WO2003022484A2

WO2003022484A2 - Gear rim and gears produced therefrom

Info

Publication number: WO2003022484A2
Application number: PCT/US2002/028541
Authority: WO
Inventors: Stephen L. Lovell; Ivo Botto; Eduardo Hurtado
Original assignee: Ffe Minerals Usa Inc.
Priority date: 2001-09-06
Filing date: 2002-09-06
Publication date: 2003-03-20
Also published as: WO2003022484A3; US20030041921A1

Abstract

Disclosed herein is a multi-segmented fabricated gear rim and gear formed therefrom. Each gear rim segment (11, 12) in the gear is formed from a single unsegmented steel plate. The steel plate has an upper surface (16) from which gear teeth are cut and a lower surface (15) on which the gear sub-structure is welded. Relative to each other, the upper surface (16) of the steel plate possesses high hardenability and poor weldability and the lower surface (15) possesses low hardenability and good weldability. The steel plate is preferably formed using a decarburization process.

Description

GEAR RIM AND GEARS PRODUCED THEREFROM SUMMARY OF THE INVENTION

In one embodiment, the present invention relates to a unique gear rim and a fabricated gear produced utilizing such a rim. BACKGROUND OF THE INVENTION

Large diameter gears are typically utilized in large comminution machines, e.g. crushing and milling machines, and in kilns. The applications in which such gears are utilized are typically high stress requiring the gear teeth and, obviously, the gear rim to have sufficient hardenability and strength to carry the transmitted load with minimum wear or reduced probability of fracture. Such large diameter gears are generally formed by a casting process, since, in a fabricated gear made according to traditional processes, the inner gear structure can not be easily welded to the hard gear rim. The reason for this is that typically the weldability of a material is inversely proportionate to its hardenability. Therefore, if hardenability is required in the gear teeth as is generally the case for large diameter gears, it would be anticipated that, for fabricated gears, the quality of welds between the inner surface of the gear rim and the substructure of the gear would be complex and risky. If the hardenability in the gear rim is compromised to allow ease of weldability, the gear teeth may not have sufficient hardness for the high stress applications. Typically the hardness and hardenability of a material is directly proportionate to its carbon content- relatively high carbon content will result in good hardness and hardenability. " Hardenability" is commonly measured as the distance below a quenched surface at which the metal exhibits a specific hardness. " Hardness" is the measure of the resistance of a material to surface indentation or abrasion.

Using a casting process to make large gears, i.e., gears having an outer diameter in excess of fourteen feet, for which the gear teeth would be subject to high Hertzian stresses, is very expensive. In addition, there are only a few foundries in the United States, and in fact, worldwide, that are capable of making such large castings. Furthermore, such large castings are potentially subject to casting defects. Finally, obtaining a large diameter cast gear is a very time consuming process for which it may take up to a year to cast a gear from the time the order was placed. For these and other reasons as described below it would be advantageous to have an alternative method of manufacturing such large gears.

An alternative method from the casting process for making gears is to manufacture gears using a fabrication process. One fabrication method is the so-called " weld overlay" technique, which utilizes a single flat plate having consistent mechanical properties throughout. The technique comprises conventionally welding a plurality of beads of a soft, weldable material over an alloy plate having suitable hardness for forming teeth therefrom. The gear internals are then welded to the softer material. Such a process is difficult and risky, involving many process parameters which have to be carefully controlled.

Another fabrication process is described in U.S. Patent 6,023,990, which describes a multi -segmented fabricated gear rim which is formed from a plurality of gear rim segments, with each gear rim segment been formed from two distinct steel plates that are bonded together- (i) an inner plate and (ii) an outer plate from which gear teeth are cut. Such plates are two separate and distinct steel plates that have significantly different properties from each other. The inner and outer plates are molecularly bonded to each other, and, relative to each other, the inner plate is a high weldability plate and the outer plate is a high hardenability plate. A gear manufactured from such a rim has been shown to function well. However, it would be desirable and worthwhile to have an alternative process for making a fabricated gear that does not have a gear rim fashioned from two distinct steel plates and does not therefore have to employ a molecular bonding or other bonding process.

It is an object of the present invention to manufacture a fabricated gear rim, and further to attach the gear rim by welding to the associated gear substructure, for use in applications, such as material comminution, where exceptional gear teeth hardness and mechanical properties are required, while not sacrificing the weld strength between the inner surface of the gear rim and the substructure of the gear. It is a further object of the present invention to manufacture such a gear rim by a fabricated process and also to manufacture a gear rim that is comprised of a distinct unsegmented single metal plate that, unlike the single metal plate used in the weld overlay process, has varying mechanical properties throughout and in fact has one surface having high weldability and an opposite surface having high hardenability and low weldability.

These and other objects are realized by the production of a metallic gear rim provided from a single metal plate, wherein the inner surface of the plate has a low carbon content, which low carbon content extends a sufficient distance into the interior of the plate from such inner surface to produce a metal plate having good weldability to such inner surface. The plate's outer surface, from which the gear teeth are cut, has a higher carbon content and correspondingly higher hardness than the inner surface, which high carbon content extends into the plate through a thickness beyond the depth to which the teeth are cut .

SUMMARY OF THE INVENTION

Generally, the present invention relates to a welded assembly comprising a flat, distinctive single steel plate having an upper surface and a lower surface. The upper surface of the steel plate has a higher hardness and carbon content than the lower surface and is not suitable for welding. The lower surface of the steel plate is suitable for welding ferrous materials and, to complete the welded assembly, a ferrous material is welded to the lower surface of the steel plate. More specifically, the present invention relates to a fabricated metallic gear rim for use in forming gears, and for use particularly in forming larger gears, and the gear which is constructed therefrom. The gear rim is fabricated from a single, preferably steel or alloy steel, metal plate having varying properties throughout, in particular an inner surface having a low carbon content and good weldability characteristics associated with such carbon content . The outer surface of the plate has a high carbon content, which gives it a high hardness suitable for large gear teeth. DESCRIPTION OF THE INVENTION

The present invention is directed to the fabrication of gears from a rim formed from a single steel or steel alloy plate, and particularly large multi-segmented gears. Such gears will typically have a diameter greater than about fourteen feet, although the present invention can be used to fabricate smaller diameter gears. Typically, the diameter of the multi-segmented gears fabricated according to the present invention will range from about fourteen to about forty-eight feet although smaller and larger diameter gears are contemplated as being embodied by the present invention. In addition, this invention is directed to large diameter gears that are used in applications in which the gear teeth must have a significant hardness. In particular, the large gears that are the subject of the present invention will have utility in crushing, milling, cement, mining, steel industry and kiln applications for which the gear teeth will be subject to high Hertzian stresses.

Specifically, the invention is preferably directed toward a large multi-segmented gear, each segment being a metallic rim consisting of a single plate comprising an inner and an outer surface, with the surfaces having distinctively different carbon content. Specifically, the inner surface of the plate will have a carbon content that is characteristic of a metal having relatively low hardness and good weldability. Such low carbon content will extend beneath the surface of the plate toward its midsection for a distance sufficient to ensure that a secure weld of the inner substructure of the gear can be made with the surface. The low carbon content of the steel plate will extend through its thickness to a depth sufficient to achieve good weld. Typi-cally, the remaining thickness of the plate will have a high carbon content sufficient to ensure good hardness. At the very least such high carbon content will extend through the thickness of the plate from the outer surface past the depth of the gear teeth. The term " good hardness" as used in reference to the outer surface of the gear rim is understood to mean having a carbon equivalent compatible to the hardness gradient, which typically is at least about 300 Brinell. Put another way, in the plate there is an area of decreased carbon profile that extends from the inner surface of the plate toward the midsection of the plate. At a point within the plate the carbon profile will revert to a maximum level which will continue until the outer surface of the plate. The area of maximum carbon profile will be greatly in excess of the area of decreased carbon profile, that is, the " maximum carbon" profile section of the plate will be much thicker than the " decreased carbon" profile section of the plate .

A preferred method of forming a single plate having an inner and outer surface with distinctively different carbon content is to subject the plate to a decarburization process.

" Decarburization" is generally defined as the loss of carbon from the surface and adjacent layers of a carbon- containing metal or alloy. It is typically due to reaction with one or more chemical substances in a medium that contacts the surface of the carbon-containing material. Decarburization is generally not considered to be a desirable phenomenon to occur in steel or alloy steel because oxygen reacts with the carbon at the surface of the steel, leaving behind a low-carbon surface layer that reduces the typically desired mechanical properties of the steel at the surface. As used herein, " mechanical properties" means the properties of a material to reveal its elastic and inelastic behavior when force is applied, thereby indicating its suitability for mechanical applications; for example, modulus of elasticity, tensile strength, elongation, hardness and fatigue limit. In the procedure of making the gear rim of the present invention, steel plate is subject to heating in an oxidizing atmosphere. The plate is heated originally at approximately 50 °C. The heating temperature is gradually increased to approximately 1200°C at which upper temperature the plate is maintained for a sufficient time to produce, on all surfaces of the plate, a low hardenability " layer" that will facilitate a low risk welding procedure. Some of the carbon at and near each surface of the plate will react with the oxygen environment to thereby reduce the carbon in the plate in such areas. Typically, after the heating and maintaining steps the entire surface of the gear has been subject to decarburization to a depth that can be confirmed by cross sectional analysis. In the decarburized areas the minimum carbon profile will be at the surface of the plate. The carbon profile of the plate will increase gradually from the surface of the plate through its thickness until it eventually reaches the plate's original carbon profile, that is, the carbon profile of the plate prior to treatment. In effect the decarburized plate will have a decarburized " skin" in any area of the plate that was exposed to the specific heat/atmosphere combination which typically will be any surface of the plate. Since the outer surface of the plate from which the gear teeth are cut should be at the plate's original carbon profile, any undesired decarburized areas (such as on the outer surface or the sides of the plate) are therefore removed, such as by mechanical machining, at some point in the process, typically after the inner sub - structure of the gear has been welded to the inner surface of the rim. The undesired decarburized areas on the outer surface are removed to a depth (typically about 5/32" ) sufficient to ensure that the " new" outer surface of the plate, i.e., the outer surface exposed on the plate after the decarburized area has been removed, possesses the desired hardness and other mechanical properties of the original steel or alloy steel plate, which for large gear applications can typically be a low alloy steel plate (although other alloy steel and steel plates having suitable mechanical properties can be utilized) .

The steel in the decarburized layer will have a low carbon content suitable for welding, the depth to which the gear teeth will be cut (which in large gears may range from 6.0cm to about 9.0cm) will have a have a high carbon content and will possess desirable hardenability and mechanical properties.

The welding operation to be performed on the gear rim of the present invention will be the welding of the inner structure of the gear to the high weldability low carbon layer on the inner surface of the steel plate, thus eliminating the need for more risky and complex welding procedures. No welding is done to the high carbon outer surface.

In the formation of gears utilizing the present invention, a large gear rim of the present invention will first be fabricated from a number of flat " decarburized" metallic plates, or segments, treated as specified above and thereafter fabricated into circular arc segments. Obviously, the geometry of the arc and the number of segments will depend on the desired design and diameter of the gear to be constructed therefrom. The high weldability inner surface of the metallic plates will comprise the inner layer of the circular arc segments and, accordingly, the inner layer of the gear rim fabricated therefrom. A corresponding portion of the inner structure of the gear, i.e., web, gusset and joint plates, will be welded to the high weldability/low carbon content layer of the circular arc segments. The segments are then joined together, such as by mechanical fastening, to each other to form a generally circular unit. The gear teeth are then cut into the high hardenability/high carbon content outer surface of the circular unit to thereby form the completed gear assembly. Generally, two or more circular arc segments will be utilized to form a circular unit. It is in this sense that the gears of the present invention are described as being "multi-segmented".

Typically, in the large cast gears of the present invention the gear rims will have a thickness ranging from 4" to about 10" and a width ranging from about 8" to about 48". In certain designs of gears the thickness of a gear rim may vary from place to place on the gear. However, the gear rim will typically be of a thickness that is at least 2.2 times the depth of the gear teeth.

DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side elevation of a multi-segmented circular assembly of the present invention from which a gear is machined.

Fig. 2 depicts a portion of the gear of Fig. 1 along lines A-A.

Fig. 3 depicts a portion of the gear of Fig. 1 along lines B-B.

Fig. 4 is a detail of Fig. 3 that depicts the gear rim.

Like numerals in different Figures refer to similar components. The Figures are not necessarily drawn to scale.

With reference to Fig. 1, there is shown a multi-segmented, fabricated, circular gear rim assembly 10 that consists of two arcs, 11 and 12. The arcs are bolted together at joint plates 11a and lib (for arc 11) and 12a and 12b (for arc 12) . Fig. 1 also depicts gussets 13 which are welded to gear rim 14. Obviously, gear rim 14 as depicted does not have any gear teeth. The gear teeth for gear rim 14 will be machined on surface 14a of gear rim 14.

Fig. 2 depicts that portion of the circular assembly 10 of Fig. 2 along lines A-A. Depicted in Fig. 1 is gear rim 14 having high weldability inner surface " layer" or " area" 15 and high hardenability outer surface " layer" or " area" 16. Also depicted is web plate 17 and joint plate lib. Bolt holes 19 are located in joint plate lib.

Fig. 3 depicts a section of gear rim 14 to which there are attached web plates 17, which are also welded to gear rim 14.

Fig. 4 shows gear rim 14 in greater detail and shows outer high hardenability/high carbon area 16 and inner high weldability/low carbon area 15 in greater detail.

While there are shown and described present preferred embodiments of the invention, it is distinctly to be understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

What is claimed is:

1. A welded assembly comprising a single distinctive steel plate having an upper surface and a lower surface, the upper surface having a higher hardness and carbon content than the lower surface, said upper surface not being suitable for welding; and said lower surface having a ferrous material welded thereto.

2. A multi -segmented fabricated gear rim, with each gear rim segment being formed from an single unsegmented steel plate, said steel plate having an upper surface from which gear teeth are cut and a lower surface, wherein, relative to each other, the upper surface possesses high hardenability and poor weldability and the lower surface possesses low hardenability and good weldability.

3. A multi -segmented fabricated gear which is formed from a plurality of gear rim segments, with each gear rim segment being formed from an single unsegmented steel plate, said steel plate having an upper surface from which gear teeth are cut and a lower surface, wherein, relative to each other, the upper surface possesses high hardenability and poor weldability and the lower surface possesses low hardenability and good weldability, wherein there is a gear substructure welded to the lower surface of each steel plate .

4. A process for forming a multi -segmented fabricated gear comprising (a) subjecting a single flat, unsegmented, steel plate having an inner layer and an outer layer to a decarburization process to thereby reduce the carbon content on all surfaces of the plate exposed to the decarburization process; (b) bending the plate to form a arc-like segment of a circular gear rim; (c) welding gear sub-structure to the decarburized inner layer of the plate; (d) attaching the ends of the welded plate to the ends of one or more similarly decarburized and welded plates to form a circular unit; (e) machining the decarburized surface from the outer layer of the plate; and (f) cutting the gear teeth from the machined outer layer of the plate.