Electrical isolator for couplings

A coupling includes a first portion configured for connection to a first shaft and having a first portion flange and a second portion configured for connection to a second shaft and having a second portion flange. The coupling also includes an isolating member disposed between the first portion flange and the second portion flange and that is formed of ceramic and electrically isolates the first portion from the second portion. The coupling also includes one or more connecting members passing through the first portion flange, the second portion flange, and the isolating member and holding the first portion flange and the second portion flange in a fixed relationship to one another.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to couplings and, in particular, to electrical isolating rings in couplings.

Many applications require that rotating shafts be coupled together. For example, generators are driven by industrial turbines to produce electricity. A load coupling is often used to connect the shafts of the generator and the turbine. The coupling is typically electrically insulated to prevent current traveling down the rotor shafts. If current is transmitted to the rotor of the turbine, there is a possibility of electrical arcing from the rotor to the bearing surfaces, which can cause damage and potentially failure of the bearings.

In more detail, in one class of couplings for rotating shafts, a drive portion of the coupling is coupled to a driving shaft by a group of circumferentially spaced fasteners. A driven portion of such a coupling is similarly coupled to a driven shaft. The two parts of the coupling are then coupled together by connecting bolts. During rotation, the driving shaft applies a force that is transmitted through the connecting bolts to the driven shaft.

Typically, the drive portion is electrically isolated from the driven portion by a spacer ring disposed between the two. The prevailing approach to providing electrical isolation is through the use of a fiberglass reinforced epoxy (FG/Ep) insulating assembly. The insulating assembly typically includes four elements: an insulating plate, bushings, washers and a pilot ring.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a coupling includes a first portion configured for connection to a first shaft and having a first portion flange and a second portion configured for connection to a second shaft and having a second portion flange. The coupling of this aspect also includes an isolating member disposed between the first portion flange and the second portion flange, the isolating member being formed of ceramic and electrically isolating the first portion from the second portion and one or more connecting members passing through the first portion flange, the second portion flange, and the isolating member and holding the first portion flange and the second portion flange in a fixed relationship to one another.

According to another aspect of the invention, an transfer device includes a first portion having a first portion flange and a second portion having a second portion flange. The transfer device of this aspect also includes an isolating member disposed between the first portion flange and the second portion flange, the isolating member being formed of ceramic and electrically isolating the first portion from the second portion. The transfer device of this embodiment also includes a fastener passing through the isolating member, the first portion flange and the second portion flange and holding the first portion flange and the second portion flange in a fixed relationship to one another.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the prevailing approach to providing electrical isolation included fiberglass reinforced epoxy (FG/Ep) components. Although FG/Ep provides good isolation, experience and analysis have shown that FG/Ep may exhibit failure due to high stress (crushing of fiberglass). In addition, FG/Ep components may experience radial growth and deformation due to non-uniform properties (properties are dependent on direction of fiberglass reinforcement); may wear and degrade over time and have a low coefficient of friction (static=0.1) when in contact with steel which may result in low torque transmission through friction which can cause loading of connecting members in shear.

Accordingly, embodiments of the present invention may provide a more wear resistant and still electrically isolating material at the junction of a driving portion and a driven portion of a coupling. In particular, the material may be in form of an isolating member formed of ceramic.

In addition, embodiments of the present invention may include radial barriers to debris liberation in the event of a fracture in or damage to the isolating member. This may include providing recesses in one or both the drive and driven portions of the coupling. While the following description is directed to a load coupling, it shall be understood that the teachings herein may be applied to any coupling100, for example a transmission joint that joins two rotating members and transfers energy, movement or the like.

FIG. 1is cross sectional view of one half of a coupling100according to one embodiment. The coupling100joins a first shaft102to a second shaft104. In one embodiment, the coupling100is a flexible load coupling and may provide a load transfer in cases where the first shaft102and the second shaft104may become slightly off-axis from one another due to factors such as thermal expansion of the shafts. In operation, the coupling100, the first shaft102and the second shaft104rotate generally around rotation axis106.

The coupling100includes a first portion108coupled to the first shaft102and a second portion110coupled to the second shaft104. The first portion108is coupled to the second portion110by one or more connection assemblies115. Each connection assembly115includes a fastener114that holds a flange116of the first portion108to a flange118of the second portion110.

In one embodiment, the first portion108is electrically isolated from the second portion110. In this manner, the first shaft102is electrically isolated from the second shaft104. In one embodiment, one or more isolating members112are disposed between the first portion108and the second portion110to electrically isolate them. In operation, the isolating members112transmit torque between the first portion108and the second portion110in addition to electrically isolating them from each other. Accordingly, in one embodiment, the isolating members112are formed of a material that is relatively stiff. In one embodiment, the isolating member is a ring. The ring may be a continuous ring or may be comprised of multiple portions as described in greater detail below.

In one embodiment, the isolating member112is formed of a ceramic material The ceramics may include high compressive strength, direction independent properties, high wear resistance and higher coefficient of friction than FG/Ep (i.e., static coefficient friction greater than or equal to about 0.22). The higher coefficient of friction may result in increased torque transmission through friction which can prevent loading of connecting members in shear. For example, the isolating members112may be formed at least one of Alumina (Al2O3), Alumina-Silica, Alumina-Carbon-SiC, Alumina-Chromium oxide, Alumina-Calcium oxide, cordierite (2MgO-2Al2O3-5SiO2), Mullite (Al6Si2O13), Silicon carbide (SiC), Silicon nitride (Si3N4), Zirconia (ZrO2), Zirconium-Silicate (ZrSiO4) and Zirconia strengthened aluminas (ZrO2—Y2O3, Al2O3—ZrO2, ZrO2—MgO, ZrO2—Y2O3—CeO2) and combinations including at least one of the foregoing. Of course, other materials may be utilized.

One concern that may exist with utilizing ceramic materials as the isolating member112include the fact that ceramics may be brittle and prone to fast-fracture raising the threat of debris or large pieces being liberated. In the case of a turbine, the debris may become foreign object debris to the critical rotating components (airfoils) with the potential for catastrophic unit damage. In one embodiment, such issues may be addressed by providing projections from or recesses within either or both the first portion108and the second portion110that offer radial barriers to debris liberation.

Another concern with using ceramics in couplings is high tensile stresses, which can initiate and grow cracks. One embodiment may, therefore, include an isolating member112comprising a ring formed of segments with an improved geometry to reduce tensile stress in the circumferential or “hoop” direction. The segmented geometry may provide the following benefits over a continuous ring: it may create discrete boundaries as crack arrestors should over-stress and fast-fracture result; segments are cheaper and easier to manufacture while controlling critical features; segments may reduce bending stress during bolt tightening; segments represent reduced volumes of brittle material (such as ceramics) for which the strength is inversely proportional to the volume of material used (the probability of finding a critical flaw is statistically greater); segments allow for higher part quality due to the ability to control processing parameters and conditions and segments may reduce tangential tensile stress due to radial growth of flanges116and118at high speeds.

Further, while all of the options for ceramics described above, it may be beneficial to select a ceramic material with a similar modulus of elasticity to the flange material (steel). The elastic modulus is a measurement of the stiffness of the material. For instance, alumina has a higher elastic modulus than steel and is stiffer. Zirconia, on the other hand, has the same elastic modulus as steel and, therefore, is approximately the same stiffness. When spinning, the flange material grows radially outward. The high friction force from the clamping load provided by fastener114prevents the isolating member112from sliding against the steel flange. Thus, the isolating member112preferably is formed of a material that expands at the same rate as the material of the flange.

InFIG. 1, the connecting members114are shown as connecting the driving portion108and the driven portion110in a region outside an outer surface of both of these portions. Of course, the connecting members114may be located with the outer surface in one embodiment.

In the following description, the first portion108is referred to as a “driving” portion the second portion110is referred as a “driven” portion for the sake of convenience and clarity. It shall be understood that this is not meant as limiting and the first portion108could be referred to as a driven portion and the second portion110could be referred to as a driving portion.

FIG. 2shows an example of an isolating member112according to one embodiment. The isolating member112includes one or more holes202to allow connecting members (not shown) to pass through. As shown, the isolating member112is formed as a continuous ring.

FIG. 3shows an exploded view of an isolating member112formed of multiple segments. In particular, the isolating member112is segmented into a first segment302, a second segment304, a third segment306, and fourth segment308. In one embodiment, radially oriented edges of the segments302,304and306may be beveled to allow the segments to overlap one another. The number of segments shown inFIG. 3is illustrative only and not meant as limiting. Indeed, in one embodiment, the isolating member112may be formed of any number of segments equal to or less than the number of holes202.

FIG. 4is a perspective view of a split segment402of an isolating member according to one embodiment. The split segment402is split along split lines404that each intersect a hole202in at least two locations. As shown, the split segment402is split at two adjacent holes202. In one embodiment, the split lines404are radial centerlines emanating from a center point of a circular isolating member.

As discussed above, embodiments of the present invention may provide radial barriers formed by the driving portion or the driven portion to prevent or reduce debris liberation in the event that an isolating member cracks or otherwise fails. In particular, either or both of the driven portion or the driving portion may include projections from or recesses within that offer radial barriers to debris liberation.

FIG. 5is a cut-away side view of a connection assembly115(FIG. 1) that provides radial barriers502to the debris that may be created in the event that the isolating member112breaks or is otherwise damaged. The isolating member112may be implemented, for example, as any of the embodiments shown inFIGS. 2-4.

The connection assembly115includes a driving flange208of the driving portion108and a driven flange210of the driven portion110. In this example, the driven flange210is separated from the driving flange208by isolating member112and by an air gap504. The air gap504may be sized to ensure that an expected current cannot arc across it and electrically couple the driving portion108to the driven portion110.

In more detail, the driving flange208includes a first face508. The radial barriers502extend beyond the first face508in the direction of the driven portion110. In one embodiment, the radial barriers502surround the isolating member112and may be formed as the result of a groove being formed in the driving flange210. In one embodiment, the width of the groove is greater than a width of the isolating member112.

InFIG. 5, a connecting member114is implemented as a bolt having a head602. The bolt600may be coupled to a nut604to hold the driving flange208in a fixed relationship to the driven flange210. The combination of the bolt600and the nut604forming connecting member114in this embodiment provide a compressive force holding the driving flange208and the driven flange210in a fixed relationship to one another. Additionally, the connecting member114may transfer torque from the driving flange208to the driven flange210during operation.

Typically, the connecting member114, the driving flange208and the driven flange210are all formed of a conducting metal such as steel. Accordingly, the isolating member112electrically isolates the driving flange208from the driven flange210. Of course, other means may be required to isolate the connecting member114from the driving flange208and the driven flange210. Otherwise, the connecting member114may provide an electrical connection between the driving flange208and the driven flange210. One example includes washers and bushings formed of electrically insulating materials that electrically isolate the connecting members form the driving flange208and the driven flange210.

FIG. 5shows the radial barriers502as formed on the driving flange208. Of course, in an alternative embodiment the radial barriers502could be formed on the driven flange210.