MXPA99003856A - Clutch-magnetic brake with center polo - Google Patents

Abstract

The present invention relates to a clutch / brake unit, comprising: a rotor disk connected to an arrow for rotation therewith, the arrow extends along a first axis and the rotor disk extends radially towards out from the arrow, an armature axially separated from the rotor disk, an output member connected to the armature, a field cover axially spaced from the armature by the rotor disk, the field cover includes a radially extending flange outward, first and second pole pieces angularly spaced from one another and connected to the flange, the first and second pole pieces have a relatively low magnetic reluctance; first and second permanent magnets disposed between the flange and the first and second parts of pole, respectively, the field cover, the first and second permanent magnets, the first and second pole pieces and the armature form a first magnetic circuit which pulls the armature in a first axial direction away from the rotor disk and a first portion of the armature in engagement with the first and second pole pieces, a third pole piece connected to the flange and disposed between the first and second pole pieces; the second pole pieces, the third pole piece has a relatively high magnetic recruitment, and means for selectively generating a second magnetic circuit between the field cover, the rotor disk and the armature, the second magnetic circuit pulls the armature to a second axial direction towards the rotor disk wherein a second portion of the armature angularly opposite the first portion engages the first rotor disk thereby thereby forcing first, the first portion to be decoupled from one of the first and second po parts.

Description

CLUTCH-MAGNETIC BRAKE WITH CENTRAL POLO BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a clutch / brake unit for use in a riding lawn mower, or a similar vehicle, and more particularly, to a unit having a pole piece having a relative magnetic reluctance. Highly high that separates a pair of pole pieces that have relatively low magnetic reluctance to allow a more efficient release of the brake and clutch engagement. 2. Description of the Related Art Clutch / brake units are commonly adapted for use in riding mowers, garden tractors or similar vehicles to selectively transmit a driving torque from a motor to an implement driver (e.g. mower blade drive) and to apply a positive braking torque to the impeller when the torque of the impeller is not being transmitted.

A conventional brake / brake unit may include an inlet member, or hub, connected to a longitudinally extending inlet arrow to rotate therewith, a rotor disk mounted on the inlet member, an axially spaced armature from the rotor disk, and an output member, such as a pulley connected to the armature. The armature and the exit member can be connected by a plurality of springs which deflect the armature in a first axial direction away from the rotor disk and into a braked position against the braking member. The unit may further include an electromagnetic clutch disposed on one side of the rotor disc opposite the armature. The clutch may include an electrical coil disposed within a fixed field cover. The energization of the coil establishes a magnetic circuit between the field cover, the rotor disc and the armature, and pulls the armature in a second axial direction towards the rotor disk and in a position engaged with the clutch, which concurrently releases the armature of the coupling with the braking member or members. De-energizing the coil cuts the magnetic circuit and the springs once the armature is pulled back in a first axial direction to a braked position against the braking member. As noted by the applicant in the applicant's prior US Patent No. 5,119,918 (hereinafter, "patent x918") the full disclosure of which is incorporated herein by reference - the embryo / brake units described above have disadvantages significant In these conventional units, the entire braking force is provided by the springs connecting the armature and the exit member. As a result, these springs must be relatively strong. A relatively high electromagnetic force must thus be generated by the electromagnetic clutch in order to release the brake and cause clutch engagement. Likewise, the force that the springs provide increases as the armature is pulled away from the braked position and into the position engaged with the clutch against the rotor disc. Because a high electromagnetic force is required, longer and / or more expensive electrical components must be used, both within the embryo / brake unit itself, and within any vehicle or machine that incorporates the unit. In the? 918 patent, the applicant describes an embedment / brake unit that is capable of overcoming the deficiencies mentioned above. In particular, the applicant describes a brake / brake unit that incorporates a plurality of permanent magnets to effect braking. The plurality of magnets is disposed between a radially extending flange of the field cover and a corresponding plurality of pole pieces having a relatively low magnetic reluctance. The magnets create a magnetic circuit between the magnets, the pole pieces, the field cover and the armor that pulls the armature in a first axial direction to a braked position against the pole pieces. Because the armature is pulled to a braking position by magnetic attraction, the springs that connect the armature and the output member need only be strong enough to transmit the torque from the armature to the output member. In addition, the braking force provided by the magnetic circuit decreases as the armature is pulled in a second axial direction towards the rotor disk and into the engaged position with the clutch (as opposed to the increasing force on the spring-set brake). conventional). Because less force is required to release the brake and bring the armature into engagement with the rotor disk, smaller and / or less expensive electrical components may be used in the embed / brake unit and in the vehicle or machine that incorporates unit. Although the clutch / brake unit described in the '918 patent represents a significant improvement over the prior art, further improvements are desirable. A desired improvement would be to further reduce the electromagnetic force required to release the brake and engage the clutch. In the embryo / brake unit described in the '918 patent, the magnets and the pole pieces extend at an angular distance less than half the circumference of the field cover. Therefore, when the coil is energized, a portion of the armature that is further away from the magnets rapidly jumps to and engages with the rotor disk. As the magnetic attraction between the armature and the rotor disk decreases, the portion of the armature adjacent to the magnets also jumps into engagement with the rotor disk. The force required to make this last portion of the armature engage the rotor disk, however, is still greater than what is expected. Another desirable improvement is to allow the application of a protective coating, such as chromium carbide, to one or more of the pole pieces in order to increase the durability and life of the pole pieces. The chromium carbide has a magnetic reluctance height. In conventional embedding / brake units, therefore, only a relatively small amount can be applied to the pole pieces without a significant reduction in magnetic pull between the frame and the pole pieces, and consequently, the torque Braking torsion. Therefore, there is a need for a brake that minimizes or eliminates one or more of the aforementioned deficiencies.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides an embed / brake unit that requires less electromagnetic force when compared to conventional units to release the brake and engage the clutch. An object of the present invention is to provide a brake / brake unit that releases the brake and achieves clutch engagement with greater efficiency than conventional units. Another object of the present invention is to provide an embedment / brake unit in which a protective coating can be applied to the components of the unit to increase the durability and life of the unit. An embedment / brake unit according to the present invention includes a rotor disk connected to an arrow to rotate therewith. The arrow extends along a first axis and the rotor disc extends radially outwardly from the arrow. The unit also includes an armature spaced axially from the rotor disk, an output member connected to the armature, and a field cover axially spaced from the armature by the rotor disk. The field cover includes a flange that extends radially outward. The unit further includes first and second pole pieces angularly spaced apart from one another and connected to the flange. The first and second pole pieces have a relatively low magnetic reluctance. The unit also includes a first and second permanent magnets disposed between the flange and the first and second pole pieces, respectively. The field cover, the first and second permanent magnets, the first and second pole pieces, and the armature form a first magnetic circuit that pulls the armature in a first axial direction away from the rotor disk and pulls a first portion of the rotor. the armature putting it in coupling with the first and second pole pieces. In this form, the armature assumes a braked position. The unit further includes a third pole piece connected to the flange disposed between the first and second pole pieces. The third pole piece has a relatively high magnetic reluctance. Finally, the unit includes means, such as a field coil, to selectively generate a second magnetic circuit between the field cover, the rotor and the armature. The second magnetic circuit pulls the armature in a second axial direction towards the rotor disk where a second portion of the armature angularly opposite the first portion couples the rotor disk with which first it forces the first portion to uncouple from one of the first and second polo pieces. In the embed / brake unit described in the '918 patent, the armature remains in engagement with both of the first and second pole pieces, even after the portion of the armature angularly opposite the pole pieces is pulled in engagement. with the rotor disc. The addition of a third pole piece between the first and second pole pieces, however, ensures that the armature remains in engagement only with one of the first and second pole pieces, once the armature portion angularly opposite to that of the first and second pole pieces. the pole pieces engage the rotor disc. The resulting air space between the armature and the uncoupled pole piece weakens or forces the magnetic circuit that generates the braking torque. As a result, less electromagnetic force is required to make the armature completely in engagement with the rotor disk. A brake / brake unit according to the present invention has increased durability and a longer life, because less electromagnetic force is needed to overcome the braking torque within the unit. Durability and life are also increased because a protective chromium carbide coating can be applied to the third pole piece. The embed / brake unit described in the '918 patent uses only two pieces of pole, of which both are part of the magnetic braking circuit. Because the chromium carbide has a high magnetic reluctance, the application of that to those pole pieces would significantly affect the braking torque of the unit. In an emulsion unit according to the present invention, however, the coating can be applied to the third pole piece, which is not part of the magnetic braking circuit. The application of the chromium carbide coating to the third pole piece will reduce the wear of all the pole pieces and, consequently, will prolong the life of the unit. Those skilled in the art will be apparent to these and other features and objects of the present invention from the following detailed description and the accompanying drawings which illustrate the features of this invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a brake / brake unit according to the present invention, illustrating the unit in a braked position. Figure 2 is a cross-sectional view of the embryo / cold unit of Figure 1 taken substantially along lines 2-2 of Figure 1. Figure 3 is a plan view of the embryo unit. Figure 1 and 2, taken substantially along the lines 3-3 of Figure 2. Figure 4 is a partial cross-sectional view of a brake / brake unit according to the drawings. with the present invention illustrating the unit in a position partially engaged with the clutch. Figure 5 is an enlarged side plan view of a portion of a brake / brake unit of Figure 4, taken substantially along lines 5-5 of Figure 4.

DETAILED DESCRIPTION OF THE PREFERRED MODE With reference now to the drawings in which similar reference numerals are used to identify identical components in various views, Figures 1 and 2 illustrate a embryo / brake unit 10 according to the present invention. The unit 10 is provided for use in a riding mower, a garden tractor, or a similar vehicle to selectively transmit the torque of an input shaft to an implement driver (e.g. mower blades). The unit 10 is also provided to apply a positive braking torque to the implement driver when the torque is not being transmitted to the impeller. It should be understood that the unit 10 can be used in a wide variety of vehicles and applications that are not specifically set forth herein. The unit 10 includes a rotor assembly 12 disposed about a longitudinally extending arrow 14, an armature assembly 16 and a field cover assembly 18. The rotor assembly 12 may include an inlet hub 20 and a rotor disk 22. The armature assembly 16 may include an armature 24, an exit member 26 such as a pulley, and means, such as leaf springs 28, for connecting the member 26 to the armature 24. The field cover assembly 18 may include a field cover 30, a pair of permanent magnets 32, 34, a corresponding pair of pole pieces 36, 38 having relatively low magnetic reluctance, a pole piece 40 having a relatively high magnetic reluctance, and means , such as field coil 42, for generating a magnetic circuit between the field cover 30, the rotor disk 22 and the armature 24 in order to cause selective engagement of the armature 24 with the rotor disk 22. The input hub 20 is used in a conventional manner to provide a structural frame and structural support for the other components of the unit 10. The hub 20 can be made of a powdered metal. The hub 20 is disposed radially outwardly of the arrow 14, which extends longitudinally along an axis 44 and includes a portion 46 of reduced diameter. The hub 20 includes the key 48 extending axially and can be coupled to the arrow 14 by inserting the key 48 into an axially extending key passage (not shown) formed along the periphery of the portion 46 of the arrow 14. The unit 10 may also include a spacer 50 axially spaced from the portion 46 of the arrow 14 and plugged into the hub 20. The spacer 50 may be used to support the armature assembly 16 in an assembled relationship with the others components of the unit 10 and may be made of conventional materials including powdered metals. The separator 50 has a generally cylindrical external surface 52, a cylindrical through hole 54 and a flange 56 at a longitudinal end. The surface 52 may include one or more keyways 58 that extend axially to receive the key 48 from the hub 20. The hole 54 is configured to receive a screw thread 60 or other fastener means that can be screwed through the hole 54 and in an opening 62 in the portion 46 of the arrow 14 in order to hold the spacer 50, the hub 20, and a bearing 64, in assembled relationship with the arrow 14. An inner ball ring 66 of the bearing 64 is secured between a shoulder 68 of the hub 20 and a flange 56 of the spacer 50. The rotor disk 22 is provided for selective engagement with the armature 24 for transmitting the torque from the arrow 14 to the exit member 26. The disk 22 of the rotor can be made of conventional metals or metal alloys, including steel. The rotor disk 22 is connected to the arrow 14 by the inlet hub 20 and extends radially outwardly from the arrow 14. As set forth in the applicant's prior US Patent No. 5,285,882 (hereinafter "patent") "882") whose full description is incorporated herein by reference - the disk 22 may include a plurality of notches (not shown). A corresponding plurality of radially outwardly extending, angularly spaced tongues (not shown) on the hub 20 can be inserted into the notches and extended in order to create a tight engagement of the disc 22 and the hub 20. The disc 22 it includes an axially extending annular member 74 disposed about the radial periphery of the disc 22. The disc 22 also includes a plurality of radially spaced rows of angularly spaced slots 76, the purpose of which is set forth in more detail hereinafter. In the illustrated embodiment, the disk 22 includes three rows of slots 76. It should be understood, however, that the number of rows, the number of slots 76 in any row, and the size and shape of the slots 76 may vary. The armature 24 is provided to transmit a braking torque of the output member 26 and to selectively transmit a pulse torque from the arrow 14 to the member 26. The armature 24 may be made from a variety of conventional metals and alloys Metals that include steel. The armature 24 is disposed radially outwardly of the arrow 14 and axially spaced from the rotor disk 22 by an air space 78. Like the disk 22, the armature 24 includes a plurality of radially spaced rows of angularly spaced grooves 80, the purpose of which is described in greater detail hereinafter. In the illustrated embodiment, the armature 24 includes two rows of slots 80. The radially internal row of the slots 80 on the armature 24 is disposed between the radially internal and radially central rows of the slots 76 on the rotor disk 22. The radially outer row of the slots 80 on the armature 24 is disposed between the radially central and radially outer rows of the slots 76 on the disk 22. Again, it should be understood that the number of rows of slots 80 on the armature 24, the number of slots 80 in any row and the size and shape of the slots 80 may vary. The exit member 26 is provided to transmit the torque to an implement such as a mower blade (not shown). The exit member 26 may comprise a pulley and may include a band (not shown), which is connected to the blade of the mower or other driving device in a conventional manner. The exit member 26 can be made of a variety of conventional metals and metal alloys, including steel. The member 26 is supported for relative rotation with respect to the hub 20 by the bearing 64. The member 26 may include one or more shoulders 82 that help restrict the axial movement of the bearing 64. The leaf springs 28 are provided to transmit the torque of torsion of the armature 24 to the exit member 26. The springs 28 are also provided to allow axial movement of the armature 24 relative to the member 26 and towards and away from the rotor disk 22. The springs 28 can be made of stainless steel. The springs 28 are connected at one end of the armature 24 and at a second end to the member 26 using the rivets 84 and other clamped means. The field cover 30 is provided to receive the field coil 42. The cover 30 is also part of the magnetic circuit that causes the selective engagement of the disk 22 and the armature 24 as described in greater detail hereinafter. The field cover 30 can be made of conventional metals and metal alloys, which include steel. The cover 30_ is generally U-shaped in cross section and includes radially internal and radially outer annular members 86, 88. The inner member 86 is disposed adjacent to, and radially outward of, the input hub 20. The inner member 86 also rests on an outer ball collar 90 of a bearing 92 which is used to support the cover 30 while allowing the arrow 14 and the hub 20 to rotate inside the inner member 86 of the cover 30. The axial movement of the bearing 92 is restricted by a shoulder 94 of the arrow 14 on one axial end of the bearing 92 and by the hub 20 and a shoulder 98 of the inner member 86 on another axial end of the bearing 92. The outer member 88 of the cover 30 it is disposed radially outwardly of the member 74 of the disc 22. The cover 30 also includes a flange 100 which is integral with the outer member 88 and which extends radially outwardly. The fasteners (not shown) extend through the notches 102 of the flange 100 and secure the cover 30 to the engine block or other fixed structure to prevent rotation of the cover 30. The pole pieces 36, 38 provide a surface of braking for the armature 24. The pole pieces 36, 38 may be made of materials having a relatively low magnetic reluctance including conventional metals and metal alloys such as steel. The pole pieces 36, 38 can be connected to the flange 100 of the cover 30 by rivets 104 or other fastening means. A radially internal portion of each of the pole pieces 36, 38 overlaps a radially external portion of the armature 24. Referring now to Figure 3, the permanent magnets 32, 34 are provided to form a magnetic circuit 106 between the field cover 30, magnets 32, 34, pole pieces 36, 38 and armature 24. The magnets 32, 34 may be made of a ceramic material. The magnets 32, 34 are disposed between the pole pieces 36, 38, respectively, and the flange 100 and are secured therebetween by the rivets 104. A thin layer of plastic can be arranged on the radially inner and outer surfaces of the magnets 32, 34. In the illustrated embodiment there are two magnets 32, 34 and two corresponding pole pieces 36, 38. The number of magnets (and corresponding pole pieces) may vary. However, as will be evident from the following discussion, the magnets (and the pole pieces) must be arranged so that the angular distance occupied by the magnets (and the pole pieces) is less than half of the distance. cunference of field cover 30. The magnets 32, 34 are magnetized so that the magnets 32, 34 have axially facing poles of opposite polarity. In the illustrated embodiment, the north pole of the magnet 34 is axially closer to the flange 100 of the cover 30, while the south pole of the magnet 32 is axially closer to the flange 100. It should be understood, however, that the The polarity of the magnets 32, 34 may vary as long as similar poles of the adjacent magnets face in axially opposite directions. Within the magnetic circuit 106, the magnetic flux flows along the following path: magnet 32 - >; piece of pole 36 - > Armor 24 - > piece 38 of pole - > magnet 34 - > tab 100 of the field cover 30 - > magnet 32. As shown in Figure 1, whenever the coil 42 is de-energized, the circuit 106 pulls the armature 24 in a first axial direction away from the rotor disk 22 and to a braked position in engagement with the parts 36, 38 of polo. Referring now to Figure 2, the pole piece 40 is provided to allow the easier release of the armature 24 from the pole pieces 36, 38 as described in greater detail hereinafter. The pole piece 40 is disposed between the pole pieces 36, 38 and may be made of materials having a relatively high magnetic reluctance, such as stainless steel powder metal. The pole piece 40 may be axially farther from the armature 24 than the pole pieces 36, 38. The pole piece 40 is connected to the flange 100 of the field cover 30 by rivets 108, thread screws, or other fastening means. A radially outer portion of the pole piece 40 may have an axial length greater than the radially inner portion of the pole piece 40 overlapped by the frame 24. Referring now to Figure 4, the field coil 42 is conventional in the This technique is provided to generate a magnetic circuit 110 between the field cover 30, the rotor disk 22 and the armature 24 to cause the coupling of the rotor disk 22 and the armature 24 and thereby to transmit the torque from the rotor. arrow 14 of entry to exit member 26. The field coil 42 is generally annular and may be encapsulated within plastic. The coil 42 is disposed between the inner and outer members 86, 88 of the field cover 30 and can be connected electrically to a power supply (not shown) such as a vehicle battery. When the coil 42 is energized, the circuit 110 is formed between the field cover 30, the rotor disc 22 and the armature 24. The magnetic flux flows from the outer member 88 of the cover 30 through an air gap to the air. member 74 of rotor disc 22. The rows of the slots 76 on the disk 22 and the rows of the slots 80 on the armature 24 then make the flow travel back and forth between the disk 22 and the armature 24 through the air space 78 as shown. This arrangement allows a high torque coupling between the disk 22 and the armature 24 even when the axial distance of the space 78 is very large. Finally, the flow returns from the disk 22 to the inner member 86 of the cover 30. The circuit 110 pulls the armature 24 in a second axial direction towards the disk 22 and in a position engaged with the clutch. In particular, a portion 112 of the armature 24 angularly opposite the magnets 32, 34 is snap-coupled to the disk 22 first. As mentioned hereinbefore, the magnets 32, 34 (and the corresponding pole pieces 36, 38) preferably extend at an angular distance of less than half the circumference of the field cover 30. As a result, the magnetic flux within the circuit 106 travels through only a portion of the field cover 30 and the interference between the circuit 110 and the circuit 106 is minimized. In addition, the arrangement of the magnets 32, 34 and the pole pieces 36, 38 allow the portion 112 of the armature 24 to snap-fit with the disk 22. Referring now to Figure 5, as the portion 112 of the armature 24 engages the disk 22, a portion 114 of the armature 24 _s-e uncouples the pole pieces 36, 38 from one. In the illustrated embodiment, the armature 24 has been decoupled from the pole piece 38. In the embedding / brake unit described in the '918 patent, the armature 24 remains coupled with both parts 36, • 38 pole until sufficient magnetic force has been generated within the circuit 110 to decouple the armature 24. Although the portion 114 of the armature 24 rapidly engages by jump with the disk 22, it is desirable to further reduce the magnetic force required to make that the armature 24 is fully engaged with the disk 22. With the addition of the pole piece 40 between the pole pieces 36, 38, once the portion 112 of the armature 24 engages the disk 22, the armature 24 can remain in contact with only one of the pole pieces 36, 38, (in addition to the pole piece 40). The resulting air space between the armature 24 and the uncoupled pole piece 38 weakens or even cuts off the circuit 106. As a result, less electromagnetic force is required to uncouple the armature 24 from the pole piece 36 and to make the armor 24 is completely coupled with the disk 22. The unit 10 can, therefore, operate more efficiently. The addition of the pole piece 40 provides an additional advantage. In order to increase the durability and life of the pole pieces 36, 38, 40, it is desirable to place a protective coating, such as chromium carbide, on at least a portion of one or more of the parts 36, 38, 40 of pole. However, chromium carbide has high magnetic reluctance. Therefore, the application of the retreat to the pole pieces 36, 38 causes a significant reduction in the magnetic attraction between the armature 24 and the pole pieces 36, 38 and consequently, the braking torque. In the present invention, however, a coating 116 of chromium carbide can be applied to the pole piece 40 (as best shown in Figure 2) since it is not part of the magnetic circuit 106. The coating application 116 the pole piece 40 helps to reduce wear on the pole pieces 36, 38, 40, thereby increasing the durability and prolonging the life of the unit 10. Although the invention has been shown and described in particular with reference to the Preferred embodiments thereof, those skilled in the art will well understand that various changes and modifications may be made to the invention without departing from the spirit and scope of the invention.

Claims (20)

1. An embedment / brake unit, comprising: a rotor disk connected to an arrow for rotation therewith, the arrow extends along a first axis and the rotor disk extends radially outwardly from the arrow; an armature axially spaced from the rotor disk; an output member connected to the armature; a field cover axially spaced from the armature by the rotor disk, the field cover includes a radially outwardly extending flange, first and second pole pieces angularly spaced apart from one another and connected to the flange, the first and second pole pieces have a relatively low magnetic reluctance; first and second permanent magnets arranged between the flange and the first and second pole pieces, respectively, the field cover, the first and second permanent magnets, the first and second pole pieces and the armature form a first magnetic circuit that pulls the armature in a first axial direction away from the rotor disk and a first portion of the armature in engagement with the first and second pole pieces; a third piece of pole connected to the flange and disposed between the first and second pieces of the pole, the third piece of pole has a relatively high magnetic reluctance; and means for selectively generating a second magnetic circuit between the field cover, the rotor disk and the armature, the second magnetic circuit pulls the armature to a second axial direction towards the rotor disk where a second portion of the armature angularly Opposite the first portion engages the first disk of the rotor thereby forcing first, the first portion to be decoupled from one of the first and second pole pieces.

2. The embryo / brake unit according to claim 1, wherein the rotor disc includes a plurality of radially spaced rows of angularly spaced grooves.

3. The embryo / brake unit according to claim 1, wherein the armature includes a plurality of radially spaced rows of angularly spaced slots.

4. The embedment / brake unit according to claim 1, wherein the armature is connected to the exit member by a plurality of leaf springs.

5. The embryo / brake unit according to the rei indication 1, wherein the field cover includes a radially outer annular member and the rotor disc includes an annular member disposed radially inwardly of the annular member radially outwardly.

6. The embryo / brake unit according to claim 1, wherein the angular distance occupied by the first and second permanent magnets is less than the diameter of a circumference of the field cover.

7. The embryo / brake unit according to claim 1, wherein a north pole of the first permanent magnet faces the first axial direction and a north pole of the second permanent magnet faces the second axial direction.

The embryo / brake unit according to claim 1, wherein the third pole piece has a chromium carbide coating.

9. A clutch / brake unit, comprising: a rotor disk connected to an arrow for rotation therewith, the arrow extends along a first axis and the rotor disk extends radially outwardly from the arrow, the rotor disk includes a first plurality of radially spaced rows of angularly spaced grooves; an armature axially spaced from the rotor disk, the armature includes a second plurality of radially spaced rows of angularly spaced slots; an output member connected to the armature; a field cover axially spaced from the armature by the rotor disk, the field cover includes a flange extending radially outwardly; first and second pole pieces angularly spaced apart from one another and connected to the flange, the first and second pole pieces have a relatively low magnetic reluctance; first and second permanent magnets arranged between the flange and the first and second pole pieces, respectively, the field cover, the first and second permanent magnets, "the first and second pole pieces and the armature form a first magnetic circuit that pulls the armature in a first axial direction away from the rotor disk and a first portion of the armature in engagement with the first and second pole pieces, a third pole piece connected to the flange and disposed between the first and second parts of pole, the third pole piece has a relatively high magnetic reluctance, and means for selectively generating a second magnetic circuit between the field cover, the rotor disk and the armature, the second magnetic circuit pulls the armature to a second axial direction towards the rotor disc wherein a second portion of the armature angularly opposite the first portion couples the first disk of the rotor forcing thereby first, the first portion to be uncoupled from one of the first and second pole pieces.

10. The embedment / brake unit according to claim 9, wherein the armature is connected to the exit member by a plurality of leaf springs.

11. The embryo / brake unit according to the rei indication 9, wherein the angular distance occupied by the first and second permanent magnets is less than half a circumference of the field cover.

12. The brake / braking unit according to claim 9, wherein a north pole of the first permanent magnet faces the first axial direction and a north pole of the second permanent magnet faces the second axial direction.

13. The embryo / brake unit according to claim 9, wherein the third pole piece has a chromium carbide coating.

14. An embryo / brake unit, which comprises: an input cube connected to an arrow for rotation with it, the arrow extends along a first axis; a rotor disk disposed radially outwardly of the input hub and rotatably connected to the input hub; a groove axially spaced from the rotor disc; an exit member; means for connecting the output member to the armature; a field cover axially spaced from the armature by the rotor disk, the field cover includes a flange extending radially outwardly; first and second pole pieces angularly spaced from one another and connected to the flange, the first and second pole pieces have a relatively low magnetic reluctance; first and second permanent magnets arranged between the flange and the first and second pole pieces, respectively, the field cover, the first and second permanent magnets, the first and second pole pieces and the armature form a first magnetic circuit that pulls the armature in a first axial direction away from the rotor disk and a first portion of the armature in engagement with the first and second pole pieces; a third pole piece connected to the flange and disposed between the first and second pole pieces, the third pole piece has a relatively high magnetic reluctance; and means for selectively generating a second magnetic circuit between the field cover, the rotor disk and the armature, the second magnetic circuit pulls the armature to a second axial direction toward the rotor disk where a second portion of the armature is angularly opposite. the first portion engages the first disk of the rotor thereby forcing first, the first portion to be uncoupled from one of the first and second pole pieces.

15. The embryo / brake unit according to the rei indication 14, wherein the rotor disc includes a plurality of radially spaced rows of angularly spaced grooves.

16. The clutch / brake unit according to claim 14, wherein the armature includes a plurality of radially spaced rows of angularly spaced slots.

17. The embryo / brake unit according to rei indication 14, wherein the field cover includes a radially outer annular member and the rotor disc includes an annular member disposed radially inwardly of the annular member radially outwardly.

18. The embryo / brake unit according to the rei indication 14, wherein the angular distance occupied by the first and second permanent magnets is less than half a circumference of the field cover.

19. The embryo / brake unit according to claim 14, wherein a north pole of the first permanent magnet faces the first axial direction and a north pole of the second permanent magnet faces the second axial direction.

20. The embedment / brake unit according to claim 14, wherein the third pole piece has a chromium carbide coating. SUMMARY An improved brake / brake unit is provided for use on a riding mower or other vehicle. The unit includes a rotor assembly disposed radially outwardly of an inlet arrow extending longitudinally, an armor assembly and a field cover assembly. The rotor assembly includes a rotor disc disposed radially outwardly of the input shaft. The armature assembly includes an armature axially spaced from the rotor disk and an output member connected to the armature. The field cover assembly includes a field cover which is fixed against rotation, a pair of permanent magnets, first and second pole pieces having relatively low magnetic reluctance, a third pole piece having a relatively high magnetic reluctance - and a field coil. The magnetic cover includes a radially outwardly extending flange on which all the pole pieces are disposed. The magnets are disposed between the flange and the first and second pole pieces while the third pole piece is disposed between the first and second pole pieces. The field cover, the magnets, the first and second pole pieces and the armature form a magnetic circuit that pulls the armature in a first axial direction to a braking position in engagement with the first and second pole pieces. After energizing the field coil, a second magnetic circuit is formed between the field cover, the rotor disk and the armature. The second magnetic circuit pulls the armature in a second axial direction towards the rotor disk and into a position engaged with the brake. In particular, a portion of the armature angularly opposite the magnets engages the rotor disk first and forces the armature to disengage from one of the first and second pole pieces.