Hardened wheel

Details Hardened wheel Hardened wheel

1999-12-23

2001-10-02

Yee, Deborah (Department: 1742)

Metal treatment

Stock

Ferrous

Reexamination Certificate

active

06296721

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates, generally, to wheels for use in overhead crane assemblies, pulley systems, or the like. More particularly, the present invention concerns crane wheels which travel along a rail in an overhead crane assembly. Specifically, the present invention pertains to case hardened crane wheels with improved wear characteristics and toughness.
BACKGROUND OF THE INVENTION
Overhead cranes which travel on wheels along spaced apart, generally parallel rails, are subject to the continuous problem of crane wheel wear and failure. In such overhead cranes, wheels roll along a rail surface such that a portion of a crane wheel comes into contact with the rail surface thereby subjecting that portion of the crane wheel to wear.
A typical prior art crane wheel
20
is shown in
FIGS. 1 and 2
. The crane wheel
20
includes a hub
22
which surrounds an axis of rotation
24
of the crane wheel
20
. The hub
22
is part of a radially inner portion
26
which consists of a body or core material
28
of the wheel
20
. The crane wheel
20
further includes a radially outer portion
30
which includes a working tread surface
32
and opposing outer flanges
34
,
36
which have respective inner surfaces
38
,
40
. The working tread surface
32
and at least portions of the flange inner surfaces
38
,
40
make up a wear area
42
of the crane wheel
20
.
As can be appreciated by those skilled in the art, certain portions of a crane wheel need different physical characteristics as compared to other portions of the crane wheel. The different physical properties are necessary because of the different conditions encountered by the different parts of the crane wheel as the crane wheel is in service. The wear area that engages a rail of an overhead crane must be resistant to wear. Thus, this portion of the wheel should be hardened. The hub of the wheel may be machined after heat treating of the wheel for the reception of an axle and of various bearing members in a crane assembly. Thus, this portion of the wheel should preferably remain machinable after heat treating of the wheel. As a result, for these types of wheels, processes have been used in an attempt to harden areas subjected to wear while attempting to maintain other areas of the wheel ductile or, as-forged.
Two prior processes used to harden wear surfaces of a crane wheel and which are capable of providing the necessary surface hardness required to support and guide heavy crane wheel loads, are generally known as the salt bath process and the gas carburizing process. The salt bath process involves heating the surface temperature of a crane wheel to roughly about 1650° F. by immersing the entire wheel or part of the wheel into a molten salt bath. When immersing only part of the wheel at any given time, the wheel is usually mounted on a rotating member such that the flanges, working tread surface and part of the body come into contact with the salt bath as the wheel is rotated. The heating process takes from one to three hours depending on the size of the crane wheel. Once the desired temperature is reached, the wheel is removed from the molten salt bath and transported to a quench bath where the wheel may be spin quenched in a manner similar to heating the wheel as outlined above. Alternatively, the entire wheel may be submerged in the quench bath.
The gas carburizing process involves securing a crane wheel in place in a gas tight box. Air in the box is evacuated and replaced with a carbon rich gas. The box is then heated to roughly about 1650° F. for six to 36 hours, depending on the size of the wheel and the desired case depth. The elevated temperature allows the crane wheel surface to accept carbon from the gas. The wheel obtains a high carbon level on the outside surfaces, including the wear area, which surfaces can then be exposed to a thermal transformation process in order to obtain high surface hardness at the exposed surfaces.
FIGS. 1 and 2
represent prior art crane wheels created according to prior methods such as those just described. As can be observed from the shaded-in portions
39
, of the crane wheels
20
, the flanges
34
,
36
are completely through hardened. As will be further explained below, these through-hardened portions are extremely brittle and subject to possible failure upon adverse impact during use.
SUMMARY OF THE INVENTION
As generally known, to heat treat and harden steel, the material must be heated beyond its critical or transformation temperature. Once past the critical or transformation temperature, the material becomes austenite. A rapid quench of the austenite material creates a hardened material called martensite. Although this hardened material is highly resistant to wear, this hardened material is generally very brittle. As will be further explained below, fully hardened or through hardened flanges of a crane wheel, although somewhat resistant to wear, are typically brittle and subject to possible failure during use in the field.
The prior salt bath process completely through hardens the flange areas of a crane wheel. Meaning, not only are the wear surfaces of the flanges hardened, the entire area of each flange is hardened. As noted, a completely through hardened flange of a crane wheel makes the flange extremely brittle and reduces the overall impact strength of the flange such that the wheel is more susceptible to failure. Thermal cracking frequently occurs in flange areas of a wheel when such flange areas are through hardened. Typically, this results in large portions of the flange area separating from the wheel rendering the wheel inoperable, and creating a dangerous situation where large portions of the wheel may fall on equipment or unsuspecting persons located below an overhead crane.
A phenomenon known as radical cracking occurs when raw material does not meet the material cleanliness specifications when a wheel is formed or when a wheel is unevenly heated during a hardening process. With the large volume of material being hardened in a salt bath process, any slag inclusions in the material or uneven heating within the core of the wheel will create internal stresses that make the wheel prone to radial crack failure. Only a slight deviation from the material cleanliness specification or slightly uneven heating makes a wheel highly susceptible to radial crack failure rendering the salt bath process less than desirable in some instances.
Another problem with the salt bath process is explained with reference to hardening a typical 500-pound crane wheel. To completely transform the wear areas of such a crane wheel using the salt bath method, the wheel must be heated generally for three hours to reach a temperature of around 1650° F. Because of the extensive time needed to reach the transformation temperature, pressures for increased productivity may result in some necessary parts of a crane wheel to not reach the proper transformation temperature. This causes what is generally known as a “butterfly” of soft material in portions of the wheel. This weakens the overall wheel structure and premature field failures may occur.
The prior carburizing process is rarely used today because of the amount of time and degree of temperature needed to obtain hardened surfaces. For example, for a typical 500-pound crane wheel, to completely transform all the wear areas of a crane wheel, the crane wheel must be heated generally for 30 hours to reach a temperature of around 1650-1750° F. This will provide a hardened surface but, for many crane wheels, the case depth is still insufficient to effectively resist wear. As with the salt bath process, and even more so because of the longer periods of heating time, production requirements may result in some crane wheels being heated at improper temperatures for too short of time. This causes shallow and irregular heat patterns which, in turn, provide improper or unsatisfactory wear characteristics. Additionally, another problem with the slow heating process of the carburizing process is that the slow heating process causes heat t

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