicate that the bearing has worn through the overlay  16  and into the bearing layer  14 . 
     Silver tracer bearings, however, cannot serve as tracer bearings in some applications such as locomotive engine applications, since there are often other sources of silver that are exposed to the engine oil, such as silver slipper bearings. There are also environmental considerations to be dealt with in plating the silver, namely the use of silver cyanide and potassium-cyanide solutions in the plating process. The requirement for an additional nickel barrier layer also adds to the cost and complexity of producing such engine bearings. Accordingly, there is not known to be a tracer bearing suitable for locomotive engine applications employing a tracer element not found in any material or components exposed to the engine oil that could contribute as a source of any detection of such tracer element. 
     The inclusion of bismuth into the liner  14  provides such a solution. Bismuth is not present in any of the engine components of a locomotive engine that the lubricating oil comes in contact with, nor is it present in any of the usual oil additives. In this respect, bismuth as a tracer element is unique. Detecting bismuth in the oil conclusively establishes wear of the liner layer  14 . There is no uncertainty with respect to the source of bismuth. If bismuth is prematurely detected in the oil (i.e., prior to the typical wear life of the bearing where the liner  14  would begin to wear), it indicates a problem with the engine and corrective action can be taken before major problems develop with the engine. 
     Substantial amounts of bismuth can be added to the copper-tin liner  14  so that detection of bismuth can be made before too much wear or damage occurs in the bearings, and without the presence of the bismuth impairing the bearing properties of the liner  14 . In fact, bismuth, which is a soft, low melting point material, has excellent bearing properties and may contribute to the bearing properties of the liner  14 . 
     Unlike silver, bismuth can be incorporated into the liner material  14  and when sintered or cast onto the backing  12 , the bismuth present itself as distinct phases  20  within a matrix of the liner material  14 , illustrated diagrammatically in FIG.  2 . The bismuth is present in the amount of about 5 to 25 wt % of the liner mates of lead containing bismuth may also be used in lieu of just bismuth. A lead alloy containing about 3±0.2 wt % by weight bismuth would be suitable as such would avoid formation of an undesirable low melting point eutectic phase of Pb—Sb. The lead in such case would be present in the amount of about 25 wt % of the liner material. The liner  14  preferably has a thickness of about 250 to 1,250 μm. 
     According to a method of the invention, a tracer bearing for locomotive engine applications is fabricated by securing a metallic functional layer  14  by either casting or sintering onto the backing end having incorporated into the matrix of the liner material  14  tracer phases  20  containing at least bismuth, and applying a overlay to the bearing liner, with all layers except the functional layer  14  being free of bismuth. The tracer phases  20  may comprise bismuth or an alloy thereof, such as lead-bismuth. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.