USING SAME

RELATED APPLICATIONS

This application claims priority to and is a continuation-in-part of U. S. serial number 61/577,863 filed December 20, 2011, and a continuation-in-part of U.S. serial number 61/601,660 filed February 22, 2012, the entire contents of both applications are incorporated by reference herein.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to methods and apparatus for machines with rotating shafts. The apparatus and methods feature a thermally conductive shaft design which materially increases the reliability and consequently the service life of the machine, by causing parts of the machine to operate at a lower internal temperature due to the distribution of heat within the motor.

BACKGROUND OF THE INVENTION

Thermal design of motors and generators is a well recognized issue of great importance to the life cycle cost of such equipment. In general, 60% of motor/generator failures are due to bearings, 30 % are due to winding insulation failure, and 10% to a number of less frequent causes. It is generally accepted that reliability of both bearings and electrical insulation is strongly related to temperature. Failure occurs sooner if the operating temperature is higher.

Extending the useful life of motors has economic value, both because of the replacement cost and because of the significant losses which can result from unscheduled down-time in a manufacturing process. BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to apparatus having an improved service life and methods of using and making such apparatus. That is, such apparatus is less likely to fail due to temperature related degradation of parts and components.

One embodiment of the present invention directed to an apparatus for use as a shaft in a machine comprising a body and a core. The body has a cylindrical wall made of a high strength material having a first thermal conductivity. The cylindrical wall has a first end, a second end, an interior surface and an exterior surface. The interior surface defines a space for a core. The core comprises a thermal transfer material. The thermal transfer material has a second thermal conductivity greater than the first thermal conductivity to facilitate the movement of thermal energy through the core. The core is thermally connected to the body. As a result, the core transfers and redistributes heat so the hottest areas within the machine become cooler, whether or not additional heat is removed from the machine. Furthermore, in situations where the core is thermally connected to an external heat sink, for example, at one or both of its ends, the core receives thermal energy from the body and removes thermal energy from the machine through said end or ends. As used herein, the term "thermally connected" is used to mean that the two objects or physical locations so referred to share the ends of a thermal path whose conductivity is low compared to other paths by which heat leaves the high temperature end of the circuit, thus allowing thermal energy to be transferred or communicated readily between the tow objects or physical locations.

One embodiment of the present invention features a high strength material comprised of steel. As used herein, steel refers to all forms of steel including but not limited to stainless steel. Further embodiments of the present invention feature a thermal transfer material comprising copper, aluminum, silver, gold, platinum and other thermally conductive metals. Those skilled in the art will recognize that cost considerations will normally favor copper and aluminum.

Embodiments of the present invention include a core thermally connected to fins, heat convection apparatus, and heat exchangers for dissipating heat energy. Embodiments of the present invention described above include an apparatus wherein the machine is an electric motor or an electric generator, however, embodiments of the present invention have applications wherever an apparatus or machine contains areas or devices such as bearings, which would last longer and be more reliable if they could were caused to operate at lower temperature. For example without limitation, such machines include the dryer rollers in paper machines, high speed printing press printing rollers, sheet metal rolling mills, and Gatling guns of the sort commonly used in combat aircraft and other weapons. A further embodiment of the present invention is directed to a method of operating a machine having a shaft. The method comprises the step of providing a shaft having a body and a core. The body has a cylindrical wall made of a high strength material having a first thermal conductivity. The cylindrical wall has a first end, a second end, an interior surface and an exterior surface. The interior surface defines a space for a core. The core comprises a thermal transfer material having a second thermal conductivity greater than said first thermal conductivity to facilitate the movement of thermal energy through said core. The core is thermally connected to the body to receive thermal energy from the body and remove the thermal energy through at least one of the ends. And, the method comprises the step of operating the machine such that the core receives thermal energy from the body, reduces peak temperatures within the body, and optionally in addition removes the thermal energy through at least one of the ends.

A further embodiment of the present invention is directed to a method of making a machine having a shaft. The method comprises the step of forming a body and a core. The body has a cylindrical wall made of a high strength material having a first thermal conductivity. The cylindrical wall has a first end, a second end, an interior surface and an exterior surface. The interior surface defines a space for a core. The core comprising a thermal transfer material having a second thermal conductivity greater than the first thermal conductivity to facilitate the movement and redistribution of thermal energy through the core. The core is thermally connected to the body to receive thermal energy from the body, to reduce peak temperatures within the body, and, optionally, to remove the thermal energy through at least one of the ends to form a shaft. The method further comprising the step of incorporating the shaft in the machine. Thus in this embodiment, for example, without limitation, a shaft is employed in the machine which is annular in construction, containing an inner core of copper surrounded by a steel annulus. This results in much greater thermal conductivity through the shaft, and as a result lower operating temperature for heat sources such as bearings, armature and stator windings. Because the torsional strength of a cylinder or tube varies as the third power of its radius, the lower strength of copper in the center of the shaft has little effect on torsional or bending strength, but has a significant effect on thermal conductivity. Increased conductivity results in a lower temperature throughout the motor. Failure rate of both bearings and winding insulation (90% of all motor failures) are strongly temperature dependent

DESCRIPTION OF FIGURES

Figure 1A shows the apparatus embodying features of the present invention in sideview; Figure IB depicts the apparatus in cross-section;

Figure 2 is a graph comparing the approximate thermal conductivity and torsion strength of a solid steel shaft as compared to an annular structure of steel and copper; Figure 3 compares the torsion strength of the hybrid shaft when the steel and copper are bonded together and when they are not;

Figure 4 shows the dominant thermal paths in a motor or generator; Figure 5 shows a possible heat exchanger system added to Figure 4;

Figure 6 depicts an embodiment of the present invention directed to a method of securing a thermally conductive shaft within a high strength outer cylinder; Figure 7 depicts bearing temperature over time recorded in tests;

Figure 8 depicts bearing temperature and shaft under bearing over time;

Figure 9 depicts average shaft temperature over time; and Figure 10 depicts temperature variation with shaft position.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of the invention is to cause the hottest parts of electromechanical devices such as motors and generators to run cooler, thereby achieving greater reliability and mean time between failures. In some embodiments, it may cause the entire motor or generator to run cooler. The invention is based on improving the conductivity of the motor/generator shaft by constructing it as a coaxial assembly. Turning now to Figure 1A and IB, an apparatus, useful as a shaft of a motor or machine, generally designated by the numeral 11, is depicted. Figure 1A depicts apparatus 11 in a substantially side view, whereas Figure IB shows apparatus 11 in cross section. The apparatus has a core 13 and an annulus or body 15. For the purpose of this discussion, the core 13 is described as copper and the body 15 as comprising steel. The core 13 comprises a material with greater thermal conductivity than the thermal conductivity of the material of the body 15.

The steel provides most of the torsional stiffness and strength of the structure, because this quantity varies as the third power of the radius. Because of this fact, a significant cross section of the shaft can be made of copper with very little impact upon the torsional stiffness and torsional yield strength of the shaft assembly. Copper has roughly ten times the specific thermal conductivity of steel (depending on the steel alloy employed) and therefore can materially increase heat conduction. For example, if one third of the diameter is copper, the yield strength of the assembly in torsion is reduced by less than 4%, whereas the thermal conductivity of the shaft is doubled. At 50% copper, the strength is reduced 12% and the heat transfer is tripled.

An approximate analysis follows. Figure 1 is a cross sectional drawing of the shaft, with various parameters used in the calculation noted. The core (13) is copper, and the outer annulus (15) is made of steel. The interface between the two (17) is also shown.

The torsional strength of a conventional steel shaft and the hybrid steel shaft is compared first, as a function of the ratio of the copper and steel diameters, or equivalently, radii. Let α = rJr _s where the radii are defined as in Figure 1. It is assumed in this analysis that the steel and copper shafts are bonded together so that there is no slip at the interface.

The first question is whether the copper or steel portion will yield first. This will depend upon the relative yield stress of the two materials. From the principles of solid mechanics (see, for example, Crandall and Dahl, An Introduction to the Mechanics of Solids, McGraw-Hill, 1959):

where

x _S y = yield stress of steel, approximately 250 GPa

Xcy = yield stress of copper, approximately 70 GPa

G _s = modulus of rigidity of steel, approximately 79 GPa

G _c = modulus of rigidity of steel, approximately 45 GPa

Ysy = yield strain of steel

Ycy = yield strain of copper

At the steel/copper interface, the strain in the copper and the strain in the steel must be the same:

and the maximum strain in copper and in steel occurs respectively at r _c an r _{s ,} so

Ycy = Xcy / Gc at r _c and _sy = x _sy / G _s at r _s

Both materials will yield at the same torque if the value of a is such that:

Xcy / Gc = r _c x _sy /r _s G _s = a x _sy / G _s

Thus, the steel will yield first, and limit shaft strength when:

a < Xcy G _s / x _sy G _c

and, the copper will yield first, and limit shaft strength when: α > x _cy G _s / x _sy G _c

Substituting the approximate values of these parameters for the two metals indicates that this inflection occurs at a diameter ratio a = 0.49.

For the case of steel yielding first (greater total strength), we now calculate the yield stress of the two rod constructions as a function of a.

Let:

M _s = twisting moment in a steel rod at which yield stress occurs

M _c = twisting moment in a hybrid rod at which yield stress occurs

Then:

M _s = π x _sy r _s ⁴ / 2 r _s = π x _sy r _s ³ / 2 M _c = π x _{sy (} r _s ³ - r _c ³ / 2 + π x _c r _c ³ / 2

Substituting for x _c = a x _sy G _c / G _s

M _c = π x _{sy (} r _s ³ - r _c ³ / 2 + π a x _sy G _c r _c ³ / 2 G _s

Therefore,

(M _c / M _s ) = 1 - a ³ + a ⁴ G _c / G _s when a < x _cy G _s / x _sy G _c For the other case (copper yielding first), a similar approach shows that: M _s = π x _s r _s ³ / 2

M _c = π Xs (r _s ³ - r _c ³ )/2 + π x _cy r _c ³ / 2 and x _s = x _cy G _s / a G _c

(M M _s ) = Xs (1 - a ³ ) / x _S y + Xcy a ³ / x _S y Since x _s = x _cy G _s / a G _c (M _c /M _s ) = x _cy G _s (1 - a ³ ) / a x _sy G _c + x _cy a ³ / x _sy when a >x _cy G _s / x _sy G _c

The thermal conductivity of the two assemblies compares as follows. The thermal conductivity of an object is:

C = σΑ/L

Where

σ = specific conductivity of the material

A = cross sectional area through which the heat flows

L = distance through which heat flows.

Define:

σ s = specific conductivity of steel, approximately 20 w/mK

specific conductivity of copper, approximately 213 w/mK

C s = thermal conductivity of all-steel shaft

C _c = thermal conductivity of hybrid shaft

Then:

C _s = o _s Ji r _s ² /L

C c = σ _s π (r _s ² - r _c ² )/L + σ _c π r _c ² /L

(C c / C s ) = 1 - a ² + a ² (a _c / a _s )

Figure 2 is a chart showing the relative performance of the hybrid vs. standard shaft, as a dimensionless ratio of shaft yield strength for the two cases and the ratio of end-to-end thermal conductivity of the shaft as a second dimensionless ratio. For an all-copper shaft, the strength ratio is 0.28, and the conductance is 10.6, both of which are ratios of the relevant properties of the two materials.

An alternative embodiment of the invention does not attempt to bond the two materials together; instead a sliding fit is employed and thermal grease is applied to the interface prior to assembly. This will provide nearly equivalent thermal conductivity across the interface, and will significantly simplify manufacturing issues. It will weaken the shaft slightly, because the copper will bear no torque. However a potential failure mode is avoided, wherein the bond between the steel and copper ruptures during the life of the motor, both weakening the shaft and reducing thermal conductivity significantly. Rupture may occur because if the materials are bonded, the strain of the copper and of the steel must be the same at the interface. However the stress differs by the ratio G _c / G _s between the two materials. The difference is borne by the interface bond, and may exceed its yield strength.

Such a failure might result from inadequate control of the heat- shrink assembly process of the two shafts or inadequate quality control of the finish of one of the materials.

From the calculations above, removing the copper from the torsion strength relationship results in the following:

(M _c / M _s ) = l - a ³

Figure 3 plots the relative strength with and without the copper bearing any torque. It is noted that for the region of interest, when a < x _cy G _s / x _sy G _c , the difference between the two cases is inconsequential.

Typically, the shaft diameter for a particular application is determined from a set of standard sizes for which fittings and bearings are easily obtained, so there is usually, but not always, a significant safety margin in strength. As a result, the sort of strength reductions noted could generally be accommodated without going to the next shaft size.

The dominant heat flow paths in a motor are shown in Figure 4. Heat is generated primarily in the armature (401) and stator (402) due to eddy current losses, and in the windings themselves due to resistance losses. This may result in hot spots in the motor, rather than a uniform temperature near the average. Heat is also generated in the bearings, (403 and 404) but much less than that due to the electrical losses. In an open frame motor, air is typically drawn into the rotating assembly, and provides a primary path for forced convective cooling. In the former case, it is drawn through openings in the case (405), some of it through the air gap (406) between armature and stator, In both open and closed motors, heat is shed by conduction through the shaft (407) to the outside world, and through the case (405) to the mounting brackets (408) which secure the motor. Conduction also occurs from the bearings to the shaft, from the armature to the shaft, and from the shaft to external air and to equipment to which the motor is attached. In some cases, bearing lubricant is circulated and cooled externally. The total thermal resistance of the path through the shaft depends on the motor's application. If a pump impeller for a liquid is directly attached to the shaft, then the total resistance will be quite low, and a significant heat transfer to the liquid will occur. A pump for air or gas will transfer less than a liquid, but still more than the heat transfer out of a closed-enclosure motor with sealed bearings.

Another option is to mount a heat exchanger on the shaft, either between the drive and the load, or on the back-end of the shaft, as shown in Figure 5. A heat exchanger (501) is in thermal contact with the shaft (502) between the motor (503) and its load (504) via a lubricated surface, through which heat is conducted from the shaft to a finned structure (505) which provides convective or water cooling.

Various suitable methods of fabrication are known to one skilled in the art, and may be found to be more suitable from the standpoint of manufacturing yield and cost. The choice may differ depending upon the specifics of the shaft materials, size, proportions, and intended operating environment.

For example, without limitation, suitable methods of making a thermally conductive shaft include: 1. Various methods of metallic bonding between core and shaft, including but not limited to a thermal shrink fit between shaft and core, compression welding and electric welding;

2. Morse taper of the high strength annulus material and high thermal conductivity core material;

3. Morse taper with full contact circumferential screw threads at one or both ends to

secure the core and body to one another;

4. Screwed contact using full-contact threads (full-length or ends only) without taper; the first end being of smaller diameter than the shaft, which is in turn of less diameter than the second screw; sliding fit for the shaft portion; 5 Screwed contact using full-contact threads (full-length or ends only) as in (3) with taper, in which relative rotation of the parts to tighten the screws increases the pressure at the high strength annulus material and high thermal conductivity core material interface, thus improving the thermal conductivity of the interface;

6 Thermal grease layer coating at the high strength annulus material and high thermal conductivity core material interface to improve thermal conductivity across it;

7 Thermal conducting epoxy (for example, silver loaded or other available types) which bonds tapered or non-tapered pieces together;

8 high strength annulus with end-caps, filled with thermally conducting liquid;

9 high strength annulus, filled with carbon composite of high thermal conductivity.

Figure 6 depicts a shaft 611 having a steel body or annulus 615 and a copper core 617 in which the copper core 617 is screwed into contact with the steel annulus 615 using full- contact threads 618 at the larger end and 619 at the smaller end. The relative rotation of the parts to tighten the screws increases the pressure at the high strength annulus material and high thermal conductivity core material interface, thus improving the thermal conductivity of the interface.

The taper of the shaft is exaggerated for clarity. Obtaining a good bond between the high strength annulus material and high thermal conductivity core material, such as copper and steel, will have a significant impact on thermal and mechanical performance. Heating the steel annulus before assembly will cause a tight fit when it cools; other methods may be appropriate as well. Embodiments of the present invention featuring a solid copper core and a copper tube insert were compared to a plain steel shaft. In this Example, three shafts, two inches in diameter and two feet long, were fabricated. One shaft was solid steel. One shaft had a one inch diameter center hollow along the axis in which a one inch outer diameter copper tube was inserted. The tube had a cylindrical wall having a 0.125 inch thickness. One shaft had a one inch diameter center hollow along the axis in which a one inch diameter solid copper rod was inserted.

Three resistors, each having a capacity to deliver 80 watts to its shaft, were clamped to one end of each shaft to simulate bearing heat. The shafts with a thermally conductive core had two to three times the thermal conductive capacity of the solid steel shaft without significantly weakening the shaft. For example, under similar thermal loads the heated end of the shaft with a solid copper core was 109 degree cooler than the heated end of the solid steel shaft. The copper tube embodiment was 71 degrees cooler than the solid steel shaft. These results over time are depicted in Figure 7.

Figure 8 depicts the results at the resistor representing the bearing. The measurements reflect surface temperature obtained with an IR probe over time. Figure 9 depicts results for the average shaft temperature. These data suggest the thermally conductive core is dissipating heat along the entire shaft. Figure 10 depicts temperature at different positions along the shaft. These data suggest the thermally conductive core is dissipating heat along the shaft and away from the "bearing".

Thus, although we have described the invention with respect to the best mode as presently contemplated by the inventors, it is understood that these embodiments can altered and modified without departing from the teaching presented herein. Therefore, the invention should not be limited to the precise details described but should encompass the subject matter of the claims that follow.