Patent Application
20070080036
1. Field of the Invention
The present invention relates to electromagnetic coupling devices.
2. Background Art
An electromagnetic (EM) coupling device may be used as a clutch or brake to selectively couple a rotating component to either another rotating component (operating as a clutch), or to a stationary component (operating as a brake). EM clutches/brakes require a wire coil to be energized by an external source of electric current to create a magnetic field used to couple the components. EM clutch/brake designs include either a rotating coil embedded in one of the rotating members, or a stationary coil placed between the selectively coupled members. In general, rotating coil designs have a more efficient magnetic flux path, but must be connected to the electric current source using brushes or slip-rings. These components add to the device complexity, cost, and part count, and can have durability issues. Alternatively, stationary coil designs have a less efficient magnetic flux path that must traverse at least one, and typically two, air gaps and therefore require expensive precision machining to minimize the air gap while still providing sufficient clearance to accommodate the rotating member(s). In addition, the small air gaps that are desirable in terms of magnetic permeability are not amenable to forced-air cooling and reduce the thermal or heat conduction efficiency of internal components, imposing additional constraints on applications, particularly those that must tolerate slip and generate significant heat in the process.
An electromagnetic coupling device includes a rotating member having an embedded coil positioned to provide a magnetic flux path through the rotating member to substantially eliminate at least one air gap. At least one bearing assembly disposed between the rotating member and a second member includes an electrically conductive lubricant to supply electrical current through the bearing and lubricant to the embedded coil during actuation of the device. A method of making an electromagnetic coupling device includes embedding a coil within a rotating member such that the rotating member contacts substantially the entire outer surface of the coil to provide an efficient magnetic flux path through the rotating member, and connecting the embedded coil to at least one conductive bearing assembly having conductive lubricating grease to provide electric current to the embedded coil. A method for selectively coupling a rotating member to a second member includes selectively supplying an electric current to a coil embedded in the rotating member through a conductive bearing having a plurality of rolling elements surrounded by electrically conductive lubricant wherein the coil is substantially surrounded by the rotating member to reduce or eliminate air gaps between the coil and the rotating member and improve effective magnetic permeability of an associated magnetic flux path through the rotating member.
Embodiments of the present invention include an electromagnetic friction clutch, and a magnetorheological (MR) fluid or magnetic particle/powder clutch. The clutches include a rotating member with a coil embedded therein such that the rotating member surrounds and contacts substantially the entire outer surface of the coil. The coil is connected via a first wire lead to a selectively actuated current source through a first bearing assembly having a conductive lubricant and rolling elements between an outer and inner race and sealed cage. In one embodiment, a return current path for the coil is provided by a second wire lead connected through a second bearing assembly electrically isolated/insulated from the first bearing assembly. In another embodiment, the return current path passes to ground through a conductive device mount or mounting assembly, such that only a single conductive bearing assembly is required for actuation.
The present invention provides a number of advantages. For example, the present invention reduces part count and variable cost by using conductive lubricating grease in a bearing to provide a direct electrical conduction path to a rotating electromagnetic coil. Elimination of parts, such as brushes and slip rings may substantially improve reliability and durability. Use of a rotating coil according to the present invention allows embedding of the coil within one of the rotating members to eliminate air gaps in the magnetic flux path to improve magnetic and corresponding electric efficiency. Improved electric efficiency facilitates use of a smaller, lighter coil, and lower power requirements with less generated heat. Elimination of air gaps reduces manufacturing costs by eliminating tight-tolerance machining operations and also improves heat conduction and clutch cooling such that higher slip tolerances may be accommodated for MR fluid and magnetic power clutch applications.
The above advantages and other advantages and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.
As those of ordinary skill in the art will understand, various features of the present invention as illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce embodiments of the present invention that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present invention may be desired for particular applications or implementations.
The electromagnetic clutch embodiments illustrated in
As illustrated in
In the embodiment of
Driving or rotating member 12 includes an embedded coil 18 having an outer surface 20 substantially entirely contacting a corresponding annular pocket or groove in member 12. In contrast to conventional fixed coil EM clutches, the present invention eliminates at least one air gap by embedding coil 18 within rotating member 12 and inserting a cover or cap 22 made of a material having desired magnetic properties that may be the same or different from the material used for other portions of driving member 12. Elimination of the air gap in the magnetic flux path of coil 18 through driving member 12 that is otherwise required for fixed coil implementations to allow rotation of rotating member 12 relative to the coil improves the magnetic and corresponding electric efficiency of EM coupling device 10. As such, reducing the number of air gaps by substantially eliminating any air gaps between coil 18 and driving member 12 requires less power to achieve a given magnetic field in the torque generating portions of device 10 relative to stationary coil designs or other designs that incorporate one or more air gaps in the magnetic flux path.
Rotating embedded coil 18 is electrically connected by first and second wire leads 30, 32 to corresponding electrically conductive outer races 40, 42 of conductive bearing assemblies 46, 48, respectively. Routing of wire leads 30, 32 to outer races 40, 42 is schematically illustrated for ease of description with actual placement or routing selected based on various application and implementation specific considerations. Outer races 40, 42 may engage rotating member 12 with an interference or press fit such that outer races 40, 42 rotate with rotating member 12 during operation. Each conductive bearing assembly 46, 48 includes a plurality of rolling elements 50 implemented by bearing balls in this embodiment. Rolling elements 50 may also be implemented by cylindrical elements, typically referred to as needle or roller bearings. Rolling elements 50 are disposed between an inner race 52 and corresponding outer race 42 with bearing balls 50 surrounded by an electrically conductive lubricant or lubricating grease 60 contained by upper and lower cages and/or seals 56, 58, respectively. Electrically conductive grease 60 should be capable of conducting an electric current capable of generating a magnetic field suitable to operate device 10 without significantly degrading or arcing. For a representative automotive application, electrically conductive lubricant 60 may have a volume resistivity of less than or equal to about 300 ohm-cm and be able to sustain a current of at least about four amperes. One such suitable lubricant is Nyogel 758G, which is commercially available from NYE Lubricants, Inc. of Fairhaven, Mass. Electrically conductive lubricant 60 provides a sufficient current carrying capacity and reduces or eliminates arcing between components of the bearing assembly, such as balls 50 and inner and outer races 42, 52, respectively.
While the representative embodiments illustrating the present invention include a bearing assembly having rolling elements between concentric inner and outer races, those of ordinary skill in the art will recognize that various applications may include devices that do not require rolling elements. For example, the present invention may be implemented by a conductive grease filled bushing assembly for various low speed applications to provide at least one conductive path to a rotating element. The particular construction of a grease filled bushing may vary by application. In general, a grease filled bushing assembly according to the present invention includes at least one bushing with at least one seal or similar device to generally contain the conductive grease to an annular gap between the bushing and rotating element. Bushing assemblies may also include two concentric annular or cylindrical members with an annular gap therebetween filled with a conductive lubricant. A seal is connected to at least one of the members to contain the conductive lubricant and allow rotation of the members relative to one another.
As also shown in
Driven member 14 of EM coupling device 10 is secured for rotation with shaft 70 via fastener 90. Driven member or armature 14 includes an annular magnetic material 92 secured to a spring 94 that biases armature 92 away from rotating member 12 to assist disengagement and reduce or eliminate drag when device 10 is deactivated. In this embodiment, armature 14 includes embedded rings or bands 96 of a friction material to provide a friction surface to increase torque carrying capacity of device 10 after engagement or contact between armature 14 and pulley 12.
In operation beginning from the disengaged position as illustrated on the right side of
According to the present invention as illustrated by the representative embodiment of an electromagnetic friction clutch in
Each conductive bearing assembly 122, 124 includes a conductive outer race 130 electrically insulated/isolated from rotating member 102 by one or more insulators 126, which also isolate conductive bearing assemblies 122, 124 from a stationary mounting shaft or assembly (not shown) that supports inner race 134. Conductive bearing assemblies 122, 124 each include a plurality of rolling or roller elements 136 surrounded by a conductive lubricant 138 and contained by corresponding seals 140. Inner race 134 of at least one bearing assembly 122, 124 is connected to a lead wire 142 that terminates at connector 144 to connect an external current source (not shown) to supply electrical current during actuation of device 100. A second lead wire 146 may also be provided and connected to a corresponding inner race and connector 144 to provide a return current path.
Depending on the particular application, a third bearing assembly 150 may be positioned between driving or rotating member 102 and second or driven member 104 to provide additional structural support and allow relative rotation therebetween. Bearing assembly 150 does not require electrical conductivity, but may also be implemented as an alternative conductive path by a conductive bearing assembly if desired.
A flowable magnetic material 154 is disposed in a space between rotating member 102 and second member 104. Flowable magnetic material 154 may be any of a number of suitable magnetic particles/powders or magnetorheological fluids that improve coupling of rotating member 102 and second member 104 during actuation of device 100. One or more seals or baffles 156, 158, 160 operate to substantially contain the flowable magnetic material in the space between members 102 and 104.
Magnetorheological fluid and magnetic particle clutches and brakes are often used in applications where slip must be tolerated, i.e. where the driven member rotates at a lower speed than the driving member when the clutch is engaged, or to provide a soft start/engagement. In these applications, heat generated by the frictional slip may be many times greater than resistive heat generated by operation of the coil and the additional heat must be dissipated. Prior art clutch/brake designs that have one or more air gaps generally try to reduce the size of the air gap due to the effect on the magnetic and corresponding electric efficiency. However, small air gaps limit air circulation and the efficacy of forced-air convective cooling. In addition, the air gaps also inhibit conductive cooling. Use of one or more conductive bearings according to the present invention allows the coil to be embedded in the rotating member and eliminates one or more air gaps to improve conductive cooling and corresponding heat dissipation so that slip tolerances can be significantly increased.
In operation, coupling device 100 provides selective coupling between rotating member 102 and second member 104 by supplying an electric current to embedded coil 110 through at least one conductive bearing assembly 124 having conductive lubricant 138. Coil 110 generates a magnetic field that passes through rotating member 102, flowable magnetic material 154, and second member 104 to couple second member 104 to rotating member 102.
As such, the present invention reduces part count and variable cost by using conductive lubricating grease in at least one bearing to provide a direct electrical current path to a rotating electromagnet coil. A direct connection through the bearing and grease eliminates brushes and/or slip rings and may therefore substantially improve reliability and durability. Positioning the coil within one of the rotating members eliminates air gaps in the magnetic flux path to improve magnetic and corresponding electric efficiency so that a smaller, lighter coil with lower power requirements and less generated heat may be used. Elimination of air gaps also reduces manufacturing costs by eliminating tight-tolerance machining operations while improving heat conduction and clutch cooling such that higher slip tolerances may be accommodated for MR fluid and magnetic power clutch applications.
While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.
